U.S. patent application number 15/511006 was filed with the patent office on 2018-04-19 for voltage-controlled resistive devices.
This patent application is currently assigned to Massachusetts Institute of Technology. The applicant listed for this patent is Massachusetts Institute of Technology. Invention is credited to Uwe Bauer, Geoffrey S.D. Beach.
Application Number | 20180108412 15/511006 |
Document ID | / |
Family ID | 59680043 |
Filed Date | 2018-04-19 |
United States Patent
Application |
20180108412 |
Kind Code |
A9 |
Bauer; Uwe ; et al. |
April 19, 2018 |
VOLTAGE-CONTROLLED RESISTIVE DEVICES
Abstract
Systems, methods, and apparatus are provided for tuning a
memristive property of a device. The device (500) includes a layer
of a dielectric material (507) disposed over and forming an
interface with a layer of an electrically conductive material
(506), and a gate electrode (508) disposed over the dielectric
material. The dielectric material layer includes at least one ionic
species (302) having a high ion mobility. The electrically
conductive material is configured such that a potential difference
applied to the device can cause the at least one ionic species to
migrate reversibly across the interface into or out of the
electrically conductive material layer, to modify the resistive
state of the electrically conductive material layer.
Inventors: |
Bauer; Uwe; (Cambridge,
MA) ; Beach; Geoffrey S.D.; (Winchester, MA) |
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Applicant: |
Name |
City |
State |
Country |
Type |
Massachusetts Institute of Technology |
Cambridge |
MA |
US |
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Assignee: |
Massachusetts Institute of
Technology
Cambridge
MA
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Prior
Publication: |
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Document Identifier |
Publication Date |
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US 20170249990 A1 |
August 31, 2017 |
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|
Family ID: |
59680043 |
Appl. No.: |
15/511006 |
Filed: |
September 11, 2015 |
PCT Filed: |
September 11, 2015 |
PCT NO: |
PCT/US2015/049667 PCKC 00 |
371 Date: |
March 13, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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PCT/US2015/020736 |
Mar 16, 2015 |
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15511006 |
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14659059 |
Mar 16, 2015 |
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PCT/US2015/049667 |
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61953689 |
Mar 14, 2014 |
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61953677 |
Mar 14, 2014 |
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62048830 |
Sep 11, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G11C 13/06 20130101;
G11C 19/0825 20130101; H01L 45/1206 20130101; G11C 13/0007
20130101; H01L 45/146 20130101; H01L 45/1253 20130101; H01L 45/14
20130101; G11C 2213/71 20130101; H01L 45/1226 20130101; G11C 11/161
20130101; G11C 11/1675 20130101; G11C 11/1697 20130101; G11C
13/0069 20130101; G11C 13/0009 20130101; B82Y 10/00 20130101; H01L
45/08 20130101; G11C 2213/15 20130101; G11C 2213/53 20130101; H01L
45/12 20130101 |
International
Class: |
G11C 13/06 20060101
G11C013/06 |
Goverment Interests
GOVERNMENT SUPPORT
[0005] This invention was made at least in part using government
support under contract nos. ECCS-1128439 and DMR-0819762, both
awarded by the National Science Foundation (NSF). The government
has certain rights in the invention.
Claims
1. A memristive element comprising: a conductive material layer
disposed in a x-y plane, the conductive material layer being
configured to reversibly uptake an amount of at least one ionic
species; a first electrode coupled proximate to a first end of the
conductive material layer; a second electrode coupled proximate to
a second end of the conductive material layer, opposite to the
first end; a gate dielectric layer disposed over the conductive
material layer, the gate dielectric layer being configured to
supply to, or receive from, the conductive material layer, an
amount of the at least one ionic species; and a gate electrode
layer disposed over, and in electrical communication with, the gate
dielectric material layer; an inert metal underlayer disposed in
electrical communication with the conductive material layer; the
gate electrode layer, the gate dielectric layer, and the conductive
material layer being configured such that: a first potential
difference applied in a first direction between the gate electrode
layer and the conductive material layer modifies a proportionate
amount of the at least one ionic species in a portion of the
conductive material layer to generate a first memristive state
comprising a first lateral resistive state between the first
electrode and the second electrode; a second potential difference
applied in a second direction between the gate electrode layer and
the conductive material layer modifies a proportionate amount of
the at least one ionic species in a portion of the conductive
material layer to generate a second memristive state comprising a
second lateral resistive state between the first electrode and the
second electrode that is different from the first lateral resistive
state; and the memristive element persists in the first memristive
state or the second memristive state in response to discontinuance
of the first potential difference or the second potential
difference, respectively.
2. The memristive element of claim 1, wherein the first memristive
state further comprises a first vertical resistive state between
the conductive material layer and the gate electrode layer, and
wherein the second memristive state further comprises a second
vertical resistive state between the conductive material layer and
the gate electrode layer that is different from the first vertical
resistive state.
3. The memristive element of claim 1, wherein the gate dielectric
layer is a bilayer comprising an ionic species storage layer
disposed over an ionic species transporting layer, and wherein the
ionic species transporting layer of the bilayer is disposed over
the conductive material layer.
4. The memristive element of claim 1, wherein the conductive
material layer has a first lateral dimension in a x-direction, and
wherein the gate electrode layer has a second lateral dimension in
the x-direction that is smaller than the first lateral
dimension.
5. The memristive element of claim 4, wherein the gate dielectric
layer has a third lateral dimension in the x-direction that
approximates the second lateral dimension.
6. The memristive element of claim 1, wherein the first lateral
dimension of the conductive material layer is configured to tune
the first memristive state and the second memristive state.
7. (canceled)
8. The memristive element of claim 1, wherein the thickness in a
z-direction of the inert metal underlayer is configured to cause a
greater proportion of current applied between the third electrode
and the fourth electrode to flow through the inert metal
underlayer, thereby modifying the first lateral resistive state and
the second lateral resistive state.
9. The memristive element of claim 1, wherein the thickness in a
z-direction of the inert metal underlayer is configured to cause a
smaller proportion of current applied between the third electrode
and the fourth electrode to flow through the inert metal
underlayer, thereby modifying the first lateral resistive state and
the second lateral resistive state.
10. The memristive element of claim 1, wherein a thickness of the
inert metal underlayer in a z-direction is configured to tune the
first memristive state and the second memristive state.
11. The memristive element of claim 1, further comprising: a third
electrode coupled to the first end of the conductive material
layer; and a fourth electrode coupled to the second end of the
conductive material layer, wherein a current is applied across the
third electrode and the fourth electrode.
12. The memristive element of claim 1, wherein the gate dielectric
layer is formed from an oxide, oxynitride, or silicate of a rare
earth metal or a transition metal, yttria-stabilized zirconia
(YSZ), or a gate oxide heterostructure.
13. The memristive element of claim 1, wherein the conductive
material layer has a longitudinal conformation in the x-y
plane.
14. The memristive element of claim 13, wherein the conductive
material layer comprises at least one nanostrip.
15. The memristive element of claim 14, wherein the at least one
nanostrip has a first end, a second end, and a central region,
wherein the first lateral dimension of the gate dielectric layer is
less than a length of the at least one nanostrip, and wherein the
gate dielectric layer is disposed over a portion of the central
region of the at least one nanostrip.
16. The memristive element of claim 1, wherein the conductive
material layer is disposed over at least one of: an electrically
conductive layer, at least one ferromagnetic material layer, at
least one oxide dielectric layer, a tunnel barrier layer, and an
integrated circuit.
17. The memristive element of claim 1, wherein: the gate dielectric
layer comprises two or more gate dielectric layers disposed over
spaced apart respective regions of the conductive material layer,
each having a respective lateral dimension in a x-direction that is
smaller than a first lateral dimension in the x-direction of the
conductive material layer; and the gate electrode layer comprises
two or more gate electrode layers, each disposed over, and in
electrical communication with, a respective gate dielectric layer
of the two or more gate dielectric layers, thereby providing a
multi-bit device.
18. The memristive element of claim 17, wherein each respective
lateral dimension has a differing value such that a potential
difference applied at each respective gate electrode modifies the
first memristive state and the second memristive state at each
respective region of the memristive element by a preselected
fractional amount, thereby providing a multi-bit storage
device.
19. The memristive element of claim 17, further comprising an inert
metal underlayer disposed in electrical communication with a
portion of the conductive material layer proximate to at least one
gate dielectric layer of the two or more gate dielectric layers,
wherein a thickness of the inert metal underlayer has a
predetermined value such that a potential difference applied at
each respective gate electrode modifies the first memristive state
and the second memristive state at each respective region of the
memristive element by a preselected fractional amount, thereby
providing a multi-bit storage device.
20. The memristive element of claim 17, wherein all bits of the
multi-bit device are addressed simultaneously by probing a
resistance state of the conductive material layer.
21. The memristive element of claim 17, wherein the conductive
material layer is disposed over at least one of: an electrically
conductive layer, at least one ferromagnetic material layer, at
least one oxide dielectric layer, a tunnel barrier layer, and an
integrated circuit.
22. The memristive element of claim 1, wherein the conductive
material layer comprises aluminum, a transition metal, a rare earth
metal, or an alloy of any of these materials.
23. An apparatus comprising: a conductive material layer disposed
on a substrate; an inert metal layer disposed in electrical contact
with the conductive material layer; first and second electrodes
disposed in contact with the conductive material layer; a gate
dielectric material layer comprising at least one ionic species,
the gate dielectric material layer being electrically in contact
with the conductive material layer; and a gate electrode disposed
in electrical contact with the gate dielectric material layer.
24. The apparatus of claim 23, wherein the inert metal layer
comprises a noble metal.
25. The apparatus of claim 23, wherein first and second electrodes
are disposed at opposed ends of the conductive material layer.
26. A method of operating a memristive device, the method
comprising: applying a potential difference at a region of a gate
electrode and a transition metal layer of a memristive device, the
memristive device comprising: the transition metal layer disposed
in a x-y plane, the transition metal layer having a first lateral
dimension in the x-direction and being configured to reversibly
uptake an amount of at least one ionic species; a first electrode
coupled proximate to a first end of the transition metal layer; a
second electrode coupled proximate to a second end of the
transition metal layer, opposite to the first end; an inert metal
underlayer disposed in electrical communication with the transition
metal layer; a gate oxide dielectric layer comprising a rare earth
oxide or a transition metal oxide, the gate oxide dielectric layer
being disposed over the transition metal layer, the gate oxide
dielectric layer having a second lateral dimension in the
x-direction that is smaller than the first lateral dimension, and
the gate oxide dielectric layer being configured to supply to, or
receive from, the transition metal layer an amount of the at least
one ionic species; and the gate electrode layer disposed over, and
in electrical communication with, the gate oxide dielectric
material layer; measuring a lateral resistive state between the
first electrode and the second electrode; wherein the gate
electrode layer, the gate oxide dielectric layer, and the
transition metal layer are configured such that: the measuring
indicates a first vertical resistive state and a first lateral
resistive state in response to applying a first potential
difference in a first direction; and the measuring indicates a
second vertical resistive state that is different from the first
vertical resistive state and a second lateral resistive state that
is different from the first lateral resistive state in response to
applying a second potential difference in a second direction that
is opposite to the first direction.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional
Application No. 62/048,830, filed on Sep. 11, 2014, entitled
"LATERAL VOLTAGE-CONTROLLED PERSISTENT RESISTIVE SWITCH AND
APPLICATIONS AS TUNABLE LATERAL MEMORY RESISTOR DEVICE," which is
hereby incorporated herein by reference in its entirety.
[0002] This application also claims priority to and benefit of U.S.
Non-provisional application Ser. No. 14/659,059, filed on Mar. 16,
2015, which claims priority to U.S. Provisional Application No.
61/953,677, filed on Mar. 14, 2014.
[0003] This application also claims priority to and benefit of
International Application No. PCT/US2015/020736, filed on Mar. 16,
2015, which claims priority to U.S. Provisional Application No.
61/953,689, filed on Mar. 14, 2014.
[0004] The entire disclosure of each of these applications is
incorporated herein by reference in its entirety, including
drawings.
BACKGROUND
[0006] Memristors or "memory resistors" are nanoscale ionic systems
that often rely on ion-migration-induced resistance changes in thin
oxide films for their nonvolatile memory functionality. Memristive
switching devices are of great interest in computer technology due
to their potential integration into next generation nonvolatile
memories. Memristor technology is nonvolatile, scalable down to
less than 10 nm, and offers low-power nanosecond-timescale
switching.
SUMMARY
[0007] The Inventors have recognized and appreciated that a novel
type of memristor would be beneficial. In view of the foregoing,
various embodiments are directed generally to methods, apparatus,
and systems for providing novel memristive elements and memristive
devices based on such memristive elements.
[0008] Example methods, apparatus, and systems are described for
providing an example memristive element that includes a conductive
material layer disposed in the x-y plane, a first electrode coupled
proximate to a first end of the conductive material layer, a second
electrode coupled proximate to a second end of the conductive
material layer, opposite to the first end, a gate dielectric layer
disposed over the conductive material layer, and a gate electrode
layer disposed over, and in electrical communication with, the gate
dielectric material layer. The conductive material layer is
configured to reversibly uptake an amount of at least one ionic
species. The gate dielectric layer is configured to supply to, or
receive from, the conductive material layer, an amount of the at
least one ionic species. The gate electrode layer, the gate
dielectric layer, and the conductive material layer are configured
such that a first potential difference applied in a first direction
between the gate electrode layer and the conductive material layer
modifies a proportionate amount of the at least one ionic species
in a portion of the conductive material layer to generate a first
memristive state including a first lateral resistive state between
the first electrode and the second electrode, and a second
potential difference applied in a second direction between the gate
electrode layer and the conductive material layer modifies a
proportionate amount of the at least one ionic species in a portion
of the conductive material layer to generate a second memristive
state comprising a second lateral resistive state between the first
electrode and the second electrode that is different from the first
lateral resistive state. The memristive element persists in the
first memristive state or the second memristive state in response
to discontinuance of the first potential difference or the second
potential difference, respectively.
[0009] In an example, the first memristive state can further
include a first vertical resistive state between the conductive
material layer and the gate electrode layer, and the second
memristive state can further include a second vertical resistive
state between the conductive material layer and the gate electrode
layer that is different from the first vertical resistive
state.
[0010] In an example, the conductive material layer can include
aluminum, a transition metal, a rare earth metal, or an alloy of
any of these materials.
[0011] In an example, the gate dielectric layer is a bilayer that
includes an ionic species storage layer disposed over an ionic
species transporting layer, and the ionic species transporting
layer of the bilayer is disposed over the conductive material
layer.
[0012] In an example, the conductive material layer has a first
lateral dimension in the x-direction, and the gate electrode layer
has a second lateral dimension in the x-direction that is smaller
than the first lateral dimension. The gate dielectric layer can
have a third lateral dimension in the x-direction that approximates
the second lateral dimension of the gate electrode layer.
[0013] In an example, the relative lateral dimensions of the
conductive material layer, gate dielectric layer, and the gate
electrode layer are configured to tune the first memristive state
and the second memristive state.
[0014] An example memristive element herein can further include an
inert metal underlayer disposed in electrical communication with
the conductive material layer. The thickness of the inert metal
underlayer relative to the thickness of the conductive material
layer can be configured to tune the first memristive state and the
second memristive state.
[0015] In an example, the thickness in a z-direction of the inert
metal underlayer can be configured to cause a greater proportion of
current applied between the third electrode and the fourth
electrode to flow through the inert metal underlayer, thereby
modifying the first lateral resistive state and the second lateral
resistive state.
[0016] In an example, the thickness in a z-direction of the inert
metal underlayer can be configured to cause a smaller proportion of
current applied between the third electrode and the fourth
electrode to flow through the inert metal underlayer, thereby
modifying the first lateral resistive state and the second lateral
resistive state.
[0017] An example memristive element herein can further include a
third electrode coupled to the first end of the conductive material
layer and a fourth electrode coupled to the second end of the
conductive material layer, such that a current can be applied
across the third electrode and the fourth electrode. Such an
example configuration allows four-point measurements of the first
lateral resistive state and the second lateral resistive state.
[0018] In an example, the gate dielectric layer is formed from an
oxide, oxynitride, or silicate of a rare earth metal or a
transition metal, yttria-stabilized zirconia (YSZ), or a gate oxide
hetero structure.
[0019] In an example, the conductive material layer can have a
longitudinal conformation in the x-y plane, such as but not limited
to being formed as a nanostrip.
[0020] Example memristive elements herein can be formed as an
example multi-bit device. In an example multi-bit device, the gate
dielectric layer can include two or more gate dielectric layers
disposed over spaced apart respective regions of the conductive
material layer, each having a respective lateral dimension in a
x-direction that is smaller than a first lateral dimension in the
x-direction of the conductive material layer. The gate electrode
layer of the multi-bit device can include two or more gate
electrode layers, each disposed over, and in electrical
communication with, a respective gate dielectric layer of the two
or more gate dielectric layers.
[0021] In an example, all bits of the example multi-bit device can
be addressed simultaneously by probing a resistance state of the
conductive material layer.
[0022] In an example, each respective lateral dimension of the two
or more gate dielectric layers can have a differing value, such
that a potential difference applied at each respective gate
electrode modifies the first memristive state and the second
memristive state at each respective region of the memristive
element by a preselected fractional amount.
[0023] In an example, the example multi-bit device can further
include an inert metal underlayer disposed in electrical
communication with a portion of the conductive material layer
proximate to at least one gate dielectric layer of the two or more
gate dielectric layers. The thickness of the inert metal underlayer
can have a predetermined value such that a potential difference
applied at each respective gate electrode modifies the first
memristive state and the second memristive state at each respective
region of the memristive element by a preselected fractional
amount.
[0024] In an example, the conductive material layer of the example
multi-bit device can be disposed over at least one of: an
electrically conductive layer, at least one ferromagnetic material
layer, at least one oxide dielectric layer, a tunnel barrier layer,
and an integrated circuit.
[0025] Example methods, apparatus, and systems are described for
operating a memristive device. The method can include applying a
potential difference at a region of a gate electrode and a
conductive material layer of the memristive device, and measuring a
lateral resistive state between the first electrode and the second
electrode. The memristive device includes a conductive material
layer disposed in an x-y plane, a first electrode coupled proximate
to a first end of the conductive material layer, a second electrode
coupled proximate to a second end of the conductive material layer,
opposite to the first end, a gate dielectric layer disposed over
the conductive material layer, and a gate electrode layer disposed
over, and in electrical communication with, the gate dielectric
material layer. The conductive material layer is configured to
reversibly uptake an amount of at least one ionic species. The gate
dielectric layer is configured to supply to, or receive from, the
conductive material layer an amount of the at least one ionic
species. The gate electrode layer, the gate dielectric layer, and
the conductive material layer are configured such that the
measuring indicates a first lateral resistive state in response to
applying a first potential difference in a first direction, and the
measuring indicates a second lateral resistive state that is
different from the first lateral resistive state in response to
applying a second potential difference in a second direction that
is opposite to the first direction.
[0026] In an example, the conductive material layer has a first
lateral dimension in a x-y plane, and the gate electrode layer has
a second lateral dimension in the x-y plane that is smaller than
the first lateral dimension.
[0027] In an example, the memristive device can further includes an
inert metal underlayer disposed in electrical communication with
the conductive material layer.
[0028] Example methods, apparatus, and systems also are described
for programming information to a memristive device. The method
includes applying a potential difference at a region of a gate
electrode and a transition metal layer of a memristive device, and
measuring a lateral resistive state between the first electrode and
the second electrode. The memristive device includes a transition
metal layer disposed in an x-y plane and having a first lateral
dimension in the x-direction, a first electrode coupled proximate
to a first end of the transition metal layer, a second electrode
coupled proximate to a second end of the transition metal layer,
opposite to the first end, a gate oxide dielectric layer disposed
over the transition metal layer, and a gate electrode layer
disposed over, and in electrical communication with, the gate oxide
dielectric material layer. The gate oxide dielectric layer includes
a rare earth oxide or a transition metal oxide, and has a second
lateral dimension in the x-direction that is smaller than the first
lateral dimension of the transition metal layer. The transition
metal layer is configured to reversibly uptake an amount of at
least one ionic species. The gate oxide dielectric layer is
configured to supply to, or receive from, the conductive material
layer an amount of the at least one ionic species. The gate
electrode layer, the gate oxide dielectric layer, and the
conductive material layer are configured such that the measuring
indicates a first vertical resistive state and a first lateral
resistive state in response to applying a first potential
difference, and the measuring indicates a second vertical resistive
state that is different from the first vertical resistive state and
a second lateral resistive state that is different from the first
lateral resistive state in response to applying a second potential
difference in a second direction that is opposite to the first
direction.
[0029] It should be appreciated that all combinations of the
foregoing concepts and additional concepts discussed in greater
detail below (provided such concepts are not mutually inconsistent)
are contemplated as being part of the inventive subject matter
disclosed herein. In particular, all combinations of claimed
subject matter appearing at the end of this disclosure are
contemplated as being part of the inventive subject matter
disclosed herein. It should also be appreciated that terminology
explicitly employed herein that also may appear in any disclosure
incorporated by reference should be accorded a meaning most
consistent with the particular concepts disclosed herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] The skilled artisan will understand that the drawings
primarily are for illustrative purposes and are not intended to
limit the scope of the inventive subject matter described herein.
The drawings are not necessarily to scale; in some instances,
various aspects of the inventive subject matter disclosed herein
may be shown exaggerated or enlarged in the drawings to facilitate
an understanding of different features. In the drawings, like
reference characters generally refer to like features (e.g.,
functionally similar and/or structurally similar elements).
[0031] FIGS. 1A-1B show conventional memristive devices, according
to principles of the present disclosure.
[0032] FIGS. 2A-2D show examples of memristive switching in a
memristor, according to principles of the present disclosure.
[0033] FIGS. 3A-3C show schematic representations of an example
device structure, according to principles of the present
disclosure.
[0034] FIG. 4A shows the layer structure of a conventional
memristive device, according to principles of the present
disclosure.
[0035] FIG. 4B shows the layer structure of an example
magneto-electric device, according to principles of the present
disclosure.
[0036] FIGS. 5A-5D show example memristive switching devices,
according to principles of the present disclosure.
[0037] FIGS. 6A-6F show the cross section of example devices that
can be implemented as memristive devices, according to principles
of the present disclosure.
[0038] FIGS. 7A and 7B illustrate two different cross-sectional
geometries of example two-terminal configurations, according to
principles of the present disclosure.
[0039] FIG. 7C illustrates an example three-terminal configuration,
according to principles of the present disclosure.
[0040] FIGS. 8A and 8B illustrate example two-dimensional arrays,
according to principles of the present disclosure.
[0041] FIG. 9 illustrates an example three-dimensional, multi-layer
array, according to principles of the present disclosure.
[0042] FIG. 10 shows an example memristive device including a
nanostrip, according to principles of the present disclosure.
[0043] FIGS. 11A-11B show plots of measurements of an example
memristive device, according to principles of the present
disclosure.
[0044] FIG. 12 shows a plot of measurements of electroforming of an
example memristive device via constant current stress (CCS),
according to principles of the present disclosure.
[0045] FIGS. 13A-13B show example of memristive switching in
example memristive devices, according to principles of the present
disclosure.
[0046] FIGS. 14A-14D show schematics of memristive switching
mechanism in example memristive devices, according to principles of
the present disclosure.
[0047] FIGS. 15A-15D schematically show an example of lateral
resistive switching in an example memristive device, according to
principles of the present disclosure.
[0048] FIGS. 16A-16C show the plots of results of computation of
lateral resistive switching
[0049] FIGS. 17A-17D shows a schematic illustration of memristive
properties of an example device, according to principles of the
present disclosure.
[0050] FIGS. 18A-18F show plots of the use of gate voltages for
control and programming of properties of example devices, according
to principles of the present disclosure.
DETAILED DESCRIPTION
[0051] Following below are more detailed descriptions of various
concepts related to, and embodiments of, inventive methods,
apparatus, and systems for novel devices based on controlled
resistive states. It should be appreciated that various concepts
introduced above and discussed in greater detail below may be
implemented in any of numerous ways, as the disclosed concepts are
not limited to any particular manner of implementation. Examples of
specific implementations and applications are provided primarily
for illustrative purposes.
[0052] As used herein, the term "includes" means includes but is
not limited to, the term "including" means including but not
limited to. The term "based on" means based at least in part
on.
[0053] With respect to layers, substrates or other surfaces
described herein in connection with various examples of the
principles herein, any references to "top" surface and "bottom"
surface are used primarily to indicate relative position, alignment
and/or orientation of various elements/components with respect to
the substrate and each other, and these terms do not necessarily
indicate any particular frame of reference (e.g., a gravitational
frame of reference). Thus, reference to a "bottom" of a substrate
or a layer does not necessarily require that the indicated surface
or layer be facing a ground surface. Similarly, terms such as
"over," "under," "above," "beneath," "underneath" and the like do
not necessarily indicate any particular frame of reference, such as
a gravitational frame of reference, but rather are used primarily
to indicate relative position, alignment and/or orientation of
various elements/components with respect to the substrate or layer
(or other surface) and each other. The terms "disposed on" and
"disposed over" encompass the meaning of "embedded in," including
"partially embedded in." In addition, reference to feature A being
"disposed on," "disposed between," or "disposed over" feature B
encompasses examples where feature A is in contact with feature B,
as well as examples where other layers and/or other components are
positioned between feature A and feature B.
[0054] The emerging field of nanoionics covers a wide range of
phenomena that result from ionic migration in solid-state nanoscale
systems. Similar to quantum confinement effects in nanoelectronics,
ionic transport is drastically changed when the material dimensions
are reduced to 10 s of nanometers. At this length scale, the
barriers for ionic transport are greatly reduced and interface
effects can become prevalent and dominate the ionic properties of
the whole material. These effects can markedly enhance ionic
conductivity, turn materials that are ionic insulators in the bulk
into good ionic conductors, and significantly reduce the
operational temperature of solid-state ionic devices.
[0055] The crossover from macroscale to nanoscale systems is
characterized by the emergence of interface effects. In nanoscale
materials, many physical and chemical properties are influenced by
the structure and composition of their interfaces. For instance, in
metal/metal-oxide heterostructures, chemical, electronic, magnetic,
and mechanical properties can emerge from interfacial oxygen
stoichiometry and defect structure. Example systems, methods,
apparatus and devices described herein allow the dynamic control of
these interface characteristics with an electric field, which can
pave the way towards voltage control of these properties in
solid-state devices. Example systems, methods, apparatus and
devices described herein demonstrate that interfacial chemistry in
metal/metal-oxide bilayers can indeed be electrically-gated using
all-solid-state devices, operating at low voltage and within the
typical operating temperature range of common semiconductor
electronics.
[0056] Metal/oxide/metal devices are pervasive in solid state
ionics and form the common basis of a wide range of applications,
from oxygen sensors, to solid oxide fuel cells, and memristive
switching memories. Memristive switching memories or memristors are
true nanoscale ionic systems and typically rely on
ion-migration-induced resistance changes in thin oxide films for
their nonvolatile memory functionality.
[0057] In nanoscale materials, many functional and structural
properties can be determined by the structure and/or composition at
the interfaces. If the dimensionality of a material is reduced and
it is scaled down from the macroscale to the nanoscale, the
structure and composition of interfaces in the material, rather
than the bulk portions of the material, can exert much greater
influence on functional and/or structural properties.
[0058] As a non-limiting example, materials formed with
metal/metal-oxide bilayers are of high interest for technological
applications due to their wide applicability. Metal/metal-oxide
bilayers have a wide range of applications, from catalysts to
coatings to semiconductor electronics. For example, in
metal/metal-oxide bilayers, physical and chemical properties can
depend strongly on oxygen stoichiometry and defect structure at the
metal/oxide interface. Some of these properties that can be
controlled and modulated by metal/metal-oxide interface include
catalytic activity, charge transport, ionic exchange, magnetic
properties, electrical properties, thermal conduction, and
mechanical behavior.
[0059] Example systems, methods, apparatus and devices according to
the principles herein provide capabilities to control the oxygen
stoichiometry at the metal oxide interface through application of a
gate voltage. Through these capabilities, properties such as, but
not limited to, catalytic activity, charge transport, ionic
exchange, magnetic properties, electrical properties, thermal
conduction, and mechanical behavior, can be controlled
electrically. The example systems, methods, apparatus and devices
herein can be integrate them into solid state devices.
[0060] A memristor (memory resistor) is a two terminal device that
can remain in either a high resistance state (R.sub.high) or a low
resistance state depending on the history of applied voltage and
current. Some existing memristive switching devices are formed from
two electrodes separated by a thin insulating layer. FIG. 1A shows
an example memristive device that includes a first electrode 110
and a second electrode 112, with an insulating layer 114 disposed
between electrodes 110 and 112. Many conventional memristive
devices are formed as a vertical stack of a bottom electrode layer
formed from platinum (Pt), an insulating layer of titanium dioxide
(TiO.sub.2), and a top electrode layer formed from platinum (Pt).
In the example of FIG. 1A, the memristive device can include a
substrate formed from silicon dioxide (Si0.sub.2) 116 over silicon
(Si) 118. Lateral geometries with two laterally separated
electrodes on top of an insulating layer have also been explored.
FIG. 1B shows an example of a memristive device with laterally
separated electrodes 160 and 162 formed from platinum, and an
insulating layer 164 formed from TiO.sub.2.
[0061] For both of the device geometries in FIGS. 1A and 1B, the
resistance state of the device is given by the resistance across
the insulating layer. For the memristive switching, application of
a voltage (Vg) or current across the insulating layer typically
results in the formation of a conductive filament 166 through the
insulating layer 164. In this example device, the filament is
formed from Ti.sub.4O.sub.7. The application of the voltage induces
migration of O.sup.2- ions and oxygen vacancies within the
insulating layer. Application of the voltage in a first direction
causes formation of the conducting filament, while application of
the voltage in a second direction breaks the conducting filament.
Conductive filament formation is a statistical process which is
hard to control, difficult to predict, and usually occurs under
condition close to dielectric breakdown of the insulating layer.
After the filament is established, the resistance state of the
device is then controlled by breaking and reestablishing the
filament (see FIG. 1B). Many such memristive switching devices are
formed using metal electrodes and oxide insulators.
[0062] FIGS. 2A-2D show examples of memristive switching in an
example device formed as a metal/oxide/metal memristor. A schematic
illustration of bipolar memristive switching in the
metal/oxide/metal memristor is provided. As shown in FIG. 2A, the
example memristive switching device is formed from two metal
electrodes 160, 162 separated by an oxide insulator 164. FIG. 2A
also shows the example memristive switching device in its virgin
state (i.e., prior to the application of a high bias voltage). FIG.
2B shows the conductive filament 166 that can form with application
of a high bias voltage (Vg>0).
[0063] The high bias voltage Vg results in the formation of the
conductive filament 166 between the two electrodes 160, 162. This
process is also referred to as electroforming. FIGS. 2C-2D show the
state of the conductive filament with further cycling of the
polarity of the voltage, from negative (Vg<0) to positive
(Vg>0). Depending on the bias polarity, the conductive filament
166 can be broken (FIG. 2C) or reestablished (FIG. 2D). This causes
switching of the resistance state of the device from high R.sub.low
to low R.sub.high and vice versa (i.e., from low R.sub.high to high
R.sub.low). Breaking and reestablishing the conductive filament
results in the characteristic hysteretic switching of the
resistance state. Memristors can be categorized by their
current-voltage characteristics (i.e., as bipolar or unipolar) and
active ionic species (anion or cation).
[0064] Similar to many existing devices, the memristors of FIGS.
1A-1B and 2A-2D are configured to prevent, or significantly reduce
the possibility of, migration of ionic species from the dielectric
material layer to an adjacent electrically conductive layer. The
migration of ionic species into any portion of an adjacent
electrically conductive layer can be a breakdown mechanism of a
device, such as a shorting. For example, diffusion barriers may be
used in these devices to prevent such ionic species migration. As
another example, the electrically conductive material layer can be
formed from a conductive material that is not conducive to ionic
species migration (such as platinum), or that reduces or prevents
the ionic species migration in normal operation (such as noble
metals). The electrically conductive layer could also be made of
other conducting material that do not strongly interact or react
with the mobile ionic species in the dielectric material layer. In
some cases, one of the terminals of the device could be replaced
with a conductor in close proximity to the device multilayer
structure, such as the tip of a scanning probe microscope.
[0065] Electric-field-driven ion transport can be exploited in
other a range of electrochemical devices, such as oxygen gas
sensors and solid oxide fuel cells. At the nanoscale,
voltage-induced O.sup.2- migration is a mechanism for resistive
switching in anionic metal/oxide/metal memristors.
[0066] The oxygen anion O.sup.2- , or equivalently, the positively
charged oxygen vacancy VO.sup.2+, is the mobile ionic species.
These devices typically rely on nanoscale metal-oxide insulators. A
wide range of metal-oxides have been explored in memristive
devices, such as magnesium oxide MgO, titanium oxide TiOx, tantalum
oxide TaOx and aluminum oxide AlOx. Memristive switching can
originate from defects in the material and not from a particular
electronic band structure. Many different oxide materials exhibit
memristive switching.
[0067] Example systems, methods, apparatus and devices of the
instant disclosure exploit voltage-driven oxygen transport to
control interfacial properties and phenomena in metal/metal-oxide
structures. According to the principles herein, the voltage-gated
control of oxygen stoichiometry allow control over not only the
magnetic properties of devices, and also is used to switch the
vertical and lateral electrical resistance in the devices. As a
non-limiting example, the devices can be based on a
transition-metal/metal-oxide bilayer structure. The example
systems, methods, apparatus and devices demonstrate the broad
applicability of solid-state switching of interface oxygen
chemistry to control material properties.
[0068] Applicants have developed novel devices, and systems,
methods and apparatus incorporating such example devices, that
exploit the reversible migration of ionic species from a dielectric
material layer to an adjacent electrically conductive layer to
regulate (and tune) the resistive state of the example devices. An
example device herein can provide for control one or both of a
vertical resistive state and a lateral resistive state.
[0069] In contrast to many existing memristors, including the
memristors of FIGS. 1A-1B and 2A-2D, an example memristive
switching device according to the principles herein is formed as a
three-terminal devices or a device with greater than three
terminals. At least two of the terminals can be used to drive a
current through an electrically conductive material layer. An
electronically insulating dielectric material layer is disposed
over and covers at least a portion of the electrically conductive
material layer. The dielectric material layer simultaneously acts
as a good ionic conductor. At least one gate electrode layer
disposed over at least a portion of the dielectric material layer
serves as the third terminal of the example memristive switching
device.
[0070] FIGS. 3A-3C show schematic representations of an example
device structure and oxygen ion motion in the device under
differing gate voltages (positive and negative gave voltages). The
example device of FIG. 3A includes a conductive material layer
(M.sub.C) that forms an interface with a dielectric material layer.
In any example, the conductive material layer (M.sub.C) can be
formed from a transition metal material, including a ferromagnetic
material. As described in greater detail hereinbelow, the
conductive material layer can include transition metals, such as
but not limited to any one or more of copper, tantalum, tin,
tungsten, titanium, tungsten, cobalt, chromium, silver, nickel,
iron, nickel, cobalt, samarium, dysprosium, yttrium, or chromium,
an alloy of one or more transition metals, or an alloy of one or
more rare earth metals, or an alloy that includes at least one
transition metal and at least one rare earth metal. The example
dielectric material layer in this example includes cations
(C.sup.x+) and oxide ions (O.sup.2-). The example device includes a
gate electrode layer (M.sub.G), which can include a noble metal, a
transition metal, or any other conductive material as described
herein. As shown in FIG. 3B, with a non-zero potential difference
applied in a first direction (a negative bias, V.sub.g<0), an
amount of the ionic species (indicated at 302) migrates into
portions of the conductive material layer (M.sub.C) proximate to
the interface. In this non-limiting example, the ionic species are
oxide ions. That is, the negative bias moves oxygen ions towards
the conductive material-dielectric oxide interface in this example.
As shown in FIG. 3C, with a non-zero potential difference applied
in a second direction that is opposite the first direction (a
positive bias, V.sub.g>0), the oxide ions that had migrated into
portions of the conductive material layer (M.sub.C) are returned to
the dielectric material layer. The positive bias moves ions away
from the interface. Accordingly, FIGS. 3A-3C illustrate the
reversible migration of the ionic species from a dielectric
material layer to the adjacent conductive material layer of an
example device.
[0071] In an example device according to the principles herein, the
conductive material layer is kept sufficiently thin, such that the
functional properties of the conductive material in the conductive
material-dielectric material bilayer is sensitive to the oxygen
stoichiometry at the interface. The dielectric material used in any
of the examples herein are a high-k dielectric materials that
includes an ionic species having a high vacancy mobility. In the
non-limiting example of FIG. 3A-3C, the high-k dielectric material
is an oxygen ion conductor with high oxygen vacancy mobility.
Application of a gate voltage across the interface results in
motion of oxygen ions in the dielectric oxide material layer. This
in turn modifies the oxygen stoichiometry at the conductive
material-dielectric material interface, and therefore changes
properties such as, but not limited to, the resistivity of the
conductive material film. For example, as illustrated in FIGS.
3A-3C, under a gate voltage in a first direction, oxygen ions move
away from the conductive material-dielectric oxide material
interface. Under a gate voltage of an opposite bias, the oxygen
ions migrate towards the conductive material-dielectric oxide
material interface. According to the example systems, methods, and
apparatus herein, the modification of the functional properties of
the conductive material film can be tuned by regulating the oxygen
stoichiometry at the conductive material-dielectric material
interface, by controlling parameters such as but not limited to,
the polarity and/or magnitude and/or dwell time of the gate
voltage, and/or the mobility of the ionic species.
[0072] FIG. 4A shows the layer structure of a conventional
memristive device 400, which includes gate electrode 402, bottom
electrode 404, and an insulating oxide layer disposed between these
two electrodes. The gate electrode 402 and bottom electrode 404 are
typically formed from a noble metal or other electrically
conductive material that is configured to prevent, or significantly
reduce the possibility of migration of ionic species from the
insulating oxide layer. The thickness of the insulating oxide layer
can range from about 10 nm to about 100 nm. Applied potential
difference (Vg) generates electric fields ( ) of about 0.1 V/m. As
described above, the switching operation of the memristive device
of FIG. 4A is based on formation and breaking of a conductive
filament.
[0073] FIG. 4B shows the layer structure of an example
magneto-electric device 450, which includes gate electrode 452,
bottom electrode 454, and a dielectric material layer 456 disposed
between these two electrodes 452 and 454, and a conductive material
layer 458 disposed between the bottom electrode 454 and forms an
interface with the dielectric material layer 456. The gate
electrode 402 and bottom electrode 404 also can be formed from a
noble metal. Conductive material layer 458 is configured to allow
migration of a proportionate amount of ionic species from the
dielectric material layer 456 to at least a portion of the
conductive material layer 458. Examples of such devices are
described in U.S. Non-provisional application Ser. No. 14/659,059
and International Application No. PCT/US2015/020736, both filed on
Mar. 16, 2015. In various non-limiting examples, the thickness of
the insulating oxide layer can range from about 1.0 nm to about 100
nm, or greater. In an example, the conductive material layer 458
can be formed as a thin ferromagnetic (FM) metal layer. The applied
potential difference (Vg) can generates electric fields ( ) ranging
from about 0.1 V/m to about 1.0 V/m. The magneto-electric effect
can be controlled through use of a voltage to cause migration of
proportionate amount of ionic species from the dielectric material
layer 456 to at least a portion of the conductive material layer
458.
[0074] FIG. 5A shows an example memristive switching device 500
according to the principles herein, formed as a three-terminal
device. Terminals 502 and 504 can be used to measure a lateral
resistive state across the electrically conductive material
(M.sub.C) layer 506. In non-limiting examples, terminals 502 and
504 can be used to drive a current through conductive material
(M.sub.C) layer 506 and/or apply a voltage across conductive
material (M.sub.C) layer 506. In any example, the conductive
material layer (M.sub.C) can be formed from a transition metal
material, including a ferromagnetic material. As described in
greater detail hereinbelow, the conductive material layer can
include transition metals, such as but not limited to any one or
more of copper, tantalum, tin, tungsten, titanium, tungsten,
cobalt, chromium, silver, nickel, iron, nickel, cobalt, samarium,
dysprosium, yttrium, or chromium, an alloy of one or more
transition metals, or an alloy of one or more rare earth metals, or
an alloy that includes at least one transition metal and at least
one rare earth metal. The conductive material is at least partially
covered by an electronically insulating dielectric material layer
507 which includes cations (C.sup.x+) and anions (A.sup.y-). As a
non-limiting example, the anions (A.sup.y-) can be oxide ions
(O.sup.2-). The electronically insulating dielectric material layer
also acts as a good ionic conductor. The gate electrode (M.sub.G)
disposed over the insulating layer provides the third terminal 508
of the device. As shown in FIG. 5A, with a non-zero potential
difference applied in a first direction across the conductive
material (M.sub.C) layer 506 and the gate electrode 508, an amount
of the ionic species (indicated at 302) migrates into portions of
the conductive material layer (M.sub.C) 506. With a non-zero
potential difference applied in a second direction (opposite to the
first direction) across the conductive material (M.sub.C) layer 506
and the gate electrode 508, an amount of the ionic species
(indicated at 302) migrates from the conductive material layer
(M.sub.C) to the dielectric material layer 507. The resulting
change in the measured value of lateral resistance of the lateral
resistive state (R.sub.L) can be probed across a segment of the
electrically conductive material (M.sub.C) layer using terminals
502 and 504. In various examples, the value of lateral resistance
can be measured: (i) across a portion of the electrically
conductive material (M.sub.C) layer that overlaps with the
dielectric material layer, or (ii) across a portion of the
electrically conductive material (M.sub.C) layer that does not
overlap with the dielectric material layer, or (iii) a segment that
encompasses both regions (i) and (ii). In some examples, the change
in the measured value of vertical resistance of the vertical
resistive state (R.sub.V) may be of interest, and may be probed
across gate electrode and conductive material layer.
[0075] As described in greater detail herein, each different
resistive state of an example memristive switching device according
to the principles herein, including example memristive switching
device 500, is achieved through changing an amount of the ionic
species that is caused to migrate into, or out of, portions of the
conductive material layer, which changes the measured value of
resistance of the conductive material layer.
[0076] With a voltage applied to the gate electrode, ionic species
in the dielectric material layer can be pumped to or away from
conductive material layer, resulting in modifications of the
interface and even the bulk composition of the conductive material
layer. The conductive material layer and dielectric material are
chosen such that these modifications in interface and bulk
chemistry of the conductive material layer result in significant
modifications in the resistance of the conductive material layer.
So in the example memristive switching device according to the
principles herein, including example memristive switching device
500, it is not the resistance change in the oxide or other form of
the dielectric material that is measured to provide the memristive
switching. Rather it is the resistance change in the conductive
material layer that gives rise to the memristive switching.
[0077] As shown in the example of FIG. 5A, the conductive material
layer 506 can be formed with a lateral dimension l.sub.1 (shown as
extending in the x-direction) that is greater than the lateral
dimension l.sub.2 of the gate electrode 508. In non-limiting
example devices, the lateral dimension of the dielectric material
layer 507 can be about the same as, or larger than, the lateral
dimension l.sub.2 of the gate electrode 508. In other non-limiting
example devices, the lateral dimension of the dielectric material
layer 507 can be smaller than, or about equal to, the lateral
dimension l.sub.1 of the conductive material layer 506. Similarly
to film thickness, the width of the conductive material layer
underneath the gate electrode could be used to tune the possible
resistance change. That is, the relative values of lateral
dimension l.sub.1 and lateral dimension l.sub.2 can be used as
additional parameters to tune the memristive properties of the
example memristive switching device. A greater overlap of the
conductive material layer and the gate electrode can result in
differing values of resistance change than a smaller overlap
between the two. Any example memristive switching device can be
fabricated to include multiple different memristive elements, two
or more of the memristive elements being configured with differing
degrees of overlap between the conductive material layer and the
gate electrode, to provide an additional tuning capability of the
functional properties of the example memristive switching device.
Such a device can be a multi-bit device.
[0078] In an example where the lateral dimension of the dielectric
material layer 507 is smaller than the lateral dimensionl l.sub.1
of the conductive material layer 506, the memristive switching
device 500 can include an insulating dielectric material having
low-mobility ionic species disposed over portions of the conductive
material layer 506 that are not in communication with the
dielectric material layer 507, to act as a passivation layer. The
passivation layer can be of any thickness in the z-direction,
including up to the thickness of the dielectric material layer 507.
In another example, a thin layer of the dielectric material layer
507 can be disposed of the portions of the conductive material
layer that extend beyond the lateral dimension of the gate
electrode 508, to act as a passivation layer.
[0079] In an example implementation, the switching behavior can be
exploited based on a lateral resistance (R.sub.L) measured across
the conductive material layer 506. In some example implementations,
the conductive material layer 506 can also serve as a bottom
electrode, or a second conductive material layer can be disposed in
electrical communication with the conductive material layer 506 to
serve as a bottom electrode. In another example implementation, the
switching behavior can be exploited based on a vertical resistance
(R.sub.V) measured between the gate electrode 508 and a bottom
electrode. In yet another example implementation, the switching
behavior can be controlled based on both the lateral resistance
(R.sub.L) measured across the conductive material layer 506, and
the vertical resistance (R.sub.V) measured between the gate
electrode 508 and a bottom electrode.
[0080] In operation of any example memristive switching device
according to the principles herein, including memristive switching
device 500, a first voltage is applied across the gate electrode at
a sufficiently high potential (or a lower potential applied for
sufficiently long duration) to switch the memristive state of the
device (referred to as a "write" function). For example, the first
voltage can be applied in a first direction to switch the example
memristive device to a first resistive state (which can be
designated as a "0" or OFF state), or in a second (opposite)
direction to switch the example memristive device to a second
resistive state (which can be designated as a "1" or ON state). A
lower second voltage (i.e., having smaller magnitude than the first
voltage) is applied to measure the memristive state of the device
(referred to as a "read" function). In various example
implementations, the read or write function can be based on a
measure of the lateral resistance (R.sub.L), or a measure of the
vertical resistance (R.sub.V), or a measure of both the lateral
resistance (R.sub.L) and the vertical resistance (R.sub.V). Such an
example memristive device according to the principles herein would
be configured as a two-state device, having a first resistive state
with a first measured value of resistance that is designated as a
"0" or OFF state, and a second resistive state with a second
measured value of resistance, different from the first measured
value of resistance, that is designated as a "1" or ON state.
[0081] Any example memristive switching device according to the
principles herein, including memristive switching device 500, can
be configured to exhibit more than two levels of lateral switching
behavior, i.e., three or more different lateral resistive states
(R.sub.L) that can be measured. For example, a memristive element
of the example memristive switching device can be designated as
being in a "L.sub.0" or OFF state based on a first measured value
of lateral resistance, in a first ON state (L.sub.1) based on a
second measured value of lateral resistance (different from the
first measured value of lateral resistance), in a second ON state
(L.sub.2) based on a third measured value of lateral resistance
(different from both the first and second measured values of
lateral resistance), up to any number of different designated
resistive states (L.sub.1, where i>2). Each different resistive
state could be correlated with a value of lateral resistance of the
conductive material layer of the example memristive switching
device (e.g., as part of a calibration of the example memristive
switching device). That is, each different discrete depth of
migration of the ionic species into the conductive material layer
can be correlated to an incremental change in the lateral
resistance of the conductive material layer, to derive multiple
resistance states from a single memristive switching device. The
magnitude and direction of the gate voltage (V.sub.G) to be applied
to the gate electrode of a memristive element, or the duration of
application of a given gate voltage, to "write" a desired state to
that memristive element would depend on the history of the
memristive element. In an example, to change a memristive element
from state L.sub.1 to state L.sub.2 may involve application of a
smaller magnitude of gate voltage (V.sub.G) than to change the
memristive element from state L.sub.0 to state L.sub.2. In another
example, to change the memristive element from state L.sub.0 to
state L.sub.2 may involve application of the same magnitude of gate
voltage (V.sub.G) that can be used to change a memristive element
from state L.sub.1 to state L.sub.2, however applying that voltage
for a longer duration of time to cause more of the ionic species to
migrate into (or out of) the conductive material layer as desired.
In an example, to change a memristive element from state L.sub.2 to
state L.sub.1 may involve changing the direction of application of
a given gate voltage (V.sub.G).
[0082] In contrast to conventional memristors, the example
memristive switching devices according to the principles herein
does not rely on the unpredictable formation of conducting
filaments in the insulating layer, but rather provides capabilities
to control the modification of the interfacial and bulk chemistry
of the conductive material layer. The memristive switching devices
according to the principles herein also avoid the damage that
typically occurs inside the dielectric layer during filament
formation (dielectric breakdown). Accordingly, the example
memristive devices according to the principles herein can exhibit
markedly enhanced reliability and predictability.
[0083] In addition, the example memristive switching device
according to the principles herein, including memristive switching
device 500, can be configured with differing "read" and "write"
paths. For example, where the memristive switching is based on a
measure of the lateral resistance, terminals coupled to the
conductive material layer (such as terminals 502 and 504) can be
used to measure the lateral resistive state in a "read" function,
while the voltage is applied across the gate electrode at a
sufficiently high potential (or a lower potential applied for
sufficiently long duration) to switch the memristive state of the
device in a "write" function. In contrast, conventional memristors
use the same paths for both "read" and "write" functions.
[0084] According to the principles of the instant disclosure, the
modification of the resistive properties of the conductive material
layer (M.sub.C) can be tuned by regulating the oxygen stoichiometry
in a portion of the conductive material layer (M.sub.C) at the
conductive material-dielectric material interface, according to the
example systems, methods, and apparatus herein.
[0085] Example systems, methods and apparatus are provided herein
that facilitate use of a voltage to control the resistive
properties of the conductive material layer (M.sub.C) in films and
devices, including nanodevices. In the context of a resistive
memory element, the oxygen stoichiometry at the conductive
material-dielectric material interface can be controlled to cause
the switching between resistive states.
[0086] As a non-limiting example, using the example systems,
methods, and apparatus herein, the change of proportionate amount
of the at least one ionic species in a portion of the conductive
material layer can be used to cause a change of a local resistive
state at differing local portions of the example device. In various
example devices, the local resistive state that is measured can be
a lateral resistive state, or a vertical resistive state, or both
vertical and lateral resistive states. Using the example systems,
methods, and apparatus herein, the change of proportionate amount
of the at least one ionic species in a portion of the conductive
material layer can be used to cause a change in magnitude of the
lateral resistive state (and in some example, the vertical
resistive state).
[0087] FIG. 5B shows another example memristive switching device
according to the principles herein, formed as a multi-terminal
device. Any description or variations described hereinabove
relative to the example memristive switching device of FIG. 5A also
apply to equivalent features and components of the example
memristive switching device of FIG. 5B. Terminals 522 and 524 can
be used to measure a lateral resistive state across the
electrically conductive material (M.sub.C) layer 526. In
non-limiting examples, terminals 522 and 524 can be used to drive a
current through conductive material (M.sub.C) layer 526 and/or
apply a voltage across conductive material (M.sub.C) layer 526. The
conductive material is at least partially covered by an
electronically insulating dielectric material layer 527 which
includes cations (C.sup.x+) and anions (A.sup.y-). The
electronically insulating dielectric material layer also acts as a
good ionic conductor. The gate electrode (M.sub.G) disposed over
the insulating layer provides the third terminal 528 of the device.
In a four-point measurement configuration, terminals 522 and 524
can be used to drive a current through conductive material
(M.sub.C) layer 526, and terminals 530 and 532 can be used to
measure, for example, the voltage across the electrically
conductive material (M.sub.C) layer, to determine the lateral
resistance of the lateral resistive state. As shown in FIG. 5B,
with a non-zero potential difference applied in a first direction
across the conductive material (M.sub.C) layer 526 and the gate
electrode 528, an amount of the ionic species (indicated at 302)
migrates into the conductive material (M.sub.C) layer 526. With a
non-zero potential difference applied in a second direction
(opposite to the first direction) across the conductive material
(M.sub.C) layer 526 and the gate electrode 528, an amount of the
ionic species (indicated at 302) migrates from the conductive
material layer (M.sub.C) to the dielectric material layer 527. The
resulting change in the measured value of lateral resistance of the
lateral resistive state (R.sub.L) can be probed across a segment of
the electrically conductive material (M.sub.C) layer. In various
examples, the value of lateral resistance can be measured: (i)
across a portion of the electrically conductive material (M.sub.C)
layer that overlaps with the dielectric material layer, or (ii)
across a portion of the electrically conductive material (M.sub.C)
layer that does not overlap with the dielectric material layer, or
(iii) a segment that encompasses both regions (i) and (ii). The
resulting change in the measured value of vertical resistance of
the vertical resistive state (R.sub.V) can be probed across gate
electrode and conductive material layer (if of interest). The
relative values of lateral dimensions of the gate electrode and
conductive material layer can be used as additional parameters to
tune the memristive properties (i.e., resistive states (L.sub.1))
of the example memristive switching device, as described
herein.
[0088] In any example memristive device according to the principles
herein, the conducting material layer could be formed with two or
more differing types of materials to control the change in
resistance that can be achieved by applying a voltage. In another
example, the conducting material layer could be coupled to an inert
metal underlayer that shunts part of the current flowing through
the device. The inert metal underlayer can be formed from any
electrically conductive material that does not admit migration of
the ionic species, such as but not limited to, a noble metal
(including ruthenium, rhodium, palladium, silver, osmium, iridium,
platinum, and gold). As a non-limiting example, a thin gold layer
can be disposed under the conductive (transition metal) material
layer. Since the inert metal layer would be essentially immune to
voltage-induced electrochemical reactions, the relative thickness
of the two layers provides a tool to tune the magnitude of the
voltage induced memristive switching effects in these example
memristive devices. Using a thicker inert metal layer shunts much
of the current through it, and therefore results in smaller
resistance changes. With a thinner inert metal layer, much larger
resistance changes could be achieved. In an example, the thickness
of the inert metal underlayer can be configured to cause a greater
proportion of current applied between the electrodes to flow
through the inert metal underlayer, and as a result modify the
measure value of resistance of the lateral resistive state. In
another example, the thickness of the inert metal underlayer can be
configured to cause a smaller proportion of current applied between
the electrodes to flow through the inert metal underlayer, and as a
result modify the measure value of resistance of the lateral
resistive state. The underlayer allows continued electrical contact
in an example memristive switching device where an applied gate
voltage causes sufficient ionic species to migrate into the
conductive material layer to cause the bulk of the conductive
material layer to change to a largely resistive state, e.g.,
becoming significantly less conductive or even non-conductive. In
any example memristive switching device according to the principles
herein, the ionic species can be caused to migrate into the
conductive material layer to depths of up to about 10 nm to about
20 nm or more, based on the magnitude, direction, and/or duration
of application of the applied gate voltage.
[0089] FIG. 5C shows an example memristive switching device
according to the principles herein, formed as a three-terminal
device that includes an underlayer. Any description or variations
described hereinabove relative to the example memristive switching
device of FIG. 5A also apply to equivalent features and components
of the example memristive switching device of FIG. 5C. Terminals
542 and 544 can be used to measure a lateral resistive state across
the electrically conductive material (M.sub.C) layer 546. In
non-limiting examples, terminals 542 and 544 can be used to drive a
current through conductive material (M.sub.C) layer 546 and/or
apply a voltage across conductive material (M.sub.C) layer 546. The
conductive material is at least partially covered by an
electronically insulating dielectric material layer 547 which
includes cations (C.sup.x+) and anions (A.sup.y-). The
electronically insulating dielectric material layer also acts as a
good ionic conductor. The gate electrode (M.sub.G) disposed over
the insulating layer provides the third terminal 548 of the device.
An underlayer 549 is disposed in electrical communication with the
conductive material (M.sub.C) layer 546. As shown in FIG. 5C, with
a non-zero potential difference applied in a first direction across
the conductive material (M.sub.C) layer 546 and the gate electrode
548, an amount of the ionic species (indicated at 302) migrates
into portions of the conductive material layer (M.sub.C) proximate
to the interface between the dielectric material layer 547 and the
conductive material (M.sub.C) layer 546. With a non-zero potential
difference applied in a second direction (opposite to the first
direction) across the conductive material (M.sub.C) layer 546 and
the gate electrode 548, an amount of the ionic species (indicated
at 302) migrates from the conductive material layer (M.sub.C) to
the dielectric material layer 547. The resulting change in the
measured value of lateral resistance of the lateral resistive state
(R.sub.L) can be probed across a segment of the electrically
conductive material (M.sub.C) layer. In various examples, the value
of lateral resistance can be measured: (i) across a portion of the
electrically conductive material (M.sub.C) layer that overlaps with
the dielectric material layer, or (ii) across a portion of the
electrically conductive material (M.sub.C) layer that does not
overlap with the dielectric material layer, or (iii) a segment that
encompasses both regions (i) and (ii). The resulting change in the
measured value of vertical resistance of the vertical resistive
state (R.sub.V) can be probed across gate electrode and conductive
material layer (if of interest). The relative thickness of the
conductive material layer 546 and the underlayer 549 can be
configured to control the resistive states (L.sub.i) of the example
memristive switching device, as described herein. In addition, the
relative values of lateral dimension l.sub.1 and lateral dimension
l.sub.2 can be used as additional parameters to tune the memristive
properties (i.e., resistive states (L.sub.1)) of the example
memristive switching device, as described herein.
[0090] FIG. 5D shows an example memristive switching device
according to the principles herein, formed as a multi-terminal
device that includes an underlayer. Any description or variations
described hereinabove relative to the example memristive switching
device of FIG. 5A or FIG. 5C also apply to equivalent features and
components of the example memristive switching device of FIG. 5D.
Terminals 552 and 554 can be used to measure a lateral resistive
state across the electrically conductive material (M.sub.C) layer
556. In non-limiting examples, terminals 552 and 554 can be used to
drive a current through conductive material (M.sub.C) layer 556
and/or apply a voltage across conductive material (M.sub.C) layer
556. The conductive material is at least partially covered by an
electronically insulating dielectric material layer 557 which
includes cations (C.sup.x+) and anions (A.sup.y-). The
electronically insulating dielectric material layer also acts as a
good ionic conductor. The gate electrode (M.sub.G) disposed over
the insulating layer provides the third terminal 558 of the device.
An underlayer 559 is disposed in electrical communication with the
conductive material (M.sub.C) layer 556. In a four-point
measurement configuration, terminals 552 and 554 can be used to
drive a current through conductive material (M.sub.C) layer 556,
and terminals 560 and 562 can be used to measure, for example, the
voltage across the electrically conductive material (M.sub.C)
layer, to determine the lateral resistance of the lateral resistive
state. As shown in FIG. 5D, with a non-zero potential difference
applied in a first direction across the conductive material
(M.sub.C) layer 556 and the gate electrode 558, an amount of the
ionic species (indicated at 302) migrates into portions of the
conductive material layer (M.sub.C) proximate to the interface
between the dielectric material layer 557 and the conductive
material (M.sub.C) layer 556. With a non-zero potential difference
applied in a second direction (opposite to the first direction)
across the conductive material (M.sub.C) layer 556 and the gate
electrode 558, an amount of the ionic species (indicated at 302)
migrates from the conductive material layer (M.sub.C) to the
dielectric material layer 557. The resulting change in the measured
value of lateral resistance of the lateral resistive state
(R.sub.L) can be probed across a segment of the electrically
conductive material (M.sub.C) layer. In various examples, the value
of lateral resistance can be measured: (i) across a portion of the
electrically conductive material (M.sub.C) layer that overlaps with
the dielectric material layer, or (ii) across a portion of the
electrically conductive material (M.sub.C) layer that does not
overlap with the dielectric material layer, or (iii) a segment that
encompasses both regions (i) and (ii). The resulting change in the
measured value of vertical resistance of the vertical resistive
state (R.sub.V) can be probed across gate electrode and conductive
material layer (if of interest). The relative thickness of the
conductive material layer 556 and the underlayer 559 can be
configured to control the resistive states (L.sub.i) of the example
memristive switching device, as described herein. In addition, the
relative values of lateral dimensions of the gate electrode and
conductive material layer can be used as additional parameters to
tune the memristive properties (i.e., resistive states (L.sub.i))
of the example memristive switching device, as described
herein.
[0091] In any example memristive device according to the principles
herein, the value of voltage (V.sub.G) that can be applied to
change the resistive state of a memristive device or memristive
element (i.e., a "write" voltage) can be on the order of about 1V
or less, about 2V, about 3V, about 5V, about 7 V, about 8 V, about
to 10V, or about 12 V, or higher. The value of voltage that can be
applied to measure the resistive state of a memristive device or
memristive element (i.e., a "read" voltage) can be on the order of
about 1V or less, such as but not limited to about 0.2V or less,
about 0.5 V, about 0.8V, about 1V, about 1.2V, about 1.5V or
higher.
[0092] A plurality of any example memristive switching device
according to the principles herein, including any of the example
memristive switching devices of FIGS. 5A-5D, can be fabricated as
part of a multi-bit device that include multiple different
memristive elements. Each of the example memristive switching
device forms a memristive element of the multi-bit device. Each
memristive element can be separately addressable, such that a
voltage and/or a current can be applied to each memristive element
separately, and each memristive element can be separately subjected
to a "read" or "write" function, as described herein. The materials
composition of the layers of two or more of the memristive elements
may differ, to allow differing portions of the multi-bit device to
exhibit differing types of resistive states. For example, the
conductive material layer and/or dielectric material layer may
differ between two of the memristive elements. As another example,
some of the memristive elements may include an underlayers while
others do not. In another example, the relative thickness of the
conductive material layer and the underlayer of some of the
memristive elements can be configured to control the resistive
states (L.sub.i) of the example multi-bit device, as described
herein. The relative values of lateral dimensions of the gate
electrode and conductive material layer also can be used as
parameters to tune the memristive properties (i.e., resistive
states (L.sub.i)), as described herein, of any two or more of the
memristive elements of the multi-bit device. Any two or more of the
memristive elements of the multi-bit device can be configured with
differing degrees of overlap between the conductive material layer
and the gate electrode, to provide an additional tuning capability
of the functional properties of the example memristive switching
device. In addition, each differing discrete depth of migration of
the ionic species into the conductive material layer can be
correlated to an incremental change in resistance, such that each
memristive element (cell) can present multiple differing resistance
states from a single cell of the multi-bit device. In an example,
the multi-bit device can be configured as a cascade of multiple
gate electrodes, each disposed over different distinct regions of a
conductive material layer, to allows for multi-bit storage, where
all bits could be read out simultaneously by probing the resistance
state of the conductive material layer.
[0093] In any example memristive device according to the principles
herein, the dielectric material-conductive material pair can be a
transition metal and a rare earth oxides with high oxygen anion
mobility. The voltage driven oxygen ion migration in the rare earth
oxide could oxidize the interface and even the bulk of the
transition metal in the conductive material layer, and thus
significantly modify its conductivity.
[0094] In any example memristive device according to the principles
herein, the mobile ionic species could also be supplied by gaseous
species in atmosphere or from the gate electrode, instead of being
entirely present in the insulating layer. If the gate electrode is
not inert, a gate voltage could be used to partially dissolve it
and transport the materials across the insulating layer.
[0095] In any example memristive device according to the principles
herein, several gate electrodes can be placed on the same
conducting material layer (e.g., a nanostrip or nanowire).
Depending on the width and layer structure of the conducting
material layer, voltage application to each gate electrode could
then be controlled to modify the resistance of the conducting
material layer in the respective gate electrode by a certain
fraction. Such a cascade of gate electrodes allows for multi-bit
storage, where all bits could be read out simultaneously by probing
the resistance state of the nanostrip. The description hereinabove
of a multi-bit device also applies to an example memristive
switching device formed with conductive material layer formed from
at least one nanostrip of conductive a material.
[0096] In any example device herein, the electrically conductive
material layer can have a thickness of about 0.5 nm, about 0.7 nm,
about 0.9 nm, about 1 nm, about 1.3 nm, about 1.5 nm, about 1.8 nm,
about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 7 nm, about
10 nm, about 12 nm, about 15 an, about 20 nm, about 25 nm, or
greater. The dielectric material layer can have a thickness of
about 1.0 nm, about 2.0 nm, about 3.0 nm, about 5.0 nm, about 7.0
nm, about 9.0 nm, about 10 nm, about 13 nm, about 15 nm, about 20
nm, about 25 nm, about 30 nm, about 35 nm, about 40 nm, about 50
nm, about 60 nm, about 70 nm, about 80 nm, or greater. References
herein to thickness of a layer are to the magnitudes in the
z-direction.
[0097] The example devices, systems, methods, and apparatus
according to the principles herein can be configured as a magnetic
recording device, a memristor, a non-volatile memory device, a
magnetoresistive random-access memory device, a voltage-controlled
magnetic memory, a voltage-tunable magnetic sensor, a
voltage-tunable inductor, a voltage-controlled resonant device, a
voltage-controlled lateral conductive device, an electrically
controllable catalysis device, a voltage controlled optical switch,
a responsive window tinting device, or a display device.
[0098] The example devices, systems, methods, and apparatus
according to the principles herein can be configured as an organic
spintronic device, or other organic device.
[0099] The example devices, systems, methods, and apparatus
according to the principles herein can be used to provide
memristors for implementation in applications such as, but not
limited to, nanoelectronic memories, computer logic, and
neuromorphic/neuromemristive computer architectures. As
non-limiting examples, the devices, systems, methods, and apparatus
according to the principles herein can be configured to provide
non-volatile computer memory and storage, flash drives, including
EEPROMs (electrically erasable programmable read-only memory),
solid-state drives (SSD), dynamic random-access memory (DRAM), and
Static random-access memory (SRAM). The example device elements can
be used in applications using different types of memory, such as
but not limited to, capacitor, variable capacitor, floating gate
transistor, four transistor feedback loop circuit, or magnetic
tunnel junction in commercialized DRAM, FeRAM, NOR flash, SRAM or
MRAM, technologies. The novel devices, systems, methods, and
apparatus according to the principles herein can be used to
removable storage devices for mobile devices and smartphones,
cameras, tablets, and other portable applications.
[0100] An example devices according to the principles herein
includes a dielectric material layer disposed in an x-y plane, and
an electrically conductive material layer over and forming an
interface with the dielectric material layer. The dielectric
material layer includes at least one ionic species having a high
ion mobility, such that exposure to electromagnetic irradiation
and/or temperature changes can cause changes in the mobility of the
least one ionic species. The electrically conductive material is
configured to reversibly uptake an amount of the at least one ionic
species.
[0101] In operation, under the directional influence of an applied
potential difference in a direction across the interface between
the dielectric material layer and the electrically conductive
material layer, the at least one ionic species are caused to
migrate into (or out of) the portions of the electrically
conductive material layer proximate to the interface. That is, the
potential difference is applied for a duration of time sufficient
to cause a change in the proportionate amount of the at least one
ionic species present in the portions of the electrically
conductive material layer proximate to the interface. Due to the
nanoscale thickness of the electrically conductive material layer,
changes to the proportionate composition at the interface can
affect the materials properties of the electrically conductive
material layer. As a result, changes in the proportionate amount of
the at least one ionic species present in the portions of the
electrically conductive material layer proximate to the interface
can cause changes in the function properties of the example
device.
[0102] In any example systems, methods, apparatus, and devices
according to the principles herein, the functional properties of
interest can be the vertical resistance, or the lateral resistance,
or both.
[0103] According to the principles of the instant disclosure,
systems, methods, and apparatus are provided for regulating (i.e.,
tuning) one or more of the functional properties of the example
device, by modifying the mobility of the at least one ionic species
in the dielectric material layer, and applying a potential
difference to cause the at least one ionic species in the desired
direction (into or out of the electrically conductive material
layer). Accordingly, the dielectric material layer serves as a
reservoir of the ionic species. Migration of the ionic species into
or out of the electrically conductive material proximate to the
interface facilitates tuning of the materials properties of the
electrically conductive material layer. This facilitates tuning of
the functional property(ies) of the example device.
[0104] As a non-limiting example, the resistive state of the
conductive material layer can be regulated based on the systems,
methods, and apparatus described herein for controlling and
regulating the migration of the ionic species into and out of the
conductive material layer. Using the example systems, methods, and
apparatus herein, the regulation of the proportionate amount of the
at least one ionic species in a portion of the conductive material
layer can be used to cause a change between a metastable state of
the conductive material layer having a first resistivity and a
metastable state of the conductive material layer having a second
resistivity. A read-out of the device can be based on a measure of
the type of resistive state of the conductive material layer (i.e.,
whether the first resistive state or the second resistive state).
This capability can be exploited to provide memristive devices,
including a memory device, by using these differing metastable
states to program information. Accordingly, the example systems,
methods, and apparatus herein can provide a memory device that is
based on use of these two metastable states, i.e., as a "1" (ON) or
as a "0" (OFF), for programming information based on any computer
logic, logic theory, or stochastic theory.
[0105] In an example implementation, the example memristive devices
according to the principles herein can be configured based on a
three-layer structure (gate electrode/dielectric material
layer/conductive material layer). In other example implementations,
the example memristive devices can be configured based on more
complex layered structures. In an example, the dielectric material
layer could be configured with multiple functional layers. In
another example, the dielectric material may be a bilayer, where
one layer acts to store the mobile ionic species of interest for
facilitating the switching behavior, and the second layer
transports the ionic species from the storage layer to the
conductive material layer.
[0106] FIGS. 6A-6F and 7A-7C describe various example layered
structures that can be implemented as memristive devices according
to the principles herein.
[0107] FIG. 6A shows the cross section of an example device 610
according to the principles of the instant disclosure. The example
device 610 includes an electrically conductive material layer 612
(a target layer) and a dielectric material layer 614 disposed in an
x-y plane. As shown in FIG. 6A, the electrically conductive
material layer 612 is disposed over and forms an interface 16 with
the dielectric material layer 614.
[0108] FIG. 6B shows the cross section of another example device
620 according to the principles of the instant disclosure. The
example device 620 includes an electrically conductive material
layer 622 (a target layer) formed from a transition metal material,
and a dielectric material layer 624 disposed in an x-y plane. As
shown in FIG. 6B, the electrically conductive material layer 622 is
disposed over and forms an interface 626 with the dielectric
material layer 624.
[0109] FIG. 6C shows the cross section of another example device
630 according to the principles of the instant disclosure. The
example device 630 includes an electrically conductive material
layer 632 formed from a transition metal material, and a dielectric
material layer 634 disposed in an x-y plane, as a gate oxide
dielectric layer. As shown in FIG. 6C, the electrically conductive
material layer 632 is disposed over and forms an interface 636 with
the dielectric material layer 634. Example device 630 includes a
gate electrode layer 638 in electrical communication with the
dielectric material layer 634 (as a gate oxide dielectric layer).
In an example, device 630 also includes an electrically conductive
material layer to serve as an electrical contact to the
electrically conductive material layer 632 (which serves as a
target layer). In another example, the electrically conductive
material layer could be excluded, and an electrical contact could
be made to the electrically conductive material layer 632.
[0110] FIG. 6D shows the cross section of another example device
640 according to the principles of the instant disclosure. The
example device 640 includes an electrically conductive material
layer 642 (a target layer) disposed in an x-y plane, and a
dielectric material bilayer that includes as a thinner intermediate
dielectric layer 643 and a thicker gate dielectric layer 645. In an
example, the thicker gate dielectric layer 45 can be formed as a
gate oxide dielectric layer. As shown in FIG. 6D, the electrically
conductive material layer 642 is disposed over and forms an
interface 646 with the intermediate dielectric layer 43 of the
dielectric material bilayer. Example device 640 includes a gate
electrode layer 48 in electrical communication with the thicker
gate dielectric layer 645. In an example, device 640 also can
include another electrically conductive material layer to serve as
an electrical contact to the electrically conductive material layer
642 (which serves as a target layer). In this example, the
electrically conductive material layer 642 can be formed from a
transition metal material. In another example implementation, the
electrically conductive material layer could be excluded, and
contact could be made to the electrically conductive material layer
642.
[0111] FIG. 6E shows an example device 650 according to the
principles of the instant disclosure. The example device 650
includes an electrically conductive material layer 652 formed from
a transition metal material disposed in an x-y plane, and a
dielectric material layer 654 disposed in an x-y plane, as a gate
oxide dielectric layer. As shown in FIG. 6E, the electrically
conductive material layer 652 forms an interface 656 with the gate
oxide dielectric layer 654. Example device 650 includes a gate
electrode layer 658 in electrical communication with the gate oxide
dielectric layer 655. As shown in example device 650, the gate
oxide dielectric layer 654 and the gate electrode layer 658 can
each have a substantially rectangular or square cross-section.
[0112] In the non-limiting examples of FIG. 6E, the lateral
dimension l.sub.2 of the gate oxide dielectric layer is
approximately equal to the lateral dimension l.sub.1 of the gate
electrode layer. In these examples, the lateral dimension l.sub.3
of the transition metal material layer is greater than the lateral
dimensions l.sub.1 and l.sub.2. In other non-limiting example
devices, the lateral dimension l.sub.1 of the gate electrode layer
can be smaller than the lateral dimension of the gate oxide
dielectric layer l.sub.2.
[0113] FIG. 6F shows an example device 660 according to the
principles of the instant disclosure. The example device 660
includes an electrically conductive material layer 662 formed from
a transition metal material disposed in an x-y plane, and a bilayer
of dielectric material formed as an intermediate oxide dielectric
layer 663 and a gate oxide dielectric layer 665. As shown in FIG.
6F, the electrically conductive material layer 662 forms an
interface 666 with the intermediate oxide dielectric layer 663.
Example device 660 includes a gate electrode layer 668 in
electrical communication with the gate oxide dielectric layer
665.
[0114] As shown in the non-limiting examples of FIGS. 6E and 6F,
the gate oxide dielectric layer and the gate electrode layer can
have a substantially rectangular or square cross-section. In
another example, the gate oxide dielectric layer and the gate
electrode layer can each be formed with an elliptical, circular, or
other polygonal cross-sections, such as but not limited to a
hexagonal cross-section. As also shown in the non-limiting examples
of FIGS. 6E and 6F, the lateral dimension l.sub.2 of the gate oxide
dielectric layer is approximately equal to the lateral dimension
l.sub.1 of the gate electrode layer. In these example, the lateral
dimension l.sub.3 of the transition metal material layer (and the
intermediate oxide dielectric layer l.sub.4 in FIG. 6F) are greater
than the lateral dimensions l.sub.1 and l.sub.2. In other
non-limiting example devices, the lateral dimension l.sub.1 of the
gate electrode layer can be smaller than the lateral dimension of
the gate oxide dielectric layer l.sub.2.
[0115] In the non-limiting example of FIG. 6F, the intermediate
oxide dielectric layer and the transition metal material layer are
shown as having similar lateral dimensions
(l.sub.3.apprxeq.l.sub.4). In other examples, the intermediate
oxide dielectric layer and the transition metal material layer can
be configured to have different lateral dimensions
(l.sub.3.noteq.l.sub.4). For example, the example device can be
fabricated such that the transition metal material layer has a
greater lateral dimension than the intermediate oxide dielectric
layer (l.sub.1<l.sub.4).
[0116] In various example implementations according to the
principles herein, including the example devices of any of FIG. 6D,
and 6F, the gate oxide dielectric layer can be configured with a
greater thickness in the z-direction than the intermediate oxide
dielectric material layer, by a factor of about 2, about 3, about
5, about 10, or higher. In some examples, the intermediate oxide
dielectric layer can be formed from a different dielectric material
than the gate oxide dielectric layer.
[0117] In various example implementations according to the
principles herein, including the example devices of any of FIGS. 5A
through 6F, any of the example devices according to the principles
herein may be configured in a two-terminal configuration, a
three-terminal configuration (illustrated in FIGS. 7A-7C).
[0118] FIGS. 7A and 7B illustrate two different cross-sectional
geometries of non-limiting example two-terminal configurations 700
and 700'. Both FIGS. 7A and 7B show example two-terminal
configuration that include electrically conductive contacts 702 and
704 coupled in electrical communication with opposite sides of
example device 706 in the z-direction. In accordance with the
principles described herein, example devices 706 include an
electrically conductive material layer 712 that forms an interface
716 with a dielectric material layer 714. In the example
two-terminal configuration 700 of FIG. 7A, the electrically
conductive contacts 702 and 704 are disposed to overlap each other.
In the example two-terminal configuration 700' of FIG. 7B, the
electrically conductive contacts 702 and 704 are disposed to have
no overlap.
[0119] FIG. 7C illustrates a non-limiting example three-terminal
configuration 750. The example three-terminal configuration
includes electrically conductive contacts 702 and 704 coupled in
electrical communication with electrically conductive material
layer 712. One side of the dielectric material layer 714-a forms an
interface 716 with the electrically conductive material layer 712.
A gate electrode 720 is disposed over the other side of the
dielectric material layer 714-a. In this example according to the
principles herein, the example device 706 includes a dielectric
material layer 714-a that forms an interface 716 with only a
portion of the electrically conductive material layer 712. In the
example of FIG. 7C, electrically conductive contacts 702 and 704
are disposed on the same side of electrically conductive material
layer 712. In another example according to the principles herein,
electrically conductive contacts 702 and 704 can be disposed on
opposite sides of electrically conductive material layer 712.
[0120] In various example devices and configurations according to
the principles herein, including the example devices of any of
FIGS. 5A through 6F or the device configurations of any of FIGS. 7A
through 7C, the dielectric material layer is formed from a
dielectric material that includes at least one ionic species having
a high ion mobility. The dielectric material of the dielectric
material layer is configured to be tunable, such that exposure to
electromagnetic radiation and/or temperature changes can cause
changes in the mobility of the least one ionic species. In an
example implementation, the mobility of the at least one ionic
species can be tuned (i.e., regulated) by increasing or decreasing
the amount of electromagnetic radiation impinging on, or otherwise
coupled to, the example device. A laser and/or another source of
electromagnetic radiation can be used to provide the
electromagnetic radiation. In another example implementation, the
mobility of the at least one ionic species can be tuned (i.e.,
regulated) by increasing the temperature (heating) or decreasing
the temperature (cooling) of a portion of the example device. As
non-limiting examples, the changes in temperature can be achieved
using a heating element, a thermoelectric element, and/or a laser
beam.
[0121] As non-limiting examples, the at least one ionic species can
be at least one of: an anion including oxygen and an anion
including hydrogen. The at least one ionic species can be, but is
not limited to, an oxide, an oxynitride, a nitride, or a
silicate.
[0122] As non-limiting examples, the dielectric material can be
based on at least one of: gadolinium, hafnium, terbium, zirconium,
yttrium, tantalum, titanium, aluminum, silicon, germanium, gallium,
indium, tin, antimony, tellurium, barium, bismuth, titanium,
vanadium, chromium, manganese, cobalt, nickel, copper, zinc,
niobium, molybdenum, palladium, cadmium, strontium, tantalum,
niobium, cerium, praesydium, or tungsten, or any combination
thereof. For example, the dielectric material can be an oxide, an
oxynitride, a nitride, or a silicate of any of these materials. As
other non-limiting examples, the dielectric material can be
aluminum oxide (AlO.sub.x), bismuth zinc niobate, hafnium oxide
(AlO.sub.x), barium strontium titanate, tantalum oxide, or
gadolinium oxide (GdO.sub.x). In any example herein, the dielectric
material can be Gd.sub.2O.sub.3 or SrTiO.sub.3.
[0123] In any example herein, the dielectric material can be formed
from any dielectric material or electrolyte having high ion
mobility that is considered for application, e.g., in fuel cells or
electrochemical metallization memory cells. For example, dielectric
material layer can be formed from any of the high ionic mobility
materials known in the art, and listed, e.g., in R. Waser et al.,
Advanced Materials, vol. 21, pp. 2632-2663 (2009), or W. Lu et al.,
Materials Research Society Bulletin, vol. 37, pp. 124-130 (2012),
each of which is incorporated herein for the disclosure of the
dielectric materials and/or electrolytes.
[0124] In any example herein, the dielectric material can be formed
to have an amorphous structure, or a semi-crystalline structure,
since such structures can facilitate higher mobility of ion
vacancies.
[0125] In any example herein, the dielectric material can be formed
from an organic material having high mobility ionic species,
including any applicable polymeric material. As non-limiting
examples, an example device that includes an organic dielectric can
be used to provide an organic memory, an organic spintronic device,
an organic magnetic recording device, an organic memristor, an
organic non-volatile memory device, an organic magnetoresistive
random-access memory device, an organic voltage-controlled magnetic
memory, an organic voltage-tunable magnetic sensor, an organic
voltage-controlled lateral conductive device, an organic
electrically controllable catalysis device, an organic voltage
controlled optical switch, an organic responsive window tinting
device, or an organic display device.
[0126] In various example devices and configurations according to
the principles herein, including the example devices of any of
FIGS. 5A through 6F or the device configurations of any of FIGS. 7A
through 7C, the electrically conductive material layer of the
active region of the device is formed from an electrically
conductive material that is configured to reversibly uptake an
amount of the at least one ionic species. The electrically
conductive material layer can include aluminum, a transition metal,
a rare earth metal, and/or an alloy of any of these conductive
materials. As non-limiting examples, the electrically conductive
material layer can include copper, tantalum, tin, tungsten,
titanium, tungsten, cobalt, chromium, silver, nickel, iron, nickel,
cobalt, samarium, dysprosium, yttrium, chromium. In various
examples, the alloy can be an alloy of one or more transition
metals, or an alloy of one or more rare earth metals, or an alloy
that includes at least one transition metal and at least one rare
earth metal. The alloy can be a binary or ternary system of any of
these conductive materials. In an example where the electrically
conductive material layer is ferromagnetic, the ferromagnetic
material can include iron, nickel, cobalt, samarium, dysprosium,
yttrium, chromium, or an alloy of at least one of iron, nickel,
cobalt, and samarium. In an example, the ferromagnetic material can
also include a non-magnetic element, such as but not limited to
boron. Non-limiting examples of such ferromagnetic materials are a
ferromagnetic alloy including any one or more of cobalt (Co),
nickel (Ni), iron (Fe), as well as at least one of boron (B),
carbon (C), copper (Cu), hafnium (Hf), palladium (Pd), platinum
(Pt), rhenium (Re), rhodium (Rh), or ruthenium (Ru). For example,
the ferromagnetic material can be formed as cobalt-iron-boron.
[0127] In an example, the electrically conductive material layer
can also be a bilayer or multilayer of several of any of the metals
and/or alloys described hereinabove. One or more functional
properties of the target layer (whether in the layer or at its
interface with the dielectric layer) can depend on the relative
thicknesses of the layers. As a non-limiting example, the
conductive material layer can be a metal bilayer of a transition
metal (such as but not limited to iron, nickel, cobalt, and
samarium) and a noble metal (such as but not limited to gold). The
resistive properties at the interface of the metal bilayer can be
controlled by using a voltage to regulate the oxide content (i.e.,
the oxidation or reduction) of the transition metal of the metal
bilayer.
[0128] In various example devices and configurations according to
the principles herein, including the example devices of any of
FIGS. 5A through 6F or the device configurations of any of FIGS. 7A
through 7C, the example electrically conductive contact herein can
be formed from gold, platinum copper, tantalum, tin, tungsten,
titanium, tungsten, cobalt, chromium, silver, nickel, rhuthenium or
aluminum, or a binary or ternary system of any of these conductive
materials.
[0129] The electrically conductive material layer of the example
devices and configurations according to the principles herein are
configured for reversible uptake of an amount of the at least one
ionic species. That is, the electrically conductive material layer
is configured to be oxidizable, or reducible, or otherwise capable
of reversibly coupling with the at least one ionic species. The
dielectric material layer serves as a reservoir of the ionic
species. The amount of the higher-mobility ionic species in the
dielectric material layer can be changed (increased or decreased)
by regulating the temperature and/or electromagnetic radiation
exposure of the dielectric material. The direction of the applied
potential difference across the interface between the electrically
conductive material layer and the dielectric material layer causes
the mobile ionic species to migrate into (or out of) the portions
of the electrically conductive material layer proximate to the
interface. The magnitude of the potential difference drives the
ionic species into the electrically conductive material layer at
interface, such that the state of electrically conductive material
layer changes proximate to the interface to change the properties
of the electrically conductive material. For example, the mobile
ionic species can be driven to a depth of up to about 0.1 nm, about
0.3 nm, about 0.5 nm, about 0.8 nm, about 1 nm, about 1.2 nm, or
more, into the electrically conductive material layer (as measured
from the interface). This change in the state of electrically
conductive material layer results in a change in the functional
property of the example device.
[0130] The example devices and configurations according to the
principles herein are capable of retaining the change of the
functional property even after discontinuance of the application of
the potential difference, the irradiating, and/or the temperature
regulating. That is, the changed state of electrically conductive
material layer (from the presence of the at least one ionic
species) is a metastable state that persists for a period of time
even after discontinuance of the applied potential difference. This
metastable state can persist for a (persistence) period of time up
to about 10 nanoseconds, about 100 nanoseconds, about 500
nanoseconds, about 1 microsecond, about 500 microseconds, about 1
millisecond, about 100 milliseconds, about 500 milliseconds, about
1 second, about 5 seconds, about 10 seconds, about 30 seconds,
about 60 seconds, about 3 minutes, about 5 minutes, about 10
minutes, about 30 minutes, about 60 minutes, for several hours, for
several days, or longer (including substantially longer periods of
time, such as weeks, or years, which might be desirable for data
storage applications). Once the ionic species are driven into the
electrically conductive material layer under an applied potential
difference in a first direction, this changed state of the
electrically conductive material layer (a first state) persists in
the metastable state (for the duration of its persistence period)
unless a potential difference having opposite polarity (i.e., in an
opposite direction) is applied. A first, non-zero amount of the
ionic species is present in the electrically conductive material
layer (e.g., as quantified by proportion or concentration) in this
first metastable state. When a potential difference of an opposite
polarity is applied, the at least one ionic species migrate out of
the electrically conductive material layer, back to the dielectric
material layer. This results in a smaller amount of the ionic
species remaining in the electrically conductive material layer (as
quantified by proportion or concentration), to provide a second
metastable state. The overall example device has different
functional properties depending on whether the electrically
conductive material layer is in the first metastable state or in
the second metastable state.
[0131] Example systems, methods, and apparatus are provided for
selectively and locally "programming" different functional
properties into different spatial regions of an example device,
configured in any applicable configuration. For example, the
reversible metastable change in state of the electrically
conductive material layer can be "programmed" at different local
spatial regions of an example device. Through discrete local
application of the potential difference, and either (i) exposure to
the electromagnetic radiation, or (ii) local temperature
regulation, or both (i) and (ii), differing regions of the example
device can be caused to exhibit different functional properties.
Accordingly, example systems, methods, and apparatus are provided
for locally and controllably modifying the state of electrically
conductive material layer, thereby reversibly, locally and
controllably changing the functional properties of the example
device. This example provides for direct "writing" of the
functional properties at different portions of the example
devices.
[0132] Example systems, methods, and apparatus are also provided
for tuning the functional properties. An example apparatus includes
a regulating element coupled to a spatial region of any example
device or configuration according to the principles herein,
including the example devices of any of FIGS. 5A through 6F or the
device configurations of any of FIGS. 7A through 7C. The example
regulating element includes a voltage applying element and at least
one of a temperature regulating element and a source of
electromagnetic radiation. The voltage applying element is
configured to apply a potential difference in a direction across
the interface between the dielectric material layer and the
electrically conductive material layer. The temperature regulating
element is configured to regulate a temperature of the spatial
region of the example device. The source of electromagnetic
radiation is configured to irradiate at least a portion of the
spatial region of the example device. The regulating element is
configured to irradiate and/or regulate the temperature of the
spatial region of the example device (to activate mobile ionic
species in the dielectric material layer), and to regulate the
applied potential difference for a duration of time sufficient to
modify a proportionate amount of the at least one ionic species in
a portion of the electrically conductive material layer proximate
to the interface, thereby causing a change of the functional
property of the device. Using the regulating element, the desired
functional property change can be "programmed" into selected
regions of example device by controlling the metastable state of
the electrically conductive material layer at that spatial region.
As described herein, the change of the functional properties can be
retained (for the lifetime of the metastable state) after
discontinuance of applying the potential difference, and the
irradiating and/or the temperature regulation.
[0133] In various example implementations, the voltage applying
element can be configured to apply a sufficiently high potential
difference of a magnitude of 50 millivolts or less, about 0.1 V,
about 0.3 V, about 0.5 V, about 0.7 V, about 1.0 V, about 1.5,
about 2.0 V, about 3.0 V, about 5.0 V, about 7.0 V, about 10 V,
about 20 V, about 50 V, about 100 V, or greater. As described
herein, the polarity of the potential difference depends on the
type of metastable state sought, the existing state of the device
at the time the potential difference is applied, and the device
layer structure.
[0134] The direction of ionic motion is determined by the charge of
the ionic species and the polarity of the applied voltage.
Properties of the electrically conductive material layer depend on
the chemical composition and defect structure at the interface
formed with the dielectric material layer, accumulation or
depletion of the mobile ionic species at this interface can
significantly modify the properties of the target material layer.
Also, motion of the mobile ionic species into a portion of the
target layer, beyond the immediate interface region, facilitates
the modification of the chemical composition and defect structure
of parts of the target material layer. This allows access to
additional material properties that might not be directly sensitive
to the interface.
[0135] In various example implementations, the temperature
regulating element can be a heating element, a thermoelectric
element, or a laser beam. The heating element may be configured as
a resistive element coupled to the spatial region of the device.
The thermoelectric element can be thin-film thermoelectric, such as
but not limited to a Bi.sub.2Te.sub.3-based film or a
CoSb.sub.3-based skutterudite material.
[0136] In an example implementation wherein the temperature
regulating element is configured to heat the spatial region of the
device, the heating is applied to heat the region to a threshold
temperature value of about 22.degree. C., about 25.degree. C.,
about 30.degree. C., about 50.degree. C., about 70.degree. C.,
about 100.degree. C., about 120.degree. C., about 150.degree. C.,
about 170.degree. C., about 200.degree. C., about 250.degree. C.,
about 300.degree. C., or about 350.degree. C., or higher. In an
example, the threshold temperature value is set to be within the
range of allowable operating temperatures of an example device.
[0137] Example, systems, methods, and apparatus are also provided
for tuning the functional properties using an apparatus that
includes a plurality of separately addressable regulating elements.
Each regulating element can be coupled to a different, respective
spatial region of an example device. In this example, each
regulating element is configured to regulate a potential difference
and either (i) the temperature, or (ii) the electromagnetic
radiation exposure, or both (i) and (ii), at each different,
respective spatial region of the device, thereby causing a
modification in proportionate amount of the at least one ionic
species in the portion of the electrically conductive material
layer proximate to the interface at each different, respective
spatial region of the example device.
[0138] Example, systems, methods, and apparatus are also provided
for tuning the functional properties of an example device that
includes a plurality of device elements (also referred to herein as
an active element) in an array, using an apparatus that includes a
plurality of regulating elements. A device element can be
configured as any of the example device or configuration according
to the principles herein, including the example devices of any of
FIGS. 5A through 6F or the device configurations of any of FIGS. 7A
through 7C. Each of the regulating elements can be coupled to a
respective one or more of the device elements. Each regulating
element is configured to regulate the potential difference and
either (i) the temperature, or (ii) the electromagnetic radiation
exposure, or both (i) and (ii), at each of the respective one or
more of the device elements, thereby causing a modification in
proportionate amount of the ionic species in a portion of the
electromagnetic material layer proximate to the interface of the
respective device element.
[0139] In any example herein, the example device could be
configured as a flexible device or a substantially rigid device. In
an example device, the target layer and/or the dielectric material
layer could be formed of a flexible material. In an example, the
example flexible device could include a flexible substrate, and the
target layer and dielectric material layer could be disposed over
at least a portion of the flexible substrate. In another example,
the example device could be configured with a combination of
flexible regions and more rigid regions. In any example herein, one
or both of the target layer and dielectric material layer could be
grown, using any deposition technique and tool in the art, on a
large area substrate that includes flexible and rigid regions.
[0140] Non-limiting examples of flexible substrates include thin
wood or paper, vinyl, leather, or other fabric (including artwork
or other works on canvas), a polymer or polymeric material.
Non-limiting examples of applicable polymers or polymeric materials
include, but are not limited to, a polyimide, a polyethylene
terephthalate (PET), a silicone, or a polyeurethane. Other
non-limiting examples of applicable polymers or polymeric materials
include plastics, elastomers, thermoplastic elastomers,
elastoplastics, thermostats, thermoplastics, acrylates, acetal
polymers, biodegradable polymers, cellulosic polymers,
fluoropolymers, nylons, polyacrylonitrile polymers, polyamide-imide
polymers, polyarylates, polybenzimidazole, polybutylene,
polycarbonate, polyesters, polyetherimide, polyethylene,
polyethylene copolymers and modified polyethylenes, polyketones,
poly(methyl methacrylate, polymethylpentene, polyphenylene oxides
and polyphenylene sulfides, polyphthalamide, polypropylene,
polyurethanes, styrenic resins, sulphone based resins, vinyl-based
resins, or any combinations of these materials.
[0141] An example array of device elements according to the
principles herein can be a configured as a two-dimensional array
(illustrated in FIGS. 8A and 8B) or a three-dimensional,
multi-layer array (illustrated in FIG. 9). Device elements of the
2-D or 3-D array can be separately addressable. The device elements
can be configured as any of the example devices or configurations
herein, including the example devices of any of FIGS. 5A through
6F, or the device configurations of any of FIGS. 7A through 7C. The
example systems and apparatus of FIGS. 8A, 8B, and 9, include
components and circuits for "writing" (e.g., setting a device
component to a first metastable state or a second metastable state)
or "reading" from device elements of the example arrays. The read
operations may vary depending on the type of application, and can
involve, e.g., detecting resistive state of a portion of a device
element, sensing the charge of a particular device element, or
passing current through the device element.
[0142] FIG. 8A shows an example 2-D array of device elements
according to the principles herein. The example 2-D array includes
a plurality of device elements 802 disposed in separately
addressable regions. The example 2-D array can include at least one
interstitial region 804 that is devoid of device elements 802. As
shown in FIG. 8A, the 2-D array can also include one or more
components 806, such as but not limited to at least one processing
unit, a power source, power circuitry, one or more sensors (such as
but not limited to at least one temperature sensor and/or at least
one electromagnetic radiation sensor), at least one wireless
communication component, or other integrated circuit (CMOS)
components. In some examples, the power source can be a wireless
power source. FIG. 8A also illustrates a regulating element 810
that can be coupled to the spatial region of a device element. The
regulating element 810 can be configured according to any example
herein.
[0143] FIG. 8B illustrates an example 2-D array of device elements,
configured in a cross-bar geometry. The example 2-D crossbar array
composed of a lower layer of approximately parallel cross-bar wires
820 that are overlain by an upper layer of approximately parallel
cross-bar wires 825. The parallel cross-bar wires of the upper
layer 825 can be oriented roughly perpendicular, in orientation, to
the parallel cross-bar wires of the lower layer 820. In another
example, although the orientation angle between the upper and lower
parallel cross-bar wires may vary. The two layers of cross-bar
wires form a lattice, or crossbar, in which each cross-bar wires of
the upper layer 825 overlies all of the cross-bar wires of the
lower layer 820. The device elements 830 are disposed between an
upper layer cross-bar wire 825 and a lower layer cross-bar wire
820, formed between the crossing nanowires at the overlap
intersection of the two layers of cross-bar wires. Consequently,
each cross-bar wire 825 in the upper layer is connected to every
cross-bar wire 820 in the lower layer through a device element and
vice versa. Each device element 830 is separately addressable
through the selection of the respective upper layer cross-bar wire
825 and lower layer cross-bar wire 820. That is, lower cross-bar
wires 820 and upper cross-bar wires 825, can be used to uniquely
address, including applying voltages to read data and/or to write
data (i.e., set to a first metastable state or a second metastable
state) to the device elements. Portions of the cross-bar wires 820,
825 between the device elements can also be configured to serve as
conductive lines to the device elements, and as portions of the
regulating elements.
[0144] FIG. 9 shows an example 3-D, multi-layer array of device
elements according to the principles herein. The 3-D multi-layer
array is configured as a base 902, a multi-layer arrangement of 2-D
arrays 904 disposed over the base, and conductive lines 906, 907
leading from the base to provide electrical communication with each
layer of the multilayer structure. At least one device element and
regulating element are positioned at the intersections 908 in each
2D array on each layer. Conductive lines 906 can be driven
independently using the external applied voltage in each layer. The
base 902 includes a wiring area 903 (including CMOS circuitry), and
contact areas 904 and 905 for the conductive lines. The multi-layer
arrangement of 2-D arrays 902 can include any number of layers
(i.e., greater or fewer than four layers). The base 902 includes
circuitry and other components for providing instructions for
writing (e.g., setting a device component to a first metastable
state or a second metastable state) or reading from the 2-D arrays
904 with outside sources. The read operations may vary depending on
the types of device, and can involve, e.g., sensing the charge of a
particular device element, passing current through the device
element, and detecting resistive state. For example, an external
voltage can be applied to respective device element(s) using
conductive lines 906 and 907. In some examples, wiring area 903 can
include a column control circuit including a column switch and/or a
row control circuit including a row decoder. The base can be
integrated with (CMOS) circuitry for selectively address device
elements, providing input/output functions, buffering, logic, or
other functionality. For example, the CMOS circuitry can be
configured to selectively address, including applying the potential
to, the targeted device element(s). The CMOS circuitry can be used
to effect the applying the read and write voltages to the
conductive lines as described herein.
[0145] In the example of FIG. 9, conductive lines 907 are
illustrated as being coupled in common in the layers. In other
examples, conductive lines 907 may be driven independently in two
or more layer using the external applied voltage. The CMOS
circuitry can be configured to selectively address (including
applying external voltages to) ones of the device elements (the
targeted device elements) using the conductive lines 906, 907.
[0146] In various example devices and configurations according to
the principles herein, including the example devices of any of
FIGS. 5A through 6F or the device configurations of any of FIGS. 7A
through 9, the electrically conductive material layer can be formed
as a nanostrip disposed in the x-y plane. A nanostrip can be
configured as a portion of the electrically conductive material
layer that is formed as a longitudinal structure. For example, the
nanostrip can be configured to have a rectangular cross-section.
The nanostrip can have a length to width aspect ratio of at least
about 3:2 (i.e., length/width.apprxeq.1.5), or higher. For example,
the aspect ratio can be about 5:1, about 10:1, about 100:1, about
1000:1, or higher. The nanostrip can have a width on the order of
nanometers, such as but not limited to about 3 nm, about 5 nm, 0
nm, about 25 nm, or about 50 nm. The thickness of the nanostrip in
the z-direction can be less than the width of the nanostrip. In an
example, the electrically conductive material layer can include two
or more nanostrips.
[0147] FIG. 10 shows an example memristive device based on a
conductive material layer formed as at least one nanstrip.
Terminals 1002 and 1004 can used to drive a current through the
electrically conductive material layer 1006. The conductive
material is at least partially covered by an electronically
insulating dielectric material layer 1007 which includes ionic
species (cations (C.sup.x+) and anions (A.sup.y-). As a
non-limiting example, the anions (AY.sup.-) can be oxide ions
(O.sup.2-). The electronically insulating dielectric material layer
also acts as a good ionic conductor. The gate electrode disposed
over the insulating layer provides the third terminal 1008 of the
device. As shown in FIG. 10, with a non-zero potential difference
applied in a first direction across the conductive material layer
1006 and the gate electrode 1008, an amount of the ionic species
migrates into portions of the conductive material layer proximate
to the interface between the dielectric material layer 1007 and the
conductive material layer 1006. The resulting change in the
measured value of vertical resistance of the vertical resistive
state (R.sub.V) can be probed across gate electrode and conductive
material layer. The resulting change in the measured value of
lateral resistance of the lateral resistive state (R.sub.L) can be
probed across a segment of the electrically conductive material
layer. In various examples, the value of lateral resistance can be
measured: (i) across a portion of the electrically conductive
material layer that overlaps with the dielectric material layer, or
(ii) across a portion of the electrically conductive material layer
that does not overlap with the dielectric material layer, or (iii)
a segment that encompasses both regions (i) and (ii).
[0148] In an example implementation, the example device of FIG. 10
can be formed as a multi-bit device, with multiple gate electrodes
forming multiple separately addressable sites to apply at least one
of a bias (write) voltage or a read voltage (for determining
vertical resistance R.sub.V). In another example implementation,
the example device can be formed with more than two terminals in
electrical communication with the nanostrip to provide for multiple
differing probes of the lateral resistance R.sub.L at separately
addressable sites.
[0149] In any example system, method, apparatus or device according
to the principles herein, at least one of a conductive contact or a
gate electrode can be formed as a mask. For example, a shadowed
mask can be used as electrodes for providing electrical contact to
the example device.
[0150] Example methods are also provided for tuning the functional
properties of an example device. An example method includes (i)
irradiating a portion of the example device using electromagnetic
radiation, and/or (ii) change the temperature of the portion of the
device. The example method includes applying a potential difference
in a direction across the dielectric material layer and the
electrically conductive material layer for a duration of time
sufficient to cause a change in the proportionate amount of the at
least one ionic species in a portion of the electrically conductive
material layer proximate to the interface. As described herein,
this causes a type of property change of the device. The type of
property change can be at least one of: magnetic anisotropy
property, a magnetic permeability property, a saturation
magnetization property, an optical property, a magneto-optical
property, an electrical property (including a resistive state), a
mechanical property, an d a thermal property of a portion of the
device. As described herein, the example device retains the type of
property change after discontinuance of the irradiating, and/or the
temperature change, of the device.
[0151] In various examples, the duration of time for applying the
potential difference can be about 1.0 nanosecond, about 10
nanoseconds, about 20 nanoseconds, about 50 nanoseconds, about 100
nanoseconds, about 1 microsecond, about 500 microseconds, about 1
millisecond, about 100 milliseconds, about 500 milliseconds, about
second, about 5 seconds, about 10 seconds, about 30 seconds, about
60 seconds, about 3 minutes, about 5 minutes, about 10 minutes,
about 30 minutes, about 60 minutes, or longer (including
substantially longer periods of time).
[0152] In various examples, changing the temperature can include
heating the portion of the device to a temperature above a
threshold temperature value. The threshold temperature value can be
about 22.degree. C., about 25.degree. C., about 30.degree. C.,
about 50.degree. C., about 70.degree. C., about 100.degree. C.,
about 120.degree. C., about 150.degree. C., about 170.degree. C.,
about 200.degree. C., about 250.degree. C., about 300.degree. C.,
or about 350.degree. C., or higher.
[0153] In various examples, the magnitude of the potential
difference can be 50 millivolts or less, about 0.1 V, about 0.3 V,
about 0.5 V, about 0.7 V, about 1V, about 2 V, about 3V, about 5 V,
about 7 V, about 10V, or greater.
[0154] According to the principles herein, example methods are also
provided for controlling materials properties of a multi-layer
device design. In this example, the material property is a
resistance state of the conductive material layer of the device. In
a non-limiting example, the device design can include three layers
and the device can function as a two terminal device. The main part
of the multilayer structure can be made of the bilayer of the
target material layer (the electrically conductive material layer)
whose properties are to be electrically regulated, and a functional
material layer (dielectric material layer) that includes the mobile
ionic species, i.e., the ionic species that can move between the
functional material and the target material in an electric field
(from the applied potential difference. The dielectric layer acts
as an electrical insulator to block the flow of electrons across
the interface between the electrically conductive material layer
(target material layer) and the dielectric material layer
(functional material layer). The layer of target material is
typically thin such that its properties are strongly influenced by
the chemical composition of its interface with the functional
material layer. In an example device configuration, the target
layer can be disposed over and in electrical communication with a
first conductive contact layer, the functional layer is disposed
over and forms an interface with the target layer, and a second
conductive contact layer can be disposed over and in electrical
communication with the functional layer. The conductive contact
layers are configured to act as the two terminals in this example
device configuration. Example implementations herein provide a
device having a layer structure for the device. The example devices
may also include one or more layers, in addition to those discussed
herein) to optimize parameters such as, but not limited to,
performance and functionality.
[0155] In an example implementation, a sufficiently high voltage is
applied between the two terminals of the multilayer structure. The
resulting electric field acts to move the mobile ionic species in
the functional layer towards (or away) from the interface with the
target material (depending on the direction of the electric field).
The direction of ionic motion is determined by the charge of the
ionic species and the polarity of the applied voltage. Since the
properties of the target material strongly depend on the chemical
composition and defect structure at the interface with the
functional layer, accumulation or depletion of the mobile ionic
species at this interface can significantly modify the properties
of the target material layer. Also, motion of the mobile ionic
species into a portion of the target layer, beyond the immediate
interface region, facilitates the modification of the chemical
composition and defect structure of parts of the target material
layer. This allows access to additional material properties that
might not be directly sensitive to the interface. In an example
implementation of a multilayer device structure, the target layer
could be a non-noble metal and the functional material could be a
metal-oxide with high oxygen ion mobility.
[0156] In a non-limiting example implementation, the example device
includes an electrically conductive material layer forming an
interface with a dielectric material layer, to provide a
metal/metal-oxide bilayer. Such example devices can be of great
commercial and technological interest, since they can be used
widely in industries such as the microelectronics and the chemical
industry. According to example systems, methods, and apparatus
herein, regulation of ionic species at the interface of
metal/metal-oxide bilayers in such example device can be caused to
regulate functional properties as varied as catalytic activity,
charge and spin transport, ionic exchange, mechanical behavior,
thermal conductivity, electrical properties, and magnetism. These
properties depend sensitively on the oxygen stoichiometry and
defect structure at the metal/metal-oxide interface. Voltage
application between the two terminals of the example device
structure facilitates regulation of the oxygen stoichiometry at the
metal/metal-oxide interface, thereby providing control over a wide
variety of material properties and device functional
properties.
[0157] According to the principles herein, an example device can be
operated at temperatures above room temperature to speed up the
motion of the mobile ionic species and increase the speed at which
the properties of the target material can be regulated electrically
(using the applied voltage). The higher ionic mobility at elevated
temperature also facilitates reduction of the voltage applied to
the two terminals of the example device. The temperature range in
which the example device is operated can be chosen such that the
elevated temperature alone does not result in permanent
modifications of the material properties of the electrically
conductive material layer. The applied voltage provides
directionality to the ionic motion (through its polarity), and the
elevated temperature supplies thermal energy to the system to
activate the motion of the mobile ionic species of the dielectric
material layer.
[0158] Example systems, methods, and apparatus according to the
principles herein also provide several ways to spatially control
the extent of the change to the properties of the target layer,
thereby controlling the functional properties of the example
device. In an example, the spatial extent of one or more of the
electrical contact terminals of the example device is patterned in
a configuration that provides spatial control over the extent of
application of the applied voltage. The electric field from the
applied voltage can be caused to act on the portion of the
dielectric material layer within the extent of the electrical
contact terminal to drive the mobile ionic species into or out of
the target layer. In other examples, the change to the properties
of the target layer can be regulated through local control of the
voltage and/or temperature of the example device. In a first
example, the example device can include means to apply a voltage
globally across two conductive contacts (terminals) of the device,
and means to regulate the temperature at local spatial regions of
the example device. As a non-limiting example, a focused laser beam
can be used to supply the thermal energy at local spatial regions
of the example device to activate motion of the mobile ionic
species in the dielectric material layer. In a first example, the
example device can include means to heat substantially the entire
example device and meant to apply a voltage at local spatial
regions of the example device. As a non-limiting example, the
voltage can be supplied using a conductive tip in close proximity
to the dielectric material layer (such as but not limited to a tip
from a scanning probe microscope). Any combination of local
temperature regulation (such as heating) and local voltage
application could be used to control the device functional
properties, according to the principles herein. In another example,
the properties of the target layer can be regulated using
electromagnetic irradiation. For example, optical exposure with
optical stimulation using electromagnetic radiation can be used to
cause changes in the optical functional properties of a device
(such as, but not limited to, light transmission characteristics
for responsive window tinting or outdoor displays). The optical
exposure can be applied to local spatial regions of the example
device, or to larger areas of or substantially the entire device.
The voltage can be applied to the example device while local
spatial regions are irradiated with electromagnetic radiation. In
another example, local optical exposure can be coupled with
temperature regulation to cause changes in the optical functional
properties. For example, the larger areas of or substantially the
entire device can be heated to activate the mobile ionic species,
and optical exposure can be made to local spatial regions of the
example device. The voltage can be applied to the example device
while local spatial regions are irradiated with electromagnetic
radiation and/or subjected to temperature regulation.
[0159] Example systems, methods and apparatus are also provided to
electrically control the properties of a thin layer of target
material with high spatial resolution. As example apparatus can be
implemented to create complex patterns of variations of material
properties across a spatially extended area of the example device,
thereby generating a device having complex functional properties.
The example apparatus can include one or more regulating elements
that can be configured to scan across the example device and
locally to perform at least one of applying a voltage, regulating
temperature, or irradiating using electromagnetic radiation. Thus,
the one or more regulating elements can be operated similarly to a
"write head" to spatially program the desired materials properties
(i.e., introduce the desired metastable state in different
spatially distinct regions of the example device). In an example,
the spatial resolution of such an example apparatus could be
determined by the minimum area at which voltage and/or thermal
energy and/or electromagnetic irradiation is supplied to the
example device structure using the at least one regulating element.
In an example implementation, the apparatus can be configured such
that the at least one regulating element scans across the different
spatial regions of the example device, to program the desired
pattern of variations of metastable state into distinct different
spatial regions of the example device. In another example, the
apparatus can be configured such that the example device is moved
(displaced) relative to a substantially stationary (or limited
displacement range) regulating element(s), to program the desired
pattern of variations of metastable state into distinct different
spatial regions of the example device. In example apparatus where
both the at least one regulating element and the example device are
configured for displacement, the at least one regulating element
could be configured to scan only in one or more directions while
the example device is driven (displaced) in a different direction.
For example, the at least one regulating element and the example
device could be moved in different, perpendicular directions. In
these example apparatus, the at least one regulating elements
function similarly to the "write head" of a property printer.
[0160] Example systems, methods and apparatus herein provided for
control of the displacement of the one or more regulating elements
and/or the example device using manual control, or control by a
control device including at least one processing unit.
[0161] Non-limiting examples of control devices include a computing
device (such as, but not limited to, a computer, a laptop, a
notebook), a smartphone (such as, but not limited to, an
IPHONE.RTM. (Apple Inc., Cupertino, Calif.), a BlackBerry.RTM.
(Blackberry Limited, Waterloo, Ontario, Canada), or an
Android.TM.-based smartphone), a tablet, a slate, an
electronic-reader (e-reader), a digital assistant, or other
electronic reader or hand-held, portable, or wearable computing
device, or any other equivalent device, or a game system (such as
but not limited to an XBOX.RTM. (Microsoft, Redmond, Wash.), a
Wii.RTM. (Nintendo of America Inc., Redmond, Wash.), or a
PLAYSTATION.RTM. (Sony Computer Entertainment America Inc., San
Diego, Calif.)).
[0162] Example systems, methods and apparatus provide a graphical
user interface configured to allow a user to use property design
files, translated into a tool path for controlling the patterning
of the metastable states to the example device (like the write head
of a property printer). For example, the property design files can
be digital files. An example digital property design files can
include processor-executable instructions, to be executed by a
processing unit, to cause an example apparatus to effect a
displacement of the one or more regulating elements and/or the
example device, to program the desired pattern of variations of
metastable state into the different spatial regions of the example
device. An example property design files can include
processor-executable instructions, to be executed by a processing
unit, to cause an example apparatus to effect the actuation of the
one or more regulating elements relative to one or more distinct
spatial regions of the example device, to program the desired
pattern of variations of metastable state into the different
spatial regions of the example device. Execution of the
processor-executable instructions of the example property design
file would cause the desired positioning of the at least one
regulating element and also determine the local dose of one or more
of the voltage, thermal energy, and electromagnetic irradiation, to
generate the desired pattern of metastable states.
[0163] An example property design file can include a
two-dimensional (2-D) map of the desired pattern of metastable
states for the desired target material properties (and hence device
functional properties). This example property design file can
include processor-executable instructions, to be executed by a
processing unit, to cause an example apparatus to effect a
displacement of the one or more regulating elements and/or the
example device, to transfer the desired pattern of metastable
states to the target material layer. An example graphical user
interface can be configured to allow a user to use the 2-D map of
the property design files, translated into a tool path for
controlling the patterning of the metastable states to the example
device (like the write head of a property printer). Execution of
the processor-executable instructions of the example property
design file would cause the desired positioning of the at least one
regulating element and also determine the local dose of one or more
of the voltage, thermal energy, and electromagnetic irradiation, to
generate the desired pattern of metastable states.
[0164] Non-limiting examples of processor-executable instructions
include software and firmware.
[0165] Example systems, methods and apparatus herein provided for
use of a reusable mask, or single use mask, that can be coupled to
a portion of a surface of the example device, to selectively couple
the one or more regulating elements relative to one or more
distinct spatial regions of the example device. For example, an
apparatus could be configured such that only exposed parts of the
example device area can be subjected to at least one of a voltage,
thermal energy, and electromagnet irradiation. Such an example
apparatus could allow provide faster throughput at reduced cost. As
a non-limiting example, an apparatus could be configured such that
a voltage could be applied globally across portions of the example
device, and use a high-power lamp (such as but not limited to an
infrared lamp) with an opaque mask that to expose selected parts of
the example device to electromagnetic radiation. The heat from the
electromagnetic radiation can cause local heating of the dielectric
material layer, activate the mobile ionic species and to generate
the desired metastable state (as described herein), i.e., modify
the materials properties only in those exposed areas.
[0166] Example systems, methods and apparatus herein provided a
platform that provides a pathway to electrically gate a wide
variety of key materials in electronics devices. The ability to
electrically gate these materials (i.e., using voltage as a
parameter to tune metastable properties) facilitates the
programming of the patterns of materials properties in the
electronic devices, according to the principles of any example
system, method and apparatus herein. The example platforms
described herein can be used to provide a wide variety of
completely new and previously unimaginable electronic devices and
applications. Non-limiting examples of envisioned applications of
the example systems, methods and apparatus herein include
electrically-controllable catalysts for the chemical industry,
voltage-controlled optical switches for the optical communications
industry, voltage-controlled low power memories for the
microelectronics industry, voltage-tunable sensors, and
voltage-controlled lateral conductive devices. According to the
principles of the example systems, methods and apparatus herein,
the capability to pattern material properties over large areas and
with high spatial resolution can be exploited to produce low-cost
sensors, electronic devices, and lab-on-a-chip systems that might
otherwise require many complex and expensive fabrication steps to
pattern materials into the desired spatial configuration. Since the
material properties changes achievable using the example systems,
methods and apparatus occur in response to application of the
regulating tools of voltage, temperature regulation, or optical
stimulation, or some combination of these regulating tools,
environmentally-responsive materials can be designed and developed
using these techniques.
[0167] Example systems, methods and apparatus are provided for
tuning the functional properties of an example device based on use
of a spacer layer. In this example, the device includes a
dielectric material layer disposed in an x-y plane, a spacer layer
disposed over and forming a first interface with the dielectric
material layer, and a target layer disposed over and forming a
second interface with the spacer layer. The layer structure of such
an example device could be described relative to the layer
structure of and of FIGS. 6C through 6F. Relative to FIG. 6C or 6E,
the electrically conductive material layers could be formed as a
bilayer of a target layer and a spacer layer. Relative to FIG. 6D
or 6F, the electrically conductive material layers could be formed
as a bilayer of a target layer and a spacer layer.
[0168] The target layer and/or the spacer layer can be configured
to reversibly uptake an amount of at least one ionic species that
migrates from the dielectric material. The spacer layer can be
formed from a transition metal, a rare earth metal, a noble metal,
or any combination thereof. The interaction between the target
layer and the spacer layer can create hybrid species at the
interface that affect the interfacial properties of the target
layer. The hybrid states can affect the device functional
properties, for example, by modifying the initial state of the
interfacial properties of the target layer. For example, the spacer
layer can affect ionic transport to the target layer, and can also
changes interface properties (to derive a different baseline of
properties). This provides additional capabilities for tuning the
functional properties of the example device, since the offset of
the initial state of a device can cause the device to function in
differing functional regimes. The regulating elements can be used
to apply a potential difference and/or irradiate and/or regulate
the temperature of the spatial region of the device, for a duration
of time sufficient to modify a proportionate amount of the at least
one ionic species in a portion of the target layer, thereby causing
a change of the functional property of the device.
[0169] In an example device, the spacer layer is configured through
selection of the type of metal material(s) used, the thickness of
the spacer layer (in the z-direction), and the conformation of the
layer (i.e., the spacer layer being formed as a discontinuous
layer, or a continuous layer). In different examples, the spacer
layer can have thicknesses ranging from about 0.2 nm, about 0.3 nm,
about 0.5 nm, about 0.8 nm, about 1 nm, about 1.3 nm, about 1.5 nm,
about 1.8 nm, about 2 nm, about 3 nm or thicker.
[0170] For example, an amount of a spacer at the interface between
the dielectric material and a transition metal layer can be used to
modify the electrical properties of the example device. The type
and thickness of the spacer can be tuned to change the starting
point at which an apparatus can exert electrical control of the
memristive device properties. While a thin spacer layer facilitates
operation of the example device in a first functional range, a
thicker spacer layer can facilitates operation of the example
device in a second functional range that may or may not overlap
with the first functional range.
[0171] The spacer layer is made sufficiently thin, or formed as a
discontinuous layer, that allows the at least one ionic species to
reversibly reach portions of the target layer when a potential
difference is applied to the device. In an example, the spacer
layer can be made of the same type of material as the cation of the
dielectric (e.g., a rare earth metal or a transition metal), or a
different type of material (including a transition metal, a rare
earth metal, or a noble metal). In an example where the spacer
layer is formed from a metal that does not support reversible
migration of the ionic species, such as but not limited to copper,
silver, or gold, the spacer layer can be formed as a discontinuous
or "dusting" layer, causing areas of a target layer to be exposed,
to allow the ionic species to reach portion of the target
layer.
[0172] Example systems, methods and apparatus are provided for
tuning the functional properties of an example device based on
regulation of optical properties of portions of the example device
near the interface between an electrically conductive material
layer and a dielectric layer. In an example implementation, the
device (or device element) can be an optical plasmonic device. In
an example device, the electrically conductive material layer can
be formed from a transition metal material (such as but not limited
to a 1 nm cobalt thin film), and the dielectric material layer
forming an interface can be, but is not limited to, a gadolinium
oxide material. The example device includes a layer of a noble
metal, such as but not limited to silver gold, platinum, palladium
(or any alloy thereof) disposed on the other surface of the
transition metal material layer. The noble metal layer facilitates
an efficient excitation of plasmonic waves at the noble metal-air
interface. The reflectivity spectrum of the example optical
plasmonic device can be controlled by the oxidation state of the
transition metal material layer proximate to the interface. The
plasmonic waves at the dielectric/ferromagnetic material interface
also can likewise be controlled by regulating the oxidation state
of the ferromagnetic layer proximate to the interface. For example,
the plasmonic and magneto-plasmonic resonances and reflectivity
characteristics at the dielectric-metal interface differs between
the state corresponding to a completely oxidized ferromagnetic
material layer and the state corresponding to a partially metallic
ferromagnetic material layer. Typically, the plasmonic resonances
in magnetic materials are weak and broad. By contrast, if the
ferromagnetic material layer includes a larger proportionate amount
of the ionic species (e.g., is completely oxidized), then portions
of the ferromagnetic material layer become dielectric, thereby
changing the nature of the modified ferromagnetic material
layer/noble metal interface. This causes portions of the example
device to exhibit the sharper plasmonic resonance and different
reflectivity characteristics of the noble metal. According to the
example systems, methods and apparatus herein, the patterning of
the metastable state of the ferromagnetic material layer proximate
to the interface can be used to control the local reflectivity of
different portions of the example optical plasmonic device. The
example systems, methods, and apparatus according to principles
described herein also apply to this example implementation.
[0173] The functional properties of an example optical device can
be tuned by using a spacer layer disposed between the target layer
and the dielectric material layer. For example, the spacer layer
can be configured to tune the surface properties of the underlying
target layer, which cause the optical device to exhibit a different
set of baseline of properties. Non-limiting example of such a
functional property can be an optical modulation, a photonic
property, a plasmonic resonance, a reflectivity, or a
magneto-optical property of the target layer and/or spacer layer.
By controlling the progression of the migration of the ionic
species into portions of the target layer and/or the spacer layer
according to the principles described herein, the functional
property of the device can be tuned. The migration of the ionic
species converts portions of the target layer and/or spacer layer
from a metal material to a dielectric material, thereby modifying
the optical properties. As a result, the optical properties of the
device can be tuned reversibly.
[0174] For example, the presence of a spacer layer can modify the
type of collective plasmonic resonance properties of the bilayer of
the target layer and the spacer layer. Where a device without a
spacer layer may exhibit a sharper plasmonic resonance, a device
with a spacer layer may exhibit a broader plasmonic resonance.
Typically, the plasmonic resonances in magnetic materials are weak
and broad, while the plasmonic resonance in a noble metal is sharp.
An example device can include a bilayer of a ferromagnetic spacer
layer and a noble metal target layer. If the spacer layer is
completely oxidized, then the spacer layer itself acts as a
dielectric material. With substantially complete oxidation of the
spacer layer, the interface between the target layer and the spacer
layer exhibits sharp plasmonic resonance and different reflectivity
characteristics. By controlling the progression of the migration of
the ionic species into portions of the target layer and/or the
spacer layer according to the principles described herein, the
optical property (including plasmonic resonance) of the device can
be tuned. For example, the plasmonic resonance property of the
example device can be tuned reversibly from a broader plasmonic
resonance to a sharper plasmonic resonance based on the
proportionate amount of the ionic species that is caused to migrate
into portions of the target layer and/or the spacer layer using the
systems, methods, and apparatus according to the principles
described herein. This is based on the presence of the ionic
species converting portions of the target layer and/or spacer layer
from a metal material to a dielectric material, thereby modifying
the optical properties. As a result, the optical properties of the
device can be tuned reversibly.
[0175] An example device having a noble metal layer over a
transparent dielectric material layer may exhibit a sharper
plasmonic resonance. An example device with a spacer layer of a
thin transition metal layer between the noble metal layer and the
transparent dielectric material layer may exhibit a broader
plasmonic resonance. As non-limiting examples, the thin transition
metal layer can be a cobalt layer and the noble metal layer can be
a gold layer. By controlling the progression of the migration of
the ionic species into portions of the thin transition metal spacer
layer, the plasmonic resonance can be tuned. The presence of the
ionic species converts portions of the transition metal spacer
layer from a metal material into a dielectric material, causing the
sharper plasmonic resonance of the noble metal layer to re-appear.
Since the migration of the ionic species can be controlled
reversibly, the example device can be controllably cycled between
the differing plasmonic resonance properties.
[0176] Example systems, methods and apparatus are provided for
tuning the functional properties of an example device based on
regulation of magneto-optical properties of portions of the example
device near the interface between an electrically conductive
material layer and a dielectric layer. In an example device, the
electrically conductive material layer can be formed from a
bi-layer of a spacer layer including at least one rare earth metal
material and a target layer including at least one ferromagnetic
transition metal material. Non-limiting examples of applicable rare
earth metals include gadolinium, terbium, dysprosium, holmium, or
neodymium. Non-limiting examples of applicable rare earth metals
include iron, cobalt, and nickel. In an example device, the
electrically conductive material layer can be formed from an alloy
including at least one rare earth metal material and at least one
ferromagnetic transition metal material. The
rare-earth/magnetic-transition-metal electrically conductive
material alloy or bi-layer can exhibit a significant
magneto-optical Kerr effect. The magneto-optical Kerr effect is the
change in the polarization and ellipticity of electromagnetic
radiation that is reflected from the electrically conductive layer.
The rare-earth/transition metal ferromagnet multilayers and alloys
exhibit large magneto-optical constants, while transition metal
ferromagnetic materials exhibit relatively smaller magneto-optical
constants. Therefore, the magneto-optical Kerr rotation and
ellipticity of light reflected at the dielectric/metal interface
can be substantially different between the device state where the
rare earth metal layer includes a large proportionate amount of the
ionic species (e.g, the rare earth metal is completely oxidized)
compared to the device state where the rare earth metal layer
includes a much smaller proportionate amount, or none, of the ionic
species (e.g, the rare earth metal remains unoxidized). Thus, the
optical functional properties of the example device, such as but
not limited to the state of rotation of the polarization of
electromagnetic radiation reflected from the example device, can be
controlled based on regulating the migration of the ionic species
into portions of the target layer and/or spacer layer according to
the principles herein. By regulating the migration of the ionic
species into portions of the electrically conductive material layer
according to the principles herein, the magneto-optical properties
of the example device can be changed and regulated. An example
device can be configured to implement this magneto-optical effect,
to allow for the control of the polarization of light, for use in
such applications as optical signal transmission and modulation in
photonics. Other non-limiting example implementations of device
include optical switching, optical filter applications, on-chip
application, electromagnetic radiation polarization rotation, and
polarizers. For example, an example device can be used to rotate
the polarization of electromagnetic radiation so that the
polarization of electromagnetic radiation can, or cannot, get
through a polarizer component coupled to the example device,
thereby providing an optical switch. A non-limiting example device
can include a dielectric layer formed from gadolinium oxide,
forming an interface with a gadolinium metal spacer layer, which
forms an interface with a cobalt target layer. The spacer layer
and/or the target layer can have a thickness in the range of about
0.5 nm, about 1.0 nm, about 1.5 nm, about 2.0 nm, about 2.5 nm,
about 3.0 nm, about 3.5 nm, about 4.0 nm, or greater.
[0177] Example systems, methods, apparatus herein provide example
devices that exhibit bistable nonpolar memristive switching. After
electroforming, the resistance of the example devices can be
electrically switched by about 5 orders of magnitude. This
switching behavior is observed to correlate with voltage-driven
motion of the migration front of the ionic species into a portion
of the conductive material layer. For example, the switching
behavior is observed to correlate with voltage-driven motion of the
oxidation front and suggests a memristive switching mechanism based
on oxygen anions.
[0178] As described hereinabove, the use of the example memristive
devices can be based on reversible switching of the resistance
across the dielectric material layer (R.sub.V), or reversible
switching of the lateral resistance in the conductive material
layer. As non-limiting examples, the memristive device can be based
on reversible switching of the vertical resistance across the oxide
layer (R.sub.V) and/or the reversible switching of the lateral
resistance a conductive material layer formed from a transition
metal material. The integration of this novel resistive switching
mechanism into transition metal nanowire and nanostrip devices show
potential for applications as novel micro and nanoscale electrical
switches.
[0179] The example systems, methods, apparatus and devices
according to the principles herein exhibit two distinct memristive
switching mechanisms with changes in resistance of up to 5 orders
in magnitude, also exhibit unprecedentedly strong voltage effects
on memristive properties.
[0180] In a non-limiting example implementation, a memristive
device can be formed from deposition of continuous thin films of
Ta(4 nm)/Pt(3 nm)/Co(0.9 nm)/GdOx(33 nm) on a Si(100) substrate.
The dielectric material layer is formed from a GdOx layer. The
conductive material layer is formed from Co. On top of the
dielectric GdOx layer, a 100 .mu.m diameter Gd(2 nm)/Au(12 nm) gate
electrodes can be deposited. Layer thicknesses can be controlled by
controlling the deposition rate of each material.
[0181] In an example, example devices can be deposited based on
conductive materials layers formed from nanostrips of conductive
materials (such as shown in FIG. 10). As described hereinabove, the
strips can be formed as nanowires. As a non-limiting example,
nanostrips that are 500 nm wide, 50 .mu.m long can be patterned to
form the example devices. The example device can be formed with
layered structure Ta(4 nm)/Pt(3 nm)/Co(0.9 nm)/GdOx(3 nm). The
dielectric material layer is formed from a GdOx layer. The
conductive material layer is formed from Co. Sets of 50 nm thick Cu
contacts can be deposited on the nanostrip, one spaced 50 .mu.m
apart and the other spaced 40 .mu.m apart to provide multiple
contact. These contacts provide the 4 terminals required to probing
the resistance of the nanowire. Next, the nanostrip is covered by
30 nm GdOx. On top of the GdOx layer, 30 .mu.m wide Ta(2 nm)/Au(12
nm) gate electrodes were deposited at the center of the nanowire.
The aim of this device structure is to avoid an open GdOx edge
right underneath the electrode perimeter, such that the oxygen
stoichiometry can be modified uniformly across the entire
electrode.
[0182] The electrical properties of the continuous thin film
samples are probed at RT, in the dark and under medium vacuum
conditions to reduce the influence of illumination and atmospheric
oxygen on device characteristics. Mechanical microprobes are used
to contact the Au top electrodes and the Ta/Pt/Co bottom electrode
which is common to the devices. Current-voltage characteristics are
measured.
[0183] The resistance of the example nanostrip samples is measured
in ambient atmosphere and in a four (4) terminal geometry. For
resistance measurements the four (4) Cu electrodes are contacted
with mechanical microprobes and sense currents of -5 .mu.A to -+5
.mu.A are applied through the two outer electrodes. The two inner
electrodes are used to measure the voltage drop along the
nanostrip, resulting from the sense current and the resistance was
extracted from linear fits of voltage versus sense current. In
order to control the oxygen stoichiometry at the Co/GdOx interface,
gate voltages Vg were applied between the Ta/Au top electrode and
the transition metal nanowire at 120.degree. C. and for each device
state, the nanowire resistance was extracted as described above. In
all cases, positive bias refers to the top Au electrode being
positive with respect to the Ta/Pt/Co bottom electrode.
[0184] Polar MOKE measurements are made on the nanowire samples,
using a 532 nm diode laser attenuated to 1 mW. The laser is focused
to a .about.3 .mu.m diameter probe spot and positioned by a high
resolution scanning stage with integrated temperature control.
[0185] Example implementation of the voltage control of vertical
resistance is as follows. The electrical properties of Ta(4
nm)/Pt(3 nm)/Co(0.9 nm)/GdOx(33 nm) with 100 .mu.m wide Gd(2
nm)/Au(12 nm) gate electrodes are measured to demonstrate the link
between memristive switching and the magneto-ionic effect. Here,
voltages Vg and currents I.sub.V are applied and measured between
the top and bottom electrode of the metal/oxide/metal devices and
the resistance state of a device then refers to the resistance
across the oxide layer, i.e., the vertical resistance R.sub.V of
the device.
[0186] In the virgin state, the devices show high vertical
resistance RV of .about.100 GOhm at a voltage of I.sub.V.
[0187] FIGS. 11A-11B show a plot of the results of the measurements
of the electrical characterization of an example device in an
unprogrammed state (i.e., prior to application of a voltage). FIG.
11A shows cyclical voltammetry curve measured at a voltage scan
rate of 0.4V/s. FIG. 11B shows capacitance frequency characteristic
measured at zero bias with an AC amplitude of 0.3V on Ta(4 nm)/Pt(3
nm)/Co(0.9 nm)/GdOx(33 nm) with 100 .mu.m wide Gd(2 nm)/Au(12 nm)
gate electrodes. The cyclical voltammetry measurements are
performed in a voltage range of .+-.2 V, and show nearly
rectangular curves indicating the devices behave like ideal passive
capacitors. The rounded corners are likely the result of power loss
during charge and discharge of the capacitor due to series
resistance originating from the electrical contacts and electrodes.
The rapid rise in current above .+-.1.5 V also indicates the
emergence of ionic processes in the GdOx layer, above that voltage.
The device capacitance C can be determined geometrically from Eq. 1
and from a fit to the data plotted in FIG. 11A using Eq. 2, as
follows:
C= .sub.0 .sub.r,F/d.sub.GdOx Eq. 1
C=I.sub.V/dV.sub.g/dt Eq. 2
[0188] Here, .sub.0 is the permittivity of free space, .sub.r is
the dielectric constant of GdOx, which typically is .about.16.
Symbol F is the electrode area, and d.sub.GdOx is the thickness of
the GdOx layer. In Eq. 2, dVg/dt corresponds to the voltage scan
rate of .about.0.4V/s. With both equations the device capacitance
is computed as similar values of .about.35 pF.
[0189] These computation results are in good agreement with the
capacitance measurements performed in FIG. 11B, which yield
capacitance C.apprxeq.40 pF across the entire frequency range from
200 Hz to 1 MHz. The flat frequency dependence of C confirms the
nearly ideal capacitor behavior at low bias voltages. These results
are consistent with a conclusion that, at low bias voltages, the
voltage-induced changes can be attributed to magnetic properties to
electron accumulation/depletion in the Co electrode.
[0190] To use the metal/oxide/metal capacitor structures into
programmable memristive switching devices initially involves
electroforming step, involving either sweeping Vg to high bias
voltages or applying constant current stress (CCS). These methods
can be used to turn the nearly insulating virgin devices into a
memristive memory with a high resistance state (OFF-state, or "0")
and a low resistance state (ON-state, or "1").
[0191] FIG. 12 shows electroforming of an example device formed as
a Pt/Co/GdOx/Gd/Au capacitor under CCS. The voltage across a Ta(4
nm)/Pt(3 nm)/Co(0.9 nm)/GdOx(33 nm)/Gd(2 nm)/Au(12 nm) capacitor is
monitored over time, while a constant currents stress of 500 pA is
applied. The inset shows the device capacitance derived from the
slope of voltage versus time in the main figure. A current stress
of 500 pA is applied and the voltage across the device is monitored
over time. We find that the voltage increases continuously over
time until it reaches a critical value of .about.3V after
.about.1.1 s. Above this critical value, the voltage required to
sustain the constant current of 500 pA suddenly drops, indicating
the formation of a conducting path through the GdOx layer. The
device has now been electroformed and is in its low resistance or
ON state.
[0192] In electroforming via CCS, the current is kept constant and
therefore changes in the slope dVg/dt correspond to changes in
device capacitance (see Eq. 2). The capacitance during CCS is
plotted in the inset of FIG. 12 and increases steadily while the
voltage approaches its critical value. This increase in capacitance
can be attributed to charge trapping and defect formation/migration
in the oxide layer, which typically precedes the formation of a
conductive path through the oxide layer.
[0193] Alternatively, the example memristive devices can be
electroformed by sweeping Vg from 0 to +12 V or 0 to -12 V while
the compliance current is set to IV=10 mA. While this method
results in switchable devices, the power dissipated during
electroforming can be much higher in the voltage sweeping method
than in the CCS method. The CCS method can be used to keep device
degradation to a minimum.
[0194] FIGS. 13A-13B shows example memristive switching in example
devices based on a Pt/Co/GdOx/Gd/Au layer structure. FIG. 13A shows
the current-voltage characteristic showing transition from low
resistance ON state to high resistance OFF state (RESET) during a
positive voltage sweep. FIG. 13B shows the current-voltage
characteristic of ON and OFF states. The Ta(4 nm)/Pt(3 nm)/Co(0.9
nm)/GdOx(33 nm)/Gd(2 nm)/Au(12 nm) devices were initially
electroformed under constant current stress of 500 pA. After
electroforming, the Pt/Co/GdOx/Gd/Au devices are in their low
resistance ON state and exhibit reliable resistive switching. In
the ON state, a sweep of Vg beyond a so called RESET voltage of
typically 2 V to 5 V results in an abrupt drop in the current
through the device, corresponding to a transition from the low
resistance ON state to the high resistance OFF state (see FIG.
13A). Similarly, in the OFF state, a sweep of Vg beyond the so
called SET voltage of 7 V to 10V results in an abrupt increase in
the current and thus a transition from the OFF back to the ON state
(not shown here). At Vg=1 V, the resistance in the OFF state is
R.sub.V.apprxeq.50 MOhm whereas in the ON state R.sub.V.apprxeq.500
Ohm, implying resistance switching by 5 orders in magnitude (see
FIG. 13B).
[0195] Although the SET and RESET processes occur reliably under
positive bias. Sweeping Vg to negative bias during the SET and
RESET process can also switch the device between the ON and OFF
state. Various combinations of bias polarities during SET/RESET,
i.e., positive/positive, negative/negative, negative/positive and
positive/negative, can give rise to resistive switching in the
example memristive devices herein. This polarity independence of
the switching characteristics suggests that the memristive
switching observed in the memristive devices according to the
principles herein are of the bistable nonpolar type.
[0196] The formation of conductive filaments in the oxide layer can
be due to the combined effect of Joule heating and electric field.
In nonpolar switching, electroforming is typically explained in a
two-step process. A purely electronic effect establishes a hot
electronic filament and the resulting high current densities then
drive changes in material composition through heat-assisted ionic
motion. The RESET switch results from thermal rupture of the
conducting filament, similar to a fuse, and is likely driven by
heat assisted ionic motion in the concentration gradient around the
filament.
[0197] In the non-limiting example Pt/Co/GdOx/Gd/Au memristive
devices according to the principles herein, there is the unique
benefit of a thin ferromagnetic layer between the oxide and the
bottom electrode. With its sensitivity to interface chemistry and
structure (i.e., the magneto-ionic effect) the ferromagnetic layer
almost acts as a sense layer that allows identification of
voltage-induced modifications of interface composition and
chemistry. Therefore, combining the results of the magneto-ionic
switching measurements with the results of the memristive switching
measurements, provides additional insight into the microscopic
mechanisms behind nonpolar switching.
[0198] In various example memristive devices according to the
principles herein, oxygen anion migration provides for
magneto-ionic coupling in Pt/Co/GdOx/Gd/Au devices, consistent with
oxygen anions giving rise to memristive switching in these
devices.
[0199] The evolution of the resistive properties during
electroforming of any example memristive device described herein,
using positive or negative bias, is as follows. Under positive
bias, modifications of the resistive properties are measured before
the device switches to the low resistance ON state. This indicates
that filaments first nucleate at the conductive material (Co) layer
and then grow towards the Gd/Au top electrode. Moreover, the
spatially extended nature of the modifications in resistive
properties suggests that modifications of the oxide composition and
therefore the interface oxygen stoichiometry are not limited to
individual, localized filaments but likely occur across the whole
electrode area. This result is very different from the simple model
of local filament growth which is usually employed to explain
electroforming in conventional memristor devices. Results also
indicate that no changes in resistive properties occur during SET
and RESET of the memristive devices, indicating that filament
rupture can occurs far away from the conductive material (Co)
layer. The structural and chemical sensitivity of transition metal
thin films can therefore indeed provide an additional window into
the microscopic processes that give rise to memristive
switching.
[0200] FIGS. 14A-14B show schematics of memristive switching
mechanism in an example memristive device based on a Ta(4 nm)/Pt(3
nm)/Co(0.9 nm)/GdOx(33 nm)/Gd(2 nm)/Au(12 nm) device structure.
FIG. 14A shows a virgin device in its insulating state. FIG. 14B
shows the result after electroforming at positive voltage Vg, where
a conductive filament (CF) made of oxygen deficient GdOx
(GdOx-.delta.) connects the two electrodes and switches the device
to the ON state. After electroforming, the filament can be broken
(RESET to OFF state) and reestablished (SET to ON state) by
applying a positive or negative bias voltage. FIG. 14C shows the
result with application of a positive bias voltage (Vg>0). FIG.
14C shows the result with application of a negative bias voltage
(Vg<0). The R.sub.V is the vertical resistance and I.sub.V is
the current flowing vertically through the device.
[0201] FIGS. 14A-14B provide a simple model for memristive
switching in Pt/Co/GdOx/Gd/Au devices. During electroforming at
positive Vg (FIG. 14B), the Gd/Au electrode likely acts as a source
for oxygen vacancies. In the applied bias, the oxygen vacancies are
driven through the GdOx layer, towards the Pt/Co electrode and
accumulate at the Co/GdOx interface. Due to oxygen vacancy
accumulation an oxygen deficient and therefore conductive GdOx
phase forms at the Co/GdOx interface and grows towards the Gd/Au
electrode, such that eventually a conducting filament connects the
electrodes and the device switches to the ON state. As the magnetic
results suggest, the oxygen deficient GdOx phase likely forms
across the whole electrode area but extended filaments grow only in
certain locations, likely where their growth is aided by the
availability of a high diffusion path.
[0202] In FIG. 14C, filament rupture is induced during the RESET
process, which switches the device to the OFF state. Due to the
growth direction of the filament, rupture likely occurs in
proximity to the Gd/Au electrode, where the filament diameter is
expected to be smaller than closer to the Pt/Co electrode. The
filament can then be reestablished by application of positive Vg.
(see FIG. 14D). This SET process transitions the device back to the
low resistance ON state.
[0203] This switching mechanism indicates that, in the example
memristive devices according to the principles herein,
magneto-ionic switching occurs during the slow electroforming step
rather than during the fast SET and RESET steps of the memristor.
In memristive switching, electroforming is considered undesirable
because it is slow, unpredictable, and potentially destructive to
the device. Electroforming in conventional memristive switching
includes growth of a filament, and therefore ionic motion, across
the whole thickness of the oxide. In magneto-ionic switching, ions
need to be displaced only by atomic distances from the electrode
interface, which should allow for switching times similar to the
fast SET and RESET steps in memristive switching.
[0204] Example implementation of the voltage control of lateral
resistance is as follows. In ultra-thin metal films and wires,
electron scattering at surfaces and interfaces can contribute
significantly to the overall resistivity. Modifying in interface
chemistry and structure can result in changes in resistivity. The
ability to electrically control the oxygen stoichiometry at the
Co/GdOx interface provides the ability to tune the lateral
resistance R.sub.L in a memristive device based on a Ta(4 nm)/Pt(3
nm)/Co(0.9 nm)/GdOx(3 nm) multilayer structure. In this example,
the conductive material layer is formed as a nanostrip.
[0205] FIGS. 15A-15D schematically show an example of lateral
resistive switching in a nanoscale memristive device formed with a
nanostrip. FIG. 15A shows the example Ta/Pt/Co nanostrip device in
the virgin state (i.e., prior to electroforming. FIGS. 15B-15D show
a schematic of the example nanostrip device after sequential
application of a negative bias voltage Vg<0 (see FIG. 15B), no
bias voltage Vg=0 (see FIG. 15C), and a positive voltage Vg>0
(see FIG. 15D), to the GdOx/Ta/Au gate electrode. In each device
state, the lateral resistance R.sub.L is measured based on the
voltage necessary to drive the current h through the nanowire. At
Vg<0, the oxidation front migrates into the Co layer and
oxidizes Co to cobalt oxide (CoOx). These modifications are
retained at zero bias voltage (see FIG. 15C) but can be reversed at
Vg>0 (see FIG. 15D). In each device state, the lateral
resistance R.sub.L is measured based on the voltage to drive the
current I.sub.L through the nanostrip. Under a negative bias
voltage Vg<0, oxygen anions are expected to migrate towards the
Co/GdOx interface, such that under extended bias application, the
Co layer itself is oxidized (see FIG. 15B). At Vg<0, the
oxidation front migrates into the Co layer and oxidizes Co to
cobalt oxide (CoOx). Due to the insulating properties of Co oxide
(CoOx), migration of the oxidation front into the Co layer reduces
the effective cross section of the metal nanostrip and increase its
resistance. Similar to magneto-ionic coupling, retention of the
resistance modifications is expected in zero bias (see FIG. 15C).
These modifications are retained at zero bias but can be reversed
at Vg>0. As shown in FIG. 15D, a positive bias reverses the
effect. In order to verify this behavior experimentally, 50 .mu.m
long and 500 nm wide Ta(4 nm)/Pt(3 nm)/Co(0.9 nm)/GdOx(3 nm)
nanowires were covered with a 30 .mu.m wide GdOx(30 nm)/Ta(2 nm)/Au
(12 nm) gate electrode, to control by voltage the oxygen
stoichiometry at the Co/GdOx interface. The lateral resistance RL
of the nanowire is then measured with the four terminal technique
and simultaneously magnetic hysteresis loops are acquired from the
nanowire, to confirm via magneto-ionic coupling that
voltage-induced changes to oxygen stoichiometry occur.
[0206] In each device state of FIG. 15B-15D, the lateral resistance
R.sub.L is measured based on the voltage necessary to drive the
current I.sub.L through the nanowire.
[0207] FIGS. 16A-16C show the plots of results of computation of
lateral resistive switching and magnetic property modifications.
FIG. 16A-16B show the evolution of the coercivity Hc, and lateral
resistance R.sub.L, respectively, under bias voltage at 120.degree.
C. for a 500 nm wide and 50 .mu.m long Ta(4 nm)/Pt(3 nm)/Co(0.9
nm)/GdOx(3 nm) nanowire with a GdOx(30 nm)/Ta(2 nm)/Au(12 nm) gate
electrode. Error bars are smaller than the data point size. FIG.
16C shows the bias voltage applied. The evolution of the resistance
RL and coercivity Hc is measured during sequential application of
Vg=-5 V for 6 minutes and Vg=+5 V for 4 minutes. Every 2 minutes
the gate voltage is momentarily removed to facilitate measurements
of RL and Hc. Under negative bias we observe a gradual reduction of
Hc from 210 Oe to 70 Oe, which is then reversed under positive bias
(FIG. 8 6(a)).
[0208] FIGS. 16A-16C show that, at the same time R.sub.L increases
by .about.4% under negative bias and then returns close to its
initial value under positive bias (see FIG. 16B). The resistance
modifications are nonvolatile and remain at zero bias. This
behavior is consistent with the projection that oxidation of the
Co/GdOx interface and the Co layer itself should result in a
measurable increase in wire resistance. Using typical bulk values
for the resistivity of Co, Pt, and Ta of 60, 100, and 2000
n.OMEGA.m respectively, and treating the three layers as a set of
three parallel resistors, it is determined that a 4% increase in
resistance corresponds to a reduction of the Co metal thickness by
.about.0.2 nm or 20%. Under negative bias, a gradual reduction of
Hc from 210 Oe to 70 Oe is measured, which is then reversed under
positive bias (FIG. 16A). This confirms that the oxidation front
migrates towards/away from the bottom electrode under
negative/positive bias, respectively.
[0209] In an example, the ionic species may migrate uniformly into
the conductive material layer, resulting in a substantially uniform
reduction in thickness of the conductive portion of the conductive
material layer, thereby resulting in the higher resistance lateral
resistive state. In another example, the ionic species may
penetrate the conductive material film primarily along fast
diffusion paths, such as grain boundaries, and therefore mainly
affect electron scattering at grain boundaries, thereby resulting
in the higher resistance lateral resistive state. In another
example, the observed increase in resistance of the lateral
resistive state can result from some combination of these
effects.
[0210] In the example of FIGS. 16A-16C, the voltage-induced effects
on R.sub.L are relatively small, but this is largely due to the
presence of the thick Ta and Pt layers which are unaffected by Vg.
Since the purpose of the Ta/Pt layer is to establish the correct
magnetic properties, which are irrelevant for resistive switching,
much larger changes in R.sub.L could readily be achieved by simply
removing those layers. Such modifications, and any other
modification for device optimization, are within the scope of this
disclosure.
[0211] Example systems, methods, and apparatus herein provide
bistable nonpolar memristive switching in the memristive devices
that also demonstrate a magneto-ionic effect. In a non-limiting
examples, the memristive devices can be based on Pt/Co/GdOx/Au
multilayers. After electroforming, these example memristive devices
can be electrically switched between a high and low resistance
state, separated by 5 orders of magnitude in resistance. The
observed switching behavior is consistent with observations of
voltage-driven motion of the oxidation front and suggests that
ionic species migration, such as but not limited to oxygen anions
as the mobile species, is responsible for memristive switching.
[0212] The example memristive devices exhibit a unique combination
of voltage-induced changes to electrical resistance and magnetic
properties and also provide a new tool to investigate the
microscopic processes responsible for memristive switching. With
its high sensitivity to interface chemistry and structure, the thin
conductive material layer could be used as a sense layer, providing
otherwise hard to access information about structural and chemical
changes at the electrode interfaces. As non-limiting examples, the
memristive device can include a conductive material layer formed
from a thin ferromagnetic layer, sandwiched between an oxide
dielectric layer and electrodes.
[0213] Bistable switching of the lateral resistance in a nanostrip
(or nanowire) device is also demonstrated. Comparison of magnetic
and electrical properties reveals that this effect is likely due to
voltage-driven modification of the Co/GdOx interface and likely
oxidation of the Co layer itself. This lateral resistive switching
mechanism provides a novel approach to realize micro and nanoscale
electrical switches.
[0214] The example systems, methods, and apparatus herein provide
memristive devices that exhibit two distinct memristive switching
effects in addition to unprecedentedly strong magneto-ionic effects
in the same device structure. The parameters that can be control by
voltage in these devices include magnetic anisotropy, coercivity,
saturation magnetization, DW motion, lateral electrical resistance,
vertical electrical resistance and chemical composition. This
multifunctionality indicates the impact and potential of solid
state switching of oxygen stoichiometry as a route towards voltage
programmable nano materials.
[0215] The example systems, methods, and apparatus herein provide
for voltage control of material properties of example memristive
devices through solid-state switching of interface oxygen
chemistry. The example memristive devices are layered structures
formed of a conductive material layer (M.sub.C), a dielectric oxide
material layer disposed over the conductive material layer, and a
gate electrode (M.sub.G) disposed over the dielectric material
layer. FIGS. 17A-17D shows a schematic illustration of a
metal/oxide/metal device and its material properties in the virgin
state (FIG. 17A) and after sequential application of negative bias
(FIG. 17B), zero bias (FIG. 17C) and positive bias (FIG. 17D).
Under negative bias, oxygen anions move towards the bottom
electrode and modify the oxygen stoichiometry at the
metal/metal-oxide interface, thereby modifying the properties of
the bottom electrode layer underneath the electrode area (FIG.
17B). The changes to the metal/metal-oxide interface and material
properties persist at zero bias (FIG. 17C) and only application of
a reverse bias return the interface and the material properties
back to their initial state (FIG. 17D).
[0216] Based on these design principles, it is observed that
relatively small changes in temperature and gate voltage can
improve device response times by orders of magnitude and that by
varying the thickness and morphology of the gate oxide and
electrode, the magneto-ionic switching time drops from hundreds of
seconds to hundreds of microseconds. Enhancement in performance and
functionality may be achieved by using oxides with higher ionic
conductivity, or designing gate oxide heterostructures that include
separately optimized oxygen storage and ion conducting layers.
[0217] Based on magneto-ionic coupling, example systems, methods,
and apparatus are provided to reversibly imprint material
properties through local activation of ionic migration, which can
be used to locally pattern magnetic anisotropy and resistive
states. Since the magneto-ionic effect does not rely on maintaining
an electrical charge, these anisotropy patterns persist in the
power-off state but can be removed on demand by applying a reverse
bias. The example systems, methods, and apparatus allow reversible
patterning of magnetic microstructures without the need for
lithography and materials processing.
[0218] Example systems, methods, and apparatus herein demonstrate
the ability to reversibly control the interfacial oxygen
stoichiometry in metal/metal-oxide bilayers at room temperature
(RT) has implications in many different areas. FIGS. 18A-18F show
plots of the use of gate voltages for control and programming of
properties such as but not limited to magnetic anisotropy,
coercivity, saturation magnetization, domain wall motion, lateral
electrical resistance, vertical electrical resistance, and chemical
composition by gate voltage. This multifunctionality across
material properties and technology fields demonstrate the potential
of solid state switching of interface oxygen chemistry as a path
towards voltage programmable materials.
[0219] Example systems, methods, and apparatus herein provide
memristive switching devices that can be used for the imaging of
memristive switching processes, section by section. By using
conductive material layers formed from magnetic layers that differ
in their magnetic properties, several magnetic layers could be
integrated simultaneously into the same memristive device. As a
non-limiting example, a memristive switching device could be formed
with two magnetic layers: (i) layer with high coercivity Hc at the
bottom electrode/oxide interface, and (ii) a layer with low Hc at
the oxide/top electrode interface. Due to the difference in Hc, the
two layers and their magnetic properties should be distinguishable
in MOKE measurements. From the evolution of the magnetic properties
at the two electrodes, it would then be possible to determine at
which electrode oxygen vacancies accumulate during electroforming,
where filament formation initiates and where filament rupture
occurs during SET and RESET of the memristive device. This example
memristive device has tremendous potential in the convergence of
magneto-electric and memristive switching devices. The capability
of imaging memristive switching through magnetic sense layers
presents a promising technique to gain additional insight into the
microscopic details of memristive switching.
[0220] Example systems, methods, and apparatus herein provide
memristive switching devices that can exhibit magneto-ionic
switching on a faster timescale. Example memristive devices
described herein are able to reduce the time required for
magneto-ionic switching by 6 orders of magnitude, from hundreds of
seconds to hundreds of microseconds. Although microsecond scale
switching is sufficient for a wide range of application, for
certain memory and logic devices, nanosecond switching is
desirable. Since magneto-ionic switching relies on voltage-driven
ion migration, further improvements in performance and
functionality can likely be achieved by increasing ionic mobility
and enhancing ionic exchange.
[0221] In some example implementations, the dielectric material
layer of the example memristive switching devices could be formed
from oxides with higher ionic conductivity, such as but not limited
to yttria-stabilized zirconia (YSZ), or gate oxide heterostructures
that include separately optimized oxygen storage and ion conducting
layers. For example, the Pt/Co/YSZ system can exhibits strong
memristive properties.
[0222] In some example implementations, the dielectric material
layer of the example memristive switching devices could be formed
with an optimized concentration of oxygen defects in the oxide
layer for maximum ion conductivity. This can be achieved either by
doping the oxide, or by carefully controlling the oxygen partial
pressure during oxide deposition. Both methods could result in
orders of magnitude improvements in magneto-ionic switching
times.
[0223] Other non-limiting example applications of systems, devices,
methods, and apparatus described herein include in security,
military, and industrial applications. The example systems,
devices, methods, and apparatus described herein can be implemented
in spectroscopic applications as well.
[0224] In another non-limiting example, systems, devices, methods,
and apparatus described herein can be made low-cost and/or
disposable.
[0225] Conclusion
[0226] While various inventive embodiments have been described and
illustrated herein, those of ordinary skill in the art will readily
envision a variety of other means and/or structures for performing
the function and/or obtaining the results and/or one or more of the
advantages described herein, and each of such variations and/or
modifications is deemed to be within the scope of the inventive
embodiments described herein. More generally, those skilled in the
art will readily appreciate that all parameters, dimensions,
materials, and configurations described herein are meant to be
exemplary and that the actual parameters, dimensions, materials,
and/or configurations will depend upon the specific application or
applications for which the inventive teachings is/are used. Those
skilled in the art will recognize, or be able to ascertain using no
more than routine experimentation, many equivalents to the specific
inventive embodiments described herein. It is, therefore, to be
understood that the foregoing embodiments are presented by way of
example only and that, within the scope of the appended claims and
equivalents thereto, inventive embodiments may be practiced
otherwise than as specifically described and claimed. Inventive
embodiments of the present disclosure are directed to each
individual feature, system, article, material, kit, and/or method
described herein. In addition, any combination of two or more such
features, systems, articles, materials, kits, and/or methods, if
such features, systems, articles, materials, kits, and/or methods
are not mutually inconsistent, is included within the inventive
scope of the present disclosure.
[0227] The above-described embodiments of the invention can be
implemented in any of numerous ways. For example, some embodiments
may be implemented using hardware, software or a combination
thereof. When any aspect of an embodiment is implemented at least
in part in software, the software code can be executed on any
suitable processor or collection of processors, whether provided in
a single computer or distributed among multiple computers.
[0228] In this respect, various aspects of the invention may be
embodied at least in part as a computer readable storage medium (or
multiple computer readable storage media) (e.g., a computer memory,
one or more floppy disks, compact disks, optical disks, magnetic
tapes, flash memories, circuit configurations in Field Programmable
Gate Arrays or other semiconductor devices, or other tangible
computer storage medium or non-transitory medium) encoded with one
or more programs that, when executed on one or more computers or
other processors, perform methods that implement the various
embodiments of the technology discussed above. The computer
readable medium or media can be transportable, such that the
program or programs stored thereon can be loaded onto one or more
different computers or other processors to implement various
aspects of the present technology as discussed above.
[0229] The terms "program" or "software" are used herein in a
generic sense to refer to any type of computer code or set of
computer-executable instructions that can be employed to program a
computer or other processor to implement various aspects of the
present technology as discussed above. Additionally, it should be
appreciated that according to one aspect of this embodiment, one or
more computer programs that when executed perform methods of the
present technology need not reside on a single computer or
processor, but may be distributed in a modular fashion amongst a
number of different computers or processors to implement various
aspects of the present technology.
[0230] Computer-executable instructions may be in many forms, such
as program modules, executed by one or more computers or other
devices. Generally, program modules include routines, programs,
objects, components, data structures, etc. that perform particular
tasks or implement particular abstract data types. Typically the
functionality of the program modules may be combined or distributed
as desired in various embodiments.
[0231] Also, the technology described herein may be embodied as a
method, of which at least one example has been provided. The acts
performed as part of the method may be ordered in any suitable way.
Accordingly, embodiments may be constructed in which acts are
performed in an order different than illustrated, which may include
performing some acts simultaneously, even though shown as
sequential acts in illustrative embodiments.
[0232] All definitions, as defined and used herein, should be
understood to control over dictionary definitions, definitions in
documents incorporated by reference, and/or ordinary meanings of
the defined terms.
[0233] The indefinite articles "a" and "an," as used herein in the
specification and in the claims, unless clearly indicated to the
contrary, should be understood to mean "at least one."
[0234] The phrase "and/or," as used herein in the specification and
in the claims, should be understood to mean "either or both" of the
elements so conjoined, i.e., elements that are conjunctively
present in some cases and disjunctively present in other cases.
Multiple elements listed with "and/or" should be construed in the
same fashion, i.e., "one or more" of the elements so conjoined.
Other elements may optionally be present other than the elements
specifically identified by the "and/or" clause, whether related or
unrelated to those elements specifically identified. Thus, as a
non-limiting example, a reference to "A and/or B", when used in
conjunction with open-ended language such as "comprising" can
refer, in one embodiment, to A only (optionally including elements
other than B); in another embodiment, to B only (optionally
including elements other than A); in yet another embodiment, to
both A and B (optionally including other elements); etc.
[0235] As used herein in the specification and in the claims, "or"
should be understood to have the same meaning as "and/or" as
defined above. For example, when separating items in a list, "or"
or "and/or" shall be interpreted as being inclusive, i.e., the
inclusion of at least one, but also including more than one, of a
number or list of elements, and, optionally, additional unlisted
items. Only terms clearly indicated to the contrary, such as "only
one of" or "exactly one of," or, when used in the claims,
"consisting of," will refer to the inclusion of exactly one element
of a number or list of elements. In general, the term "or" as used
herein shall only be interpreted as indicating exclusive
alternatives (i.e. "one or the other but not both") when preceded
by terms of exclusivity, such as "either," "one of," "only one of,"
or "exactly one of." "Consisting essentially of," when used in the
claims, shall have its ordinary meaning as used in the field of
patent law.
[0236] As used herein in the specification and in the claims, the
phrase "at least one," in reference to a list of one or more
elements, should be understood to mean at least one element
selected from any one or more of the elements in the list of
elements, but not necessarily including at least one of each and
every element specifically listed within the list of elements and
not excluding any combinations of elements in the list of elements.
This definition also allows that elements may optionally be present
other than the elements specifically identified within the list of
elements to which the phrase "at least one" refers, whether related
or unrelated to those elements specifically identified. Thus, as a
non-limiting example, "at least one of A and B" (or, equivalently,
"at least one of A or B," or, equivalently "at least one of A
and/or B") can refer, in one embodiment, to at least one,
optionally including more than one, A, with no B present (and
optionally including elements other than B); in another embodiment,
to at least one, optionally including more than one, B, with no A
present (and optionally including elements other than A); in yet
another embodiment, to at least one, optionally including more than
one, A, and at least one, optionally including more than one, B
(and optionally including other elements); etc.
[0237] In the claims, as well as in the specification above, all
transitional phrases such as "comprising," "including," "carrying,"
"having," "containing," "involving," "holding," "composed of," and
the like are to be understood to be open-ended, i.e., to mean
including but not limited to. Only the transitional phrases
"consisting of" and "consisting essentially of" shall be closed or
semi-closed transitional phrases, respectively, as set forth in the
United States Patent Office Manual of Patent Examining Procedures,
Section 2111.03.
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