U.S. patent application number 15/994589 was filed with the patent office on 2019-12-05 for resistive switching memory device.
The applicant listed for this patent is UCHICAGO ARGONNE, LLC. Invention is credited to Bhaswar Chakrabarti, Supratik Guha, Leonidas E. Ocola, Sushant Sonde.
Application Number | 20190372001 15/994589 |
Document ID | / |
Family ID | 68536172 |
Filed Date | 2019-12-05 |
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United States Patent
Application |
20190372001 |
Kind Code |
A1 |
Chakrabarti; Bhaswar ; et
al. |
December 5, 2019 |
RESISTIVE SWITCHING MEMORY DEVICE
Abstract
Provided herein are resistive switching devices comprising a
nanocomposite, an inert electrode and an active electrode. Also
provided are methods for preparing and using the disclosed
resistive switching devices.
Inventors: |
Chakrabarti; Bhaswar;
(Westmont, IL) ; Ocola; Leonidas E.; (Oswego,
IL) ; Guha; Supratik; (Lemont, IL) ; Sonde;
Sushant; (Lemont, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
UCHICAGO ARGONNE, LLC |
Chicago |
IL |
US |
|
|
Family ID: |
68536172 |
Appl. No.: |
15/994589 |
Filed: |
May 31, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 45/1253 20130101;
H01L 45/146 20130101; H01L 45/1608 20130101 |
International
Class: |
H01L 45/00 20060101
H01L045/00 |
Goverment Interests
STATEMENT OF US GOVERNMENT SUPPORT
[0001] The United States Government has rights in this invention
pursuant to Contract No. DE-AC02-06CH11357 between the United
States Government and UChicago Argonne, LLC representing Argonne
National Laboratory.
Claims
1. A resistive switching device comprising a nanocomposite
comprising a polymer with a metal oxide infiltrated throughout the
polymer; an inert electrode and; an active electrode, wherein the
inert electrode and the active electrode are separated by the
nanocomposite, and the active electrode has an area of 1,000
.mu.m.sup.2 to 20,000 .mu.m.sup.2.
2. The resistive switching device of claim 1, wherein the polymer
comprises poly(methyl methacrylate), polydimethyl siloxane,
polyimide, bisphenol A novolac epoxy, or a mixture thereof.
3. The resistive switching device of claim 2, wherein the polymer
comprises poly(methyl methacrylate).
4. The resistive switching device of claim 1, wherein the metal
oxide comprises aluminum oxide (Al.sub.2O.sub.3), titanium oxide
(TiO.sub.2), hafnium oxide (HfO.sub.2), zinc oxide (ZnO), or a
mixture thereof.
5. The resistive switching device of claim 4, wherein the metal
oxide comprises Al.sub.2O.sub.3.
6. The resistive switching device of claim 1, wherein the
nanocomposite has a thickness of 10 nm to 50 nm.
7. The resistive switching device of claim 6, wherein the
nanocomposite has a thickness of 10 nm.
8. The resistive switching device of claim 1, wherein the
nanocomposite comprises a porous section.
9. The resistive switching device of claim 8, wherein the porous
section of the nanocomposite comprises pore sizes of about 1
nm.
10. The resistive switching device of claim 1, wherein the inert
electrode comprises platinum, palladium, titanium, gold, or a
mixture thereof.
11. The resistive switching device of claim 10, wherein the inert
electrode comprises platinum, titanium, or a combination of.
12. The resistive switching device of claim 1, wherein the inert
electrode has a thickness of 30 to 70 nm.
13. The resistive switching device of claim 1, wherein the active
electrode comprises gold or platinum and either silver or
copper.
14. The resistive switching device of claim 13, wherein the active
electrode comprises a film of silver or copper with a gold layer on
top.
15. The resistive switching device of claim 14, wherein the film of
silver or copper has a thickness of 5 to 20 nm and the gold layer
has a thickness of 50 nm to 100 nm.
16. (canceled)
17. The resistive switching device of claim 1, further comprising a
silicon substrate under the inert electrode, optionally where the
silicon substrate comprises a p-type dopant with resistivity of 1
to 10 .OMEGA./cm.
18. The resistive switching device of claim 1 having a switching
current of 500 nA to 100 .mu.A, or an operating voltage of about
400 mV.
19. A method of preparing the resistive switching device of claim 1
comprising (a) depositing the polymer on the first electrode by
spin-coating to form a polymer layer, baking the polymer layer at a
temperature of 180.degree. C., and optionally irradiating a portion
of the polymer layer with an electron beam to form a higher
porosity polymer; (b) infiltrating the polymer layer with a metal
oxide using sequential infiltration synthesis via atomic layer
deposition to form the nanocomposite; and (c) depositing the second
electrode on the nanocomposite by photolithography followed by
electron beam evaporation of the second electrode metal or
metals.
20. The method of claim 19, wherein the second electrode is
prepared by electron beam evaporation of silver or copper to a
thickness of 10 nm then electron beam evaporation of gold to a
thickness of 70 nm.
21. A resistive switching device comprising a nanocomposite
comprising a polymer with a metal oxide infiltrated throughout the
polymer, and the nanocomposite having a porous section with pore
sizes of about 1 nm; an inert electrode and; an active electrode,
wherein the inert electrode and the active electrode are separated
by the nanocomposite.
22. A resistive switching device comprising a nanocomposite
comprising a polymer with a metal oxide infiltrated throughout the
polymer; an inert electrode and; an active electrode comprising
gold or platinum and either silver or copper, wherein the inert
electrode and the active electrode are separated by the
nanocomposite.
Description
BACKGROUND
[0002] Recent scaling trends in semiconductor technologies
including both logic and memory indicate that the current
technologies will soon reach their performance limits. As a result,
not only alternative technologies are being actively sought but
entirely new computational frameworks are being investigated.
Resistance switching devices have been investigated recently not
only as a replacement for current state-of-the art flash memory
technology but also for neuromorphic computation. The emerging
field of neuromorphic computation which is expected to be orders of
magnitude more efficient at analyzing increasingly complex
exa-scale data compared to conventional Von-Neumann computation.
Large volumes of data are needed to be analyzed in many different
private and public sectors including social media, search engines,
public health, national security and many more. Resistive switching
memories are ideal candidates for creating such neuromorphic
hardware due to their scalability and ease of 3D integration for
achieving extremely high device density. However, resistive
switching memories developed so far suffer from a variety of issues
including high operating voltage and current values as well as
large device-to-device and cycle-to-cycle variability. There is a
need for low operating voltage and current as well as limited
variability for resistive switching memory technology.
SUMMARY
[0003] Provided herein are resistive switching devices comprising
nanocomposites comprising a polymer with a metal oxide infiltrated
throughout the polymer, an inert electrode, and an active
electrode, wherein the inert electrode and the active electrode are
separated by the nanocomposite.
[0004] In embodiments, the polymer comprises poly(methyl
methacrylate), polydimethyl siloxane, polyimide, bisphenol A
novolac epoxy, or a mixture thereof. In some cases, the polymer
comprises poly(methyl methacrylate).
[0005] In various embodiments, the metal oxide comprises aluminum
oxide (Al.sub.2O.sub.3), titanium oxide (TiO.sub.2), hafnium oxide
(HfO.sub.2), zinc oxide (ZnO), or a mixture thereof. In some cases,
the metal oxide comprises Al.sub.2O.sub.3.
[0006] In various cases, the nanocomposite has a thickness of 10 nm
to 50 nm. In some cases, the nanocomposite has a thickness of 10
nm. In various embodiments, the nanocomposite comprises a porous
section. In some embodiments, the porous section of the
nanocomposite comprises pore sizes of about 1 nm.
[0007] In various cases, the inert electrode comprises platinum,
palladium, titanium, gold, or a mixture thereof. In some cases, the
inert electrode comprises platinum, titanium, or a combination of.
In embodiments, the inert electrode has a thickness of 30 to 70
nm.
[0008] In various embodiments, the active electrode comprises gold
or platinum and either silver or copper. In some embodiments, the
active electrode comprises a film of silver or copper with a gold
layer on top. In some cases, the film of silver or copper has a
thickness of 5 to 20 nm and the gold layer has a thickness of 50 nm
to 100 nm. In some embodiments, the active electrode has an area of
1,000 .mu.m.sup.2 to 20,000 .mu.m.sup.2.
[0009] In various embodiments, the resistive switching device
disclosed herein, can further comprise a silicon substrate under
the inert electrode, optionally where the silicon substrate
comprises a p-type dopant with resistivity of 1 to 10 .OMEGA./cm.
In some embodiments, the resistive switching device described
herein can have a switching current of 500 nA to 100 .mu.A, or an
operating voltage of about 400 mV.
[0010] Another aspect of the disclosure provides a method of
preparing the resistive switching device as described herein
comprising (a) depositing the polymer on the first electrode by
spin-coating to form a polymer layer, baking the polymer layer at a
temperature of 180.degree. C., and optionally irradiating a portion
of the polymer layer with an electron beam to form a higher
porosity polymer, (b) infiltrating the polymer layer with a metal
oxide using sequential infiltration synthesis via atomic layer
deposition to form the nanocomposite, and (c) depositing the second
electrode on the nanocomposite by photolithography followed by
electron beam evaporation of the second electrode metal or
metals.
[0011] In some embodiments, the second electrode is prepared by
electron beam evaporation of silver or copper to a thickness of 10
nm then electron beam evaporation of gold to a thickness of 70
nm.
BRIEF DESCRIPTION OF FIGURES
[0012] FIG. 1 is a depiction of the fabrication process details of
a resistive switching device disclosed herein.
[0013] FIG. 2 shows a 3D schematic of a sample with
PMMA-Al.sub.2O.sub.3 nanocomposite switching layer, an inert
electrode and an active electrode on a p-type silicon substrate
layer in (a) and shows an Atomic force microscope image of the
PMMA-Al.sub.2O.sub.3 nanocomposite showing a homogenous film in
(b).
[0014] FIG. 3 shows the switching characteristics of the device
with 100 .mu.A operating current in (a). At this current, the
device shows bipolar resistive memory behavior. Under a positive
voltage sweep the device first turns on from an off state at
V.sub.ON.about.500 mV (path 1) and stays on as the voltage is then
reduced from V.sub.ON to zero (path 2). The device can be turned
off by applying a negative voltage sweep (path 3 and 4). The
switching characteristics of the device on a linear scale are shown
in (b).
[0015] FIG. 4 shows cycle-to-cycle statistics of (a) Set voltage,
(b) Reset voltage and (c) peak reset current of devices disclosed
herein. Statistics is taken over 80 Set and Reset cycles.
[0016] FIG. 5 exhibits the device-to-device statistics of the
set/reset voltages for the fabricated resistive switching devices
measured over 20 different devices disclosed herein.
[0017] FIG. 6 Non-volatile resistive switching at ultra-low current
in Ag/PMMA-Al.sub.2O.sub.3 (10 nm)/Pt devices after baking at
180.degree. C., for 6 minutes.
[0018] FIG. 7 shows resistive switching device structure with
PMMA-Al.sub.2O.sub.3 nanocomposite in (a); modified device
structure where PMMA is selectively exposed to E-beam irradiation
before Al.sub.2O.sub.3 infiltration, resulting in enhanced porosity
in the irradiated region in (b); crossbar architecture for
high-density nanocomposite memory development in (c). This
particular embodiment has 20.times.20 array of devices with each
device having a footprint of 200.times.200 nm.sup.2; a schematic of
the layout showing the region of the polymer (indicated by the
arrows) that will be irradiated with E-beam in (d).
[0019] FIG. 8 shows the switching characteristics of the
Ag/PMMA-Al.sub.2O.sub.3 (10 nm)/Pt devices disclosed herein at
ultra-low operating current (100 nA) in (a); Switching in
Ag/Al.sub.2O.sub.3 (10 nm)/Pt devices showing high switching
voltage (4 V) and unstable switching (device shorts after
switching) in (b).
[0020] FIG. 9 shows switching characteristics of resistive
switching devices with two different active electrode thicknesses
(Ag). The switching voltages of a 15 nm Ag electrode in (a) are
less compared to that of a 5 nm Ag electrode in (b).
DETAILED DESCRIPTION
[0021] Provided herein are resistive switching devices which can be
used, e.g., as solid-state memories or neuromorphic circuits. The
disclosure herein relates to resistive switching devices comprising
(i) a nanocomposite comprising a polymer with a metal oxide
infiltrated throughout the polymer, (ii) an inert electrode and
(iii) an active electrode, wherein the inert electrode and active
electrode are separated by the nanocomposite.
[0022] The resistive switching device disclosed herein can have
several advantages, for example, low power consumption and reduced
variability from device to device. In some cases, the device can
operate at ultra-low voltages (e.g., about 400 mV), achieve
ultra-low current operation (e.g., about 500 nA), and/or
demonstrate high device-to-device and cycle-to-cycle
uniformity.
[0023] A resistive switching device disclosed herein is a device
having two resistance states, a high resistance state (HRS) and a
low resistance state (LRS), that can be achieved by an appropriate
electric stimulus such as a certain voltage that "switches" the
device from one state to the other. The HRS is a state where the
resistive switching device is non-conducting, and can be achieved
upon application of a voltage of greater than or equal to
|V.sub.OFF| about 400 mV. The LRS is a state where the resistive
switching device is conducting, and can be achieved upon
application of a voltage of greater than or equal to |V.sub.ON|
about 400 mV.
[0024] The resistive switching devices presented herein can further
comprise a substrate layer under the inert electrode. The substrate
layer can comprise any suitable resistive substrate. In
embodiments, the substrate layer can be doped with p-type or n-type
semiconductors. In embodiments, the substrate layer comprises
silicon. In embodiments, the silicon can be doped with p-type or
n-type semiconductors. In embodiments, the substrate comprises a
p-type dopant. In embodiments, the substrate comprises silicon and
a p-type dopant. In embodiments, the substrate comprises a dopant
with resistivity of 1 to 100 .OMEGA./cm. In embodiments, the
substrate comprises a dopant with resistivity of 1 to 50
.OMEGA./cm. In embodiments, the substrate comprises a dopant with
resistivity of 1 to 10 .OMEGA./cm, such as 1 .OMEGA./cm, 2
.OMEGA./cm, 3 .OMEGA./cm, 4 .OMEGA./cm, 5 .OMEGA./cm, 6 .OMEGA./cm,
7 .OMEGA./cm, 8 .OMEGA./cm, 9 .OMEGA./cm or 10 .OMEGA./cm.
[0025] The resistive switching devices disclosed herein can operate
at ultra-low voltages and can switch at ultra-low currents without
sacrificing the low-voltage operability. In embodiments, the
resistive switching devices herein can operate at voltages of about
200 mV to about 10 V. In embodiments, the resistive switching
devices herein can operate at voltages of about 200 mV to about 700
mV, such as about 200 mV, about 300 mV, about 400 mV, about 500 mV,
about 600 mV, and about 700 mV. In embodiments, the resistive
switching devices herein can operate at voltages of about 400 mV.
In embodiments, the resistive switching devices herein can switch
at currents of 100 nA to 1 mA. In embodiments, the resistive
switching devices herein can switch at currents of 100 nA to 100
.mu.A. In embodiments, the resistive switching devices herein can
switch at currents of 500 nA to 100 .mu.A. In embodiments, the
resistive switching devices herein can switch at currents of 500 nA
or greater. In embodiments, the resistive switching devices herein
can switch at currents of about 500 nA without sacrificing the
low-voltage operability.
[0026] The use of the terms "a," "an," "the," and similar referents
in the context of describing the invention (especially in the
context of the claims) are to be construed to cover both the
singular and the plural, unless otherwise indicated. Recitation of
ranges of values herein merely are intended to serve as a shorthand
method of referring individually to each separate value falling
within the range, unless otherwise indicated herein, and each
separate value is incorporated into the specification as if it were
individually recited herein. The use of any and all examples, or
exemplary language (e.g., "such as") provided herein, is intended
to better illustrate the invention and is not a limitation on the
scope of the invention unless otherwise indicated. No language in
the specification should be construed as indicating any non-claimed
element as essential to the practice of the invention.
[0027] As used herein, the terms "about" and "approximately"
generally mean plus or minus 10% of the stated value. For example,
about 0.5 would include 0.45 and 0.55, about 10 would include 9 to
11, about 1000 would include 900 to 1100.
Nanocomposite
[0028] The nanocomposites used herein are composites comprising a
polymer with metal oxide nanoparticles infiltrated throughout the
polymer. The nanocomposites present herein can be prepared by any
suitable method to one skilled in the art wherein the metal oxide
is dispersed throughout the polymer. In embodiments, the
nanocomposite can be prepared by any chemical vapour deposition
method, for example atomic layer deposition. In embodiments, the
nanocomposite can be prepared by an atomic layer deposition method,
such as sequential infiltration synthesis ("SIS"). In embodiments,
the nanocomposite can be prepared by sequential infiltration
synthesis. In embodiments, the metal oxide nanoparticles are
"infiltrated" throughout the polymer via a method called sequential
infiltration synthesis ("SIS"), wherein the polymer is exposed to
various gas phase precursors to synthesize the metal oxides in
situ. This synthetic method allows for the metal oxide to coat the
surfaces of the polymer and infiltrate into the bulk polymer. The
term "infiltrate" herein refers to the coating and/or filling of
the free volume available in the polymer matrix of the polymer in a
uniform manner such as to form a thin film covering the inner and
outer walls of the space between polymer chains of the polymer. The
SIS process uses a pair of gaseous precursors to form the
infiltrated metal oxide disclosed herein. A first precursor
comprises a desired metal or metal-containing compound suitable for
SIS, such as trimethyl aluminum, titanium tetrachloride, or diethyl
zinc, selected to infiltrate the polymer. A second precursor
comprises a compound suitable for SIS selected to react with the
first precursor or a portion thereof, such as an oxygen source
(e.g., H.sub.2O, O.sub.2, O.sub.3, H.sub.2O.sub.2) or a reducing
agent (e.g., H.sub.2, H.sub.2S.sub.2Si.sub.2H.sub.6). For example,
a metal oxide can be synthesized via reaction of a first precursor
and a second precursor while infiltrated in the polymer using SIS,
thereby resulting in the metal oxide infiltrated throughout the
polymer.
[0029] In embodiments where the nanocomposite is prepared via SIS,
the polymer of the nanocomposite is exposed to gas phase precursors
of the metal oxides, in situ. Using SIS allows for the metal oxides
to coat the surfaces of the polymer including infiltrating the
polymer. Examples of contemplated metal oxides include
Al.sub.2O.sub.3, TiO.sub.2, ZnO, SiO.sub.2, HfO.sub.2, ZrO.sub.2,
and oxides of cerium, lanthanum, yttrium, erbium, terbium,
ytterbium, scandium, praseodymium, gadolinium, and samarium. In
some cases, the metal oxide is Al.sub.2O.sub.3, TiO.sub.2, ZnO,
SiO.sub.2, HfO.sub.2, or ZrO.sub.2. In embodiments, the gas phase
precursors used during SIS comprise water. In embodiments, the gas
phase precursors used during SIS comprise water and one or more
aluminum containing precursors, such as trimethyl aluminum. The SIS
can be performed at temperatures of 80.degree. C. to 150.degree.
C., such as 80.degree. C., 85.degree. C., 90.degree. C., 95.degree.
C., 100.degree. C., 105.degree. C., 110.degree. C., 115.degree. C.,
120.degree. C., 125.degree. C., 130.degree. C., 135.degree. C.,
140.degree. C., 145.degree. C., and 150.degree. C. In embodiments,
SIS can be performed at a temperature of 95.degree. C. The SIS can
be performed in any number of cycles suitable to one of ordinary
skill in the art. In embodiments, the SIS can be performed in 15 to
35 cycles or 20 to 30 cycles. Specific cycles contemplated include
15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31,
32, 33, 34, and 35 cycles.
[0030] The nanocomposite disclosed herein can be porous. In
embodiments, the polymer comprises pores of a size from 0.1 nm to 5
nm such as 0.1 nm, 0.2 nm, 0.3 nm, 0.4 nm, 0.5 nm, 0.6 nm, 0.7 nm,
0.8 nm, 0.9 nm, 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm, 4.5
nm, or 5 nm. In embodiments, the polymer comprises pores of a size
of about 1 nm. In embodiments, the porosity of the polymer can be
controlled by exposure to electron beam or by baking the
nanocomposite after SIS. In embodiments, the nanocomposite
comprises locally enhanced porosity. The term "locally enhanced
porosity" refers to one particular section of the nanocomposite
that is more porous than the rest of the nanocomposite (for
example, as shown in FIG. 7 in (b)). Without intending to be bound
by theory, it is thought the locally enhanced porosity section of
the nanocomposite will have higher porosity and thereby will allow
higher diffusivity of metal ions, such as metal cations, in that
particular section of the nanocomposite during operation of the
resistive switching device. As a result, resistive switching device
variability can be further reduced as operating voltages can be
decreased even further. In embodiments, the polymer comprises less
than 10% by volume locally enhanced porosity. In embodiments, the
polymer comprises less than 25% by volume locally enhanced
porosity. In embodiments, the polymer comprises less than 50% by
volume locally enhanced porosity. In embodiments, the polymer
comprises less than 75% by volume locally enhanced porosity.
[0031] The nanocomposite can undergo a baking step after the SIS.
The baking step can comprise a temperature of 150.degree. C. to
180.degree. C. such as, 150.degree. C., 155.degree. C., 160.degree.
C., 165.degree. C., 170.degree. C., 175.degree. C., or 180.degree.
C. In embodiments, the baking step can comprise a temperature of
about 180.degree. C. The baking step can occur for 1 to 10 minutes,
such as, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 minutes. In embodiments,
the baking step can occur for about 3 minutes.
[0032] In embodiments where the nanocomposite has undergone a
baking step, the resistive switching device can demonstrate
non-volatile switching at ultra-low currents, for example 500 nA
(see, e.g., FIG. 6). Without intending to be bound by theory, the
ability for the device to demonstrate non-volatile switching at
such a low current is thought to be possible due to microstructural
change of the nanocomposite after baking.
[0033] The nanocomposite layer of the devices disclosed herein can
have a thickness of up to 100 nm, e.g., 10 nm to 100 nm. In
embodiments, the nanocomposite layer can have a thickness of up to
50 nm, e.g., 10 nm to 50 nm. In embodiments, the nanocomposite can
have a thickness of 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40
nm, 45 nm or 50 nm.
[0034] Polymer
[0035] The polymer of the nanocomposite described herein can
comprise any polymer or blend of polymers known to one skilled in
the art. For example, the polymer can comprise poly(methyl
methacrylate) ("PMMA"), polydimethyl siloxane, polyimide, bisphenol
A novolac epoxy or a mixture thereof. In embodiments, the polymer
comprises poly(methyl methacrylate). PMMA can include a PMMA
homopolymer, a PMMA copolymer, a PMMA terpolymer or a mixture
thereof. In embodiments, the polymer comprises a PMMA
homopolymer.
[0036] When the PMMA is a copolymer or terpolymer, the other
monomers to be copolymerized with are generally carboxyl
group-containing ethylenically unsaturated monomers, e.g., methyl
(meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate,
isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl
(meth)acrylate, tert-butyl (meth)acrylate, 2-ethylhexyl acrylate,
n-octyl (meth)acrylate, lauryl (meth)acrylate, tridecyl
(meth)acrylate, octadecyl (meth)acrylate, isostearyl
(meth)acrylate, C.sub.1-24-alkyl (meth)acrylate and the like;
2-hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, the
polyhydric; DOO, 2,3-dihydroxy-butyl (meth)acrylate, 4-hydroxybutyl
(meth)acrylates such as, polyethylene glycol mono(meth)acrylate,
polyhydric monoesters of alcohols with acrylic acid or methacrylic
acid hydroxyl group-containing monomers such as -caprolactone
monoester of alcohol and acrylic acid or methacrylic acid and
ring-opening polymerization; styrene, acrylonitrile,
methacrylonitrile, and vinyl acetate. These monomers can be used
alone or can be used in combination of two or more different
monomers.
[0037] The PMMA weight average molecular weight ("Mw") can be tuned
based on the application needs. In embodiments, the PMMA Mw is
300,000 or less. In embodiments, the PMMA Mw can be 100,000 or
less. In some embodiments, the PMMA Mw can be between 100,000 to
250,000. When a polymer is described as having (or not having) a
particular Mw, unless specified otherwise, it is intended that the
specified molecular weight is the weight average molecular weight
for the resin, which inherently can have a molecular weight
distribution.
[0038] The polymer of the nanocomposite described herein can
undergo a baking step prior to the SIS. In embodiments, the baking
step can comprise a temperature of 150.degree. C. to 180.degree. C.
such as, 150.degree. C., 155.degree. C., 160.degree. C.,
165.degree. C., 170.degree. C., 175.degree. C., or 180.degree. C.
In embodiments, the baking step can comprise a temperature of
180.degree. C.
Metal Oxide
[0039] The resistive switching devices herein comprise a
nanocomposite comprising a metal oxide infiltrated throughout the
polymer. In embodiments, the metal oxide can be aluminum oxide
(Al.sub.2O.sub.3), titanium oxide (TiO.sub.2), hafnium oxide
(HfO.sub.2), zinc oxide (ZnO) or a mixture thereof. In various
embodiments, the metal oxide comprises Al.sub.2O.sub.3. In
embodiments, the metal oxide can comprise cerium, lanthanum,
yttrium, erbium, terbium, ytterbium, scandium, praseodymium,
gadolinium, samarium, or a mixture thereof.
Electrodes
[0040] The resistive switching device described herein can comprise
two electrodes. In some embodiments, the resistive switching device
can comprise an inert electrode and an active electrode.
Inert Electrode
[0041] The inert electrode can comprise any metal known to one
skilled in the art. As used herein, the term "inert electrode"
refers to an electrode composed of one or more metals that serves
as a source of electrons but may not participate in a chemical
reaction. In embodiments, the inert electrode can comprise more
than one metal. In embodiments, the inert electrode comprises
platinum, palladium, titanium, gold, or a mixture thereof. In
embodiments, the inert electrode comprises platinum, titanium or
combinations thereof. In embodiments, the inert electrode comprises
platinum and titanium.
[0042] In embodiments, the inert electrode can have a thickness of
100 nm or less, for example, 30 nm to 100 nm, 30 nm to 80 nm, or 30
nm to 70 nm. In embodiments, the inert electrode can have a
thickness of 30 nm to 70 nm, or 50 to 80 nm. Contemplated thickness
of the inert electrode include, for example, 30 nm, 35 nm, 40 nm,
45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, or 80 nm.
[0043] In some cases, the inert electrode comprises titanium and
platinum. In such embodiments, the platinum can have a thickness of
30 nm to 100 nm, such as 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm,
90 nm, or 100 nm. In some embodiments, the platinum can have a
thickness of 40 nm to 50 nm. In some embodiments, the titanium can
have a thickness of 1 nm to 15 nm, such as 1 nm, 2 nm, 3 nm, 4 nm,
5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, or 15 nm. In some embodiments,
the titanium can have a thickness of 5 nm. In some cases, the
platinum has a thickness of 40 to 50 nm (e.g., 50 nm) and the
titanium has a thickness of 1 to 10 nm (e.g., 5 nm).
Active Electrode
[0044] As used herein, the term "active electrode" refers to an
electrode composed of one or more metals that may participate in a
chemical reaction either by oxidation or reduction. In embodiments,
the active electrode can comprise more than one metal. In
embodiments, the active electrode comprises gold or platinum and
either silver or copper. In embodiments, the active electrode
comprises a film. In embodiments, the active electrode comprises a
metallic film and another metal. In embodiments, the active
electrode comprises a film of silver or copper and a gold layer. In
embodiments, the active electrode comprises a film of silver or
copper and a gold layer on top of the film. In embodiments, the
active electrode can comprise a chemically reactive metal. In
embodiments, the chemically reactive metal can be Ag, Cu, or a
mixture thereof.
[0045] In embodiments, the active electrode comprises more than one
metal and the metals are each at least 5 nm thick. In embodiments,
the active electrode comprises more than one metal and each metal
has a thickness of 100 nm or less. In embodiments, the active
electrode comprises more than one metal wherein one metal has a
thickness of 1 nm to 50 nm (e.g., 5 to 20 nm) and a second metal
has a thickness of 30 nm to 150 nm (e.g., 50 to 100 nm). In
embodiments, the active electrode comprises a film of silver or
copper wherein the film can have a thickness of 5 to 20 nm and a
gold layer wherein the gold layer can have a thickness of 30 to 100
nm. In embodiments, the active electrode comprises more than one
metal, wherein a first metal comprises a chemically reactive metal
and a second metal comprises an inert metal. In embodiments, the
chemically reactive metal comprises silver or copper and the inert
metal comprises gold or platinum. The inert metal of the active
electrode can prevent the chemically reactive metal from
oxidation.
[0046] In embodiments, the active electrode, inert electrode, or a
mixture thereof can have an area (length.times.width, not
thickness) of 100 nm.sup.2 to 50,000 .mu.m.sup.2. In embodiments,
the active electrode can have an area of 100 .mu.m.sup.2 to 20,000
.mu.m.sup.2. In embodiments, the active electrode can have an area
of 1,000 .mu.m.sup.2 to 20,000 .mu.m.sup.2, such as 1,000
.mu.m.sup.2, 5,000 .mu.m.sup.2, 10,000 .mu.m.sup.2, 15,000
.mu.m.sup.2, or 20,000 .mu.m.sup.2. In embodiments, the active
electrode can have an area of 10,000 .mu.m.sup.2. In embodiments,
the inert electrode can have an area of 100 .mu.m.sup.2 to 20,000
.mu.m.sup.2. In embodiments, the inert electrode can have an area
of 1,000 .mu.m.sup.2 to 20,000 .mu.m.sup.2, such as 1,000
.mu.m.sup.2, 5,000 .mu.m.sup.2, 10,000 .mu.m.sup.2, 15,000
.mu.m.sup.2, or 20,000 .mu.m.sup.2. In embodiments, the inert
electrode can have an area of 10,000 .mu.m.sup.2.
Method of Preparing the Resistive Switching Device
[0047] The methods herein provide a disclosure for the preparation
of the resistive switching devices herein. In embodiments, a
resistive switching device as disclosed herein can be prepared by a
method comprising (a) depositing the polymer on the first electrode
by spin-coating to form a polymer layer, baking the polymer layer
at a temperature of 180.degree. C. and optionally irradiating a
portion of the polymer layer with an electron beam to form a higher
porosity polymer; (b) infiltrating the polymer layer with a metal
oxide using sequential infiltration synthesis via atomic layer
deposition to form the nanocomposite and optionally baking the
nanocomposite after SIS at a temperature of 180.degree. C.; and (c)
depositing the second electrode on the nanocomposite by
photo/electron-beam lithography followed by electron beam
evaporation of the second electrode metal or metals. In some
embodiments, the resistive switching device can be prepared by
comprising steps (a)-(c) and additionally baking the nanocomposite,
irradiating a portion of the polymer layer with an electron beam to
form a locally enhanced porous section of the polymer layer, or a
combination thereof. In embodiments, previous to step (a), the
first electrode can be deposited onto a silicon substrate disclosed
herein by electron beam evaporation. In some embodiments, the
methods (a)-(c) comprise the additional baking of the polymer layer
of the nanocomposite after the SIS and the temperature at which the
polymer layer is baked can be 180.degree. C. In embodiments, the
baking of the polymer layer of the nanocomposite after the SIS can
be performed in multiple steps, for example, baking the
nanocomposite film at 180.degree. C. for 6 min can be performed in
two separate baking steps with each step lasting for 3 minutes. A
particular embodiment of a method of preparing the resistive
switching device can be seen in FIG. 1.
EXAMPLES
Example 1--Synthesis of a Resistive Switching Device
[0048] The method of preparing a resistive switching device is
illustrated in FIG. 1. The devices in this embodiment were
fabricated on a Silicon substrate (p-type doped with resistivity
between 1-10 .OMEGA./cm). The inert electrode consisted of a
bi-layer stack of thin films of Ti (5 nm)/Pt (50 nm). A polymer
layer (10 nm) was deposited on top by spin-coating. The polymer
used was Poly(methyl methacrylate). After baking the polymer film
at 180.degree. C., the polymer film was infiltrated with
Al.sub.2O.sub.3 using atomic layer deposition. The particular
method used was sequential infiltration synthesis. Next, the active
electrodes were defined by photolithography followed by electron
beam evaporation of Ag or Cu (10 nm) followed by Au (70 nm). In
other embodiments of the device the thickness of the Ag or Cu thin
film can be changed to alter the switching characteristics. Without
intending to be bound by theory, by increasing the thickness of the
chemically active metal, such as Ag or Cu, the switching voltage
can be reduced. Further, if the thickness of the active electrode
is reduced, the switching voltages increase. For example, FIG. 9
compares switching characteristics of two different active
electrode thicknesses (Ag). The switching voltages with 15 nm Ag
(FIG. 9 in (a)) are less compared to that of 5 nm Ag electrode
(FIG. 9 in (b)). The device in FIG. 9 in (a) also has reduced
switching voltages compared to devices with 10 nm Ag electrode
(FIG. 6). Resistive switching device area is defined by the size of
the active electrodes which can vary from 50 .mu.m to 200 .mu.m in
this particular embodiment. An atomic force microscopy image of the
nanocomposite can be seen in FIG. 2 in (b).
Example 2--Operating Resistive Switching Devices
[0049] In embodiments, a memory window of an order of magnitude was
observed (FIG. 3 in (a)) with a resistive switching device
disclosed herein. However this window was further increased by
increasing the operating current. The voltage required in the first
cycle (Set 1) to turn the device on was the same as the turn-on
voltage for the subsequent cycles (Set 2 and 3). This was in
contrast with most resistive switching devices which require a
significantly higher voltage in the first cycle (also called
Forming process) compared to the regular on/off voltages. The data
in FIG. 3 in (a) indicates that the device is a "Forming-free"
resistive switching device.
[0050] Notably in FIG. 3 in (b), the turn-on process for the
resistive switching device under the positive sweep (path 1) was
not abrupt. The continuous turn-on procedure made the resistive
switching device an attractive candidate for neuromorphic
applications. Also notable is the maximum reset current when the
device turns off was the same as the operating current for the
turn-on (Set) process. This indicated that the switching is
filamentary, i.e. the set/reset processes were controlled by
creation/rupture of a conductive bridge path through the
dielectric. Since the active electrode in this embodiment of the
resistive switching device is Ag, the filament was created under
positive voltage sweep by diffusion of Ag ions into the
PMMA-Al.sub.2O.sub.3 nanocomposite.
[0051] The cycle-to-cycle switching statistics of the resistive
switching device was demonstrated in FIG. 4. The distribution of
the set and reset voltages measured over 80 consecutive cycles show
excellent uniformity. The average set and reset voltages are 0.53 V
and 0.84 V respectively. The standard deviations for the set and
reset voltages were found to be 0.08 V and 0.04 V respectively. The
distribution of peak reset current also showed that the peak reset
current closely corresponded to the set current (100 RA). This also
proves that the resistive switching devices herein do not exhibit
significant current overshoot during the reset process. Excellent
distribution is observed for device-to-device statistics as well
(FIG. 5).
[0052] The resistive switching devices after the additional baking
step of the nanocomposite demonstrated switching at ultra-low
current of 500 nA (FIG. 6). Even at the ultra-low current of
operation a memory window of an order of magnitude is observed. The
ability for the device to demonstrate non-volatile switching at
such a low current possibly occurred due to microstructural change
of the polymer nanocomposite after baking.
[0053] In embodiments, the resistive switching devices herein,
wherein the polymer film will be selectively irradiated within a
nanoscale region (.about.10 nm) using electron beam (FIG. 7). As a
result after the infiltration with Al.sub.2O.sub.3 the irradiated
zone of 10 nm within the film had higher porosity compared to the
rest of the nanocomposite film. As a result, resistive switching
device variability can be further reduced as operating voltages can
be decreased even further. FIG. 7 in (c) shows the 20.times.20
crossbar architecture for implementation of the memory arrays with
the polymer-oxide nanocomposite resistive switching devices.
Example 3--Comparative Example
[0054] The switching characteristics of a Ag/PMMA-Al.sub.2O.sub.3
(10 nm)/Pt devices at 100 nA operating current were shown in FIG. 8
in (a). At this current level the resistive switching device acted
as a selector switch, i.e., it turns on at a positive threshold
voltage but turns off after removal of the applied bias. This was
seen from the two subsequent cycles of operation (cycle 1 and 2
respectively). After the completion of the first cycle the
resistive switching device turns off when the bias was removed. As
a result the resistive switching device in cycle 2 was initially
turned off and turns on at .about.450 mV but turns off again after
the removal of bias. Note that the switching voltage in the
resistive switching device was .about.450 mV which is almost an
order of magnitude lower than a control, which consisted of a
different device with a pure Al.sub.2O.sub.3 switching layer of
same thickness (10 nm). The device with pure Al.sub.2O.sub.3 (10
nm) as the switching layer (deposited also by atomic layer
deposition) switched at .about.4 V and shows unstable switching
(i.e., shows tendency to form a permanent short after switching).
Therefore the disclosed resistive switching devices herein provide
a significant advantage over conventional metal oxide based devices
in terms of the operating voltage.
[0055] The foregoing description is given for clearness of
understanding only, and no unnecessary limitations should be
understood therefrom, as modifications within the scope of the
disclosure herein may be apparent to those having ordinary skill in
the art.
[0056] Throughout this specification and the claims which follow,
unless the context requires otherwise, the word "comprise" and
variations such as "comprises" and "comprising" will be understood
to imply the inclusion of a stated integer or step or group of
integers or steps but not the exclusion of any other integer or
step or group of integers or steps.
* * * * *