U.S. patent application number 11/046114 was filed with the patent office on 2006-08-03 for chemical vapor deposition of chalcogenide materials.
This patent application is currently assigned to Energy Conversion Devices, Inc. Invention is credited to Smuruthi Kamepalli, Stanford R. Ovshinsky.
Application Number | 20060172067 11/046114 |
Document ID | / |
Family ID | 36756891 |
Filed Date | 2006-08-03 |
United States Patent
Application |
20060172067 |
Kind Code |
A1 |
Ovshinsky; Stanford R. ; et
al. |
August 3, 2006 |
Chemical vapor deposition of chalcogenide materials
Abstract
A chemical vapor deposition (CVD) process for preparing
electrical and optical chalcogenide materials. In a preferred
embodiment, the instant CVD-deposited materials exhibit one or more
of the following properties: electrical switching, accumulation,
setting, reversible multistate behavior, resetting, cognitive
functionality, and reversible amorphous-crystalline
transformations. In one embodiment, a multilayer structure,
including at least one layer containing a chalcogen element, is
deposited by CVD and subjected to post-deposition application of
energy to produce a chalcogenide material having properties in
accordance with the instant invention. In another embodiment, a
single layer chalcogenide material having properties in accordance
with the instant invention is formed from a CVD deposition process
including three or more deposition precursors, at least one of
which is a chalcogen element precursor. Preferred materials are
those that include the chalcogen Te along with Ge and/or Sb.
Inventors: |
Ovshinsky; Stanford R.;
(Bloomfield Hills, MI) ; Kamepalli; Smuruthi;
(Rochester, MI) |
Correspondence
Address: |
ENERGY CONVERSION DEVICES, INC.
2956 WATERVIEW DRIVE
ROCHESTER HILLS
MI
48309
US
|
Assignee: |
Energy Conversion Devices,
Inc
|
Family ID: |
36756891 |
Appl. No.: |
11/046114 |
Filed: |
January 28, 2005 |
Current U.S.
Class: |
427/248.1 ;
257/E45.002; 427/402; 427/532 |
Current CPC
Class: |
C23C 16/305 20130101;
H01L 45/04 20130101; H01L 45/144 20130101; H01L 45/1616 20130101;
H01L 45/06 20130101 |
Class at
Publication: |
427/248.1 ;
427/402; 427/532 |
International
Class: |
C23C 16/00 20060101
C23C016/00; B05D 1/36 20060101 B05D001/36; B29C 71/04 20060101
B29C071/04 |
Claims
1. A method for forming a chalcogenide material comprising the
steps of: Providing a substrate; Placing said substrate in a
deposition chamber; Delivering one or more deposition precursors to
said deposition chamber, said deposition precursors being delivered
in vapor phase form, at least one of said deposition precursors
comprising a chalcogen element; said deposition precursors reacting
to form a solid phase thin film on said substrate, said thin film
comprising said chalcogen element, said thin film having a
threshold voltage; Wherein said thin film switches from a resistive
state to a conductive state upon applying said threshold voltage to
said thin film.
2. The method of claim 1, wherein said one or more deposition
precursors includes at least two deposition precursors.
3. The method of claim 1, wherein said one or more deposition
precursors includes at least three deposition precursors.
4. The method of claim 1, wherein each of said one or more
deposition precursors provides at least one element to said thin
film during said reaction.
5. The method of claim 1, wherein said thin film comprises at least
2 elements.
6. The method of claim 1, wherein said thin film comprises at least
3 elements.
7. The method of claim 1, wherein said chalcogen element is Te.
8. The method of claim 1, wherein said thin film further comprises
Ge.
9. The method of claim 1, wherein said thin film further comprises
Sb.
10. The method of claim 1, wherein said thin film comprises Te and
Ge.
11. The method of claim 1, wherein said thin film comprises Te, Sb,
and Ge.
12. The method of claim 1, wherein said thin film comprises
GeTe.
13. The method of claim 1, wherein said thin film comprises
Ge.sub.2Sb.sub.2Te.sub.5.
14. The method of claim 1, wherein said thin film returns to said
resistive state when said threshold voltage is removed.
15. The method of claim 1, wherein said substrate is in motion
during said thin film formation step.
16. The method of claim 15, wherein said moving substrate is a
continuous web substrate.
17. A method for forming a chalcogenide material comprising the
steps of: Providing a substrate; Placing said substrate in a
deposition chamber; Delivering one or more deposition precursors to
said deposition chamber, said deposition precursors being delivered
in vapor phase form, at least one of said deposition precursors
comprising a chalcogen element; said deposition precursors reacting
to form a solid phase thin film on said substrate, said thin film
comprising said chalcogen element, said thin film having a
plurality of states, said thin film being transformable among said
states upon application of electrical energy, said states including
a plurality of states having distinguishable resistances; Wherein
said plurality of states includes states whose resistances differ
by at least a factor of two.
18. The method of claim 17, wherein said plurality of states
includes states whose resistances differ by at least a factor of
four.
19. The method of claim 17, wherein said plurality of states
includes states whose resistances differ by at least a factor of
eight.
20. The method of claim 17, wherein said plurality of states
includes at least three states.
21. The method of claim 17, wherein said plurality of states
includes at least four states.
22. The method of claim 17, wherein said one or more deposition
precursors includes at least two deposition precursors.
23. The method of claim 17, wherein said one or more deposition
precursors includes at least three deposition precursors.
24. The method of claim 17, wherein each of said one or more
deposition precursors provides at least one element to said thin
film during said reaction.
25. The method of claim 17, wherein said thin film comprises at
least 2 elements.
26. The method of claim 17, wherein said thin film comprises at
least 3 elements.
27. The method of claim 17, wherein said chalcogen element is
Te.
28. The method of claim 17, wherein said thin film further
comprises Ge.
29. The method of claim 17, wherein said thin film further
comprises Sb.
30. The method of claim 17, wherein said thin film comprises Te and
Ge.
31. The method of claim 17, wherein said thin film comprises Te,
Sb, and Ge.
32. The method of claim 17, wherein said thin film comprises
GeTe.
33. The method of claim 17, wherein said thin film comprises
Ge.sub.2Sb.sub.2Te.sub.5.
34. A method for forming an electrical switching material
comprising the steps of: Providing a substrate; Depositing a first
layer on said substrate; Depositing a second layer on said first
layer to form a penultimate multilayer structure; Applying energy
to said multilayer structure, said energy transforming said
penultimate multilayer structure into an ultimate multilayer
structure, said ultimate multilayer structure having a threshold
voltage; Wherein said ultimate multilayer structure switches from a
resistive state to a conductive state upon applying said threshold
voltage to said ultimate multilayer structure.
35. The method of claim 34, wherein application of a voltage to
said penultimate multilayer structure does not induce switching
from a resistive state to a conductive state.
36. The method of claim 34, wherein said energy is applied in the
form of electrical energy.
37. The method of claim 34, wherein at least one of said deposition
steps is a chemical vapor deposition step.
38. The method of claim 34, wherein said first layer or said second
layer comprises a chalcogen element.
39. A method for forming a chalcogenide material comprising the
steps of: Providing a substrate; Placing said substrate in a
deposition chamber; Delivering one or more deposition precursors to
said deposition chamber, said deposition precursors being delivered
in vapor phase form, at least one of said deposition precursors
comprising a chalcogen element; said deposition precursors reacting
to form a solid phase thin film on said substrate, said thin film
comprising said chalcogen element, said thin film comprising one or
more additional elements; Wherein said one or more additional
elements does not include Zn, Cd, Hg or Mg.
40. The method of claim 39, wherein said one or more additional
elements includes at least two elements.
41. The method of claim 39, wherein said chalcogen element is Se or
Te.
42. The method of claim 39, wherein said one or more additional
elements include Ge.
Description
FIELD OF INVENTION
[0001] This invention relates to a process for preparing
chalcogenide materials. More particularly, this invention relates
to the formation of thin film chalcogenide materials through a
metalorganic chemical vapor deposition process. Most particularly,
this invention relates to the metalorganic chemical vapor
deposition of chalcogenide materials comprising Ge, Sb and Te.
BACKGROUND OF THE INVENTION
[0002] Chalcogenide materials are materials that contain a
chalcogen element (O, S, Se, Te) and typically one or more
additional elements that serve to modify electronic or structural
properties. The II-VI semiconductors (e.g. CdS, ZnTe etc.) are a
well-known class of chalcogenide materials. These materials have
been widely investigated for their wide bandgap properties and
their potential for providing short wavelength light emission for
LED and laser applications. Another important class of chalcogenide
materials includes the expansive series of chalcogenide materials,
initially developed by S. R. Ovshinsky, that are currently being
used in optical and electrical memory and switching applications.
These chalcogenide materials may be referred to herein as Ovonic
chalcogenide materials. Among the Ovonic chalcogenide materials are
chalcogenide phase change materials that are currently widely in
use in optical recording technologies. The active materials in CD
and DVD applications are chalcogenide materials that have a
crystalline state and an amorphous state whose relative proportions
can be reversibly and reproducibly varied through the application
of optical energy. These materials can be used to store information
by defining a series of two or more distinct structural states,
each of which is defined by a characteristic proportion of
crystalline and amorphous phase domains within a given volume, and
associating a distinct information value to each structural state.
Storage of an information value occurs by applying optical energy
to the phase change material in an amount necessary to convert the
material to the structural state associated with the information
value.
[0003] The optical phase change chalcogenide materials are
reversibly transformable between different structural states
through the judicious application of energy. The proportion of
amorphous phase can be increased by applying energy sufficient to
create a local temperature in the phase change material that
exceeds the melting temperature and removing the energy at a rate
sufficient to prevent crystallization upon cooling. The proportion
of crystalline phase can be increased by applying energy sufficient
to create a local temperature in the phase change material that
exceeds the crystallization temperature so that a controlled
transformation of amorphous phase material to crystalline phase
material is induced. Reading of the information content of the
phase change material occurs through the detection of a physical
characteristic of the structural state of the material. In optical
recording, for example, reflectivity is a widely used as a
parameter for detecting the structural state. The reflectivity
difference between the crystalline and amorphous states provides
sufficient contrast to permit clear resolution of structural states
that differ with respect to the relative proportions of crystalline
and amorphous phase volume fractions.
[0004] Two other important types of Ovonic chalcogenide materials
are the electrical switching and electrical memory materials. The
Ovonic electrical switching chalcogenide materials are switchable
between a resistive state and a conductive state upon application
of a threshold voltage. In the resistive state, the materials
inhibit the flow of electrical current and upon application of the
threshold voltage, the material switches nearly instantaneously to
its conductive state to permit the flow of current. In the Ovonic
electrical memory materials, application of electrical energy
(typically in the form of current pulses) induces changes in the
structural state of the chalcogenide material. The relative volume
fraction of crystalline and amorphous phase domains can be
continuously varied through judicious control of the duration and
magnitude of a series of one or more applied current pulses. Each
structural state has a unique resistance and each resistance value
can be associated with a distinct information value. By applying an
appropriate current pulse, the electrical chalcogenide memory
material can be programmed into the resistance state that
corresponds to a particular information value to write that value
to the material. The electrical memory material can be transformed
among its different resistance states to provide erasing and
rewriting capabilities. Both the electrical and optical
chalcogenide memory materials can be incorporated into arrays to
provide advanced, high density memory capability.
[0005] As the appreciation of the range of applications of
available from chalcogenide materials grows, greater attention is
being placed on further understanding their properties and on
developing new chalcogenide materials that exhibit a wider range of
properties. The development of new materials requires the synthesis
or deposition of either new compositions or existing compositions
having unique microstructures. The primary preparation methods for
the optical and electrical chalcogenide materials are sputtering
and physical vapor deposition. Although these techniques have
provided for a number very interesting and useful materials, it is
expected that the development of new synthetic or preparation
methods will expand the range of compositions and properties of
chalcogenide materials and will further the objective of expanding
the applications of chalcogenide materials.
[0006] Chemical vapor deposition, hereinafter referred to as CVD,
is a widely used technique for the synthesis of materials. In the
CVD process, precursors of the constituent elements of a material
are reacted to produce a thin film on a substrate. The reaction of
the CVD precursors occurs either homogeneously in the gas phase or
heterogeneously at the solid-gas interface of the substrate
surface. Precursors for many elements are available and a variety
of thin film compositions can be synthesized using CVD.
[0007] In CVD processing, precursors are introduced into the
reactor in gas phase form. Precursors that are in the gas phase at
room conditions are directly introduced into the reactor, typically
in diluted form via a carrier gas. Liquid and solid phase
precursors are vaporized or sublimed and then introduced into the
reactor, also typically in diluted form in the presence of a
carrier gas. Upon introduction into the reactor, precursors
containing the chemical constituents of the desired material are
decomposed (thermally, photochemically, or in a plasma) to provide
intermediate species of the constituents that subsequently react to
form a thin film of desired composition. The rate of deposition,
stoichiometry, composition and morphology of the film can be varied
through appropriate control over process parameters such as
reaction temperature; substrate; selection of precursor; reactor
pressure; and the rate of introduction of precursors into the
reactor. CVD offers the advantages of providing high purity thin
films at relatively low temperatures.
[0008] Although CVD, has been widely used for II-VI materials that
contain chalcogenide elements and simple binary chalcogenides such
as Sb.sub.2Te.sub.3, its use for the Ovonic family of optical and
electrical chalcogenide materials has been virtually non-existent
due to the anticipated difficulties associated with producing the
multiple element (ternary and higher) compositions typically
associated with the most effective optical and electrical switching
and memory chalcogenide materials. CVD synthesis of the optical and
electrical switching and memory chalcogenides is an outstanding
challenge that remains to be addressed. Successful development of
the CVD synthesis of these materials is expected to provide a wider
range of compositions with more diverse switching, memory and phase
change characteristics and accordingly will provide new materials
that can fulfill the ever-increasing expectations for chalcogenide
materials.
SUMMARY OF THE INVENTION
[0009] This invention provides a chemical vapor deposition (CVD)
process for preparing chalcogenide materials suitable for use in
optical and electrical switching and memory applications. A
chalcogenide precursor is reacted with one or more precursors
containing other elements to produce a chalcogenide thin film in a
CVD process.
[0010] In one embodiment, the chalcogenide thin film is an optical
phase change material that is reversibly transformable between a
high reflectivity state and a low reflectivity state upon
application of optical energy, where the high reflectivity and low
reflectivity states differ in fractional crystallinity.
[0011] In another embodiment, the chalcogenide thin film is an
electrical switching material that can be switched from a high
resistance state to a low resistance state upon application of a
threshold voltage, where the low resistance state includes at least
a filamentary portion that exhibits high conductivity.
[0012] In another embodiment, the chalcogenide thin film is an
electrical memory material in which the relative proportions of
crystalline and amorphous phase volumes can be varied through the
application of an electrical signal.
[0013] In a preferred embodiment, the instant CVD-prepared
chalcogenide material comprises Te. In another preferred
embodiment, the instant CVD-prepared chalcogenide material
comprises Te and Ge.
[0014] In yet another preferred embodiment, the instant
CVD-prepared chalcogenide material comprises Te and Sb.
[0015] In still another preferred embodiment, the instant
CVD-prepared chalcogenide material is GeTe.
[0016] In still another preferred embodiment, the instant
CVD-prepared chalcogenide material is Sb.sub.2Ge.sub.2Te.sub.5.
[0017] The instant invention provides for chalcogenide deposition
onto stationary or continuous web substrates.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1. Current-voltage characteristics of a chalcogenide
material exhibiting a switching transformation.
[0019] FIG. 2. Resistance characteristics of a chalcogenide
material as a function of applied energy or power.
[0020] FIG. 3. Schematic molecular depictions of an Sb precursor
and a Te precursor suitable for CVD deposition of chalcogenide
materials.
[0021] FIG. 4. Depth profile obtained from Auger emission
spectroscopy of a CVD-deposited Sb.sub.2Te.sub.3 thin film.
[0022] FIG. 5. Scanning electron micrograph of a CVD-deposited
Sb.sub.2Te.sub.3 thin film.
[0023] FIG. 6. Schematic molecular depictions of a Ge precursor
suitable for CVD deposition of chalcogenide materials.
[0024] FIG. 7. Resistance as a function of current characteristics
of a two-layer CVD deposited thin film structure.
[0025] FIG. 8. Current as a function of voltage characteristics of
a two-layer CVD deposited thin film structure.
[0026] FIG. 9. Resistance as a function of current characteristics
of a two-layer CVD deposited thin film structure.
[0027] FIG. 10. Low magnification (2000.times.) image of a
Ge--Sb--Te ternary chalcogenide material deposited by CVD.
[0028] FIG. 11. High magnification (6000.times.) image of a
Ge--Sb--Te ternary chalcogenide material deposited by CVD.
[0029] FIG. 12. Depth profile obtained from Auger emission
spectroscopy of a CVD-deposited ternary Ge--Sb--Te thin film.
[0030] FIG. 13. Resistance as a function of current characteristics
of a CVD-deposited ternary Ge--Sb--Te thin film.
[0031] FIG. 14. Low magnification (2000.times.) image of a Ge--Te
binary chalcogenide material deposited by CVD.
[0032] FIG. 15. Resistance as a function of current characteristics
of a Ge--Te binary chalcogenide material deposited by CVD.
[0033] FIG. 16. Current as a function of voltage characteristics of
a Ge--Te binary chalcogenide material deposited by CVD.
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
[0034] The instant invention demonstrates the chemical vapor
deposition (CVD) synthesis of optical and electrical chalcogenide
materials in thin film form. As used herein, CVD encompasses all
variations of chemical vapor deposition including those generally
referred to in the art as VPE, MOVPE, MOCVD, OMVPE, OMCVD, PECVD
and RPCVD.
[0035] A chalcogenide material within the scope of the instant
invention is a material that includes at least one chalcogen
element (S, Se, or Te) in an oxidized, reduced or neutral state. In
a preferred embodiment, the chalcogenide materials include one or
more non-chalcogen elements in combination with a chalcogen
element.
[0036] In a preferred embodiment of the instant invention, a
chalcogenide precursor is combined with one or more additional
precursors in a CVD process to produce solid phase chalcogenide
materials containing two or more elements. As used herein, a
chalcogenide precursor is a chemical species that includes a
chalcogen element and is able to contribute a chalcogen element
during the formation and growth of a chalcogenide material in the
instant CVD deposition process. The CVD deposition occurs in a CVD
reactor or chamber. The CVD reactor includes a substrate onto which
deposition occurs. The substrate can be a stationary substrate
(e.g. a wafer) or a moving substrate (e.g. continuous web). The
substrate can be lattice-matched to the CVD-deposited thin film or
not. Growth precursors for the deposition are introduced into the
CVD reactor and the reaction is commenced. During deposition, the
reactor pressure and temperature are adjusted to optimize the
deposition rate and purity of the thin film that is formed.
Depending on the composition, substrate, reactor conditions,
precursors etc. the thin film formed can be epitaxial, crystalline,
polycrystalline, amorphous, homogeneous, heterogeneous etc. Two CVD
processing strategies are employed in the instant invention. In one
embodiment, the instant chalcogenides are prepared through a direct
CVD process, in which precursors for each element to be included in
the ultimate thin film material are introduced simultaneously into
a CVD reactor to form a multi-element chalcogenide material. In
another embodiment, the instant chalcogenides are prepared through
an alternating CVD process in which a penultimate multilayer
structure is deposited, where each of the alternately deposited
layers includes a different subset of the elements to be included
in the intended ultimate composition and a post-CVD processing step
is used to induce a transformation of the penultimate multilayer
structure into the ultimate film.
[0037] Successful CVD synthesis of multiple element materials
requires careful design of the precursor species. The CVD reaction
is a gas phase reaction of precursors. It is therefore necessary to
utilize gas phase precursors directly or to transform liquid and
solid phase precursors into the gas phase prior to reaction. An
important attribute of a precursor is the ability to introduce it
at a steady and reproducible rate during the CVD reaction. Gas
phase precursors are convenient for this purpose since they can be
released and delivered to the reactor at a constant flow rate with
a high degree of reproducibility. Oftentimes, gas phase precursors
are diluted in a carrier gas such as He or Ar to control
concentration in the reactor. Liquid and solid phase CVD precursors
are also suitable, but require pre-delivery vaporization or
sublimation prior to introduction into the CVD reactor.
Vaporization or sublimation can be accomplished thermally or
through entrainment in a carrier gas. Bubblers, for example, often
used to deliver liquid phase precursors to CVD reactors. Solid
phase precursors are often the most problematic in terms of
achieving uniform precursor delivery rates because the surface area
of a solid varies over the course of a deposition run. Mass flow
controllers can be used to insure uniform delivery of vaporized or
sublimed precursors into the CVD reactor.
[0038] Once the precursor is introduced into the CVD reactor, it
reacts with other precursors to form a thin film. The reaction can
occur through a gas phase reaction followed by deposition onto the
substrate surface. Alternatively, the precursors can be decomposed
(e.g. thermally or through plasma excitation) into reactive
intermediate species (frequently including free radical species)
that can combine in the gas phase or on the surface of the
substrate to form the desired thin film. Many CVD reactions occur
through decomposition of one or more precursors into reactive
intermediate species that adsorb onto the substrate surface. Once
on the surface, reactive species formed from different precursors
react to form a multielement thin film.
[0039] When binary or multi-element materials as ultimate thin
films or layers within a multilayer penultimate structure are to be
prepared, two or more precursors are introduced simultaneously into
the CVD reactor. The complexity of the process increases due to the
need to insure comparable rates of reaction or decomposition of the
different precursors in the gas phase reaction environment of the
reactor. When a multi-element material is prepared, it is
beneficial for the precursors to provide the necessary elements at
similar rates so that more nearly uniform and homogeneous thin
films are formed. If one precursor reacts at a significantly faster
rate than other precursors, the possibility arises that a film of
non-uniform or undesired composition forms. A faster reacting
precursor, for example, may deposit a mono-elemental layer onto the
substrate before appreciable reaction or decomposition of slower
reacting precursors has occurred. As a result, the stoichiometric
ratio desired in the deposited material may be lacking. In the case
of ternary and higher compositions, preferential reactions between
a subset of the precursors may also occur and lead to the formation
of a thin film that is depleted with respect to the element(s) of
the non-preferentially reacting precursor(s). A further
complication arises if the elements (or reactive species containing
the elements) desired in the deposited film differ appreciably in
volatility. Volatility is a relevant consideration because surface
desorption of the desired elements (or species containing the
desired elements) can occur during CVD deposition. If the different
elements of a multi-element composition desorb at appreciably
different rates from the surface, the intended stoichiometry may
not be achieved.
[0040] The CVD preparation of multi-element compositions therefore
requires careful selection of precursors and reaction conditions.
The reactivity of CVD precursors is influenced by the conditions in
the reactor (e.g. temperature, pressure, and concentration) as well
as by the chemical features of the precursor itself. The conditions
within the reactor can be varied to optimize the quality of
deposited thin films for a given combination of precursors and the
individual precursors can be optimized with respect to their
intrinsic reactivity through control of the structure and bonding
of the precursor. Most precursors include a central element or
elements that one wishes to incorporate into a CVD thin film along
with peripheral elements or groups that are bonded to the central
element or elements. Many precursors, for example, include a
central metal or non-metal atom that is bonded by one or more
ligands that decompose in the CVD deposition during the formation
of the reactive intermediate that contains the central element. The
bond strength between such ligands and the central atom is
typically an important contributing factor in the rate of reaction
or decomposition of the precursor. Through judicious control of the
ligands or other substituents, the reactivity of a precursor with
respect to the delivery of elements desired in the deposited thin
film can be controlled through control of relevant factors such as
decomposition rate, reaction rate and desorption rate. Chemical
tuning of the properties of CVD precursors is an important degree
of freedom in multi-element depositions. Such chemical tuning can
be used to identify and optimize combinations of precursors to
improve the quality of multi-element films and to minimize
incorporation of impurity elements into the deposited film.
[0041] The instant invention focuses on the CVD synthesis of
chalcogenide materials in thin film form. In a preferred
embodiment, the chalcogenide material is an optical or electrical
chalcogenide material that is useful in optical and electrical
memory and switching applications. In another preferred embodiment,
the chalcogenide material is not a II-VI material and accordingly
lack a column II element (Zn, Cd, or Hg) or Mg in the composition.
In another preferred embodiment, the chalcogenide material includes
Te and one or more non-chalcogen elements. In another preferred
embodiment, the chalcogenide material includes a chalcogen element
and Sb. In another preferred embodiment, the chalcogenide material
includes a chalcogen element and Ge.
[0042] In one embodiment of the instant invention, chalcogenide
materials that exhibit electrical switching are prepared in a CVD
process. The switching properties of chalcogenide materials have
been previously exploited in OTS (Ovonic Threshold Switch) devices.
The OTS has been described in U.S. Pat. Nos. 5,543,737; 5,694,146;
and 5,757,446; the disclosures of which are hereby incorporated by
reference, as well as in several journal articles including
"Reversible Electrical Switching Phenomena in Disordered
Structures", Physical Review Letters, vol. 21, p.1450-1453 (1969)
by S. R. Ovshinsky; "Amorphous Semiconductors for Switching,
Memory, and Imaging Applications", IEEE Transactions on Electron
Devices, vol. ED-20, p. 91-105 (1973) by S. R. Ovshinsky and H.
Fritzsche; the disclosures of which are hereby incorporated by
reference.
[0043] The electrical switching properties of the chalcogenide
materials used in the instant devices are schematically illustrated
in FIG. 1, which shows the I-V (current-voltage) characteristics of
a chalcogenide electrical switching material. The illustration of
FIG. 1 corresponds to a two-terminal device configuration in which
two spacedly disposed electrodes are in contact with a chalcogenide
material and the current I corresponds to the current passing
between the two electrodes. The I-V curve of FIG. 1 shows the
current passing through the chalcogenide material as a function of
the voltage applied across the material by the electrodes. The I-V
characteristics of the material are symmetric with respect to the
polarity of the applied voltage. For convenience, we consider the
first quadrant of the I-V plot of FIG. 1 (the portion in which
current and voltage are both positive) in the brief discussion of
chalcogenide switching behavior that follows. An analogous
description that accounts for polarity applies to the third
quadrant of the I-V plot.
[0044] The I-V curve includes a resistive branch and a conductive
branch. The branches are labeled in FIG. 1. The resistive branch
corresponds, to the branch in which the current passing through the
material increases only slightly upon increasing the voltage
applied across the material. This branch exhibits a small slope in
the I-V plot and appears as a nearly horizontal line in the first
and third quadrants of FIG. 1. The conductive branch corresponds to
the branch in which the current passing through the material
increases significantly upon increasing the voltage applied across
the material. This branch exhibits a large slope in the I-V plot
and appears as a nearly vertical line in the first and third
quadrants of FIG. 1. The slopes of the resistive and conductive
branches shown in FIG. 1 are illustrative and not intended to be
limiting, the actual slopes will depend on the chemical composition
of the chalcogenide material. Regardless of the actual slopes, the
conductive branch necessarily exhibits a larger slope than the
resistive branch. When device conditions are such that the
chalcogenide material is described by a point on the resistive
branch of the I-V curve, the chalcogenide material or device may be
said to be in a resistive state. When device conditions are such
that the chalcogenide material is described by a point on the
conductive branch of the I-V curve, the chalcogenide material or
device may be said to be in a conductive state.
[0045] The switching properties of the electrical switching
chalcogenide material used can be described by reference to FIG. 1.
We consider a two-terminal device configuration and begin with a
device that has no voltage applied across it. When no voltage is
applied across the chalcogenide material, the material is in a
resistive state and no current flows. This condition corresponds to
the origin of the I-V plot shown in FIG. 1. The chalcogenide
remains in a resistive state as the applied voltage is increased,
up to a threshold voltage (labeled Vt in the first quadrant of FIG.
1). The slope of the I-V curve for applied voltages between 0 and
V, is small in magnitude and indicates that the chalcogenide
material has a high electrical resistance, a circumstance reflected
in the terminology "resistive branch" used to describe this portion
of the I-V curve. The high resistance implies low electrical
conductivity and as a result, the current flowing through the
material increases only weakly as the applied voltage is increased.
Since the current through the material is very small, the resistive
state of the chalcogenide may be referred to as the OFF state of
the material.
[0046] When the applied voltage equals or exceeds the threshold
voltage, the chalcogenide material transforms (switches) from the
resistive branch to the conductive branch of the I-V curve. The
switching event occurs nearly instantaneously and is depicted by
the dashed line in FIG. 1. Upon switching, the device voltage
decreases significantly and the device current becomes much more
sensitive to changes in the device voltage. The chalcogenide
material remains in the conductive branch as long as a minimum
current, labeled I.sub.h in FIG. 1, is maintained.
[0047] In another embodiment, the material prepared by the instant
CVD process is a chalcogenide material having one or more high
resistance accumulation states, a detectably distinct low
resistance state and one or more greyscale states having
intermediate resistance. As used herein, high and low resistance
states refer to physical states characterized by high and low
electrical resistances, respectively, where the electrical
resistances of the high and low electrical resistance states are
relative to and detectably distinct from each other. The greyscale
states have electrical resistance values intermediate between the
high and low resistance states.
[0048] FIG. 2 disclosed herein is a plot of the electrical
resistance as a function of energy or power of a representative
chalcogenide material of this embodiment. The application of energy
to the chalcogenide material permits interconversion among the
different states as described hereinbelow. The electrical
resistance plot can be broadly classified into an accumulation
region and a greyscale region where the two regions are separated
by a nearly discontinuous change in electrical resistance. The
accumulation region corresponds to the high resistance plateau
shown on the left side of FIG. 2 herein and the greyscale region
corresponds to the remaining portion of the electrical resistance
response shown on the right side of FIG. 2 herein.
[0049] The accumulation region includes a plurality of high
resistance states, each of which has a similar electrical
resistance. The slope in the accumulation region can be nearly
horizontal, as shown in FIG. 2, or may exhibit a gradual slope.
States in the accumulation region may be referred to as
accumulation states. If the chalcogenide material is initially in a
high resistance state, the application of small amounts of energy
leaves the material in a high resistance state. This behavior is
depicted by the high resistance plateau region shown on the left
side of FIG. 2. If a sufficient amount of energy is applied,
however, the chalcogenide material transforms from its high
resistance state to its low resistance state. This transformation
is depicted by the steep reduction in electrical resistance
immediately to the right of the high resistance plateau region of
FIG. 2. This transformation of the material from its high
resistance state to its low resistance state may be referred to as
"setting" or "to set" the phase change material. The low resistance
state produced upon setting may be referred to as the "set state"
of the phase change material. An amount of energy sufficient to set
the material may be referred to as the "set energy" or "setting
energy". Note that the set energy is different for each position or
state along the high resistance plateau. The reset state may be
viewed as the accumulation state having the highest setting
energy.
[0050] The right side of FIG. 2 corresponds to the behavior of the
chalcogenide material when it has been set. Once set, the material
is in its low resistance state and is influenced by the application
of power or energy according to the post-setting region shown in
right side of FIG. 2. This portion of the electrical resistance
response curve may be referred to as the analog, multistate or
greyscale region of the curve. The application of energy to a
material in a greyscale state may produce changes in its electrical
resistance. The change in electrical resistance is determined by
the amount of energy applied and the rate at which the energy is
applied. The rate at which energy is provided corresponds to power
and is an important factor in the behavior of a material in the
post-setting, greyscale region.
[0051] Depending on the power and the state of the chalcogenide
material in the greyscale region of FIG. 2, an increase or decrease
in electrical resistance may occur. Furthermore, the behavior in
the greyscale region is reversible. This reversibility is depicted
by the two arrows shown in the greyscale region of FIG. 2 and
underlies the direct overwrite feature of the material in the
greyscale region. A power and electrical resistance may be
associated with each point in the greyscale region. If the applied
power exceeds the power associated with the point describing a
material in the greyscale region, the electrical resistance of the
material increases. Conversely, if the applied power is less than
the power associated with the point describing a material in the
greyscale region, the electrical resistance decreases.
[0052] The reversibility is limited to the greyscale region of FIG.
2. It is not possible to reverse the setting transformation by
applying an energy corresponding to a point in the high resistance
accumulation region of FIG. 2 that precedes (i.e. is to the left
of) the setting transformation. It is possible, however, to restore
the high resistance state of the material by applying a
sufficiently high power to a material described by a point in the
greyscale region of FIG. 2. The application of such power
corresponds to moving in the rightward direction in FIG. 2, rather
than in the direction of reversing the setting transformation. As
shown in the greyscale region of FIG. 2, the application of
continually increasing amounts power leads to a continual increase
in electrical resistance. Upon application of sufficient power to
drive the material to the far right side of FIG. 2, the material
returns to its high resistance state and renews its high resistance
plateau of accumulation states.
[0053] The power or rate of energy needed to transform a
chalcogenide material of this embodiment from a greyscale state to
a high resistance state may be referred to as the "reset power",
"resetting power", "reset energy", "resetting energy" or the like.
The low resistance set state corresponds to the greyscale state
having the maximum reset energy. The state of the material at the
conclusion of the application of the reset energy may be referred
to as the "reset state". The application of the reset power
"resets" the material to produce a high resistance reset state and
places the material in its accumulation region. The behavior
observed upon further application of energy after resetting is
corresponds to that described hereinabove for the accumulation
region of FIG. 2.
[0054] The behavior (including switching, memory, accumulation and
cognitive operation) and chemical compositions of chalcogenide
materials included within the scope of this invention have been
described, for example, in the following U.S. Pat. Nos. 6,671,710;
6,714,954; 6.087,674; 5,166,758; 5,296,716; 5,534,711; 5,536,947;
5,596,522; 5,825,046; 5,687,112; 5,912,839; 3,271,591 and
3,530,441, the disclosures of which are hereby incorporated by
reference. These references also describe proposed mechanisms that
govern the behavior of the electrical and optical chalcogenide
materials. The references also describe the structural
transformations from the crystalline state to the amorphous state
(and vice versa) via a series of partially crystalline states in
which the relative proportions of crystalline and amorphous regions
vary underlying the operation of electrical and optical
chalcogenide materials.
[0055] Representative chalcogenide materials are those that include
one or more elements from column VI of the periodic table (the
chalcogen elements) and optionally one or more chemical modifiers
from columns HI. IV or V. One or more of S, Se, and Te are the most
common chalcogen elements included in the active material of the
instant devices. The chalcogen elements are characterized by
divalent bonding and the presence of lone pair electrons. The
divalent bonding leads to the formation of chain and ring
structures upon combining chalcogen elements to form chalcogenide
materials and the lone pair electrons provide a source of electrons
for forming a conducting filament. Trivalent and tetravalent
modifiers such as Al, Ga, In, Ge, Sn, Si, P, As and Sb enter the
chain and ring structures of chalcogen elements and provide points
for branching and crosslinking.
[0056] Suitable deposition precursors for the instant invention
include gas or vapor phase molecular compounds comprising a
chalcogen element or liquid or solid phase compounds that are
capable of being converted to a gas or vapor phase through, for
example, evaporation or sublimation. Representative precursors
include alkyl compounds of chalcogen elements, chalcogen compounds
including a chalcogen-carbon bond, amine compounds of chalcogen
elements, and chalcogen compounds including a chalcogen-nitrogen
bond. Specific representative illustrations are provided in further
detail in the examples that follow hereinbelow.
EXAMPLE 1
[0057] In this example, the CVD synthesis of Sb.sub.2Te.sub.3 on a
silicon nitride substrate is demonstrated. The CVD reactor includes
a substrate mount, multiple precursor inlets for delivering
precursors in vapor or gas phase form directly or diluted in a
carrier gas as well as separate overhead showerhead and backfill
lines for providing background pressure of an inert ambient
gas.
[0058] A silicon nitride wafer substrate was placed in a CVD
reaction chamber. Tris(dimethylamino)antimony
(Sb(N(CH.sub.3).sub.2).sub.3)was used as the antimony (Sb)
precursor to provide the Sb necessary for film formation.
Diisopropyltellurium (Te(CH(CH.sub.3).sub.2).sub.2) was used as the
tellurium (Te) precursor to provide the Te necessary for film
formation. The molecular forms of the two precursors are shown in
FIG. 3 herein. Both precursors are liquids at ambient condition and
were delivered to the CVD reactor in a vapor phase form through use
of a bubbler. The Sb-precursor and the Te-precursor were placed in
separate bubblers connected through separate lines to the CVD
reactor. Each bubbler and its delivery lines were heated to
75.degree. C. N.sub.2 was used as a carrier gas for delivering each
of the precursors to the CVD reactor. N.sub.2 was bubbled through
each bubbler at a flow rate of 300 sccm to produce a gas stream
containing each precursor in a vapor phase form diluted in N.sub.2,
which serves as a carrier gas. Each of these gas streams was
further diluted in another 200 sccm of N.sub.2 and then introduced
into the CVD reactor to undergo a film formation reaction. During
the deposition, 250 sccm of N.sub.2 was delivered from the
showerhead from above the substrate and 250 sccm of N.sub.2 was
delivered from below the substrate through the backfill line. The
total pressure in the CVD reactor during deposition was
approximately 3 Torr.
[0059] The substrate was heated to 350.degree. C. and was rotated
at 50 rpm during the CVD reaction. Rotation of the substrate
promotes uniformity of deposition across the substrate. The
reaction was permitted to run for .about.30 minutes and on
conclusion of the reaction, a film of about 3000 .ANG. in thickness
had been prepared on the substrate.
[0060] The film was analyzed using Auger emission spectroscopy. The
results of Auger depth profiling are shown in FIG. 4 herein. The
Auger analysis confirmed the presence of Sb and Te in the deposited
film and further showed that the Sb:Te atomic ratio was
approximately 36:56 or 2:3.1, which is in agreement with the
expected ratio for Sb.sub.2Te.sub.3. The depth profiling further
shows the uniformity of the composition of the film in the
thickness direction. This indicates that a uniform binary film was
deposited instead of separate layers or regions of Sb and Te.
[0061] FIG. 5 herein shows a scanning electron micrograph of a
portion of the deposited film. The micrograph was obtained at a
magnification of 4000.times.. The micrograph indicates that the
deposited film is polycrystalline in nature. A typical grain size
in the film is on the order of microns.
EXAMPLE 2
[0062] In this example, a two layer structure including solid phase
layers of Sb.sub.2Te.sub.3 and Ge is prepared in a CVD process. The
deposition was performed on a SiN substrate that was rotated at 50
rpm. The CVD reactor, the Sb-precursor and Te-precursor used in
this example are as described in EXAMPLE 1 hereinabove. The
Ge-precursor was isobutylgermane, H.sub.3Ge(i-C.sub.4H.sub.9), and
has the molecular form shown in FIG. 6 herein. The Ge-precursor is
a high vapor pressure liquid at ambient conditions and was
delivered to the CVD reactor through a bubbler.
[0063] The deposition began with deposition of a Ge layer. The
Ge-precursor was placed in a bubbler. 200 sccm of He was bubbled
through the Ge-precursor to provide a gas stream containing the
Ge-precursor in a vapor phase form in He as a carrier gas. This gas
stream was further diluted with 300 sccm of He and then injected
into the reactor. During deposition of the Ge layer, 400 sccm He
was injected through the showerhead and 250 sccm He was injected
through the backfill line. The reactor pressure during deposition
of the Ge layer was approximately 6 Torr and the substrate
temperature was approximately 400.degree. C. The deposition was
allowed to proceed for 15 minutes and was then terminated. The
reactor was purged without removing the substrate containing the Ge
layer and readied for deposition of an Sb.sub.2Te.sub.3 layer.
[0064] The Sb.sub.2Te.sub.3 layer was deposited directly onto the
Ge layer under conditions as described in EXAMPLE 1 hereinabove.
The deposition was permitted to run for 25 minutes and then
terminated.
[0065] The resulting two-layer structure may be referred to herein
as the as-deposited structure, as deposited multilayer structure,
penultimate structure, penultimate multilayer structure, or the
like. The penultimate structure subsequently subjected to
electrical test measurements. Two electrical probes were placed in
contact with the upper Sb.sub.2Te.sub.3 layer of the structure and
the current-voltage (I--U(V)) and resistance-current (R--I)
responses of the two-layer material were measured. The probe tips
had a diameter of -2.5 .mu.m and were separated by a distance of a
few hundred microns. In the current-voltage measurements, the
current passing between the probes was measured as a function of
the voltage applied between the probes. In the resistance-current
measurements, current pulses having different amplitudes were
applied and the resistance of the sample following termination of
the pulse was measured.
[0066] The results of the measurements are summarized in FIGS. 7
and 8 herein. FIG. 7 shows the R--I response and FIG. 8 shows the
I--U(V) response of the as-deposited (penultimate) structure. The
response of the penultimate structure is given by the set of points
collectively labeled 100 in the R--I measurement shown in FIG. 7
and by the set of points collectively labeled 200 in the I--U(V)
measurement shown in FIG. 8. The response curve 100 shown in FIG. 7
indicates that the as-deposited (penultimate) structure has a low
resistance and undergoes no significant transformation in structure
over the range of currents investigated. This behavior is
consistent with a crystalline state for the material. The I--U(V)
response curve 200 shown in FIG. 8 is also consistent with a
crystalline state for the as-deposited (penultimate) structure.
[0067] Upon completion of the R--I and I--V measurements of the
as-deposited (penultimate) structure, a high amplitude current
pulse was applied to the structure to form an ultimate structure.
After application of the pulse, the resistance between the
electrical probes was measured at low current and was observed to
be approximately 1 M.OMEGA. (10.sup.6 .OMEGA.). The result of this
measurement is indicated by the point 10 in FIG. 7. Following the
resistance measurement, the current-voltage characteristics of the
ultimate structure were tested. The results of this measurement are
shown by the set of points collectively labeled 400 in FIG. 8. The
current-voltage response following application of the high
amplitude current pulse shows an electrical switching behavior. At
low voltages, the response is linear with a small slope that is
indicative of a high resistance structure. Upon reaching a voltage
of slightly above 2.5V (i.e. upon reaching the threshold voltage of
the structure), the structure switches from a high resistance state
to a low resistance state as evidenced by the switchback behavior
of the current-voltage response curve. The switching transition
demonstrated by the set of points 400 in FIG. 8 is analogous to the
transition from a resistive regime to a conductive regime as
described in U.S. patent applications Pub. No. 20040178401, the
disclosure of which is hereby incorporated by reference herein.
[0068] Upon completion of the current-voltage measurement, the
resistance-current characteristics of the ultimate structure were
measured to examine the effect of the application of the high
amplitude current pulse. The results of the measurement are shown
by the points collectively labeled 300 in FIG. 7. At low current,
the resistance of the structure is about 1 M.OMEGA., as described
above. A series of current pulses of progressively higher current
amplitude was subsequently applied to the ultimate structure.
Application of current pulses up to .about.3 mA resulted in a
marked decrease in the resistance of the ultimate structure. For
current pulses between .about.3 mA and .about.5 mA, the resistance
leveled off at a value in the 10.sup.3-10.sup.4 .OMEGA. range.
Above -5 mA, the resistance increased and ultimately was restored
to its initial value of .about.1 M.OMEGA..
[0069] The resistance-current behavior depicted by the set of
points 300 illustrates the setting and resetting characteristics of
the ultimate structure. The decrease in resistance observed for
current pulses up to .about.3 mA corresponds to the accumulation
behavior and setting transformation of chalcogenide materials. The
leveling and increase in resistance observed above .about.3 mA
correspond to the multistate, direct overwrite behavior of
chalcogenide materials and attainment of a resistance comparable to
the initial resistance corresponds to the resetting transformation
of a chalcogenide material. The energy accumulation capability, set
transformation, multistate regime and reset transformation are
described, for example, in U.S. Pat. Nos. 5,912,839; 6,141,241;
6,714,954; and 6,671,710; the disclosures of which are hereby
incorporated by reference herein.
[0070] While not wishing to be bound by theory, the instant
inventors believe that application of a high current amplitude
pulse to the as-deposited (penultimate) structure induces a
diffusion or interdiffusion of elements between the individual
layers such that a three-element chalcogenide composition is formed
in the ultimate structure, where the three-element composition is
one that exhibits electrical switching upon application of a
threshold voltage. The ultimate structure may additionally exhibit
accumulation, setting and resetting characteristics. The instant
inventors believe that a three-element composition is formed
through the post-deposition application of energy, such that the
electrical switching characteristics analogous to that shown by the
set of points 400 in FIG. 8 and accumulation, setting and resetting
characteristics analogous to those shown by the set of points 300
in FIG. 7 for the ultimate structure are induced.
[0071] FIG. 9 shows a further measurement, depicted by the set of
points collectively labeled 350, of the resistance-current
characteristics of the ultimate structure. The lower set of points
collectively labeled 100 duplicates the points labeled 100 in FIG.
7. The initial state 20 of the structure for this measurement
differed from the initial state 10 of the experiment shown in FIG.
7. Specifically, the initial resistance was about a factor of 1000
smaller for the experiment of FIG. 9 than for the experiment of
FIG. 7. As can be seen in FIG. 9, accumulation, setting, reversible
multistate, and resetting characteristics are exhibited by the
structure. The lower initial resistance leads to a smaller decrease
in resistance upon setting. Such behavior is consistent with the
expected behavior of the cognitive chalcogenide materials discussed
in the prior art. The reversible multistate region extends from
about 2 mA up to the reset state (.about.5.8 mA). The reversible
multistate region includes a plurality of states distinguishable
based on resistance. The range of resistances within the plurality
exceeds a factor of 10.
[0072] This example thus demonstrates that post-deposition
application of a high amplitude current pulse is capable of
transforming an as-deposited penultimate multilayer structure that
shows no electrical switching, accumulation, setting and/or
resetting characteristics in its as-deposited form into an ultimate
structure which shows one or more of such characteristics.
EXAMPLE 3
[0073] In this example, a single layer three-element solid phase
chalcogenide thin film is deposited by chemical vapor deposition.
The deposition was performed on a SiN substrate that was rotated at
50 rpm. The CVD reactor, the Sb-precursor, Te-precursor and
Ge-precursor used in this example are as described in EXAMPLE 1 and
EXAMPLE 2 hereinabove.
[0074] The deposition in this example was accomplished through a
reaction of the Sb-precursor, Te-precursor, and Ge-precursor, where
all three precursors were present simultaneously in the CVD
reactor. The precursors were introduced into the CVD reactor
through separate feed lines. Helium (He) was used as a carrier gas
for all three precursors. The Sb-precursor and Te-precursor were
placed in separate bubblers heated to 75.degree. C. and delivered
to the CVD reactor through separate feed lines, also heated to
75.degree. C. He was bubbled through the Sb-precursor bubbler at a
flow rate of 200 sccm to produce a gas stream containing the
Sb-precursor in a vapor phase form diluted in He, which serves as a
carrier gas. This gas stream was further diluted in another 100
sccm of He and then introduced into the CVD reactor to provide the
Sb-precursor in a vapor phase form to the film formation reaction.
He was bubbled through the Te-precursor bubbler at a flow rate of
200 sccm to produce a gas stream containing the Te-precursor in a
vapor phase form diluted in He, which serves as a carrier gas. This
gas stream was further diluted in another 100 sccm of He and then
introduced into the CVD reactor to provide the Te-precursor in a
vapor phase form to the film formation reaction. The Ge-precursor
was placed in a separate bubbler. 200 sccm of He was bubbled
through the Ge-precursor bubbler to provide a gas stream containing
the Ge-precursor in a vapor phase form in He as a carrier gas. This
gas stream was further diluted with 300 sccm of He and then
injected into the CVD reactor to provide the Ge-precursor in a
vapor phase form to the film formation reaction.
[0075] During the deposition, 400 sccm of He was delivered from the
showerhead from above the substrate and 250 sccm of He was
delivered from below the substrate through the backfill line. The
total pressure in the CVD reactor during deposition was
approximately 6 Torr. The substrate was heated to 400.degree. C.
during the CVD reaction. The reaction was permitted to run for
.about.15 minutes and on conclusion of the reaction, a film of
about 3000 .ANG. in thickness had been prepared on the
substrate.
[0076] A scanning electron microscopy analysis of the film was
completed and selected results are presented in FIG. 10 and FIG.
11. FIG. 10 is a low magnification (2000.times.) image of the film
and shows several larger crystallites 650 in the presence of a
finer grain background material 600. FIG. 11 shows a high
magnification image (6000.times.) of one of the larger crystallites
650. Elemental analysis of the background material 600 and
crystallites 650 were completed using EDS. The EDS results
indicated that the ratio of Ge:Sb:Te in the background material 600
was 1:2:3, thus indicating a stoichiometric GeSb.sub.2Te.sub.3
composition. The EDS results indicated that the ratio of Ge:Sb:Te
in the crystallites 650 was 2:2:5, thus indicating a stoichiometric
Ge.sub.2Sb.sub.2Te.sub.5 composition.
[0077] The film was further analyzed using Auger emission depth
profiling and representative results are shown in FIG. 12 herein.
The Auger analysis confirmed the presence of Ge, Sb and Te in the
film and further showed that Ge, Sb and Te atomic compositions were
fairly uniform with some fluctuation in the depth direction. This
result confirms the formation of a ternary composition throughout
the thin film, rather than multiple binary or single element
regions, layers or domains.
[0078] The ternary chalcogenide film was subsequently subjected to
electrical test measurements. Two electrical probes were placed in
contact with the film and the current-voltage (I--U(V)) and
resistance-current (R--I) responses of the film were measured as
described in EXAMPLE 2 hereinabove. The current-voltage results
show electrical switching similar to that described in EXAMPLE 2
hereinabove with a threshold voltage above 2V.
[0079] The resistance-current results are shown in FIG. 13. Current
pulses of .about.500 ns were used in this experiment. The ternary
chalcogenide film exhibited a high initial resistance and displayed
a high resistance plateau upon application of current pulses have
amplitudes up to about 0.5 mA. The behavior observed in the high
resistance plateau corresponds to the accumulative or cognitive
functionality described in EXAMPLE 2 hereinabove. In the current
pulse amplitude region between about 0.5 mA and about 1 mA, the
film exhibited a sharp decrease in resistance, behavior that
corresponds to the setting transformation of the ternary
chalcogenide film as described in EXAMPLE 2 hereinabove. In the
current pulse amplitude region between about 1 mA and about 4 mA,
the resistance leveled. Above about 4 mA, a sharp increase in the
resistance of the film was observed, behavior that corresponds to a
resetting of the film as described in EXAMPLE 2 hereinabove. The
resistance-current characteristics shown in FIG. 13 were
reproducible upon repeated performance of the experiment.
[0080] This resistance-current results of FIG. 13 show that the
ternary chalcogenide thin film formed through the chemical vapor
deposition process of this example has a series of states, which
may be characterized according to resistance. Each of the data
points shown in FIG. 13 corresponds to representative states of the
chalcogenide thin film and a resistance value can be associated
with each state. The states include a plurality of states having
distinguishable resistances. The resistances of the states within
the plurality differ by a factor of up to eight.
[0081] The electrical measurement results show that the ternary
chalcogenide material formed in this experiment displays the
electrical switching, cognitive, accumulation, setting, resetting
and multistate memory functionality described in the patents
incorporated by reference herein.
EXAMPLE 4
[0082] In this example, a single layer two-element (GeTe) solid
phase chalcogenide thin film is deposited by chemical vapor
deposition. The deposition was performed on a SiN substrate that
was rotated at 75 rpm. The CVD reactor, Te-precursor and
Ge-precursor used in this example are as described in EXAMPLE 1,
EXAMPLE 2 and EXAMPLE 3 hereinabove.
[0083] The deposition in this example was accomplished through a
reaction of the Te-precursor and the Ge-precursor, where both
precursors were present simultaneously in the CVD reactor. The
precursors were introduced into the CVD reactor through separate
feed lines. Helium (He) was used as a carrier gas for both
precursors. The Te-precursor was placed in a bubbler heated to
75.degree. C. and delivered to the CVD reactor through separate
feed lines, also heated to 75.degree. C. He was bubbled through the
Te-precursor bubbler at a flow rate of 100 sccm to produce a gas
stream containing the Te-precursor in a vapor phase form diluted in
He, which serves as a carrier gas. This gas stream was further
diluted in another 50 sccm of He and then introduced into the CVD
reactor to provide the Te-precursor in a vapor phase form to the
film formation reaction. The Ge-precursor was placed in a separate
bubbler. 100 sccm of He was bubbled through the Ge-precursor
bubbler to provide a gas stream containing the Ge-precursor in a
vapor phase form in He as a carrier gas. This gas stream was
further diluted with 150 sccm of He and then injected into the CVD
reactor to provide the Ge-precursor in a vapor phase form to the
film formation reaction.
[0084] During the deposition, 500 sccm of N.sub.2 was delivered
from the showerhead from above the substrate and 250 sccm of
N.sub.2 was delivered from below the substrate through the backfill
line. The substrate was heated to 400.degree. C. during the CVD
reaction. The reaction was permitted to run for .about.15 minutes
and on conclusion of the reaction, a film with an estimated
thickness of about 1000-2000 .ANG. had been formed on the
substrate.
[0085] A scanning electron microscopy analysis of the film was
completed and a selected result is presented in FIG. 14. FIG. 14 is
a 2000.times. image of the film and shows several larger
crystallites 750 in the presence of a finer grain background
material 700. Elemental analysis of the background material 700 and
crystallites 750 were completed using EDS. The EDS results
indicated that the ratio of Ge:Te in the background material 700
was approximately 1:1, thus indicating a stoichiometric GeTe
composition. The EDS results indicated that the ratio of Ge:Te in
the crystallites 750 was also approximately 1:1, thus indicating a
stoichiometric GeTe composition.
[0086] The film was further analyzed using Auger emission depth
profiling. The Auger analysis confirmed the presence of Ge and Te
in the film and further showed that the Ge and Te atomic
compositions were uniform in the depth direction. This result
confirms the formation of a binary GeTe composition throughout the
thin film.
[0087] The GeTe chalcogenide film was subsequently subjected to
electrical test measurements. Two electrical probes were placed in
contact with the film and the current-voltage (I--U(V)) and
resistance-current (R--I) responses of the film were measured as
described in EXAMPLE 2 hereinabove.
[0088] The results of the measurements are summarized in FIGS. 15
and 16 herein. FIG. 15 shows the R--I response and FIG. 16 shows
the I--U(V) response of the GeTe film. The response of the
as-deposited GeTe film is given by the set of points collectively
labeled 810 in the R--I measurement shown in FIG. 15 and by the set
of points collectively labeled 850 in the I--U(V) measurement shown
in FIG. 16. The response curve 810 shown in FIG. 15 indicates that
the as-deposited GeTe film has a low resistance and undergoes no
significant transformation in structure over the range of currents
investigated. This behavior is consistent with a crystalline state
for the as-deposited material. The I--U(V) response curve 850 shown
in FIG. 16 is also consistent with a crystalline state for the
as-deposited material.
[0089] Upon completion of the R--I and I--V measurements of the
as-deposited film, a high amplitude current pulse (.about.40 mA
applied for .about.1 .mu.s) was applied between the probe tips to
induce amorphization and to establish an initial state for
subsequent measurements. A similar procedure was used to establish
various initial states in the examples described hereinabove. After
application of the pulse, the resistance between the electrical
probes was measured at low current and was observed to be
approximately 1 Mn (10.sup.6 .OMEGA.). The result of this
measurement is indicated by the point 30 in FIG. 15. Following the
resistance measurement, the current-voltage characteristics of the
GeTe material were tested. The results of this measurement are
shown by the set of points collectively labeled 860 in FIG. 16. The
current-voltage response following application of the high
amplitude current pulse shows an electrical switching behavior. At
low voltages, the response is linear with a small slope that is
indicative of a high resistance material. Upon reaching a voltage
of about 3.25 V (a voltage corresponding to the threshold voltage
of the material), the GeTe material switches from a high resistance
state to a low resistance state as evidenced by the switchback
behavior of the current-voltage response curve. The switching
transition demonstrated by the set of points 860 in FIG. 16 is
analogous to the transition from a resistive regime to a conductive
regime as described in EXAMPLE 2 hereinabove.
[0090] Upon completion of the current-voltage measurement, the
resistance-current characteristics of the GeTe material were
measured to examine the effect of the amorphizing pulse on these
characteristics. The results of the measurement are shown by the
points collectively labeled 820 in FIG. 15. At low current, the
resistance of the structure is about 1 M.OMEGA. and corresponds to
the point labeled 30, as described above. A series of current
pulses of progressively higher current amplitude was subsequently
applied. Application of a current pulse of close to .about.3 mA
resulted in a marked decrease in the resistance of the GeTe film
and demonstrates the accumulation, cognitive, and setting
functionality of the film. The points collectively labeled 830 in
FIG. 15 show the results of another resistance-current measurement
in which the initial state of the GeTe film was adjusted through
use of a current pulse to provide the state indicated at 40 in FIG.
15. A series of additional current pulses of increasingly higher
amplitude were applied to the material in small incremental steps
to map out the variation of resistance with current. The points 830
show a high resistance plateau for current amplitudes up to about
0.75 mA, followed by a transformation region and leveling of
resistance above about 0.75 mA. These data also demonstrate the
accumulative, cognitive and setting functionality of the GeTe
film.
[0091] The instant invention extends generally to the chemical
vapor deposition of chalcogenide thin films exhibiting electrical
switching, accumulation, setting, resetting and/or memory
functionality as described hereinabove. In one embodiment, the
deposition occurs on a stationary substrate. In another embodiment,
the deposition occurs on a moving substrate, such as a continuous
web substrate, discrete substrates positioned on a moving conveyor
or other transported substrates. The latter embodiment provides for
the continuous deposition of a chalcogenide material according to
the chemical vapor deposition process of the instant invention. The
deposition chamber in the embodiment which includes a moving
substrate includes a substrate inlet port into which the substrate
is fed. The deposition chamber further includes means for
delivering deposition precursors and the rate of delivery of
deposition precursors and rate of transportation of the moving
substrate are optimized to insure adequate residence time of the
substrate in the growth environment of the chamber to insure
deposition of a chalcogenide thin film. The deposition chamber
further includes a substrate outlet port out of which the
substrate, now containing the deposited thin film, is withdrawn.
Deposition onto a moving substrate can occur through the formation
of a multilayer structure as described in EXAMPLE 2 hereinabove or
through the simultaneous introduction of multiple deposition
precursors to form a single layer, multielement chalcogenide thin
film as described in EXAMPLE 3 and EXAMPLE 4 hereinabove. The scope
of this embodiment includes deposition onto a substrate that is
continuously in motion during deposition as well as deposition onto
substrates that are stationary during deposition, but which are
transported sequentially into the deposition chamber for deposition
in, for example, a "start-stop" or intermittent motion mode of
operation in which substrate motion is interrupted during
deposition and resumed upon completion of the deposition.
[0092] The foregoing discussion and description are not meant to be
limitations upon the practice of the present invention, but rather
illustrative thereof. It is to be appreciated by persons of skill
in the art that numerous equivalents of the illustrative
embodiments disclosed herein exist. It is the following claims,
including all equivalents and obvious variations thereof, in
combination with the foregoing disclosure which define the scope of
the invention.
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