TRANSFER BETWEEN TWO META-STABLE PHASES

Field of the Invention

[0001] The present invention relates to memristive devices and more particularly, to memristive devices based on reversible ion transfer between two meta-stable phases of differing conductivities in an ion intercalated material.

Background of the Invention

[0002] Memristive devices are electrical switches that can alter their state of internal resistance based on an applied voltage and current. See, for example, Yang et al., "Memristive Devices for Computing: Mechanisms,

Applications and Challenges," USLI Process Integration 8 at the 224 ^th Electrochemical Society Meeting (October 27 - November 1 , 2013) (7 pages). Memristive devices have gained significant interest for accelerated machine learning applications.

[0003] Memristive devices need to have the following characteristics. Memristive devices have to be non-volatile and capable of storing a variable resistance value. This resistance can be tuned up and down using current or voltage pulses. Memristive device resistance needs to be symmetrically tunable, meaning that when positive (+) or negative (-) voltage pulses are applied to the device, the resistance moves up or down by roughly the same magnitude.

[0004] Current nanoionic memristor devices (e.g., resistive random-access memory (RRAM), conductive bridging random access memory (CBRAM), etc.) however do not show symmetric modulation between stored resistance states. This asymmetry precludes as is current technologies for use in the implementation of the resistive processing unit (RPU) for accelerated machine learning which involves performing a large number of computations in parallel.

[0005] Therefore, techniques for achieving symmetric modulation of resistances in a memristive device would be desirable.

Summary of the Invention

[0006] The present invention provides memristive devices based on alkali-transfer between two meta-stable phases in an ion intercalated material. In one aspect of the invention, a memristive device is provided. The memristive device includes: a first inert metal contact; a layer of a phase separated material disposed on the first inert metal contact, wherein the phase separated material includes interstitial ions; and a second inert metal contact disposed on the layer of the phase separated material. The first phase of the phase separated material can have a different concentration of the interstitial ions from the second phase of the phase separated material such that the first phase of the phase separated material has a different electrical conductivity from the second phase of the phase separated material.

[0007] In another aspect of the invention, a method of operating a memristive device is provided. The method includes: providing the memristive device having a first inert metal contact; a layer of a phase separated material disposed on the first inert metal contact, wherein the phase separated material includes interstitial ions, and a second inert metal contact disposed on the layer of the phase separated material, wherein a first phase of the phase separated material is a cathode of the memristive device, and a second phase of the phase separated material is an anode of the memristive device; applying a positive voltage pulse to the first inert metal contact to transfer some of the interstitial ions from the cathode to the anode; and applying a negative voltage pulse to the first inert metal contact to return the transferred interstitial ions back to the cathode.

[0008] A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings.

Brief Description of the Drawings

[0009] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 A is a schematic diagram illustrating asymmetric device resistance modulation;

FIG. 1 B is a schematic diagram illustrating symmetric device resistance modulation according to an embodiment of the present invention;

FIG. 2 is a diagram illustrating an exemplary memristive device according to an embodiment of the present invention;

FIG. 3 is a diagram illustrating an exemplary methodology for forming the present memristive device according to an embodiment of the present invention;

FIG. 4 is a diagram illustrating an exemplary methodology for operating the present memristive device according to an embodiment of the present invention;

FIG. 5 is a diagram illustrating how lithium titanium oxide (LTO) resistance can be modulated through the introduction of conduction band electrons from lithium ions (Li+) according to an embodiment of the present invention;

FIG. 6 is a diagram illustrating phase separation in an LTO material in terms of Li concentration x as a function of both distance and time according to an embodiment of the present invention; FIG. 7 is a diagram illustrating the phase separation in an LTO material in terms of Li concentration x as a function of time according to an embodiment of the present invention;

FIG. 8 is a diagram illustrating symmetric transfer of Li+ ions between the phases in terms of Li concentration x as a function of both distance and time according to an embodiment of the present invention; and

FIG. 9 is a diagram illustrating symmetric transfer of Li+ ions between the phases in terms of Li concentration x as a function of time according to an embodiment of the present invention.

Detailed Description of Preferred Embodiments

[0010] Provided herein is a bipolar, analog memristive device that enables symmetric switching between a multitude of resistance states. As provided above, device resistance must be symmetrically modulated, e.g., when provided n positive pulses, n negative pulses returns the device to the same resistance. Current nanoionics (e.g., RRAM, CBRAM, etc.) do not show symmetric modulation of resistance due to their principles of operation, which consist of forming and breaking a conductive filament of material within the device. See, for example, FIGS. 1 A and 1 B which illustrate asymmetric and symmetric modulation of resistance, respectively, as a function of pulse number (#). Further, as shown in the two examples presented in FIG. 1 B, the potentiation/depotentiation paths do not have to be linear (i.e., one path shown in FIG. 1 B is linear and the other is not), however they need to be symmetric.

[0011] In order to achieve symmetric modulation between resistance states, an alternative to filamentary based memristive devices is proposed herein, where phase separation is used to achieve constant μυ ₊ (chemical potential of lithium (Li) or other intercalated ions) in each phase, and thus reversible modulation of lithium (or other intercalated ions) between phases. Specifically, in the present memristive device a phase-separated material with interstitial ions (i.e., a two-phase material) is placed between electrically conductive, inert electrodes. The phase separated material has two phases, each phase having a different concentration of the interstitial ions and hence a different electrical conductivity. Specifically, the two-phase material has an electrically conductive phase, and the second phase that is (electrically) insulating but becomes electrically conductive upon adding (intercalating) the ions (see, for example, FIGS. 6 and 7, described below). Physics allows transfer of ions between these two meta-stable phases up to the stability limit (second derivative of Gibb's free energy), thus increasing or decreasing the resistance of the device. The extent of non-volatility is determined by the kinetics of ion diffusion and the

Coulombic repulsive forces between mobile ions. Symmetric switching is achieved because in a phase-separation material, the chemical potential of lithium ions (Li ⁺ ) (or other intercalated ions) is equal in both phases and so transfer to and fro is reversible.

[0012] An exemplary configuration of the present memristive device is shown in FIG. 2. As shown in FIG. 2, memristive device 200 includes a substrate 202 on which a first inert metal contact 204 is disposed. Suitable substrates include, but are not limited to, glass (e.g., soda lime glass (SLG)), ceramic, metal foil, or plastic substrates. According to an exemplary embodiment, the substrate is first coated with a thin adhesion layer 203 to aid in deposition of the inert metal contact 204 onto the substrate 202. By way of example only, the adhesion layer 203 is formed having a thickness of from about 1 nanometer (nm) to about 5 nm, and ranges therebetween.

Suitable materials for the adhesion layer 203 include, but are not limited to, chromium (Cr), titanium (Ti), and/or nickel (Ni).

[0013] Suitable materials for the inert metal contact 204 include, but are not limited to, ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), gold (Au) and/or platinum (Pt). By 'inert' it is meant that the metal is unreactive under normal operating conditions and blocks ion transport.

[0014] A phase separated material is disposed on the inert metal contact 204. As described above, the phase separated material includes two phases. One of the phases serves as a cathode 206 and another of the phases serves as an anode 208 of the memristive device. As such, according to an exemplary embodiment, the cathode 206 and anode 208 contain different phases (i.e., Phase 1 and Phase 2, respectively) of the phase separated intercalated material, i.e., the cathode 206 and anode 208 are formed from disproportionate mixtures of a common material (e.g., lithium titanium oxide (LTO), lithium cobalt oxide (LCO), etc. - see below) having different concentrations of lithium (Li) interstitial ions.

[0015] Using Li as an example, a suitable ion intercalated material is lithium (Li) titanium (Ti) oxide or LTO. The LTO spinel can exist in two phases, Li Tis0i2 and LizTisO^. During Li intercalation into the Li Tis0i2 spinel, rearrangement of Li ions occurs filling different sites in the sub-lattices, forming LizTisO^. See, for example, Tsai et al., "Atomic Structure and Ab Initio Electrochemical Properties of Li Tis0i2 Defect Spinel for Li Batteries," Journal of the Electrochemical Society, 161 (3) A439-A444 (January 2014), the contents of which are incorporated by reference as if fully set forth herein. Another suitable ion intercalated material is lithium cobalt (Co) oxide or LCO.

The LCO spinel can exist in two phases, Lio.7 Co02 and U0.95C0O2. Like with LTO, during Li intercalation into the Lio.7 Co02 spinel, rearrangement of Li ions occurs filling different sites in the sub-lattices, forming Lio.95Co02.

[0016] As will be described in detail below, Li ion intercalation occurs by applying electric pulses to the device to transfer interstitial Li ions between the two phases. Li intercalation involves phase transformations. See, for example, Anton Van der Ven et al., "Understanding Li Diffusion in Li-Intercalation Compounds," Acc. Chem. Res., 2013, 46(5), pp. 1216-1225 (May 2012), the contents of which are incorporated by reference as if fully set for the herein. Accordingly, the cathode 206 and anode 208 in the present device structure can be phase-separated from one another. According to one exemplary embodiment, the device is constructed with LizTisO^ or L10.95C0O2 as the cathode 206 and Li Tis0i2 or Lio.7 Co02 as the anode 208. However, the cathode and anode are in practice interchangeable as this is a zero-volt battery structure. Thus, according to an exemplary embodiment, the cathode 206 includes LTO or LCO having the formula Li Tis0i2 or Lio.7 Co02, and the anode 208 includes LTO or LCO having the formula LizTisO^ or U0.95C0O2.

[0017] Optionally, the cathode 206 and/or the anode 208 can be doped with a transition metal to tune the resistance. Suitable transition metal dopants include, but are not limited to nickel (Ni), tungsten (W), iron (Fe), and/or tantalum (Ta). Doping the cathode 206 and/or anode 208 with a transition metal can be used to ensure that either the anode or cathode is consistently conductive to electrons. For instance, Li ion diffusion out of a phase into the other phase disproportionately affects conductivity of each phase. It is desirable for the overall phase- separation material to remain somewhat conductive to electrons (<gigaohms (GOhm)) during operation so that memristive properties are apparent over capacitive effects. Thus, employing non-diffusing dopants (such as transition metal ions) to tune the base resistance can insure that both the anode and cathode are of appropriate resistances. For instance, the Li TisOi2 may be doped with a transition metal M to make it more conductive, for example, Li Ti5- _x M _x Oi2, wherein x is 0-0.3, e.g., x = 0.1. The notion here is that there are two phases, one electrically conductive and one electrically insulating. The overall resistance across the two layers is a combination of the two resistances. If this base resistance is too high (>10 ^Λ 9 ohms), the device will act as a capacitor with a high RC time constant which prevents an accurate read on the device. Hence, resistance tuning via transition metal doping can be implemented.

[0018] A second inert metal contact 210 is disposed on the phase-separated material. Suitable materials for the inert metal contact 210 include, but are not limited to, Ru, Rh, Pd, Ag, Au and/or Pt, same as for the inert metal contact 204. In fact, it is preferable that the inert metal contact 204 and the inert metal contact 210 are formed from the same material or combination of materials to insure symmetric Ohmic behavior in reverse and forward bias. However, as shown in FIG. 2, the inert metal contact 204 and the inert metal contact 210 do not have to have the same shape/dimensions as one another. For instance, in the example depicted, the inert metal contact 210 covers only a portion of the top of the phase-separated material, while a complete layer of the inert metal contact 204 covers the substrate 202 beneath the phase-separated material. For practical reasons, since the device is built sequentially from the bottom up (see below), each layer provides structural support for the immediately preceding layer in the stack. Engineering, e.g., the inert metal contact 210, to be smaller in size than the underlying layers, enables convenient access to the underlying anode 208, cathode 206, and inert metal contact 204, etc. during operation.

[0019] FIG. 3 is a diagram illustrating an exemplary methodology 300 for forming the present memristive device 200. In step 302, the thin adhesion layer 203 is coated on the substrate 202. Suitable substrate and adhesion layer materials were provided above. As also provided above, the adhesion layer 203 is formed as a coating on the substrate 202 having a thickness of from about 1 nm to about 5 nm, and ranges therebetween. The adhesion layer 203 can be formed on the substrate using a process such as evaporation or sputtering. While an optional feature, the adhesion layer 203 serves as a nucleation center and thus aids in deposition of the inert metal contact 204 onto the substrate 202.

[0020] In step 304, the (first) inert metal contact 204 is formed on the substrate 202 (e.g., on the optional adhesion layer 203). Suitable materials for the inert metal contacts were provided above. The inert metal contact 204 can be formed on the substrate 202 using a process such as evaporation, sputtering, or electrochemical deposition (which is enabled by use of the optional adhesion layer 203).

[0021] In step 306, a layer of a phase-separated ion intercalated material is next formed on the inert metal contact 204. As provided above, the phase-separated ion intercalated material is a single layer that includes two distinct phases of the material, one of which will serve as the cathode 206, and the other which will serve as the anode 208 of the memristive device. For instance, the cathode/anode layer of phase-separated ion intercalated material formed in step 306 can include, for example, LizTisO^/LUTisO^ or Lio.7 Co02/Lio.95Co02, respectively. Thus, the phase-separated ion intercalated material is formed as a single layer having two phases, one serving as the cathode and the other as the anode of the device.

[0022] According to an exemplary embodiment, the phase-separated ion intercalated material is formed by depositing a single layer at one stoichiometry and then either adding or removing lithium to the layer to push the stoichiometry into the miscibility gap (i.e., where the mixture exists as two phases). This mixture is unstable and will phase separate into two compositions within the layer.

[0023] For instance, the lower lithium concentration LTO phase (i.e., Li TisOi2) can be deposited as a layer on the inert metal contact 204 followed by lithiation to add lithium to the layer, whereby the material phase separates into two phases, one being (electrically) conducting and the other (electrically) insulating. For example, in the case of anodic the lower concentration Li TisOi2 can be made via solution or physical vapor deposition

(PVD) methods. Then the high concentration (i.e., LizTisO^) is created by chemical lithiation with n-butyllithium which works for any materials with above ~1 volt (V) Li+/Li relative potential.

[0024] Lithium may also be added by electrochemical lithiation where, for example, the memristive device (i.e., substrate 202/inert metal contact 204/LTO or LCO) is submerged in a liquid electrolyte along with a lithium- containing electrode. A bias voltage is applied to the device (e.g., via the inert metal contact 204) and the lithium- containing electrode to drive Li ions from the lithium-containing electrode into the LTO or LCO. Following lithiation, the device is removed from the liquid electrolyte and dried. Suitable liquid electrolytes include, but are not limited to, lithium hexafluorophosphate (LiPFe) in ethylene carbonate. Suitable lithium-containing electrodes include, but are not limited to, a lithium cobalt oxide (L1C0O2) electrode. [0025] In the case of cathodic UC0O2 the high concentration LCO phase (i.e., U0.95C0O2) can be made via solution or PVD methods as a layer on the inert metal contact 204. Then the low concentration phase (i.e., Lio.7 Co02) is created in that layer by chemical delithiation via warm (e.g., at a temperature of from about 50°C to about 120°C, and ranges therebetween) aqueous K2S2O8 (potassium persulfate) solution (e.g., for a duration of from about 1 hour to about 30 hours, and ranges therebetween depending on desired thickness). Chemical delithiation may also be done with dilute hydrochloric acid etching or hydrogen peroxide.

[0026] For a solution-based deposition process, the constituent components of the respective phase-separated ion intercalated material (such as lithium hydroxide and titanium dioxide for LTO or lithium hydroxide and cobalt oxide for LCO) are mixed a? the giver; Li/Ti or Li/Co molar ratios (e.g., lo form the lower concentration Li TisOi2 or the higher concentration LCO phase (i.e., U0.95C0O2) - see above) in a suitable solvent (such as ethano! or water) to form a solution. Optionally, as provided above, transition metal dopants can be incorporated into the solution to tune the cathode/anode resistance, The solution can then be cast (e.g., using spin-coating, spray-coating, ink jet printing, etc.) onto the inert metal contact 204 and permitted to dry. Delithiation or lithiation is then performed as described above in order to obtain a phase-separated material, e.g., or Lio.7 Co02/Lio.95Co02, serving as the cathode/anode, respectively.

[0027] Following delithiation or lithiation, an anneal is then performed, Suitable annealing conditions Include, but are not limited to, a temperature of from about 500°C to about 1,500°C, and ranges therebetween, for a duration of from about 5 hours to about 30 hour, and ranges therebetween,

[0028] With PVD, deposition is carried out in a vacuum chamber in which the device is placed along with appropriate sources of the LTO or LCO material (and optional transition metal dopant) in the appropriate ratios. Following PVD, delithiation or lithiation, is carried out as described above, followed by the anneal using the above- described conditions.

[0029] As described above, an optional transition metal dopant can be added to the cathode 206 and/or anode 208 to tune the resistance. Suitable transition metal dopants were provided above. This doping can be accomplished by incorporating the dopant into the starting solution, prior to casting.

[0030] The layer of a phase-separated ion intercalated material formed as per step 306 produces a cathode 206 (i.e., as a first phase of the phase-separated ion intercalated material) and an anode 208 (i.e., as a second phase of the phase-separated ion intercalated material) on the inert metal contact 204. In step 308, the (second) inert metal contact 210 is formed on the phase-separated ion intercalated material. Suitable materials for the inert metal contacts were provided above, and the inert metal contact 210 may be formed in the same manner as described above for forming the inert metal contact 204. As previously noted, it is preferable that the inert metal contact 204 and the inert metal contact 210 are formed from the same material or combination of materials to insure symmetric Ohmic behavior in reverse and forward bias.

[0031] An exemplary methodology 400 for operating the present memristive device 200 is now described by way of reference to FIG. 4. The following description uses a phase separated ion intercalated material layer that includes

Li7Ti50i2/Li Ti50i2 or Lio.7 Co02/Lio.95Co02 as examples. The present teachings, however, are not limited to LTO, LCO or any other particular configuration, as long as the cathode 206 and anode 208 include a phase separated ion intercalated material.

[0032] In step 402, with no applied voltage memristive device 200 is present in its as-fabricated state described above. Specifically, in this particular example, the cathode 206 and anode 208 include LizTisO^ and Li Tis0i2, or Lio.7 Co02 and L10.95C0O2, respectively.

[0033] As shown in step 404, a first (positive) pulse is applied to the (first) inert metal contact 204. This first pulse serves to transfer some lithium ions (Li ⁺ ) from the cathode 206 to the anode 208. As a result of this Li intercalation into the anode 208 (from the cathode 206), a resistance across the device changes.

[0034] Reversing the pulse reverses the flow of lithium ions back towards the cathode 206. See step 406. As shown in step 406, applying a second (negative) pulse to the (first) inert metal contact 204 serves to return the transferred lithium ions (Li ⁺ ) from the anode 208 back to the cathode 206, restoring the resistance back to its original state (prior to application of the first pulse). The first (positive) and second (negative) pulses are of the same duration and magnitude.

[0035] The resistance state of the memristive device would be "read" by applying a much smaller voltage pulse than is needed for Li intercalation (e.g., less than 0.5 volts (V)) and measuring the current. Higher current measurements at the lower-voltage "read" condition would correspond to a lower resistance state, and vice versa.

[0036] FIG. 5 is a diagram illustrating how LTO resistance can be modulated through the introduction of conduction band electrons (e-) from Li+. Specifically, as shown in FIG. 5, conductivity (measured in Siemens per centimeter (S cm ^"1 )) increases with an increase in intercalated Li ions. With LTO, Ti ⁺ is reduced to Ti ³⁺ with Li ⁺ intercalation, introducing conduction band electrons. FIG. 5 also illustrates that there is a non-linear dependence of resistance on composition. [0037] As provided above, the ion-intercalated layer is phase separated, e.g., into a cathode and anode. For an illustration of an LTO sample separated into an LizTisO^ phase and an Li Tis0i2 phase see, for example, figure 9(b) of Li et al., "Two-phase transition of Li-intercalation compounds in Li-ion batteries," Materials Today, vol. 17, no. 9, pgs. 451-463 (November 2014), the contents of which are incorporated by reference as if fully set forth herein. Li ions can be moved back and forth between the phases using a voltage potential. As long as the same potential is used (i.e., same duration and magnitude - see above), Li intercalation between the phases will be symmetric and reversible.

[0038] A phase separation simulation for an LTO material is shown in FIGS. 6 and 7. Referring first to FIG. 6, it can be seen that a positive voltage pulse (in this case a +1 volt (V) pulse for 50 nanoseconds (ns)) transfers Li+ ions from the higher Li+ ion phase (i.e., the Li-LTO phase) to the lower Li+ ion phase (i.e., the LTO phase). FIG. 6 provides a three-dimensional projection wherein Li concentration x is illustrated as a function of both distance (measured in nanometers (nm)) and time (measured in seconds (s)). FIG. 7 projects in two dimensions Li concentration x as a function of time. The LTO phase with higher Li+ concentration is now conductive, and the Li- LTO phase remains conductive. As a result, the net conductivity across the device increases. Resistance is Ω ~ 52 meV.

[0039] As highlighted above, an important characteristic of the present memristive devices is symmetric transfer of Li+ ions between the phases. As shown in FIGS. 8 and 9, a first positive pulse (in this case a +1V pulse for 50 ns) transfers some Li+ ions and the second negative pulse of the same duration and magnitude (i.e., a -1 V pulse for 50 ns) transfers the Li+ ions back. Thus, the phases can be reversibly changed back to their original state. FIG. 8 provides a three-dimensional projection wherein Li concentration x is illustrated as a function of both distance (measured in nm) and time (measured in s). FIG. 9 projects in two dimensions Li concentration x as a function of time.

[0040] Although illustrative embodiments of the present invention have been described herein, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be made by one skilled in the art without departing from the scope of the invention.