Resistive switching element, memory device, and neural network device

Abstract

To provide a resistance change element improved in repetition characteristics and linearity.SOLUTION: A resistance change element 10 includes: a first electrode 21; a second electrode 22; a first transition metal compound layer 23 which is provided between the first electrode and the second electrode and is a metal compound containing lithium ion at lattice positions; a second transition metal compound layer 24 which is provided between the first transition metal compound layer and the second electrode and is a metal compound containing lithium ions at lattice positions; and a lithium ion conductor layer 25 which is provided between the first transition metal compound layer and the second transition metal compound layer and is a solid material which passes lithium ions and hardly passes electrons.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] Embodiments of the present invention relate to resistive switching elements, memory devices, and neural network devices. [Background technology]

[0002] In recent years, hardware-based neural network devices have been studied. Furthermore, neuromorphic neural networks, or brain-inspired neural networks, that mimic the human brain are known. Brain-inspired neural networks operate with low energy consumption and possess high error tolerance, mimicking the human brain.

[0003] In the field of brain-inspired neural networks, the development of new hardware is desired alongside algorithms. In the development of new hardware, the development of novel resistive random-access memory (RRAM) is particularly desired. Various types of resistive random-access memory (ReRAM) have been proposed as elements for neurons or synaptic circuits. However, all types of resistive random-access memory exhibit significant variation in characteristics from element to element. For this reason, large-scale brain-inspired neural networks using resistive random-access memory have not yet been developed.

[0004] For example, one type of conventional ReRAM is known to have a structure in which metal electrodes are provided at both ends of a transition metal oxide such as TiO2. In such a ReRAM, the amount or distribution of oxygen vacancies present in the transition metal oxide is changed by applying a voltage or current pulse between the metal electrodes. In such a ReRAM, if the oxygen vacancies in the transition metal oxide increase, electrons are introduced near the oxygen vacancies to maintain electrical neutrality, resulting in a low-resistance state (LRS). Conversely, in such a ReRAM, if the oxygen vacancies in the transition metal oxide decrease, it enters a high-resistance state (HRS). However, such a ReRAM exhibits large variations in its characteristics. Therefore, it is difficult to apply such a ReRAM to large brain-inspired neural networks. In recent years, thin-film battery-type ReRAMs using lithium ions instead of oxygen vacancies have been proposed, but further improvements in the linearity and repeatability characteristics of the device are desired. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2023-43142 [Non-patent literature]

[0006] [Non-Patent Document 1] T. Marukame et. al., “Lithium-ion-based resistive devices of LiCoO2 / LiPON / Cu with ultrathin interlayers of titanium oxide for neuromorphic computing”, IEEE J. El. Dev. Soc. Special Issue, 2023, p.1-9 [Non-Patent Document 2] MG Verde et. al., “Elucidating the Phase Transformation of Li4Ti5O12 Lithiation at the Nanoscale”, ACS Nano 2016, 10, p.4312-4321. [Non-Patent Document 3] Juan Carlos Gonzalez-Rosillo1 et. al., “Lithium-Battery Anode Gains Additional Functionality for Neuromorphic Computing through Metal-Insulator Phase Separation”, Adv. Mater. 2020, doi: 10.1002 / adma.201907465, p.1-27 [Non-Patent Document 4] K. Mukai et.al., “Understanding the Zero-Strain Lithium Insertion Scheme of Li[Li1 / 3Ti5 / 3]O4: Structural Changes at Atomic Scale Clarified by Raman Spectroscopy”, J. Phys. Chem. C 2014, 118, p.2992-2999 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] The problem that this invention aims to solve is to provide a resistive switching element, a memory device, and a neural network device that exhibit excellent repeatability and linearity. [Means for solving the problem]

[0008] The resistive switching element according to the embodiment includes a first electrode, a second electrode, a first transition metal compound layer provided between the first electrode and the second electrode and being a metal compound containing lithium ions in lattice positions, a second transition metal compound layer provided between the first transition metal compound layer and the second electrode and being a metal compound containing lithium ions in lattice positions, and a lithium ion conductor layer provided between the first transition metal compound layer and the second transition metal compound layer and being a solid material that allows lithium ions to pass through but does not readily allow electrons to pass through. The second transition metal compound layer is a λ-type cubic metal oxide λMnO 2 and spinel-type transition metal oxide Li 1 / 2 MnO 2 It is a mixture of the above. [Brief explanation of the drawing]

[0009] [Figure 1] A diagram showing an example of the cross-sectional structure of a resistance-changing element according to the first embodiment. [Figure 2] This figure shows an example of a case where the resistance-changing element according to the first embodiment is in a low-resistance state. [Figure 3] This figure shows an example of a case where the resistance-changing element according to the first embodiment is in a high-resistance state. [Figure 4] This figure shows the electromotive force relative to metallic lithium during the discharge of the transition metal compound Li4+εTi5O12 (0<ε<3). [Figure 5] This figure shows the state change of the second transition metal compound layer when a set pulse is applied to the resistance switching element according to the first embodiment. [Figure 6] This figure shows the state change of the second transition metal compound layer when a set pulse is applied to the resistance switching element according to the first embodiment. [Figure 7] This figure shows the state change of the second transition metal compound layer when a set pulse is applied to the resistance switching element according to the first embodiment. [Figure 8] This figure shows the state of a resistive switching element to which an input pulse is applied when the resistive switching element according to the first embodiment is in a low-resistance state. [Figure 9] A diagram showing an example of the configuration of a storage device according to the second embodiment. [Figure 10]A diagram showing an example of the configuration of a storage device according to the third embodiment. [Figure 11] A diagram showing an example of the configuration of a storage device according to the fourth embodiment. [Figure 12] A diagram showing an example of the configuration of a storage device according to the fifth embodiment. [Figure 13] A diagram showing an example of the configuration of a neural network device according to the sixth embodiment. [Figure 14] A diagram illustrating one layer of the neural network in the neural network device according to the sixth embodiment. [Figure 15] A diagram illustrating the sum-of-accumulate operation performed by a sum-of-accumulate circuit included in the neural network device according to the sixth embodiment. [Figure 16] A diagram showing an example of the configuration of a neural network device according to the seventh embodiment. [Modes for carrying out the invention]

[0010] Embodiments will be described below with reference to the drawings. In the following description, components having the same function and configuration will be denoted by the same reference numerals.

[0011] (1. First Embodiment) (1.1 Configuration of resistive switching elements) First, the configuration of the resistive switching element 10 according to the first embodiment will be described. Figure 1 is a diagram showing an example of the cross-sectional structure of the resistive switching element 10 according to the first embodiment.

[0012] The resistive switching element 10 is an element whose electrical resistance changes. More specifically, the resistive switching element 10 changes to a low-resistance state (LRS) where current can flow from one electrode to the other, or a high-resistance state (HRS) where almost no current flows, due to the movement of lithium ions inside the resistive switching element 10. For example, by assigning data "0" and data "1" according to the state of the resistive switching element 10, the resistive switching element 10 can store data. Hereinafter, the operation of changing the state of the resistive switching element 10 from a high-resistance state to a low-resistance state, or from a low-resistance state to a high-resistance state, will be referred to as a write operation. Also, the operation of reading the state of the resistive switching element 10 will be referred to as a read operation. The resistive switching element 10 can be used, for example, as a memory element for neuron circuits or synaptic circuits in a brain-inspired neural network.

[0013] As shown in Figure 1, the resistive switching element 10 includes a first electrode 21, a second electrode 22, a first transition metal compound layer 23, a second transition metal compound layer 24, and a lithium ion conductor layer 25. The resistive switching element 10 has a stacked structure in which the first electrode 21, the first transition metal compound layer 23, the lithium ion conductor layer 25, the second transition metal compound layer 24, and the second electrode 22 are stacked in that order from the first electrode 21 side. The resistive switching element 10 may further include layers not shown between the first electrode 21 and the second electrode 22. Hereinafter, the direction along the junction surface of each layer is defined as the X direction. The direction intersecting the X direction and along the junction surface is defined as the Y direction. The direction intersecting the X and Y directions and perpendicular to the junction surface is defined as the Z direction. The Z direction can also be expressed as the "stacked direction" in which each layer is stacked.

[0014] Within the resistive switching element 10, lithium ions can move between the first transition metal compound layer 23 and the second transition metal compound layer 24 via the lithium ion conductor layer 25. The direction of lithium ion movement is the stacking direction (i.e., the Z direction) in which the first electrode 21, the first transition metal compound layer 23, the lithium ion conductor layer 25, the second transition metal compound layer 24, and the second electrode 22 are stacked.

[0015] For example, the junction surfaces of the first electrode 21, the first transition metal compound layer 23, the lithium ion conductor layer 25, the second transition metal compound layer 24, and the second electrode 22 are approximately the same in shape and size. More specifically, for example, the junction surfaces of the first electrode 21 and the first transition metal compound layer 23, the junction surface of the first transition metal compound layer 23 and the lithium ion conductor layer 25, the junction surface of the lithium ion conductor layer 25 and the second transition metal compound layer 24, and the junction surface of the second transition metal compound layer 24 and the second electrode 22 are approximately the same in shape and size. Note that when we say "approximately the same," it may include errors due to manufacturing variations.

[0016] Let the film thickness of the first transition metal compound layer 23 in the Z direction (stacking direction) be defined as film thickness T1, and the film thickness of the second transition metal compound layer 24 in the Z direction (stacking direction) be defined as film thickness T2. For example, in this embodiment, the film thickness T2 of the second transition metal compound layer 24 is less than the film thickness T1 of the first transition metal compound layer 23 (T1 > T2). Note that the film thickness T2 of the second transition metal compound layer 24 may be the same as the film thickness T1 of the first transition metal compound layer 23, or it may be thicker than the film thickness T1 of the first transition metal compound layer 23. In other words, the relationship between film thicknesses T1 and T2 may be T1 ≤ T2.

[0017] The first electrode 21 and the second electrode 22 are connected, for example, to an external circuit (not shown). A voltage is applied between the first electrode 21 and the second electrode 22 from the external circuit. The first electrode 21 and the second electrode 22 include a conductive material. The configurations of the first electrode 21 and the second electrode 22 may be the same or different.

[0018] The first transition metal compound layer 23 is connected to (in contact with) the first electrode 21. The second transition metal compound layer 24 is connected to (in contact with) the second electrode 22.

[0019] The first transition metal compound layer 23 and the second transition metal compound layer 24 each contain lithium ions (Li 1+is a metal compound containing [[ID=]] at lattice positions. For example, through observation using AFM (Atomic Force Microscopy), it can be confirmed that lithium ions are arranged at lattice positions. The lithium ions contained in the first transition metal compound layer 23 and the second transition metal compound layer 24 move between the first transition metal compound layer 23 and the second transition metal compound layer 24 when a voltage is applied between the first electrode 21 and the second electrode 22. Also, in each of the first transition metal compound layer 23 and the second transition metal compound layer 24, electrons can move internally when a voltage is applied between the first electrode 21 and the second electrode 22. In this case, electrons can move between the first electrode 21 and the first transition metal compound layer 23. Also, electrons can move between the second electrode 22 and the second transition metal compound layer 24.

[0020] Each of the first transition metal compound layer 23 and the second transition metal compound layer 24 may be, for example, a metal oxide containing lithium ions at lattice positions. Hereinafter, in this embodiment, the case where the first transition metal compound layer 23 and the second transition metal compound layer 24 are oxides containing lithium and titanium will be described. The first transition metal compound layer 23 is represented as Li 4+ε1 Ti5O 12 The second transition metal compound layer 24 is represented as Li 4+ε2 Ti5O 12 Here, the variables ε1 and ε2 are in the relationship of 0 ≦ ε2 < ε1 ≦ 3. The composition 4 + ε1 represents the composition (concentration) of lithium ions in the first transition metal compound layer 23. The composition 4 + ε2 represents the composition of lithium ions in the second transition metal compound layer 24. Li 4+ε1 Ti5O 12 and Li 4+ε2 Ti5O 12 are not solid solutions but mixtures of Li4Ti5O 12 and Li7Ti5O 12 For example, Li4Ti5O 12 and Li7Ti5O 12 can be confirmed by analysis such as X-ray diffraction method (XRD: X-Ray Diffraction).

[0021] Li4Ti5O12 It is a spinel-type transition metal oxide and has insulating properties. Li7Ti5O 12 It is a rock salt type transition metal oxide and exhibits metallic conductivity. Spinel type transition metal oxide Li4Ti5O 12 and rock salt type transition metal oxide Li7Ti5O 12 In both cases, lithium ions are positioned at lattice locations. Compositions 4+ε1 and 4+ε2 are Li4Ti5O present in the first transition metal compound layer 23 and the second transition metal compound layer 24, respectively. 12 and Li7Ti5O 12 It changes depending on the ratio (volume ratio) of the first transition metal compound layer 23. 12 As the proportion of increases, the composition 4+ε1 increases. Similarly, in the second transition metal compound layer 24, Li7Ti5O 12 As the proportion of increases, the composition 4+ε2 increases. The first transition metal compound layer 23 and the second transition metal compound layer 24 have different compositions of lithium ions arranged in the lattice positions. The variables ε1 and ε2 have the relationship ε2 < ε1. That is, the composition 4+ε1 and the composition 4+ε2 have the relationship 4+ε2 < 4+ε1. Therefore, the first transition metal compound layer 23 has a higher proportion of Li7Ti5O than the second transition metal compound layer 24. 12 The proportion of lithium ions is high. In other words, the first transition metal compound layer 23 has a larger composition of lithium ions positioned in the lattice than the second transition metal compound layer 24.

[0022] For example, the composition of the first transition metal compound layer 23, 4 + ε1, can be up to 7. That is, there are cases where ε1 = 3. In other words, the first transition metal compound layer 23 is Li7Ti5O 12 Includes Li4Ti5O 12 It may be in a state that does not contain it. Also, the composition of the second transition metal compound layer 24, 4 + ε2, is at least 4. That is, there are cases where ε2 = 0. In other words, the second transition metal compound layer 24 is Li4Ti5O 12 Includes Li7Ti5O 12 It may result in a state that does not include it.

[0023] The first transition metal compound layer 23 may be realized from the same base material as the second transition metal compound layer 24, as described above, but spinel-type transition metal oxides such as LiTi2O4 and Li4Ti5O, which contain lithium ions in lattice positions, may also be used. 12 , Li[CrTi]O4, (Li 1 / 2 Fe 1 / 2 )[Li 1 / 2 Fe 1 / 2 Ti]O4, or Li 1 / 2 It may be any of Mn2O4 or a mixture thereof. In addition, the first transition metal compound layer 23 may be LiTiO2, Li7Ti5O, or other rock salt type transition metal oxides that contain lithium ions in lattice positions. 12 , Li2[CrTi]O4, or (Li 3 / 2 Fe 1 / 2 )[Li 1 / 2 Fe 1 / 2 It may be any of Ti]O4. Furthermore, the first transition metal compound layer 23 may be a mixture of the spinel-type transition metal oxide and the rock salt-type transition metal oxide described above.

[0024] Furthermore, the second transition metal compound layer 24 is a mixture of spinel-type transition metal oxides and rock salt-type transition metal oxides that contain lithium ions in their lattice positions. For example, spinel-type transition metal oxide Li 0.5 It may also be a mixture of oxides having different crystal structures, such as MnO2 and λ-type cubic metal oxide λMnO2.

[0025] The lithium-ion conductor layer 25 is formed between the first transition metal compound layer 23 and the second transition metal compound layer 24. For example, two surfaces of the lithium-ion conductor layer 25 facing the Z direction (stacking direction) are in contact with the first transition metal compound layer 23 and the second transition metal compound layer 24, respectively. The lithium-ion conductor layer 25 is a solid material that allows lithium ions to pass through but makes it difficult for electrons to pass through. In other words, the lithium-ion conductor layer 25 is a solid material that almost completely prevents electrons from passing through, compared to how easily lithium ions pass through it. That is, the lithium-ion conductor layer 25 is a solid material that makes it more difficult for electrons to pass through than lithium ions. For example, the lithium-ion conductor layer 25 can be realized using LiPON. Note that the lithium-ion conductor layer 25 may be made of any material having the above-described properties, not just LiPON. The lithium-ion conductor layer 25 is also called a solid electrolyte.

[0026] (1.2 State of resistive switching element) Next, we will describe the state of the resistive switching element 10. First, we will describe the resistive switching element 10 in the low resistance state (LRS). Figure 2 shows an example of the case when the resistive switching element 10 is in the low resistance state.

[0027] As shown in Figure 2, when the resistive switching element 10 is in a low-resistance state, the second transition metal compound layer 24 contains a predetermined amount or more of lithium ions. The resistive switching element 10 changes state based on the composition of lithium ions in the second transition metal compound layer 24. When the resistive switching element 10 is in a low-resistance state, the second transition metal compound layer 24 contains an insulator, spinel-type transition metal oxide Li4Ti5O 12 And, the rock salt type transition metal oxide Li7Ti5O exhibits metallic conductivity. 12 It is a mixture of the following. For example, within the second transition metal compound layer 24 in a low-resistance state, there is Li7Ti5O which is stretched in the Z direction, with one end in contact with the lithium ion conductor layer 25 and the other end in contact with the second electrode 22. 12 One or more filaments are formed.

[0028] The second transition metal compound layer 24, which is in a low-resistance state, contains a predetermined amount or more of lithium ions, which are positive ions. 12 The filament of the first electrode is in contact with the second electrode 22. In this state, a voltage is applied to the resistive switching element 10 such that the voltage on the first electrode 21 side is higher than the voltage on the second electrode 22 side. For example, a positive voltage pulse is applied to the first electrode 21, where the voltage on the first electrode 21 side is higher than the voltage on the second electrode 22 side. As a result, lithium ions move from the first transition metal compound layer 23 to the second transition metal compound layer 24. Electrons are supplied to the second electrode 22 from an external circuit. The electrons supplied from the second electrode 22 are converted into Li7Ti5O 12 The current is incorporated into the second transition metal compound layer 24 via the filament. Therefore, when a voltage higher than the voltage on the second electrode 22 side is applied to the low-resistance resistive element 10 (for example, when a positive voltage pulse is applied to the first electrode 21), the resistive element 10 can conduct current in the direction from the first electrode 21 to the second electrode 22.

[0029] Thus, the resistive switching element 10 enters a low-resistance state when the second transition metal compound layer 24 contains a predetermined amount or more of lithium ions.

[0030] The resistance value of the resistive switching element 10 in the low-resistance state is determined by the combined resistance value of the first transition metal compound layer 23, the second transition metal compound layer 24, and the lithium-ion conductor layer 25. In most cases, the resistance value of the second transition metal compound layer 24 is sufficiently greater than the resistance values ​​of the first transition metal compound layer 23 and the lithium-ion conductor layer 25. Therefore, the resistance value of the resistive switching element 10 in the low-resistance state is substantially determined by the resistance value of the second transition metal compound layer 24.

[0031] For example, the resistance of the second transition metal compound layer 24 is that of the spinel-type transition metal oxide Li4Ti5O, which is an insulator. 12 And, the rock salt type transition metal oxide Li7Ti5O exhibits metallic conductivity. 12It depends on the volume ratio of the two. Therefore, the resistance value of the resistance-changing element 10 in the low-resistance state changes linearly with respect to the lithium ion composition of the second transition metal compound layer 24.

[0032] Next, we will describe the resistance changing element 10 in the high resistance state (HRS). Figure 3 shows an example of the case where the resistance changing element 10 is in the high resistance state.

[0033] As shown in Figure 3, when the resistive switching element 10 is in a high-resistance state, the second transition metal compound layer 24 contains fewer lithium ions than a predetermined amount. For example, in this case, the variable ε2 of the second transition metal compound layer 24 is almost zero. The composition of the second transition metal compound layer 24 is a spinel-type transition metal oxide Li4Ti5O, which is an insulator. 12 It is mostly composed of rock salt type transition metal oxides Li7Ti5O, which exhibit metallic conductivity. 12 It contains almost no electrons. In this state, when a voltage higher than the voltage on the second electrode 22 side is applied to the resistive switching element 10 (for example, when a positive voltage pulse is applied to the first electrode 21), an electric field is applied to the second electrode 22 that moves electrons toward the second transition metal compound layer 24. However, since the second transition metal compound layer 24 is an insulating layer, electrons supplied from the second electrode 22 side are not taken into the second transition metal compound layer 24. Therefore, when the resistive switching element 10 is in a high-resistance state, even if a voltage higher than the voltage on the second electrode 22 side is applied to the resistive switching element 10, it is not possible to pass current from the first electrode 21 toward the second electrode 22.

[0034] Thus, when the amount of lithium ions contained in the second transition metal compound layer 24 is less than a predetermined amount, the resistive element 10 enters a high-resistance state in which no current flows from the first electrode 21 to the second electrode 22.

[0035] The resistance value of the resistive switching element 10 in the high-resistance state is substantially determined by the resistance value of the second transition metal compound layer 24. It is preferable that the resistance value of the resistive switching element 10 in the high-resistance state be higher, so that the difference between it and the resistance value in the low-resistance state is larger.

[0036] For example, if the film thickness T2 of the second transition metal compound layer 24 is thinner than the film thickness T1 of the first transition metal compound layer 23, the resistive switching element 10 can keep the lithium ions contained in the second transition metal compound layer 24 below a certain value in the high-resistance state, thereby stably achieving a high resistance value. Furthermore, a resistive switching element 10 with such a configuration can contain a relatively small amount of lithium ions in the second transition metal compound layer 24. Therefore, when changing the resistive switching element 10 from a low-resistance state to a high-resistance state, lithium ions can be moved from the second transition metal compound layer 24 to the first transition metal compound layer 23 in a relatively short time. Consequently, a resistive switching element 10 with such a configuration is preferable in that it can speed up the writing operation. In addition, a resistive switching element 10 with such a configuration can contain a relatively large amount of lithium ions in the first transition metal compound layer 23 in the low-resistance state, thus increasing the amount of current that the resistive switching element 10 can supply. Therefore, when a resistive switching element 10 with such a configuration is used, for example, as a memory element for a neuron circuit or synaptic circuit in a brain-inspired neural network, a relatively large amount of current can be rapidly supplied from the resistive switching element 10 in a low-resistance state to the neuron circuit or synaptic circuit. Thus, since only a small amount of lithium ions need to be moved under a large amount of current, it is preferable in that the operating speed of the resistive switching element 10 can be made faster. Furthermore, a resistive switching element 10 with such a configuration (T1>T2) is preferable in that it can suppress the operating delay of the neuron circuit or synaptic circuit. In other words, a resistive switching element 10 with such a configuration (T1>T2) is preferable in that it can improve the processing capacity of the neuron circuit or synaptic circuit.

[0037] The film thickness T2 of the second transition metal compound layer 24 may be the same as the film thickness T1 of the first transition metal compound layer 23, or it may be thicker than the film thickness T1 of the first transition metal compound layer 23. The thicker the film thickness of the second transition metal compound layer 24 in the direction of lithium ion movement, the higher the resistance value that can be achieved in the high-resistance state. In this way, the difference between the resistance value in the high-resistance state and the resistance value in the low-resistance state of the resistive switching element 10 can be increased. Therefore, when a resistive switching element 10 with such a configuration is used, for example, as a memory element for a neuron circuit or synapse circuit in a brain-type neural network, leakage current from the resistive switching element 10 in the high-resistance state to the neuron circuit or synapse circuit can be blocked with greater precision. For this reason, a resistive switching element 10 with such a configuration (T1 ≤ T2) is preferable because it can suppress malfunctions of the neuron circuit or synapse circuit.

[0038] In the resistive switching element 10, the lithium ion concentration in the first transition metal compound layer 23 and the second transition metal compound layer 24 changes each time the resistance changes. As a result, each layer expands and contracts repeatedly with each resistance change. Consequently, the lifespan of the resistive switching element 10 is shortened. In this embodiment, by selecting materials with low expansion and contraction for the first transition metal compound layer 23 and the second transition metal compound layer 24, and by reducing the film thickness of each layer, a high-performance resistive switching element 10 that is resistant to expansion and contraction can be realized. For example, the film thickness of the first transition metal compound layer 23 and the second transition metal compound layer 24 can each be reduced to 100 nm or less.

[0039] Furthermore, the conductivity of the lithium-ion conductor layer 25 is 10 -2 Scm -1 To that extent, if the thickness of the lithium-ion conductor layer 25 is increased, the resistance value of the resistance-changing element 10 in the low-resistance state is determined by the resistance value of the lithium-ion conductor layer 25. For this reason, in order to utilize the resistance value of the second transition metal compound layer 24 down to the low-resistance state, the thinner the lithium-ion conductor layer 25 is preferable. For example, in this embodiment, the thickness of the lithium-ion conductor layer 25 can be 100 nm or less.

[0040] (1.3 Relationship between lithium ion composition and electromotive force) Next, we will explain the relationship between lithium ion composition and electromotive force. Figure 4 shows the transition metal compound Li 4+ε Ti5O 12 This figure shows the electromotive force against metallic lithium during discharge (0 < ε < 3).

[0041] As shown in Figure 4, Li 4+ε Ti5O 12 For 0 < ε < 3, the electromotive force related to metallic lithium is constant. Therefore, when the resistance switching element 10 is in a low resistance state, no electromotive force is generated between the first electrode 21 and the second electrode 22, regardless of the difference in the composition of lithium ions contained in the first transition metal compound layer 23 and the second transition metal compound layer 24, respectively.

[0042] (1.4 Writing Operation) Next, the writing operation of the resistive switching element 10 will be described.

[0043] First, we will explain the case in which the resistive switching element 10 is changed from a high-resistance state to a low-resistance state during the writing operation. Figures 5 to 7 show the state change of the second transition metal compound layer 24 when a set pulse is applied to the resistive switching element 10.

[0044] In the following description, the voltage pulse applied to the resistive switching element 10 to change it from a high-resistance state to a low-resistance state will be referred to as the "set pulse." Similarly, the voltage pulse applied to the resistive switching element 10 to change it from a low-resistance state to a high-resistance state will be referred to as the "reset pulse." During the writing operation, either the set pulse or the reset pulse is applied to the resistive switching element 10.

[0045] When changing the resistance changing element 10 from a high resistance state to a low resistance state, the control circuit that controls the resistance changing element 10 applies a set pulse of a positive voltage higher than the voltage of the second electrode 22 to the first electrode 21. Applying a set pulse of a positive voltage higher than the voltage of the second electrode 22 to the first electrode 21 is equivalent to applying a set pulse of a negative voltage lower than the voltage of the first electrode 21 to the second electrode 22.

[0046] Insulator: spinel-type transition metal oxide Li4Ti5O 12 The rock salt type transition metal oxide Li7Ti5O, which exhibits metallic conductivity, is formed within it. 12 Although detailed studies on its growth process have not been conducted, phase-separation type growth under an external electric field is hypothesized.

[0047] As shown in state (a) of Figure 5, first, a set pulse is applied to the resistive switching element 10 such that the voltage at the first electrode 21 becomes V + ΔV and the voltage at the second electrode 22 becomes V. That is, a voltage higher than the voltage at the second electrode 22 is applied to the resistive switching element 10. As a result, the Li4Ti5O of ​​the second transition metal compound layer 24 12 Inside, Li7Ti5O is located along the electric field lines from the lithium ion conductor layer 25 side. 12 The filaments penetrate (grow). The penetration site depends on the microstructure of the second transition metal compound layer 24 and is a region where the electric field is relatively strong.

[0048] As shown in state (b) of Figure 5, Li7Ti5O 12 The growth of the filament stops when it reaches the second electrode 22. Li7Ti5O 12 The filament is saturated with lithium ions. Therefore, Li7Ti5O 12 No more lithium ions will flow into the filament. This is because the lithium ion conductor layer 25 and Li7Ti5O 12 This means that an effective high-resistance interface is created between the filament and the material.

[0049] As shown in state (c) of Figure 6, a high electric field region is generated in the interface region of the high-resistance interface.

[0050] As shown in state (d) of Figure 6, in the high electric field region, i.e., the region with a strong electric field, the new Li7Ti5O 12 The filament begins to grow.

[0051] As shown in state (e) of Figure 7, Li7Ti5O 12 The region increases, and the conductivity of the resistive switching element 10 increases. The conductivity of the resistive switching element 10 is due to the spinel-type transition metal oxide Li4Ti5O, which is an insulator in the second transition metal compound layer 24. 12 And, the rock salt type transition metal oxide Li7Ti5O exhibits metallic conductivity. 12 It is determined by the ratio of these factors. Therefore, the conductivity of the resistive switching element 10 depends linearly on the composition of lithium ions in the second transition metal compound layer 24.

[0052] Note that Li7Ti5O 12 There can be multiple locations where the filament can penetrate. Therefore, Li7Ti5O, which was explained using states (a) to (e) in Figures 5 to 7, 12 The growth of the filaments can occur at multiple locations within the second transition metal compound layer 24.

[0053] Next, we will explain the case in which the resistive element 10 is changed from a low-resistance state to a high-resistance state during the writing operation. When the resistive element 10 is changed from a low-resistance state to a high-resistance state, the control circuit that controls the resistive element 10 applies a reset pulse to the first electrode 21 that is a negative voltage lower than the voltage of the second electrode 22. Applying a reset pulse to the first electrode 21 that is a negative voltage lower than the voltage of the second electrode 22 is equivalent to applying a reset pulse to the second electrode 22 that is a positive voltage higher than the voltage of the first electrode 21.

[0054] The lithium ions contained in the second transition metal compound layer 24 move through the lithium ion conductor layer 25 to the first transition metal compound layer 23 due to the electric field created by the reset pulse. As a result, the second transition metal compound layer 24 changes from a state containing lithium ions to a state without lithium ions. In other words, the second transition metal compound layer 24 becomes a rock salt type transition metal oxide Li7Ti5O 12 The state changes to one that does not contain electrons. As a result, the second transition metal compound layer 24 becomes unable to absorb electrons from the second electrode 22. Consequently, the second transition metal compound layer 24 changes from a conductor to an insulator. As a result, the resistance-changing element 10 changes to a high-resistance state.

[0055] (1.5 Specific examples of input pulse operation to resistive switching elements) Next, we will explain the input pulse to the resistive switching element 10. Figure 8 shows the state of the resistive switching element 10 when an input pulse is applied, while the resistive switching element 10 is in a low-resistance state.

[0056] In the following explanation, when reading data from the resistive switching element 10, or when using the resistive switching element 10 as a resistive element, the voltage pulse applied to the resistive switching element 10 will be referred to as the "input pulse." For example, the input pulse is a voltage pulse with a lower voltage than the set pulse.

[0057] When a circuit (memory device) incorporating the resistive switching element 10 reads data from the resistive switching element 10, or when the resistive switching element 10 is used as a resistive element, a positive voltage input pulse higher than the voltage of the second electrode 22 is applied to the first electrode 21. Applying a positive voltage input pulse higher than the voltage of the second electrode 22 to the first electrode 21 is equivalent to applying a negative voltage input pulse lower than the voltage of the first electrode 21 to the second electrode 22.

[0058] Lithium ions contained in the first transition metal compound layer 23 move through the lithium ion conductor layer 25 to the second transition metal compound layer 24 due to the electric field created by the input pulse. When lithium ions reach the second transition metal compound layer 24, it absorbs electrons from the second electrode 22. As a result, the low-resistance resistive element 10 can conduct current when an input pulse is applied.

[0059] Furthermore, there is an upper limit to the amount of charge of lithium ions contained in the first transition metal compound layer 23. That is, there is an upper limit to the amount of lithium ions that can move from the first transition metal compound layer 23 to the second transition metal compound layer 24. For example, 100 × 100 × 100 nm 3 The amount of charge of lithium ions contained in a Li7Ti5O2 of this size is 1.5 × 10⁻⁶. -11 It is on the order of Coulombs. In this case, the resistive switching element 10, which includes a first transition metal compound layer 23 made of Li7Ti5O2 of this size as the base material, can handle current pulses of 1 nA with a pulse time width of 1 μs, with an upper limit of 1.5 × 10⁻¹⁰ 4 It can handle up to a certain number of current pulses. The control circuit that controls the resistive switching element 10 may perform control to read the state of the resistive switching element 10 so as not to exceed the upper limit of the current (number of current pulses) that can be supplied to the resistive switching element 10.

[0060] In contrast, when the resistive switching element 10 is in a high-resistance state, even if lithium ions reach the second transition metal compound layer 24 due to the electric field created by the input pulse, the layer does not contain a sufficient amount of lithium ions at the lattice positions, and therefore cannot absorb electrons from the second electrode 22. As a result, the resistive switching element 10 in a high-resistance state cannot conduct current even when an input pulse is applied.

[0061] (1.6 Effects according to this embodiment) In the configuration according to this embodiment, the resistive switching element 10 can achieve low-resistance and high-resistance states by changing the composition of lithium ions contained in the lattice positions of the first transition metal compound layer 23 and the second transition metal compound layer 24. This improves the repeatability and linearity of the resistive switching element.

[0062] For example, conventional thin-film battery-type resistive switching elements using lithium ions achieve low-resistance and high-resistance states by changing the composition of lithium ions contained in the interlattice gaps of the transition metal compound layer. In this case, the volume of the transition metal compound layer changes depending on the composition of lithium ions contained in the interlattice gaps. For example, the volume change can reach several percent. If the volume expands and contracts repeatedly as the resistance state of the resistive switching element changes, the resistive switching element deteriorates. As a result, sufficient repeatability cannot be obtained. In other words, the lifespan of the resistive switching element is relatively short. Furthermore, the state of the resistive switching element depends on the composition of lithium ions in the transition metal compound layer. However, the resistance value of the resistive switching element does not change linearly with respect to the lithium ion composition. That is, the conductivity of the resistive switching element does not change linearly with respect to the number of voltage pulses applied to the resistive switching element.

[0063] In contrast, the resistive switching element 10 according to this embodiment includes a first electrode 21, a first transition metal compound layer 23, a lithium ion conductor layer 25, a second transition metal compound layer 24, and a second electrode 22. The first transition metal compound layer 23 and the second transition metal compound layer 24 contain lithium ions at their lattice positions. The resistive switching element 10 can change the composition of lithium ions contained at the lattice positions of the first transition metal compound layer 23 and the second transition metal compound layer 24 based on the voltage applied to the resistive switching element 10. As a result, the resistive switching element 10 can achieve low resistance and high resistance states. The resistive switching element 10 changes the composition of lithium ions at the lattice positions. Therefore, the resistive switching element 10 can suppress the volume change of the first transition metal compound layer 23 and the second transition metal compound layer 24 due to the change in state to 1 percent or less. The resistive switching element 10 can suppress degradation of the resistive switching element 10 due to volume change and improve the repeatability of the resistive switching element 10. Therefore, the lifespan of the resistive switching element 10 can be improved.

[0064] Furthermore, the second transition metal compound layer 24 of the resistivity changing element 10 according to this embodiment is, for example, a spinel-type transition metal oxide Li4Ti5O, which is an insulator. 12 And, the rock salt type transition metal oxide Li7Ti5O exhibits metallic conductivity. 12 The resistance of the second transition metal compound layer 24 is the spinel-type transition metal oxide Li4Ti5O 12 And, rock salt type transition metal oxide Li7Ti5O 12 It depends on the volume ratio of the two components. Therefore, the resistance value of the resistive switching element 10 in the low-resistance state changes linearly with respect to the composition of lithium ions contained in the second transition metal compound layer 24. Thus, the resistance value of the resistive switching element 10 in the low-resistance state can be controlled by the composition of the second transition metal compound layer 24. Consequently, the conductivity of the resistive switching element 10 can be changed linearly with respect to the number of voltage pulses applied to the resistive switching element 10, that is, its linearity can be improved.

[0065] (2. Second Embodiment) Next, a storage device 30 according to the second embodiment will be described. The storage device 30 according to the second embodiment uses the resistive switching element 10 described in the first embodiment as a storage element.

[0066] (2.1 Storage Device Configuration) First, an example of the configuration of the storage device 30 according to this embodiment will be described. Figure 9 is a diagram showing an example of the configuration of the storage device 30 according to the second embodiment. In the example of Figure 9, the first transition metal compound layer 23, the second transition metal compound layer 24, and the lithium ion conductor layer 25 are omitted for the sake of simplicity.

[0067] As shown in Figure 9, the memory device 30 includes a resistive switching element 10, a switch 31, a control circuit 32, and an output circuit 33.

[0068] Switch 31 is a circuit that short-circuits or opens the second electrode 22 of the resistive switching element 10 and ground. One end of the transmission path of switch 31 is connected to the second electrode 22 of the resistive switching element 10 and the output circuit 33, and the other end of the transmission path is connected to ground. Switch 31 receives a switching signal from the control circuit 32. The switching signal is a signal that switches switch 31 to a short-circuit state (on state) or an open state (off state). Switch 31 is controlled to be in a short-circuit state or an open state based on the switching signal. For example, switch 31 is in a short-circuit state during a write operation and in an open state during a read operation. Switch 31 is implemented by, for example, a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor).

[0069] The control circuit 32 controls the write and read operations of the resistive element 10. The control circuit 32 is connected, for example, to the first electrode 21 and switch 31 of the resistive element 10. The control circuit 32 is also connected to a circuit (not shown) that manages the state of the resistive element 10. For example, during a write operation, the control circuit 32 receives a state setting signal from the circuit that manages the state of the resistive element 10. The state setting signal is a signal that instructs the resistive element 10 to enter a high-resistance state or a low-resistance state. Based on the state setting signal, the control circuit 32 applies a set pulse or a reset pulse to the first electrode 21.

[0070] During a write operation, the control circuit 32 shorts the switch 31. The second electrode 22 of the resistive element 10 is connected to ground. This puts the resistive element 10 in a state where a set pulse or reset pulse can be applied. The control circuit 32 then applies a set pulse or reset pulse to the first electrode 21 based on the state setting signal. This puts the resistive element 10 into a low-resistance state or a high-resistance state.

[0071] More specifically, when the control circuit 32 receives a state setting signal corresponding to a low-resistance state during a write operation, for example, it applies a set pulse to the resistive element 10. That is, the control circuit 32 applies a positive voltage to the first electrode 21 that is higher than the voltage of the second electrode 22, causing lithium ions contained in the first transition metal compound layer 23 to move to the second transition metal compound layer 24. As a result, the resistive element 10 is placed in a low-resistance state.

[0072] Furthermore, during a write operation, if the control circuit 32 receives a state setting signal corresponding to a high-resistance state, for example, it applies a reset pulse to the resistance-changing element 10. That is, the control circuit 32 applies a negative voltage to the first electrode 21 that is lower than the voltage of the second electrode 22, causing lithium ions contained in the second transition metal compound layer 24 to move to the first transition metal compound layer 23. As a result, the resistance-changing element 10 is placed in a high-resistance state.

[0073] During the read operation, the control circuit 32 opens the switch 31. The second electrode 22 of the resistive element 10 is then disconnected from ground. As a result, the resistive element 10 becomes readable by an external circuit (not shown).

[0074] In this state, for example, an input pulse is applied to the first electrode 21 of the resistive switching element 10 from an external circuit. More specifically, for example, an input pulse with a voltage higher than the voltage of the second electrode 22 is applied from the external circuit to the first electrode 21 of the resistive switching element 10. The memory device 30 may further include a pulse generation circuit that generates an input pulse when it receives a read instruction from the external circuit. For example, the pulse generation circuit is connected to the first electrode 21.

[0075] The output circuit 33 outputs an output signal indicating whether or not current flows through the resistive switching element 10 at the timing when an input pulse is applied to the resistive switching element 10. The output circuit 33 is connected to the second electrode 22 of the resistive switching element 10 and the switch 31. For example, the output circuit 33 has a capacitor 41. One electrode of the capacitor 41 is connected to the second electrode 22 of the resistive switching element 10, and the other electrode is connected to ground. The output circuit 33 then outputs the voltage at the connection point between the capacitor 41 and the second electrode 22 of the resistive switching element 10 as an output signal. For example, data can be read by comparing the voltage of the output signal with a predetermined value (reference voltage).

[0076] When an input pulse is applied to the low-resistance resistive element 10 during the read operation, current flows through the resistive element 10. As a result, the output circuit 33 outputs an output signal with a voltage higher than a predetermined value.

[0077] Furthermore, if an input pulse is applied to the resistance changing element 10 in a high-resistance state during the read operation, no current flows through the resistance changing element 10. As a result, the output circuit 33 outputs an output signal with a voltage below a predetermined value.

[0078] (2.2 Effects according to this embodiment) With the configuration according to this embodiment, the same effects as in the first embodiment can be obtained.

[0079] Furthermore, the storage device 30 according to this embodiment includes a resistive element 10, a switch 31, a control circuit 32, and an output circuit 33. When the control circuit 32 receives an instruction to change the resistive element 10 to a low-resistance state (a state setting signal corresponding to the low-resistance state) during a write operation, it can change the resistive element 10 to a low-resistance state. Also, when the control circuit 32 receives an instruction to change the resistive element 10 to a high-resistance state (a state setting signal corresponding to the high-resistance state) during a write operation, it can change the resistive element 10 to a high-resistance state.

[0080] Furthermore, in the storage device 30 according to this embodiment, an input pulse is applied to the resistive switching element 10 during the read operation. At this time, if the resistive switching element 10 is in a low-resistance state, the output circuit 33 can output an output signal with a voltage higher than a predetermined value. If the resistive switching element 10 is in a high-resistance state, the output circuit 33 can output an output signal with a voltage lower than or equal to a predetermined value. The state of the resistive switching element 10 can be determined by the voltage of the output signal. That is, data can be read from the resistive switching element 10.

[0081] The storage device 30 according to this embodiment can change the state of the resistive element 10 in response to instructions from an external circuit, and can also output the state of the resistive element 10 to the external circuit. Therefore, the storage device 30 can function as a storage device that stores binary information.

[0082] (3. Third Embodiment) Next, the storage device 30 according to the third embodiment will be described. In the third embodiment, the configuration of the storage device 30 will be described in a way that differs from that of the second embodiment. The following description will focus on the differences from the second embodiment.

[0083] (3.1 Storage Device Configuration) First, an example of the configuration of the storage device 30 according to this embodiment will be described. Figure 10 is a diagram showing an example of the configuration of the storage device 30 according to the third embodiment. In the example of Figure 10, the first transition metal compound layer 23, the second transition metal compound layer 24, and the lithium ion conductor layer 25 are omitted for the sake of simplicity in the explanation.

[0084] As shown in Figure 10, the memory device 30 includes a resistive switching element 10, a switch 31, a control circuit 32, an output circuit 33, and a positive / negative inversion circuit 51.

[0085] The positive / negative inversion circuit 51 is a circuit that switches the connection between the resistive element 10 and the external circuit, switch 31, and control circuit 32 based on a control signal received from the control circuit 32. In other words, the positive / negative inversion circuit 51 reverses the connection relationship between the first electrode 21 and the second electrode 22 and the other circuits.

[0086] The positive / negative inversion circuit 51 includes terminals TN1 to TN6. Terminal TN1 is connected to the output terminals of the set pulse and reset pulse of the external circuit and the control circuit 32. Terminal TN1 is also configured to be connectable to either terminal TN2 or terminal TN3. Terminal TN2 is connected to terminal TN6 and the first electrode 21 of the resistive element 10. Terminal TN3 is connected to terminal TN5. Terminal TN4 is connected to the switch 31 and the output circuit 33. Terminal TN3 is also configured to be connectable to either terminal TN5 or terminal TN6. Terminal TN5 is connected to terminal TN3 and the second electrode 22 of the resistive element 10.

[0087] The positive / negative inversion circuit 51 is set to either a combination of connecting terminals TN1 and TN2 and terminals TN4 and TN5, or a combination of connecting terminals TN1 and TN3 and terminals TN4 and TN6, based on the control signal received from the control circuit 32. For example, if the combination of connecting terminals TN1 and TN2 and terminals TN4 and TN5 is set, an input pulse, set pulse, or reset pulse can be applied to the first electrode 21. Then, the second electrode 22 is connected to the switch 31 and the output circuit 33. Alternatively, if the combination of connecting terminals TN1 and TN3 and terminals TN4 and TN6 is set, an input pulse, set pulse, or reset pulse can be applied to the second electrode 22. Then, the first electrode 21 is connected to the switch 31 and the output circuit 33.

[0088] For example, during a readout operation, the positive / negative inversion circuit 51 switches between applying the input pulse applied from an external circuit to the first electrode 21 as a first input pulse with a voltage higher than the voltage of the second electrode 22, or applying it to the second electrode 22 as a second input pulse with a voltage higher than the voltage of the first electrode 21. As a result, during a readout operation, the first input pulse and the second input pulse are alternately applied to the resistive switching element 10.

[0089] The switch 31 in this embodiment is a circuit that short-circuits or opens the connection between terminal TN4 of the positive / negative inversion circuit 51 and ground. One end of the transmission path of the switch 31 is connected to terminal TN4 of the positive / negative inversion circuit 51 and the output circuit 33, and the other end of the transmission path is connected to ground. The other configurations are the same as described in the second embodiment.

[0090] The control circuit 32 of this embodiment is further connected to the positive / negative inversion circuit 51. The other connections are the same as described in the second embodiment. The control circuit 32 transmits a control signal to the positive / negative inversion circuit 51. The control signal is a signal that controls the connection of the positive / negative inversion circuit 51.

[0091] Furthermore, during a read operation, the control circuit 32 can switch between the first input pulse and the second input pulse each time the first input pulse or the second input pulse is applied a predetermined number of times. In other words, it switches the connection of the positive / negative inversion circuit 51.

[0092] For example, the control circuit 32 switches the connection of the positive / negative inversion circuit 51 so that a first input pulse is applied to the resistive element 10 after the state of the resistive element 10 has been changed (after the write operation). Subsequently, in the read operation, the control circuit 32 switches the connection of the positive / negative inversion circuit 51 so that a second input pulse is applied to the resistive element 10 after the first input pulse has been applied to the resistive element 10 a predetermined number of times. Subsequently, in the read operation, the control circuit 32 switches the connection of the positive / negative inversion circuit 51 so that a first input pulse is applied to the resistive element 10 after the second input pulse has been applied to the resistive element 10 a predetermined number of times. The control circuit 32 repeats this switching process in the read operation. For example, when a first input pulse is applied to the first electrode 21, lithium ions move from the first transition metal compound layer 23 to the second transition metal compound layer 24. On the other hand, when a second input pulse is applied to the second electrode 22, lithium ions move from the second transition metal compound layer 24 to the first transition metal compound layer 23. During the readout operation, when a first input pulse and a second input pulse are alternately applied to the resistive switching element 10, lithium ions that have moved from the first transition metal compound layer 23 to the second transition metal compound layer 24 due to the first input pulse move back to the first transition metal compound layer 23 due to the second input pulse.

[0093] The output circuit 33 outputs an output signal indicating whether or not current flows through the resistive switching element 10 when a first input pulse or a second input pulse is applied to the resistive switching element 10 from an external circuit. The configuration of the output circuit 33 is the same as described in the second embodiment. For example, when a first input pulse is applied to the resistive switching element 10, the output circuit 33 outputs the voltage of the second electrode 22 of the resistive switching element 10 as an output signal. Also, for example, when a second input pulse is applied to the resistive switching element 10, the output circuit 33 outputs the voltage of the first electrode 21 of the resistive switching element 10 as an output signal.

[0094] (3.2 Effects according to this embodiment) With the configuration according to this embodiment, the same effects as those of the first and second embodiments can be obtained.

[0095] Furthermore, the memory device 30 according to this embodiment includes a resistive switching element 10, a switch 31, a control circuit 32, an output circuit 33, and a positive / negative inversion circuit 51. The positive / negative inversion circuit 51 can switch the connection of the resistive switching element 10. The positive / negative inversion circuit 51 can switch between a case where a first input pulse is applied to the first electrode 21 and a case where a second input pulse is applied to the second electrode 22 during a read operation. When the first input pulse is applied to the first electrode 21, lithium ions move from the first transition metal compound layer 23 to the second transition metal compound layer 24. When the second input pulse is applied to the second electrode 22, lithium ions move from the second transition metal compound layer 24 to the first transition metal compound layer 23. Therefore, during a read operation, by switching the connection of the resistive switching element 10, lithium ions that have moved from the first transition metal compound layer 23 to the second transition metal compound layer 24 can be returned to the first transition metal compound layer 23. As a result, the memory device 30 can supply current to the resistive switching element 10 in such a way that it does not exceed the upper limit of the current that can flow through the resistive switching element 10 (the upper limit of movable lithium ions) when the resistive switching element 10 is in a low-resistance state. Therefore, the memory device 30 can perform a read operation of the resistive switching element 10 by an external circuit without limiting the number of times an input pulse is applied.

[0096] (4. Fourth Embodiment) Next, the storage device 30 according to the fourth embodiment will be described. In the fourth embodiment, the configuration of the storage device 30 will be described in a way that differs from that of the second and third embodiments. The storage device 30 of this embodiment is effective when an electromotive force is generated between the first transition metal compound layer 23 and the second transition metal compound layer 24 of the resistive switching element 10. The following description will focus on the differences from the second and third embodiments.

[0097] (4.1 Storage Device Configuration) First, an example of the configuration of the storage device 30 according to this embodiment will be described. Figure 11 is a diagram showing an example of the configuration of the storage device 30 according to the fourth embodiment. In the example of Figure 11, the first transition metal compound layer 23, the second transition metal compound layer 24, and the lithium ion conductor layer 25 are omitted for the sake of simplicity in the explanation.

[0098] As shown in Figure 11, the memory device 30 includes a resistive switching element 10, a switch 31, a control circuit 32, an output circuit 33, and a short-circuit switch 52.

[0099] The short-circuit switch 52 is a circuit that short-circuits or opens the first electrode 21 and the second electrode 22 of the resistive switching element 10 based on a control signal received from the control circuit 32. One end of the transmission path of the short-circuit switch 52 is connected to the first electrode 21, and the other end of the transmission path is connected to the second electrode 22. The short-circuit switch 52 is implemented, for example, by a MOSFET.

[0100] One end of the transmission path of the switch 31 in this embodiment is connected to the second electrode 22, the short-circuit switch 52, and the output circuit 33, while the other end of the transmission path is connected to ground. The other configurations are the same as those described in the second embodiment.

[0101] The control circuit 32 of this embodiment is further connected to the short-circuit switch 52. Other connections are the same as described in the second embodiment. The control circuit 32 transmits a control signal to the short-circuit switch 52. The control signal of this embodiment is a signal that switches the short-circuit switch 52 to a short-circuit state (on state) or an open state (off state). In the read operation, for example, each time an input pulse is applied a predetermined number of times, the control circuit 32 short-circuits the short-circuit switch 52 for a predetermined time, thereby short-circuiting the first electrode 21 and the second electrode 22. For example, by short-circuiting the first electrode 21 and the second electrode 22, an electromotive force may be generated in the resistive switching element 10. In this case, lithium ions that have moved from the first transition metal compound layer 23 to the second transition metal compound layer 24 due to the input pulse move back to the first transition metal compound layer 23 due to the electromotive force generated in the resistive switching element 10.

[0102] The output circuit 33 of this embodiment is connected to the second electrode 22, the short-circuit switch 52, and the switch 31. The other configurations are the same as those described in the second embodiment.

[0103] (4.2 Effects according to this embodiment) With the configuration according to this embodiment, the same effects as those of the first and second embodiments can be obtained.

[0104] Furthermore, the memory device 30 according to this embodiment includes a resistive switching element 10, a switch 31, a control circuit 32, an output circuit 33, and a short-circuit switch 52. The short-circuit switch 52 can short-circuit the first electrode 21 and the second electrode 22 of the resistive switching element 10. In a read operation, the control circuit 32 short-circuits the short-circuit switch 52 for a predetermined time each time an input pulse is applied a predetermined number of times, thereby electrically connecting the first electrode 21 and the second electrode 22. When the first electrode 21 and the second electrode 22 are short-circuited, an electromotive force is generated in the resistive switching element 10. Due to the electromotive force, lithium ions move from the second transition metal compound layer 24 to the first transition metal compound layer 23. Therefore, by short-circuiting the first electrode 21 and the second electrode 22 using the short-circuit switch 52, lithium ions that have moved from the first transition metal compound layer 23 to the second transition metal compound layer 24 due to the input pulse can be returned to the first transition metal compound layer 23. As a result, the memory device 30 can supply current to the resistive switching element 10 in such a way that it does not exceed the upper limit of the current that can flow through the resistive switching element 10 (the upper limit of movable lithium ions) when the resistive switching element 10 is in a low-resistance state. Therefore, the memory device 30 can perform a read operation of the resistive switching element 10 by an external circuit without limiting the number of times an input pulse is applied.

[0105] (5. Fifth Embodiment) Next, the storage device 30 according to the fifth embodiment will be described. In the fifth embodiment, the configuration of the storage device 30 will be described in a way that differs from that of the second to fourth embodiments. The following description will focus on the differences from the second to fourth embodiments.

[0106] (5.1 Storage Device Configuration) First, an example of the configuration of the storage device 30 according to this embodiment will be described. Figure 12 is a diagram showing an example of the configuration of the storage device 30 according to the fifth embodiment.

[0107] As shown in Figure 12, the memory device 30 includes a resistive switching element 10, a switch 31, a control circuit 32, and an output circuit 33.

[0108] In this embodiment, during the readout operation, an input pulse with a voltage higher than the voltage of the second electrode 22 (hereinafter referred to as the "forward rotation pulse") and an input pulse with a voltage lower than the voltage of the second electrode 22 (hereinafter referred to as the "reverse rotation pulse") are alternately applied to the first electrode 21 of the resistive switching element 10 from an external circuit. That is, the input pulse includes both the forward rotation pulse and the reverse rotation pulse. While the forward rotation pulse is applied to the resistive switching element 10, lithium ions move from the first transition metal compound layer 23 to the second transition metal compound layer 24. Also, while the reverse rotation pulse is applied to the resistive switching element 10, lithium ions move from the second transition metal compound layer 24 to the first transition metal compound layer 23. Therefore, by alternately applying the forward rotation pulse and the reverse rotation pulse, lithium ions that have moved from the first transition metal compound layer 23 to the second transition metal compound layer 24 by the forward rotation pulse move back to the first transition metal compound layer 23 by the reverse rotation pulse. Furthermore, the resistive switching element 10 may be subjected to a forward rotation pulse followed by an inversion pulse, or vice versa. The memory device 30 may also include a pulse generation circuit that receives a read instruction from an external circuit and generates a forward rotation pulse and an inversion pulse in succession when a read instruction is received. For example, the pulse generation circuit is connected to the first electrode 21.

[0109] The configuration of the switch 31 and the control circuit 32 is the same as described in the second embodiment.

[0110] The output circuit 33 of this embodiment outputs an output signal indicating whether or not current flows through the resistive switching element 10 when a forward rotation pulse is applied to the resistive switching element 10. The output circuit 33 outputs an output signal indicating that no current flows when an inverting pulse is applied to the resistive switching element 10.

[0111] For example, the output circuit 33 includes an N-type MOSFET 60, a first resistive element 61, a second resistive element 62, a third resistive element 63, and a capacitor 41.

[0112] The gate of the N-type MOSFET 60 is connected to the second electrode 22 of the resistive switching element 10 and the switch 31.

[0113] One terminal of the first resistor 61 is connected to the gate of the N-type MOSFET 60. The other terminal of the first resistor 61 is connected to ground.

[0114] The power supply voltage Vdd is applied to one terminal of the second resistor 62. The other terminal of the second resistor 62 is connected to the drain of the N-type MOSFET 60.

[0115] One terminal of the third resistor 63 is connected to the source of the N-type MOSFET 60. The other terminal of the third resistor 63 is connected to ground.

[0116] One electrode of capacitor 41 is connected to the source of N-type MOSFET 60. The other electrode of capacitor 41 is connected to ground.

[0117] The output circuit 33 outputs the voltage at the connection point between the capacitor 41 and the source of the N-type MOSFET 60 as an output signal.

[0118] The output signal during the read operation is briefly explained below. For example, while a forward rotation pulse is applied to the low-resistance resistive element 10, current flows through the resistive element 10. During this time, a voltage higher than the threshold voltage is applied to the gate of the N-type MOSFET 60. As a result, the N-type MOSFET 60 is turned on, and the drain-source connection is made conductive. Consequently, the output circuit 33 outputs an output signal at the power supply voltage level during the period when a forward rotation pulse is applied to the low-resistance resistive element 10.

[0119] Furthermore, when a forward rotation pulse is applied to the high-resistance resistive element 10, no current flows through the resistive element 10. Therefore, a voltage lower than the threshold voltage is applied to the gate of the N-type MOSFET 60. As a result, the N-type MOSFET 60 is turned off, and the drain-source connection is non-conductive. Consequently, the output circuit 33 outputs a ground-level output signal when a forward rotation pulse is applied to the high-resistance resistive element 10.

[0120] When an inverting pulse is applied to the resistive switching element 10, no current flows through the resistive switching element 10, just as when a forward rotation pulse is applied to the resistive switching element 10 in a high-resistance state. Therefore, a voltage lower than the threshold voltage is applied to the gate of the N-type MOSFET 60. As a result, the N-type MOSFET 60 is turned off, and the drain-source connection becomes non-conductive. Consequently, when an inverting pulse is applied to the resistive switching element 10, the output circuit 33 outputs a ground-level output signal.

[0121] (5.2 Effects according to this embodiment) With the configuration according to this embodiment, the same effects as those of the first and second embodiments can be obtained.

[0122] Furthermore, the memory device 30 according to this embodiment includes a resistive switching element 10, a switch 31, a control circuit 32, and an output circuit 33. In a read operation, the memory device 30 can alternately apply forward rotation pulses and reverse rotation pulses to the resistive switching element 10. When a forward rotation pulse is applied to the resistive switching element 10, lithium ions move from the first transition metal compound layer 23 to the second transition metal compound layer 24. On the other hand, when a reverse rotation pulse is applied to the resistive switching element 10, lithium ions move from the second transition metal compound layer 24 to the first transition metal compound layer 23. Therefore, by alternately applying forward rotation pulses and reverse rotation pulses to the resistive switching element 10, lithium ions that have moved from the first transition metal compound layer 23 to the second transition metal compound layer 24 can be returned to the first transition metal compound layer 23. As a result, when the resistive switching element 10 is in a low-resistance state, the memory device 30 can supply current to the resistive switching element 10 in such a way that it does not exceed the upper limit of the current that can flow through the resistive switching element 10 (the upper limit of movable lithium ions). Therefore, the storage device 30 can read the state of the resistive element 10 by an external circuit without imposing a limit on the number of read operations.

[0123] (6. Sixth Embodiment) Next, a neural network device 70 according to the sixth embodiment will be described. The neural network device 70 according to the sixth embodiment uses the memory device 30 described in the second to fifth embodiments as a memory circuit.

[0124] (6.1 Configuration of a neural network device) First, let's describe an example of the configuration of the neural network device 70. Figure 13 shows an example of the configuration of the neural network device 70 according to the sixth embodiment.

[0125] As shown in Figure 13, the neural network device 70 includes an arithmetic circuit 71, an inference weight memory circuit 72, a learning weight memory circuit 73, and a learning control circuit 74.

[0126] The arithmetic circuit 71 performs arithmetic processing according to the neural network. The arithmetic circuit 71 is implemented by an electrical circuit including, for example, an analog circuit. For example, the arithmetic circuit 71 takes M input signals (x1, ..., x) (where M is an integer of 2 or more) M The arithmetic circuit 71 receives a signal and outputs an output signal (z). Note that the arithmetic circuit 71 may output multiple output signals.

[0127] The inference weight storage circuit 72 stores, for example, multiple inference weights used in computational processing according to the neural network by the arithmetic circuit 71. The inference weight storage circuit 72 includes multiple memory devices 30 corresponding to the multiple inference weights. That is, the inference weight storage circuit 72 includes multiple resistive change elements 10 that store the multiple inference weights. The inference weight storage circuit 72 stores, for example, L (where L is an integer of 2 or more) inference weights (w d1 ,…,w dL The inference weights are stored. Each of the multiple inference weights is an N value (where N is an integer greater than or equal to 2). In other words, each of the multiple inference weights is a digital value. The inference weight storage circuit 72 converts the output voltage of the storage device 30 into an N value inference weight and transmits it to the arithmetic circuit 71. For example, the inference weight storage circuit 72 converts the output voltage of the storage device 30 into an inference weight w di When converting to (where i is an integer between 1 and L), a pre-set weight conversion threshold (T0, T1, ..., T) is used. N-2 Using ), set weights (S0, S1, S2, ..., S N-1 Converts to one of the following: For example, the inference weight memory circuit 72 converts the voltage value V of the output voltage of the memory device 30. i However, T k ≤V i <T k+1 When k is an integer greater than or equal to 0 and less than or equal to N-3, the inference weight w di =S kOutput it. For example, if N = 2, the inference weight storage circuit 72 can be used as a binary weight circuit. Thereby, the arithmetic circuit 71 can rapidly execute arithmetic processing according to a neural network by an analog circuit using a plurality of inference weights each represented by N values. Note that the inference weight storage circuit 72 may be incorporated inside the arithmetic circuit 71.

[0128] The learning weight storage circuit 73 stores a plurality of weights corresponding to a plurality of inference weights in the learning process of the neural network. The learning weight storage circuit 73 stores, for example, L weights (w1, …, w L ) that correspond one-to-one to L inference weights. Each of the plurality of weights is a continuous value (for example, an analog quantity or a digital value of a predetermined number of bits). The learning weight storage circuit 73 transmits the plurality of weights (w1, …, w L ) to the inference weight storage circuit 72.

[0129] The learning control circuit 74 causes the learning weight storage circuit 73 to store initial values of a plurality of weights in the learning process of the neural network. The learning control circuit 74 repeats the update process a plurality of times. In the update process, the learning control circuit 74 generates update amounts (Δw1, …, Δw L ) corresponding to each of the plurality of weights based on the arithmetic result by the arithmetic circuit 71, and transmits them to the learning weight storage circuit 73. The learning weight storage circuit 73 updates the plurality of stored weights based on the update amounts. Note that the learning control circuit 74 may execute the update process only once. Then, after the learning process, the learning control circuit 74 causes the inference weight storage circuit 72 to store a plurality of values corresponding to the plurality of weights stored in the learning weight storage circuit 73 as a plurality of inference weights.

[0130] As described above, the learning control circuit 74 executes the learning process of the neural network using a plurality of weights represented by continuous values. Thereby, the learning control circuit 74 can increase or decrease each of the plurality of weights by a minute amount in the learning process, so that the neural network can be accurately learned.

[0131] (6.2 Configuration of Neural Network Layers) Next, an example of the configuration of the neural network layers will be described. FIG. 14 is a diagram for explaining one layer of the neural network in the neural network device 70.

[0132] As shown in FIG. 14, the neural network includes, for example, one or more layers. The arithmetic circuit 71 includes a circuit that executes an arithmetic operation corresponding to the layer as shown in FIG. 14.

[0133] The arithmetic circuit 71 has N (N is an integer of 2 or more) product-sum arithmetic circuits 80 (80_1 to 80_N) corresponding to N intermediate signals (y1 to y N ) in order to execute the arithmetic operation of the layer. The j-th (j is an integer from 1 to N) product-sum arithmetic circuit 80_j among the N product-sum arithmetic circuits 80 corresponds to the j-th intermediate signal (y j ). Also, each of the N product-sum arithmetic circuits 80 receives M input signals (x1 to x M ).

[0134] (6.3 Product-Sum Arithmetic by Product-Sum Arithmetic Circuit) Next, the product-sum arithmetic by the product-sum arithmetic circuit 80 will be described. FIG. 15 is a diagram for explaining the product-sum arithmetic by the product-sum arithmetic circuit 80.

[0135] As shown in FIG. 15, each of the N product-sum arithmetic circuits 80 is given M inference weights (w 1j , w 2j , …, w ij , …, w Mj ) corresponding to the M input signals from the inference weight storage circuit 72.

[0136] Each of the N product-sum arithmetic circuits 80 outputs a binarized intermediate signal that is the value obtained by performing a product-sum arithmetic on the M input signals and the M inference weights. For example, the product-sum arithmetic circuit 80_j corresponding to the j-th intermediate signal executes the arithmetic operation of Equation (1) analogously. <00006​​

number

[0138] In equation (1), y j x represents the j-th intermediate signal. i This represents the i-th input signal (where i is an integer greater than or equal to 1 and less than or equal to M). ij represents the inference weight multiplied by the i-th input signal among the M inference weights. In equation (1), f(X) represents a function that binarizes the value X in parentheses by a predetermined threshold. Furthermore, y j A constant bias may be added to this.

[0139] (6.4 Effects according to this embodiment) The neural network device 70 according to this embodiment stores inference weights used in calculations in the neural network using a memory device 30 that utilizes the resistive switching element 10 described in the first to fifth embodiments. As a result, the neural network device 70 can store inference weights using the resistive switching element 10, which has small variations in characteristics, and can perform calculations with high accuracy.

[0140] (7. Seventh Embodiment) Next, the neural network device 70 according to the seventh embodiment will be described. In the seventh embodiment, the configuration of the neural network device 70 will be described in a way that differs from that of the sixth embodiment. The following description will focus on the differences from the sixth embodiment.

[0141] (7.1 Configuration of the neural network device) First, let's describe an example of the configuration of the neural network device 70. Figure 16 shows an example of the configuration of the neural network device 70 according to the seventh embodiment.

[0142] As shown in Figure 16, the neural network device 70 includes an arithmetic circuit 71, a learning weight memory circuit 73, and a learning control circuit 74.

[0143] The arithmetic circuit 71 performs arithmetic processing according to the neural network. The arithmetic circuit 71 is implemented by an electrical circuit including, for example, an analog circuit. For example, the arithmetic circuit 71 takes M input signals (x1, ..., x) (where M is an integer of 2 or more) M The arithmetic circuit 71 receives the signal and outputs an output signal (z). Note that the arithmetic circuit 71 may output multiple output signals.

[0144] The learning weight memory circuit 73 stores multiple weights corresponding to multiple inference weights during the neural network learning process. The learning weight memory circuit 73 includes multiple memory devices 30 corresponding to multiple weights. That is, the learning weight memory circuit 73 includes multiple resistive change elements 10 that store multiple inference weights. The learning weight memory circuit 73 stores L weights (w1, ..., w) that correspond one-to-one to L inference weights. L The learning weight storage circuit 73 stores the weights. Each of the multiple weights is a continuous value (e.g., an analog quantity or a digital value of a predetermined number of bits). The learning weight storage circuit 73 transmits the multiple weights to the arithmetic circuit 71. The arithmetic circuit 71 uses the output values ​​(weights) of the learning weight storage circuit 73 as they are.

[0145] The learning control circuit 74 stores initial values ​​for multiple weights in the learning weight memory circuit 73 during the neural network learning process. Subsequently, the learning control circuit 74 repeats the update process multiple times. In the update process, the learning control circuit 74 updates each of the multiple weights (Δw1, ..., Δw) based on the calculation results from the arithmetic circuit 71. L The learning weights are generated and sent to the learning weights memory circuit 73. The learning weights memory circuit 73 updates each of the multiple weights it has stored based on the update amount. The learning control circuit 74 may perform the update process only once.

[0146] (7.2 Effects according to this embodiment) The neural network device 70 according to this embodiment stores the weights used for calculations in the neural network using a memory device 30 that utilizes the resistive switching element 10 described in the first to fifth embodiments. The learning control circuit 74 performs the learning process of the neural network using a plurality of weights represented as continuous values. As a result, the learning control circuit 74 can increase or decrease each of the plurality of weights by a small amount during the learning process, thereby enabling the neural network to be learned with high accuracy.

[0147] (8. Variations, etc.) While several embodiments of the present invention have been described, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These novel embodiments can be carried out in a variety of other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims of the invention and its equivalents. [Explanation of Symbols]

[0148] 10… Resistive switching element 21...1st electrode 22…Second electrode 23...First transition metal compound layer 24...Second transition metal compound layer 25…Lithium-ion conductor layer 30…Storage device 31…Switch 32...Control circuit 33…Output circuit 41…Capacitor 51...Positive / Negative Inversion Circuit 52... Short-circuit switch 60…N-type MOSFET 61...First resistive element 62...Second resistor element 63...Third resistive element 70...Neural network device 71...Arithmetic circuit 72...Inference weight memory circuit 73…Learning weight memory circuit 74...Learning control circuit 80... Sum-accumulate circuit

Claims

1. First electrode and, The second electrode and A first transition metal compound layer, which is a metal compound containing lithium ions in lattice positions, is provided between the first electrode and the second electrode. A second transition metal compound layer is provided between the first transition metal compound layer and the second electrode, and the second transition metal compound layer is the metal compound containing the lithium ions at the lattice positions, A lithium ion conductor layer is provided between the first transition metal compound layer and the second transition metal compound layer, and is a solid material that allows lithium ions to pass through but does not readily allow electrons to pass through. Equipped with, The second transition metal compound layer is a mixture of λ-type cubic metal oxide λMnO₂ and spinel-type transition metal oxide Li¹ / ²MnO₂. Resistivity switching element.

2. The first transition metal compound layer is either a rock salt type transition metal oxide or a spinel type transition metal oxide, or a mixture of the rock salt type transition metal oxide and the spinel type transition metal oxide. The resistive switching element according to claim 1.

3. The above-mentioned rock-salt type transition metal oxide is LiTiO ２ , Li ７ Ti ５ O １２ , Li ２ [CrTi]O ４ , (Li ３／２ Fe １／２ )[Li １／２ Fe １／２ Ti]O ４ and is composed of any one of them, or a mixture thereof, The resistive switching element according to claim 2.

4. The spinel-type transition metal oxide in the first transition metal compound layer is LiTi ２ O ４ Li ４ Ti ５ O １２ , Li[CrTi]O ４ , (Li １／２ Fe １／２ ) [Li １／２ Fe １／２ Ti]O ４ It consists of one of the following, or a mixture thereof. The resistive switching element according to claim 2.

5. The composition of lithium ions in the second transition metal compound layer is lower than the composition of lithium ions in the first transition metal compound layer. The resistive switching element according to claim 1.

6. The thickness of the second transition metal compound layer is thinner than the thickness of the first transition metal compound layer. The resistive switching element according to claim 1.

7. The thickness of the second transition metal compound layer is greater than or equal to the thickness of the first transition metal compound layer. The resistive switching element according to claim 1.

8. Based on the composition of the lithium ions contained in the second transition metal compound layer, the resistance changes to either a low-resistance state or a high-resistance state. The resistive switching element according to claim 1.

9. The first transition metal compound layer is in contact with the first surface of the lithium ion conductor layer, The second transition metal compound layer is in contact with the second surface of the lithium ion conductor layer that is opposite to the first surface. The resistive switching element according to claim 1.

10. The shape of the first surface is the same as the shape of the second surface. The resistive switching element according to claim 9.

11. First electrode and, The second electrode and A first transition metal compound layer is provided between the first electrode and the second electrode, containing lithium ions and composed of either a rock salt type transition metal oxide or a spinel type transition metal oxide, or a mixture of the rock salt type transition metal oxide and the spinel type transition metal oxide, A second transition metal compound layer is provided between the first transition metal compound layer and the second electrode, and is composed of a mixture of λ-type cubic metal oxide λMnO₂ and spinel-type transition metal oxide Li¹ / ²MnO₂, The device comprises a lithium ion conductor layer provided between the first transition metal compound layer and the second transition metal compound layer, which is a solid material that allows lithium ions to pass through but has difficulty passing electrons through, Resistivity switching element.

12. The resistive switching element according to any one of claims 1 to 11, A control circuit connected to the resistive element and configured to control writing and reading operations using the resistive element, An output circuit connected to the resistive element and configured to output an output voltage based on the read operation, Equipped with, storage device.

13. In the writing operation, the control circuit applies a pulse of a positive voltage higher than the voltage of the second electrode to the first electrode when the resistance changing element is brought to a low-resistance state. The storage device according to claim 12.

14. The output circuit is further comprising a first circuit configured to connect either the first electrode or the second electrode of the resistance changing element, In the readout operation, the first circuit switches from a state in which an input pulse is applied to the first electrode and the second electrode is connected to the output circuit to a state in which the input pulse is applied to the second electrode and the first electrode is connected to the output circuit. The storage device according to claim 12.

15. The system further includes a second circuit configured to enable short-circuiting of the first electrode and the second electrode, In the read operation, the second circuit short-circuits the first electrode and the second electrode after the application of the input pulse to the resistance change element has finished. The storage device according to claim 12.

16. The output circuit includes a transistor whose gate is connected to the second electrode of the resistive element, In the readout operation, the resistance change element is subjected to alternating application of a forward rotation pulse and an inverted pulse obtained by reversing the forward rotation pulse to the first electrode. The output circuit outputs a first voltage when the resistance changing element is in a low resistance state when the forward rotation pulse is applied to the first electrode, and outputs a second voltage lower than the first voltage when the resistance changing element is in a high resistance state. The storage device according to claim 12.

17. An arithmetic circuit configured to perform computational processing according to a neural network, A storage circuit comprising a resistive element according to any one of claims 1 to 11, wherein the resistive element is configured to store weights used in the calculation process, Equipped with, A neural network device.

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