Abstract
A lead-free metallic halide memristor is disclosed. The lead-free metallic halide memristor comprises a first electrode layer, an active layer and a second electrode layer, of which the active layer is made of a metallic halide material. Experimental data have proved that the lead-free metallic halide memristor possesses synaptic plasticity because of showing characteristics of short-term potentiation, short-term depression, long-term potentiation, long-term depression during the experiments. Therefore, the lead-free metallic halide memristor has significant potential for being used as an artificial synaptic element so as to be further applied in the manufacture of a reservoir computing chip. Moreover, experimental data have also proved that the lead-free metallic halide memristor also shows the characteristics of multi-level resistive switching, whereupon the lead-free metallic halide memristor can be further used as analog non-volatile memory so as to be further applied in the manufacture of a neuromorphic computing chip.
Claims
1. A lead-free metallic halide memristor, comprising: a first electrode layer; an active layer, being formed on the first electrode layer; and a second electrode layer, being formed on the active layer; wherein the active layer is made of a metallic halide material comprising a general formula MX.sub.n; wherein M is selected from a group consisting of Li, Na, K, Rb, Cs, Mg, and X being selected from a group consisting of F, Cl, Br, and I; wherein n is 1 or 2.
2. The lead-free metallic halide memristor of claim 1, wherein the first electrode layer and the second electrode layer are both made of a material selected from a group consisting of silver, gold, platinum, copper, indium tin oxide, fluorine-doped tin oxide, indium zinc oxide, gallium doped zinc oxide, and aluminum-doped zinc oxide.
3. The lead-free metallic halide memristor of claim 1, wherein the first electrode layer is made of a material, and the material being a compound of silver and titanium carbide.
4. The lead-free metallic halide memristor of claim 1, wherein there is an interfacial modification layer formed between the first electrode layer and the active layer, and the interfacial modification layer being made of a material selected from a group consisting of oxide semiconductor and organic semiconductor.
5. The lead-free metallic halide memristor of claim 1, wherein there is an interfacial modification layer formed between the active layer and the second electrode layer, and the interfacial modification layer being made of a material selected from a group consisting of oxide semiconductor and organic semiconductor.
6. The lead-free metallic halide memristor of claim 1, wherein a first interfacial modification layer is formed between the first electrode layer and the active layer, a second interfacial modification layer being formed between the active layer and the second electrode layer, and the first interfacial modification layer and the second interfacial modification layer being both made of a material selected from a group consisting of oxide semiconductor and organic semiconductor.
7. The lead-free metallic halide memristor of claim 1, wherein the first electrode layer is formed on a substrate.
8. An electronic element, being selected from a group consisting of artificial synapse, two-level resistive non-volatile memory and multi-level resistive non-volatile memory; characterized in that the electronic element comprises: a first electrode layer; an active layer, being formed on the first electrode layer; and a second electrode layer, being formed on the active layer; wherein the active layer is made of a metallic halide material comprising a general formula MX.sub.n; wherein M is selected from a group consisting of Li, Na, K, Rb, Cs, Mg, and X being selected from a group consisting of F, Cl, Br, and I; wherein n is 1 or 2.
9. An electronic chip, being selected from a group consisting of neuromorphic computing chip and reservoir computing chip; characterized in that the electronic chip comprises a plurality of memristor elements, and each said memristor element comprising: a first electrode layer; an active layer, being formed on the first electrode layer; and a second electrode layer, being formed on the active layer; wherein the active layer is made of a metallic halide material comprising a general formula MX.sub.n; wherein M is selected from a group consisting of Li, Na, K, Rb, Cs, Mg, and X being selected from a group consisting of F, Cl, Br, and I; wherein n is 1 or 2.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The invention as well as a preferred mode of use and advantages thereof will be best understood by referring to the following detailed description of an illustrative embodiment in conjunction with the accompanying drawings, wherein:
[0027] FIG. 1 shows a block diagram for describing a traditional Von Neumann architecture;
[0028] FIG. 2 shows a block diagram for describing a computer having neuromorphic computing architecture;
[0029] FIG. 3 shows a schematic elevation view of a lead-free metallic halide memristor according to the present invention;
[0030] FIG. 4A shows a first cross-sectional side of the lead-free metallic halide memristor according to the present invention;
[0031] FIG. 4B shows a second cross-sectional side of the lead-free metallic halide memristor according to the present invention;
[0032] FIG. 4C shows a third cross-sectional side of the lead-free metallic halide memristor according to the present invention;
[0033] FIG. 5 shows a curve graph for showing I-V characteristics of a sample 1 of the lead-free metallic halide memristor;
[0034] FIG. 6 shows a curve graph for showing I-V characteristics of a sample 2 of the lead-free metallic halide memristor;
[0035] FIG. 7 shows a curve graph of current versus time measured from a sample 6 of the lead-free metallic halide memristor that is applied with numbers of successive identical pulses;
[0036] FIG. 8 shows a scatter plot of conductance versus number of pulses measured from the sample 6 of the lead-free metallic halide memristor;
[0037] FIG. 9 shows a curve graph of current versus time measured from the sample 6 that is applied with numbers of successive identical pulses;
[0038] FIG. 10 shows a scatter plot of conductance versus number of pulses measured from the sample 6;
[0039] FIG. 11 shows a scatter plot of normalized conductance versus normalized number of pulses measured from the sample 6;
[0040] FIG. 12 shows a curve graph for showing I-V characteristics of a sample 4 of the lead-free metallic halide memristor;
[0041] FIG. 13 shows a curve graph for showing I-V characteristics of a sample 5 of the lead-free metallic halide memristor;
[0042] FIG. 14 shows a curve graph for showing I-V characteristics of the sample 6 of the lead-free metallic halide memristor;
[0043] FIG. 15 shows a curve graph of current versus reading time measured from the sample 6;
[0044] FIG. 16 shows a curve graph of conductance versus compliance current measured from the sample 6;
[0045] FIG. 17 shows a curve graph for showing I-V characteristics of a sample 7 of the lead-free metallic halide memristor; and
[0046] FIG. 18 shows a scatter plot of conductance versus number of pulses measured from the sample 6.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0047] To more clearly describe a lead-free metallic halide memristor and an electronic element comprising the same, embodiments of the present invention will be described in detail with reference to the attached drawings hereinafter.
[0048] The present invention discloses a lead-free metallic halide memristor, which possesses synaptic plasticity because of showing characteristics of short-term potentiation (STP), short-term depression (STD), long-term potentiation (LTP), long-term depression (LTD) during operation. Moreover, the lead-free metallic halide memristor also shows characteristic of multi-level variable resistive memory. Therefore, the lead-free metallic halide memristor has a wide variety of uses, including being used as an electronic element like artificial synapse, two-level resistive non-volatile memory and multi-level resistive non-volatile memory, being applied in the manufacture of neuromorphic computing chip, and being applied in the manufacture of reservoir computing chip.
[0049] With reference to FIG. 3, there is shown a schematic elevation view of a lead-free metallic halide memristor according to the present invention. As FIG. 3 shows, the lead-free metallic halide memristor 1 comprises: a first electrode layer 11, an active layer 12 and a second electrode layer 13. To fabricate this novel lead-free metallic halide memristor 1, it is firstly formed the first electrode layer 11 on a substrate 10, and then the active layer 12 and the second electrode layer 13 are formed on the first electrode layer 11 in turns. It is imaginable that there is a circuit topology formed on the substrate 10, whereupon a control circuit is coupled to the circuit topology, so as to drive at least one said lead-free metallic halide memristor 1 that is disposed on the substrate 10. According to the present invention, the active layer 12 is made of a metallic halide material comprising a general formula MX.sub.n, of which M is any one of Li, Na, K, Rb, Cs, or Mg, and X is any one of F, Cl, Br, or I. In other words, M is an alkali metal element or an alkaline earth metal element, and X is a halogen element. On the other hand, n is 1 or 2.
[0050] As described in more detail below, the first electrode layer 11 and the second electrode layer 13 are both made of a material, and the material can be any one of silver (Ag), gold (Au), platinum (Pt), copper (Cu), indium tin oxide (ITO), fluorine-doped tin oxide (FTO), indium zinc oxide (IZO), gallium doped zinc oxide (GZO), or aluminum-doped zinc oxide (AZO). It is worth mentioning that, in a particular embodiment, the first electrode layer 11 can be made of a compound of silver (Ag) and titanium carbide (TiC).
[0051] FIG. 4A, FIG. 4B, FIG. 4C show a first cross-sectional side, a second cross-sectional side and a third cross-sectional side of the lead-free metallic halide memristor, respectively. It is imaginable that there may be interfacial defects existing between the first electrode layer 11 and the MX.sub.n-made active layer 12 and/or between the MX.sub.n-made active layer 12 and the second electrode layer 13. In some applications, the interfacial defects would degrade the performance of the lead-free metallic halide memristor 1. Accordingly, for significantly reducing the impact of the interfacial defects, it is able to form a first interfacial modification layer 14 (as shown in FIG. 4A) between the first electrode layer 11 and the active layer 12. Moreover, as FIG. 4B shows, it may also dispose the first interfacial modification layer 14 between the active layer 12 and the second electrode layer 13. Furthermore, as FIG. 4C shows, it may also form a first interfacial modification layer 14 between the first electrode layer 11 and the active layer 12, and simultaneously form a second interfacial modification layer 15 between the active layer 12 and the second electrode layer 13. In practicable embodiments, the first interfacial modification layer 14 and the second interfacial modification layer 15 can both be made of an oxide semiconductor like MoOx or an organic semiconductor such as PEDOT:PSS.
[0052] Samples of the Lead-Free Metallic Halide Memristor
[0053] For proving that the lead-free metallic halide memristor 1 having the structure as shown in FIG. 3, FIG. 4A, FIG. 4B, or FIG. 4C can indeed show characteristics of multi-level resistive switching, short-term potentiation (STP), short-term depression (STD), long-term potentiation (LTP), and long-term depression (LTD), several samples of the lead-free metallic halide memristor 1 are made, and electrical characteristic measurements of these samples are all completed. There are ten samples of the lead-free metallic halide memristor 1 fabricated, and the basic information of the ten samples are provided in following Table (1).
TABLE-US-00001 TABLE (1) First Second interfacial interfacial Sample First modification Active modification Second No. electrode layer layer layer electrode 1 ITO — CsI — Ag (200 nm) (130 nm) 2 ITO — CsBr — Ag (100 nm) (130 nm) 3 TiC/Ag MoOx NaCl MoOx Ag (180 nm) (20 nm) (100 nm) (20 nm) (140 nm) 4 ITO — CsC1 — Ag (100 nm) (130 nm) 5 Ag — CsBr MoOx Ag (100 nm) (100 nm) (20 nm) (130 nm) 6 Ag MoOx CsBr — Ag (100 nm) (20 nm) (100 nm) (130 nm) 7 Ag MoOx CsI — Ag (100 nm) (20 nm) (200 nm) (130 nm) 8 TiC/Ag — NaCl — Ag (180 nm) (100 nm) (130 nm) 9 TiC/Ag — MgF2 MoOx Ag (180 nm) (100 nm) (20 nm) (130 nm)
[0054] In neuroscience, synaptic plasticity is the ability of synapses to strengthen or weaken over time, in response to increases or decreases in their activity. Since memories are postulated to be represented by vastly interconnected neural circuits in the brain, synaptic plasticity is one of the important neurochemical foundations of learning and memory. Data of electrophysiological experiments have indicated that characteristics of synaptic plasticity include facilitation, potentiation and depression. Moreover, the synaptic plasticity can further classified into short-term plasticity and long-term plasticity. Therefore, as long as the electrical characteristics of an electronic element exhibit the short-term plasticity and the long-term plasticity, the electronic element is regarded as an artificial synapse.
[0055] FIG. 5 shows a curve graph for showing I-V characteristics of sample 1 of the lead-free metallic halide memristor 1. According to the I-V curve of FIG. 5, it is found that the conductance of sample 1 increases by strengthening the applied negative voltage, but the conductance of sample 1 declines by enhancing the applied positive voltage. Therefore, I-V curve of FIG. 5 has proved that the electrical characteristics of sample 1 exhibit the short-term potentiation (STP) and short-term depression (STD), i.e., short-term plasticity. On the other hand, FIG. 6 shows a curve graph for showing I-V characteristics of sample 2 of the lead-free metallic halide memristor 1. Similarly, I-V curve of FIG. 6 has proved that the electrical characteristics of sample 2 also exhibit the short-term potentiation and short-term depression.
[0056] Furthermore, FIG. 7 shows a curve graph of current versus time measured from sample 6 of the lead-free metallic halide memristor 1. According to FIG. 7, it is observed that the conductance of sample 6 ascends gradually by successively applying identical negative pulses to sample 6 and exhibiting great retention in every state, further showing a positive correlation between the current readout from sample 6 and the number of pulses. FIG. 8 shows a scatter plot of conductance versus number of pulses measured from sample 6, which also indicates that it is positively correlated between the two.
[0057] FIG. 9 also shows a curve graph of current versus time measured from sample 6 that is applied with numbers of successive identical pulses. According to FIG. 9, it is observed that the conductance of sample 6 descends gradually by successively applying identical positive pulses to sample 6 and exhibiting great retention in every state, further showing a negative correlation between the current read out from sample 6 and the number of pulses. FIG. 10 shows a scatter plot of conductance versus number of pulses measured from sample 6, which also indicates that it is negatively correlated between the two. As a result, measurement data of FIG. 7, FIG. 8, FIG. 9, and FIG. 10 have proved that the electrical characteristic of sample 6 of the lead-free metallic halide memristor 1 exhibit long-term potentiation (LTP) and long-term depression (LTD), i.e., long-term plasticity. As described in more detail below, the electrical characteristic of gradually-increasing conductance and the electrical characteristic of gradually-decreasing conductance are corresponding to LTP behavior and LTD behavior of synapse, respectively, and these two electrical characteristics can be applied in achieving the weight updating (changing) operation of an artificial neural network (ANN).
[0058] In summary, experimental data of FIG. 5, FIG. 6, FIG. 7, FIG. 8, FIG. 9, and FIG. 10 have proved that the lead-free metallic halide memristor 1 of the present invention indeed possesses synaptic plasticity because of showing characteristics of short-term potentiation (STP), short-term depression (STD), long-term potentiation (LTP), long-term depression (LTD) during the experiments. Therefore, this novel lead-free metallic halide memristor 1 has significant potential for being used as an artificial synaptic element so as to be further applied in the manufacture of a neuromorphic computing chip.
[0059] Furthermore, in order to facilitate the calculation of the conductance linearity of sample 6 of the lead-free metallic halide memristor 1, experimental data of FIG. 16 are further normalized, whereupon FIG. 11 correspondingly shows a scatter plot of normalized conductance versus normalized number of pulses measured from the sample 6. Therefore, according to the normalized data of FIG. 11, the nonlinearity of the conductance states of sample 6 is calculated to be 0.0269, which is much better than conventional memristors. In other words, the lead-free metallic halide memristor exhibits a near-linear conductance modulation.
[0060] Engineers skilled in design and manufacture of memristors certainly know that, as long as an operation voltage applied to the memristor exceeds a threshold voltage, a memristor is allowed to be switched from a low resistance state (LRS) to a high resistance state (HRS), or be switched from HRS to LRS. As described in more detail below, driving the memristor to complete a write (set) operation can make the memristor switch from HRS to LRS. On the contrary, driving the memristor to achieve an erase (reset) operation makes the memristor switch from LRS to HRS. With reference to FIG. 12, there is a curve graph for showing I-V characteristics of sample 4 of the lead-free metallic halide memristor 1. According to I-V curve of FIG. 12, it is found that sample 4 exhibits the electrical characteristic (behavior) of bistable resistive switching. Moreover, there is almost 1-order resistance ratio between HRS and LRS of sample 4.
[0061] Furthermore, FIG. 13 shows a I-V curve graph of sample 5 of the lead-free metallic halide memristor 1, which exhibits bistable, unipolar switching characteristics. According to I-V curve of FIG. 13, it is observed that sample 5 can be programmed to HRS around ±0.08V, and LRS around ±0.15V. Moreover, the resistance ratio between HRS and LRS of sample 5 reaches 6 orders. As a result, experimental data of FIG. 12 and FIG. 13 have proved that the lead-free metallic halide memristor 1 of the present invention indeed can be used as a two-level resistive non-volatile memory, despite different operation methods to some extents.
[0062] It is worth mentioning that, during a SET (write) operation of the lead-free metallic halide memristor 1, a larger current of the lead-free metallic halide memristor 1 can be limited by an external compliance current provided by a control circuit. For example, after applying a pulse greater than set voltage to the lead-free metallic halide memristor 1, the lead-free metallic halide memristor 1 switches from HRS to LRS, and then the current readout from the lead-free metallic halide memristor 1 ascends from ˜10.sup.−7 A to ˜10.sup.−3 A. In such case, the control circuit would adaptively adjust the level of the pulse voltage to make the current be eventually limited in the compliance current (e.g. 100 μA). Therefore, make the memristor stay in various conductance states.
[0063] FIG. 14 depicts a curve graph for showing I-V characteristics of the sample 6 of the lead-free metallic halide memristor under different compliance currents, and FIG. 15 illustrates a curve graph of current versus reading time measured from the sample 6. As FIG. 14 and FIG. 15 show, during the set operation of sample 6, the control circuit adaptively modulates the current of sample 6 according to a designated compliance current, thereby making the current read out from sample 6 approach the compliance current. Therefore, experimental data of FIG. 14 and FIG. 15 have proved that, by using a designated compliance current to limit the current of the lead-free metallic halide memristor 1, the conductance of the lead-free metallic halide memristor 1 becomes discretely adjustable, whereupon the lead-free metallic halide memristor 1 shows the characteristics of multi-level resistive switching.
[0064] FIG. 16 shows a curve graph of conductance versus compliance current measured from the sample 6. According to FIG. 16, during successively applying identical negative pulses (e.g., −0.1V) with gradually increasing compliance current (e.g., from 160 μA to 600 μA) so as to make the conductance of sample 6 be correspondingly getting high. Therefore, experimental data of FIG. 16 have proved that the lead-free metallic halide memristor 1 of the present invention shows long-term potentiation (LTP) characteristic with high linearity and high dynamic range.
[0065] Furthermore, FIG. 17 shows a curve graph for showing I-V characteristics of sample 7 of the lead-free metallic halide memristor 1. According to FIG. 17, during the period of more than 100 times of set/reset cycling characterization, sample 7 of the lead-free metallic halide memristor 1 shows good stability. Moreover, the resistance ratio between HRS and LRS of sample 7 is still greater than 6 orders. On the other hand, FIG. 18 depicts a scatter plot of conductance versus number of pulses measured from the sample 6. After applying a pulse voltage with a low level of −0.1V to sample 6 of the lead-free metallic halide memristor 1, sample 6 switches from HRS to LRS. On the contrary, sample 6 reversely switches from LRS to HRS after receiving a pulse voltage with a level of 0.05V. According to FIG. 18, after more than 150 times of set/reset cycling characterization, the resistance ratio between HRS and LRS of sample 6 is still greater than 6 orders and shows a narrow distribution.
[0066] As a result, experimental data of FIG. 17 and FIG. 18 have proved that the lead-free metallic halide memristor can be used as a resistive non-volatile memory including advantages of high resistance ratio (i.e., dynamic range) and outstanding endurance. It is worth mentioning that, following Table (2) lists operation voltages and characteristics of samples 1-9 of the lead-free metallic halide memristor 1.
TABLE-US-00002 TABLE (2) Sample Write Erase Dynamic Multi-level Active No. Voltage Voltage Range switching layer 1 −0.15 V 0.07 V >10.sup.6 multiple CsI states 2 −0.1 V 0.05 V >10.sup.6 multiple CsBr states 3 |V| > 0.2 V >10.sup.6 >3 states NaCl 4 −0.1 V 0.1 V ~10.sup. 2 states CsCl 5 −0.1 V 0.05 V >10.sup.6 multiple CsBr states 6 −0.1 V 0.05 V >10.sup.6 multiple CsBr states 7 −0.15 V 0.07 V >10.sup.6 multiple CsI states 8 |V| > 0.2 V >10.sup.6 >3 states NaCl 9 −0.1 V 0.05 V >10.sup.5 >6 states MgF2
[0067] In summary, experimental data have proved that the lead-free metallic halide memristor 1 of the present invention indeed possesses synaptic plasticity because of showing characteristics of short-term potentiation (STP), short-term depression (STD), long-term potentiation (LTP), long-term depression (LTD) during the experiments. Therefore, the lead-free metallic halide memristor 1 has significant potential for being used as an artificial synaptic element so as to be further applied in the manufacture of a reservoir computing chip. Moreover, experimental data have also proved that the lead-free metallic halide memristor 1 of the present invention also shows the characteristics of multi-level resistive switching, whereupon the lead-free metallic halide memristor can be further used as non-volatile memory so as to be further applied in the manufacture of a neuromorphic computing chip.
[0068] Therefore, through the above descriptions, all embodiments of the lead-free metallic halide memristor and the electronic element comprising the same according to the present invention have been introduced completely and clearly. Moreover, the above description is made on embodiments of the present invention. However, the embodiments are not intended to limit the scope of the present invention, and all equivalent implementations or alterations within the spirit of the present invention still fall within the scope of the present invention.
