Patent Publication Number: US-2023165172-A1

Title: Lead-free metallic halide memristor and electronic element comprising the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the technology field of memristor elements, and more particularly to a lead-free metallic halide memristor. 
     2. Description of the Prior Art 
     With reference to  FIG.  1   , there is shown a block diagram for describing a traditional Von Neumann architecture. Von Neumann architecture  1   a  is also called Princeton architecture, and comprises a control unit  11   a , an arithmetic logic unit (ALU)  12   a , a memory  13   a , an input unit  14   a , and an output unit  15   a . Nowadays, a computer having the Von Neumann architecture  1   a  is regarded as a stored program computer, whereof at least one application program and/or the data are stored in the memory  13   a , and the control unit  11   a  control the ALU  12   a  to access the memory  13   a  to conduct the application program, read out the stored data, or write new data into the memory  13   a.    
     Engineers skilled in field of computer science certainly know that there is a system bottleneck existing in the computer having the Von Neumann architecture  1   a.  In case of the occurrence of the system bottleneck, the computing throughput of the computer is limited due to inadequate rate of data transfer between the memory  13   a  and the ALU  12   a . It is worth explaining that, the term “Von Neumann bottleneck” was coined by John Backus in 1978 for being representative of the forgoing system bottleneck. 
     Therefore, because of failing to conduct AI computing and store huge data simultaneously, the Von Neumann architecture  1   a  is found not applicable to being the principal framework of a specific computer that is particularly used in conducting image recognition and/or voice recognition. Accordingly, a fast and low power consumption computing framework using artificial neural network (ANN) is developed, and is further adopted for replacing the ALU  12   a  and the memory  13   a  as shown in  FIG.  1   . For example,  FIG.  2    shows a block diagram for describing a computer having neuromorphic computing architecture  1   b,  comprising: a control unit  11   b,  an ANN unit  12   b,  an input unit  14   b , and an output unit  15   b . During a normal operation of the neuromorphic computing architecture  1   b,  the ANN unit  12   b  succeeds in conducting AI computing and store huge data simultaneously according to the controlling command transmitted from the control unit  11   b.  As a result, compared to the computer having Von Neumann architecture  1   a  (as shown in  FIG.  1   ), the computer having neuromorphic computing architecture  1   b  exhibits advantages of high performance, low computing time, and low power consumption. 
     Synapses are specialized to transduce signals from one neuron to another, either via chemical neurotransmitters or, less commonly, by electrical coupling. A typical neuron in the mammalian brain may receive and extend 10,000 or more synapses, connecting it with numerous neurons close by or far away. The adult human brain is estimated to contain more than 10 11  neurons and 10 14  (100 trillion) synapses, with a density of approximately one billion synapses per cubic millimeter of cerebral cortex. Recently, a special type of memristor was considered to be able to mimic the behavior of neural synapses. In particular, attributed to the short-term and long-term memory of weight changes, the memristor is found to possess the synaptic plasticity. Furthermore, a variety of memristors including advantages of small size, high switching speed and low power consumption have been developed, thereby being used as artificial synapses for constituting the ANN unit  12   b  as shown in  FIG.  2   . 
     It is worth mentioning that, literature 1 has disclosed a computing system having aforesaid neuromorphic computing architecture. The disclosed computing system is called reservoir computing system, and is able to conduct AI computing and store huge data simultaneously. Herein, literature 1 is written by Du et.al, and is entitled with “reservoir computing using dynamic memristors for temporal information processing” so as to be published on Nat Commun 8, 2204 (2017). According to the disclosures of literature 1, the reservoir computing system has a dynamic reservoir comprising a plurality of memristors, and exhibits short-term memory to project features from the temporal inputs into a high-dimensional feature space. A readout function layer can then effectively analyze the projected features for tasks, such as classification and time-series analysis. The reservoir computing system can efficiently compute complex and temporal data with low-training cost, since only the readout function needs to be trained. Experimental data have proved that, even though the dynamic reservoir only consists of 88 memristors, the internal ionic dynamic processes of memristors allow the dynamic reservoir to directly process information in the temporal domain. Consequently, the reservoir computing system can already be used for tasks, such as handwritten digit recognition. 
     Engineers skilled in design and manufacture of non-volatile memories (NVM) should know that a memristor principally comprises a bottom electrode, an active layer and a top electrode. For example, U.S. patent publication No. 2018/0351095 Al has disclosed a memristor, which consists of a Pt-made bottom electrode, an active layer comprising an amorphous SrTiO film and an amorphous SrTiO 3-x  film, and a Pt-made top electrode. According to the disclosures of U.S. patent publication No. 2018/0351095 A1, as long as an operation voltage applied to the memristor exceeds 1 V, the memristor is allowed to be switched from a low resistance state (LRS) to a high resistance state (HRS). In other words, the aforesaid conventional memristor shows a practical drawback of high switching voltage. 
     On the other hand, U.S. patent No. 10,186,660 B2 has disclosed another one type of memristor which consisting of a Pt-made bottom electrode, an active layer made of HfO 2 , and a Ta-made top electrode. According to FIG. 3C of U.S. Pat. No. 10,186,660 B2, it is found that, after the memristor receives a treatment of continuously multi-level resistance modulation, there is merely one order of resistance ratio between the HRS and the LRS of the memristor In other words, the aforesaid conventional memristor shows a practical drawback of narrow dynamic range. 
     According to above descriptions, it is understood that there are rooms for improvement in the conventional memristor-based artificial synapse and the conventional memristor-based non-volatile memory. In view of that, the inventor of the present application have made great efforts to make inventive research and eventually provided a lead-free metallic halide memristor. 
     SUMMARY OF THE INVENTION 
     The primary objective of the present invention is to disclose a lead-free metallic halide memristor. 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 (STP), short-term depression (STD), long-term potentiation (LTP), long-term depression (LTD) 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. 
     For achieving the primary objective mentioned above, the present invention provides an embodiment of the 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 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. 
     In one embodiment, 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. 
     In one embodiment, the first electrode layer is made of a material, and the material is a compound of silver and titanium carbide. 
     In one embodiment, the first electrode layer is formed on a substrate. 
     In one practicable embodiment, there is an interfacial modification layer formed between the first electrode layer and the active layer, and the interfacial modification layer is made of a material selected from a group consisting of oxide semiconductor and organic semiconductor. 
     In another one practicable embodiment, there is an interfacial modification layer formed between the active layer and the second electrode layer, and the interfacial modification layer is made of a material selected from a group consisting of oxide semiconductor and organic semiconductor. 
     In other practicable embodiments, a first interfacial modification layer is formed between the first electrode layer and the active layer, a second interfacial modification layer is formed between the active layer and the second electrode layer, and the first interfacial modification layer and the second interfacial modification layer are both made of a material selected from a group consisting of oxide semiconductor and organic semiconductor. 
     Moreover, the present invention also discloses an electronic element, which is selected from a group consisting of artificial synapse, two-level resistive non-volatile memory and multi-level resistive non-volatile memory, and is characterized in comprising the aforesaid first electrode layer, active layer and second electrode layer. 
     Furthermore, the present invention also discloses an electronic chip, which is selected from a group consisting of neuromorphic computing chip and reservoir computing chip, and is characterized in comprising multiple aforesaid lead-free metallic halide memristors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG.  1    shows a block diagram for describing a traditional Von Neumann architecture; 
         FIG.  2    shows a block diagram for describing a computer having neuromorphic computing architecture; 
         FIG.  3    shows a schematic elevation view of a lead-free metallic halide memristor according to the present invention; 
         FIG.  4 A  shows a first cross-sectional side of the lead-free metallic halide memristor according to the present invention; 
         FIG.  4 B  shows a second cross-sectional side of the lead-free metallic halide memristor according to the present invention; 
         FIG.  4 C  shows a third cross-sectional side of the lead-free metallic halide memristor according to the present invention; 
         FIG.  5    shows a curve graph for showing I-V characteristics of a sample 1 of the lead-free metallic halide memristor; 
         FIG.  6    shows a curve graph for showing I-V characteristics of a sample 2 of the lead-free metallic halide memristor; 
         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; 
         FIG.  8    shows a scatter plot of conductance versus number of pulses measured from the sample 6 of the lead-free metallic halide memristor; 
         FIG.  9    shows a curve graph of current versus time measured from the sample 6 that is applied with numbers of successive identical pulses; 
         FIG.  10    shows a scatter plot of conductance versus number of pulses measured from the sample 6; 
         FIG.  11    shows a scatter plot of normalized conductance versus normalized number of pulses measured from the sample 6; 
         FIG.  12    shows a curve graph for showing I-V characteristics of a sample 4 of the lead-free metallic halide memristor; 
         FIG.  13    shows a curve graph for showing I-V characteristics of a sample 5 of the lead-free metallic halide memristor; 
         FIG.  14    shows a curve graph for showing I-V characteristics of the sample 6 of the lead-free metallic halide memristor; 
         FIG.  15    shows a curve graph of current versus reading time measured from the sample 6; 
         FIG.  16    shows a curve graph of conductance versus compliance current measured from the sample 6; 
         FIG.  17    shows a curve graph for showing I-V characteristics of a sample 7 of the lead-free metallic halide memristor; and 
         FIG.  18    shows a scatter plot of conductance versus number of pulses measured from the sample 6. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. 
     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. 
     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 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. 
     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). 
       FIG.  4 A ,  FIG.  4 B ,  FIG.  4 C  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 n -made active layer  12  and/or between the MX 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.  4 A ) between the first electrode layer  11  and the active layer  12 . Moreover, as  FIG.  4 B  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.  4 C  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. 
     Samples of the Lead-Free Metallic Halide Memristor 
     For proving that the lead-free metallic halide memristor  1  having the structure as shown in  FIG.  3   ,  FIG.  4 A ,  FIG.  4 B , or  FIG.  4 C  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 (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 
                 MoO x   
                 Ag 
               
               
                   
                 (180 nm) 
                 (20 nm) 
                 (100 nm) 
                 (20 nm) 
                 (140 nm) 
               
               
                 4 
                 ITO 
                 — 
                 CsC1 
                 — 
                 Ag 
               
               
                   
                   
                   
                 (100 nm) 
                   
                 (130 nm) 
               
               
                 5 
                 Ag 
                 — 
                 CsBr 
                 MoO x   
                 Ag 
               
               
                   
                 (100 nm) 
                   
                 (100 nm) 
                 (20 nm) 
                 (130 nm) 
               
               
                 6 
                 Ag 
                 MoO x   
                 CsBr 
                 — 
                 Ag 
               
               
                   
                 (100 nm) 
                 (20 nm) 
                 (100 nm) 
                   
                 (130 nm) 
               
               
                 7 
                 Ag 
                 MoO x   
                 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 
                 MoO x   
                 Ag 
               
               
                   
                 (180 nm) 
                   
                 (100 nm) 
                 (20 nm) 
                 (130 nm) 
               
               
                   
               
            
           
         
       
     
     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. 
       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. 
     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. 
       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). 
     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. 
     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. 
     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. 
     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. 
     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 −7  A to ˜10 −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. 
       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. 
       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. 
     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. 
     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 (2) 
               
               
                   
               
               
                 Sample 
                 Write 
                 Erase 
                 Dynamic 
                 Multi-level 
                 Active 
               
               
                 No. 
                 Voltage 
                 Voltage 
                 Range 
                 switching 
                 layer 
               
               
                   
               
             
            
               
                 1 
                 −0.15 V  
                 0.07 V 
                 &gt;10 6   
                 multiple 
                 CsI 
               
               
                   
                   
                   
                   
                 states 
                   
               
               
                 2 
                 −0.1 V 
                 0.05 V 
                 &gt;10 6   
                 multiple 
                 CsBr 
               
               
                   
                   
                   
                   
                 states 
                   
               
            
           
           
               
               
               
               
               
            
               
                 3 
                 |V| &gt; 0.2 V 
                 &gt;10 6   
                 &gt;3 states 
                 NaCl 
               
            
           
           
               
               
               
               
               
               
            
               
                 4 
                 −0.1 V 
                  0.1 V 
                 ~10     
                   2 states 
                 CsCl 
               
               
                 5 
                 −0.1 V 
                 0.05 V 
                 &gt;10 6   
                 multiple 
                 CsBr 
               
               
                   
                   
                   
                   
                 states 
                   
               
               
                 6 
                 −0.1 V 
                 0.05 V 
                 &gt;10 6   
                 multiple 
                 CsBr 
               
               
                   
                   
                   
                   
                 states 
                   
               
               
                 7 
                 −0.15 V  
                 0.07 V 
                 &gt;10 6   
                 multiple 
                 CsI 
               
               
                   
                   
                   
                   
                 states 
                   
               
            
           
           
               
               
               
               
               
            
               
                 8 
                 |V| &gt; 0.2 V 
                 &gt;10 6   
                 &gt;3 states 
                 NaCl 
               
            
           
           
               
               
               
               
               
               
            
               
                 9 
                 −0.1 V 
                 0.05 V 
                 &gt;10 5   
                 &gt;6 states 
                 MgF2 
               
               
                   
               
            
           
         
       
     
     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. 
     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.