Patent Publication Number: US-10768502-B2

Title: Synaptic electronic devices with electrochromic device

Description:
PRIORITY 
     This application is a division of and claims priority from U.S. patent application Ser. No. 14/744,764, filed on Jun. 19, 2015, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     The present invention generally relates to synaptic electronic devices, and more specifically, to electronic devices with electrochromic stacks. 
     Physical scaling limits and the relatively low efficiency of the current computing architecture (Von Neumann architecture) limits current semiconductor technology. Accordingly, alternative structures and architectures, for example, neuromorphic systems, have become attractive areas of research. 
     Neuromorphic systems are brain-inspired systems that model neurons and synapses. Neuromorphic engineering/computing uses very-large-scale integration (VLSI) systems containing electronic analog circuits to mimic neuro-biological architectures present in the nervous system. One aspect of neuromorphic engineering is to understand how neuron morphology and overall architectures creates desirable computations. 
     Neuromorphic computing systems utilize various technologies. Silicon complementary metal oxide semiconductor (CMOS) technology is one example. Nanoscale phase-change memory (PCM) or resistive random access memory (RRAM) technologies also are used to implement biological synapses. 
     SUMMARY 
     In one embodiment of the present invention, a synaptic electronic device includes a substrate including a one or more of a semiconductor and an insulator; a photosensitive layer disposed on a surface of the substrate; an electrochromic stack disposed on the photosensitive layer, the electrochromic stack including a first transparent electrode layer, a cathodic electrochromic layer, a solid electrolyte layer, an anodic electrochromic layer, and a second transparent electrode layer; and a pair of electrodes disposed on the photosensitive layer and on opposing sides of the electrochromic stack. 
     In another embodiment, a synaptic electronic device includes a substrate including one or more of a semiconductor and an insulator; an electrochromic stack disposed on a surface of the substrate, the electrochromic stack includes a first transparent electrode layer, a cathodic electrochromic layer, a solid electrolyte layer, an anodic electrochromic layer, and a second transparent electrode layer; a photosensitive layer disposed on the electrochromic stack; and a pair of electrodes disposed on and at opposing end of the photosensitive layer. 
     Yet, in another embodiment, a synaptic electronic device includes a substrate including one or more of a semiconductor and an insulator; an electrochromic stack disposed on a surface of the substrate, the electrochromic stack includes a first transparent electrode layer, a cathodic electrochromic layer, a solid electrolyte layer, an anodic electrochromic layer, and a second transparent electrode layer; a photosensitive layer disposed on the electrochromic stack; and a pair of electrodes disposed on and at opposing end of the photosensitive layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The forgoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIGS. 1A-1D  illustrate a first embodiment of a sensor first electronic device, in which: 
         FIG. 1A  illustrates a cross-sectional side view of a photosensitive layer over a substrate; 
         FIG. 1B  illustrates a cross-sectional side view of a pair of electrodes formed over the photosensitive layer; 
         FIG. 1C  illustrates a cross-sectional side view of an electrochromic stack formed between the electrodes; 
         FIG. 1D  illustrates light activation of the device in  FIG. 1C ; 
         FIGS. 2A-2F  illustrate a second embodiment of a sensor last electronic device, in which: 
         FIG. 2A  illustrates a cross-sectional side view of a substrate; 
         FIG. 2B  illustrates a cross-sectional side view of an electrochromic stack formed over the substrate; 
         FIG. 2C  illustrates a cross-sectional side view of a photosensitive layer formed over the electrochromic stack; 
         FIG. 2D  illustrates a cross-sectional side view of a pair of electrodes formed over the electrochromic stack; 
         FIG. 2E  illustrates a cross-sectional side view of an optional passivation layer formed on the photosensitive layer; 
         FIG. 2F  illustrates a cross-sectional side view of light activation of the device in  FIG. 2E . 
         FIG. 3A  illustrates a schematic diagram for operating an electronic device with an electrochromic stack and a photoresistor as the photosensitive layer; and 
         FIG. 3B  illustrates a schematic diagram for operating an electronic device with an electrochromic stack and a photodiode as the photosensitive layer. 
     
    
    
     DETAILED DESCRIPTION 
     Multi-level storage capability of PCM and RRAM devices is a challenge. Such devices may also suffer from high power consumption. Therefore, it is desirable to develop new electronic synaptic devices that have wide operation windows for multilevel operation, as well as low power requirements. 
     Accordingly, the present invention provides electronic devices with electrochromic stacks and photosensitive layers (e.g., photoresistors and photodiodes) that consume relatively low power and have wide operation windows for multilevel operation. The electronic devices are now described in detail with accompanying figures. It is noted that like reference numerals refer to like elements across different embodiments. 
     The following definitions and abbreviations are to be used for the interpretation of the claims and the specification. As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having,” “contains” or “containing,” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a composition, a mixture, process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but can include other elements not expressly listed or inherent to such composition, mixture, process, method, article, or apparatus. 
     As used herein, the articles “a” and “an” preceding an element or component are intended to be nonrestrictive regarding the number of instances (i.e. occurrences) of the element or component. Therefore, “a” or “an” should be read to include one or at least one, and the singular word form of the element or component also includes the plural unless the number is obviously meant to be singular. 
     As used herein, the terms “invention” or “present invention” are non-limiting terms and not intended to refer to any single aspect of the particular invention but encompass all possible aspects as described in the specification and the claims. 
     As used herein, the term “about” modifying the quantity of an ingredient, component, or reactant of the invention employed refers to variation in the numerical quantity that can occur, for example, through typical measuring and liquid handling procedures used for making concentrates or solutions. Furthermore, variation can occur from inadvertent error in measuring procedures, differences in the manufacture, source, or purity of the ingredients employed to make the compositions or carry out the methods, and the like. In one aspect, the term “about” means within 10% of the reported numerical value. In another aspect, the term “about” means within 5% of the reported numerical value. Yet, in another aspect, the term “about” means within 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1% of the reported numerical value. 
     As used herein, the term “electrochromic” refers to the property of changing light transmission properties in response to voltage. 
     Turning now to the Figures,  FIGS. 1A-1D  illustrate a first embodiment of a sensor first synaptic electronic device.  FIG. 1A  illustrates a cross-sectional side view of a photosensitive layer  130  over a substrate  110 . The substrate  110  has a surface  112 . The substrate includes a semiconducting material or an insulating material. Non-limiting examples of materials for the substrate  110  include silicon, glass, GaAs, GaN, or any combination thereof. The substrate has a surface  110 , and the photosensitive material  130  is formed over the surface of the substrate  110 . When the substrate  110  includes silicon, the substrate  110  may include a silicon oxide layer  120  (native oxide layer) between the photosensitive layer  130  and the substrate  110 . 
     The thickness of the substrate  110  is not intended to be limited. In one aspect, the thickness of the substrate  110  is in a range from about 10 micrometers (m) to about 10 millimeters (mm). In another aspect, the thickness of the substrate  110  is in a range from about 100 μm to about 500 μm. 
     The photosensitive layer  130  is formed over the substrate  110  by chemical vapor deposition, physical vapor deposition, or epitaxial growth. The photosensitive layer  130  includes a photoresistor or a photodiode. Non-limiting examples of suitable materials for the photosensitive layer  130  include silicon, germanium, cadmium sulfide, cadmium selenide, lead sulfide, indium antimonide, indium gallium arsenide, mercury cadmium telluride, or any combination thereof. 
     The thickness of the photosensitive layer  130  is not intended to be limited. In one aspect, the thickness of the photosensitive layer  130  is in a range from about 10 nm to about 100 μm. In another aspect, the thickness of the photosensitive layer  130  is in a range from about 100 nm to about 10 μm. Yet, in another aspect, the thickness of the photosensitive layer  130  is about or in any range from about 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1 μm, 3 μm, 5 μm, 8 μm, and 10 μm. 
       FIG. 1B  illustrates a cross-sectional side view of a pair of electrodes  140  (source and drain) formed over the photosensitive layer  130 . The pair of electrodes  140  may include, for example, molybdenum, titanium, indium tin oxide (ITO), indium zinc oxide (IZO), aluminum, gold, silver, palladium, silicon, boron, phosphorus, arsenic, gallium, or any combination thereof. The pair of electrodes  140  can be formed by lithography and etching techniques known in the art. The pair of electrodes  140  may have any suitable dimensions, depending on the size of the device. 
       FIG. 1C  illustrates a cross-sectional side view of an electrochromic stack  102  formed between the pair of electrodes  140 . The pair of electrodes  140  are on opposing sides of the the electrochromic stack  102 . The electrochromic stack  102  includes a first transparent electrode layer  150 , a cathodic electrochromic layer  160 , a solid electrolyte layer  170 , an anodic electrochromic layer  180 , and a second transparent electrode layer  190 . The first transparent electrode  150  is formed on the photosensitive layer  130 , the cathodic electrochromic layer  160  is formed on the first transparent electrode layer  150 , the solid electrolyte layer  170  is formed on the cathodic electrochromic layer  160 , the anodic electrochromic layer  180  is formed on the solid electrolyte layer, and the second transparent electrode layer  190  is formed on the anodic electrochromic layer  180 . 
     Non-limiting examples of suitable materials for the first and second transparent electrode layers  150  and  190  include indium tin oxide, graphene, or any combination thereof. Non-limiting examples of suitable materials for the cathodic electrochromic layer  160  include tungsten oxide (WO 3 ), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), polycyclic aromatic hydrocarbon (PAH), graphene, or any combination thereof. The solid electrolyte layer  170  can include any proton conducting solid electrolyte material. A non-limiting example of a suitable material for the solid electrolyte layer  170  is zirconium oxide (ZrO 2 ). The anodic electrochromic layer  180  can be an ion storage material. A non-limiting example of suitable material for the anodic electrochromic layer  180  is nickel oxide (NiO). 
     Solid electrolyte layer  170  and anodic electrochromic layer  180  are optional and enhance the electrochromic effect. In one embodiment, an electrochromic includes a first transparent electrode layer  150 , a cathodic electrochromic layer  160 , and a second transparent electrode layer  190 . 
     Although  FIGS. 1C and 1D  illustrate the pair of electrodes  140  as being formed before the electrochromic stack  102 , the electrochromic stack  102  may be formed over the photosensitive layer  130  before the pair of electrodes  140  in some embodiments. 
     The layers in the electrochromic stack  102  may be formed, for example, by physical vapor deposition (PVD) or other methods known in the art. Any of the layers in the electrochromic stack  102  (the first and second transparent electrode layers  150  and  190 , the cathodic electrochromic layer  160 , the solid electrolyte layer  170 , or the anodic electrochromic layer  180 ) can have a thickness in a range from about 10 nm to about 500 μm. In another aspect, the thickness of the individual layers in the electrochromic stack  102  is in a range from about 100 nm to about 10 μm. Yet, in another aspect, the thickness of the layers in the electrochromic stack  102  is about or in any range from about 10 nm, 100 nm, 500 nm, 1 μm, 50 μm, 100 μm, 200 μm, 300 μm, 400 μm, and 500 μm. 
       FIG. 1D  illustrates light  101  activation of the device in  FIG. 1C  (see also  FIGS. 3A and 3B ). The electrochromic stack  102  serves as a visible light filter. When a voltage is applied, and under exposure to visible light  101 , the cathodic electrochromic layer  160  and the anodic electrochromic layer  180  in the electrochromic stack  102  switch between bleached (colorless) states and colored states. For example, when the cathodic electrochromic layer  160  is WO 3 , and the anodic electrochromic layer  180  is NiO, the following redox reactions occur in the cathodic electrochromic layer  160  and the anodic electrochromic layer  180 : 
     
       
         
         
             
             
         
       
     
     Both the cathodic electrochromic layer  160  and the anodic electrochromic layer  180  are colored and bleached simultaneously, which increases optical modulation. Under a voltage bias, optical properties of the device are altered. In particular, the transmittance changes between “1” and “0” by electrically controlled electrochemical redox reactions in the electrochromic stack  102 . 
     The electrochromic stack  102  serves as a visible light  101  filter. The applied voltage determines how much light  101  passes through the electrochromic stack  102 . When the photosensitive layer  130  is a photoresistor, the photosensitive layer  103  changes resistance based on how much light passes through the electrochromic stack  102 . 
       FIGS. 2A-2F  illustrate a second embodiment of a sensor last electronic device. In contrast to the sensor first structure of  FIGS. 1A-1D  where the photosensitive layer  130  is disposed over the substrate  110  before the electrochromic stack  102 , the photosensitive layer  130  is disposed on the electrochromic stack  102 , and light  101  passes through the substrate  110 .  FIG. 2A  illustrates a cross-sectional side view of a substrate  110 . The substrate  210  has a surface  112 . The substrate  110  includes a transparent material, for example, quartz. 
       FIG. 2B  illustrates a cross-sectional side view of an electrochromic stack  102  formed on the first surface  112  of the substrate  110 . The electrochromic stack  102  includes a first transparent electrode layer  150 , an anodic electrochromic layer  180 , a solid electrolyte layer  170 , a cathodic electrochromic layer  160 , and a second transparent electrode layer  190 . The first transparent electrode layer  150  is formed on the substrate  110 , the anodic electrochromic layer  180  is formed on the first electrode layer  180 , the solid electrolyte layer  170  is formed on the anodic electrochromic layer  180 , the cathodic electrochromic layer  160  is formed on the solid electrolyte layer  170 , and the second transparent electrode is formed on the cathodic electrochromic layer  160 . 
       FIG. 2C  illustrates a cross-sectional side view of a photosensitive layer  130  formed over the electrochromic stack  102 . The photosensitive layer  130  includes a photoresistor or a photodiode. 
       FIG. 2D  illustrates a cross-sectional side view of a pair of electrodes  140  (source and drain) formed over the electrochromic stack  102 . 
       FIG. 2E  illustrates a cross-sectional side view of an optional passivation layer  220  formed on the photosensitive layer  130 . The passivation layer  220  can be any protective material, which depends on the composition of the photosensitive layer  130 . Non-limiting examples of suitable materials for the passivation layer include oxides, e.g., silicon dioxide, titanium dioxide, and nitrides, e.g., silicon nitride, or any combination thereof. Other suitable methods for forming a passivation layer  130  include alclading, chromate conversion coating, anodizing, rouging, or any combination thereof. The passivation layer  220  has a thickness in a range from about 5 nm to about 100 μm. In another aspect, the thickness of the passivation layer  220  is in a range from about 20 nm to about 1 μm. Yet, in another aspect, the thickness of the passivation layer  220  is about or in any range from about 5 nm, 50 nm, 100 nm, 500 nm, 1 μm, 25 μm, 50 μm, and 100 μm. 
       FIG. 2F  illustrates a cross-sectional side view of light  101  activation of the device in the sensor last device  FIG. 2E . The electrochromic stack  102  serves as a visible light filter. When a voltage is applied, and the under exposure to visible light  101 , the cathodic electrochromic layer  160  and the anodic electrochromic layer  180  in the electrochromic stack  102  switch between bleached (colorless) states and colored states, as described above for  FIG. 1D . 
       FIG. 3A  illustrates a schematic diagram for operating a synaptic electronic device  301 . The sensor first structure described in  FIGS. 1A-1D  or the sensor last structure described in  FIGS. 2A-2F  can be used as the synaptic electronic device  301 . The synaptic electronic device  301  includes an electrochromic stack  310  between a pair of electrodes  140  and a photoresistor  340  as the photosensitive layer. A combination of a first voltage pulse  330  from a pre-synaptic neuron  320  and a second voltage pulse  332  from a post-synaptic neuron  322  determines the amount of visible light  101  transmitted through the device  301  (transmitted light  102 ). The difference between the first voltage pulse  330  and the second voltage pulse  332  provides a larger bias, which results in a larger difference in transmittance between the bleached and colored states in the electrochromic stack  102 . The photoresistor  340  changes its resistance based on the amount of transmitted light  102 . A third voltage pulse  332  is used to read the resistance of photoresistor  340 . 
     Applying different biases can be used to control the amount of light  101  passing through the device  101 . After removing the bias, the resistance remains substantially the same. Therefore, a continuous bias is not needed in the synaptic electronic device  301 . 
       FIG. 3B  illustrates a schematic diagram for operating a synaptic electronic device  302  with an electrochromic stack  310  between a pair of electrodes  140  and a photodiode  342  as the photosensitive layer. The sensor first structure described in  FIGS. 1A-1D  or the sensor last structure described in  FIGS. 2A-2F  can be used as the synaptic electronic device  302 . The synaptic electronic device  302  includes an electrochromic stack  310  between a pair of electrodes  140  and a photodiode  342  as the photosensitive layer. A combination of a first voltage pulse  330  from a pre-synaptic neuron  320  and a second voltage pulse  332  from a post-synaptic neuron  322  determines the amount of visible light  101  transmitted through the device  302  (transmitted light  102 ). The difference between the first voltage pulse  330  and the second voltage pulse  332  provides a larger bias, which results in a larger difference in transmittance between the bleached and colored states in the electrochromic stack  102 . When the photodiode  342  absorbs the transmitted light  102 , it produces a current. A third voltage pulse  332  is used to turn on the selecting transistor in order to read the current signal from the photodiode  342 . 
     Applying different biases can be used to control the amount of light  101  passing through the device  101 . After removing the bias, the resistance remains substantially the same. Therefore, a continuous bias is not needed in the synaptic electronic device  302 . The pair of electrodes  140  are analogous to the axons and dendrites of synapses in a neurosystem. 
     The above described devices are analogous to synaptic junctions. The connection strength between two neurons is determined by the weight of the synapse connecting them. The higher weight means the stronger connection. In the present invention, the weight is recorded into the transmittance of electrochromic stack  310 , which determines the resistance of photoresistor  340  or current generated by photodiode  342 . 
     The above disclosed devices and methods provide various advantages. The devices and methods enable decoupled training and reading elements, which substantially eliminates interference. The devices do not need a selector, and an ultralow programming current can be used (e.g., sub-pico-ampere (pA), compared to micro-amperes (μA) in PCM and RRAM devices). 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, element components, and/or groups thereof. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. 
     The flow diagrams depicted herein are just one example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention. 
     The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.