Abstract:
An ionic device includes a layer ( 220 ) of an ionic conductor containing first and second species ( 222, 224 ) of impurities. The first species ( 222 ) of impurity in the layer ( 220 ) is mobile in the ionic conductor, and a concentration profile of the first species ( 222 ) determines a functional characteristic of the device ( 200 ). The second species ( 224 ) of impurity in the layer ( 220 ) interacts with the first species ( 222 ) within the layer ( 220 ) to create a structure ( 226 ) that limits mobility of the first species ( 222 ) in the layer ( 220 ).

Description:
BACKGROUND 
       [0001]    Recently developed ionic devices rely on the movement of ions in ionic conductors to change electrical or other properties of the ionic devices. For example,  FIG. 1A  shows an ionic device  100 , which includes a layer or film  120  of an ionic conductor that is sandwiched between two electrodes  110  and  130 . For example, ionic conductor  120  may be a layer of titanium dioxide (TiO 2 ), while ions  126  are oxygen vacancies, i.e., gaps in the crystal structure where oxygen ions are missing. With titanium dioxide and oxygen vacancies, ionic device  100  can behave as a memristor because a voltage difference applied between electrodes  110  and  130  can cause ion currents that move oxygen vacancies and significantly alter the electrical resistance of ionic conductor  120 . For a display device, ionic conductor  120  can be a layer of tungsten trioxide (WO 3 ), while ions  126  are lithium ions which are sufficiently mobile in tungsten trioxide to move in response to an applied voltage. Pure tungsten trioxide is clear, but lithium impurities give tungsten trioxide a blue color. Accordingly, ion currents that move lithium ions to or away from a display surface can change the color of ionic device  100 . 
         [0002]      FIG. 1A  shows a configuration of device  100  where ions  126  are concentrated near one electrode  110 . Layer  120  may initially be formed in this configuration by forming two layers  122  and  124  with distinct compositions, e.g., where one layer  122  is of a primary material such as titanium dioxide TiO 2  and the other layer  124  is of a source material such as oxygen-depleted titanium dioxide TiO 2-x . Application of a voltage having the proper polarity and sufficient magnitude between electrodes  110  and  130  can then drive an ion current that moves ions  126  from layer  122  into layer  124  to switch from the state of device  100  shown in  FIG. 1A  where ions concentrated near electrode  110  to the state of device  100  in  FIG. 1B  where more ions  126  are dispersed throughout ionic conductor  120  or even to the state of  FIG. 1C  where the ions are highly concentrated near electrode  120 . Device  100  can similarly switch back from the state of  FIG. 1B  or  1 C to the state of  FIG. 1A  by application of an opposite polarity voltage of sufficient magnitude to drive an ion current that moves ions  126  toward electrode  110 . This operation is possible because ionic conductor  120  provides sufficient mobility for ions  126  that are capable of significantly altering the properties of ionic conductor  120  and device  100  as a whole. 
         [0003]    Non-volatile operation of ionic devices such as device  100  is often desired. For example, for use as a non-volatile memristive memory, device  100  might have a high voltage applied with a polarity selected to switch device  100  to the high resistance state corresponding to  FIG. 1A  or  1 B or a low resistance state corresponding to  FIG. 1B  in order to write a binary value 0 or 1 to device  100 . A lower voltage that causes an electron current but minimal ion movement can then be used to detect or measure the resistance of device  100  and read the binary value previously written. However, the mobility of ions in ionic conductor  120  permits some movement of ions when read voltage is applied for a read operation and even when no external voltage is applied. Typically, an ionic device has only one stable ionic concentration profile (e.g., uniform distributed ions of  FIG. 1B ) corresponding to the thermodynamic equilibrium and an ionic device tends to relax, e.g. by diffusion, toward the stable concentration profile. The rate at which an ionic device will relax can be significant. For example, drift-diffusion, which controls the relaxation time, may be just V times slower than the ion current during switching, where V is the applied switching voltage in units of thermal voltage V T =k B T/e where k B  is the Boltzmann constant, e is the electron charge and T is the temperature). For typical voltages used for the thin film ionic devices, the ratio of relaxation time to switching time may only be a few thousands, so that fast switching devices may have poor non-volatile retention. In many applications, both fast switching and long retention times are desired. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIGS. 1A ,  1 B, and  1 C are cross-sectional views of a known ionic device in different conductivity states. 
           [0005]      FIGS. 2A ,  2 B, and  2 C are cross-sectional views of an ionic device in accordance with an embodiment of the invention employing interacting species of ions. 
           [0006]      FIGS. 3A ,  3 B, and  3 C are cross-sectional views of an ionic device in accordance with an embodiment of the invention employing interactions of mobile ions with a species that is either uncharged or immobile. 
       
    
    
       [0007]    Use of the same reference symbols in different figures indicates similar or identical items. 
       DETAILED DESCRIPTION 
       [0008]    In accordance with an aspect of the invention, an ionic device can employ two or more interacting species of impurities including at least one ionic species that migrates in response to an applied voltage to change a state and operating characteristic of the ionic device. Further, the interaction of the species in an ionic conductor creates an immobile or less mobile structure (e.g., a molecule) that effectively changes the mobility of the active ions and improves the non-volatile characteristics of the ionic device. For example, two species of ions that are mobile within the ionic conductor can form a dumbbell (double) stable defect or a molecule that is relatively immobile in the ionic conductor. As a result, the state of the ionic device can be highly stable. However, when a high enough voltage is applied, the bonds between the interacting species are broken, freeing mobile ions and facilitating fast switching. Accordingly, such devices can combine both fast, low-power switching with long retention times. 
         [0009]      FIG. 2A  shows a cross sectional view of an ionic device  200  including parallel electrodes  210  and  230  with a thin intervening layer of an ionic conductor  220  containing two species of reactants  222  and  224 . Species  222  and  224  can be different types of ions, particularly ions of opposite charge, but in some embodiments one species  224  may be an uncharged chemical reactant or have the same charge as ions  222 . Species  222 , however, is, in some embodiments, charged and mobile in layer  220  so that applied electric fields or bias voltages can move ions  222  and change an operational characteristic of device  200 . Species  222  and  224  should further be such that the operational characteristic of device  200  depends on the concentration profile of species  222 , and ions  224  do not interfere with desired operation of device  200 . For example, device  200  could have one electronic or optical characteristic when the concentration of species  222  near an electrode  210  or  230  is high and a very different electronic or optical characteristic when species  222  is more uniformly distributed in ionic conductor  220  or when species  222  is concentrated near the opposite electrode  230  or  210 . Additionally, species  222  and  224  can form bonds creating atomic structures  226 , e.g., a molecule or dumbbell detect, that is immobile in ionic conductor  220  or at least has a much less mobility in ionic conductor  220  than does species  220 . In general, the interaction of species forms structure  226  while releasing a corresponding energy of reaction ΔU and breaking the bonds of structures  226  to free species  222  and  224  requires input of energy ΔU. 
         [0010]    In the embodiment of  FIGS. 2A ,  2 B, and  2 C, both species  222  and  224  are ions, and the charge of ions  222  is opposite in polarity to the charge of ions  224 . Ions  222  are sometimes referred to herein as active ions  222  because ions  222  are required to change the operational characteristic of ionic device  222 . Ions  224  are sometimes referred to herein as binding ions  224  since ions  224  are not required to directly change the functional characteristic of ionic device  200  except by binding to active ions  222  and restricting the mobility of active ions  222 . 
         [0011]    In an exemplary memristive embodiment of device  200 , layer  220  is a substance such as titanium dioxide and active ions  220  are oxygen vacancies in the titanium dioxide. Pure titanium dioxide (TiO 2 ) is an insulator, but introduction of oxygen vacancies, even at relatively low concentrations, causes titanium dioxide to be a semiconductor. Accordingly, as a memristor, ionic device  200  has a low resistance state associated with the distribution of ions  222  extending across the thickness of layer  220  as shown in  FIG. 2A . In contrast, a high resistance state results when ions  222  are concentrated near an electrode as shown in  FIGS. 2B and 2C  because an extended thickness of layer  220  is then an insulator or high resistance semiconductor. The oxygen vacancies can effectively be bound to negative ions (acceptor impurities) such, as carbon or silicon in layer  220 . However, silicon is an immobile impurity in titanium dioxide TiO 2 , so that carbon may be preferred in embodiments where species  224  is mobile. 
         [0012]    In an exemplary display application, device  200  is a cell in a display, and ionic conductor  230  is a material that changes color when ions  220  are introduced. For example, pure tungsten trioxide is transparent but turns blue when lithium ions are introduced. Accordingly, for a display, different colors can be produced by ion device  200  depending on whether ions  222  are more uniformly dispersed in layer  220  as shown in  FIG. 2A , concentrated near the bottom electrode  230  as shown in  FIG. 2B  or  2 C, or concentrated near the top electrode (not shown). For example, one electrode  210  or  230  can be a transparent conductor such as indium tin oxide and the color of layer  220  viewed through the transparent electrodes will depend on the concentration of ions  222  near the display electrode  210  or  230 . Lithium as active ions  224  can be bound by species  222  when species  222  are, negative ions or acceptor atoms such as Niobium (Nb) in a tungsten trioxide layer. 
         [0013]      FIG. 2B  shows a state of ionic device  200  after a voltage V has been applied to device  200  between electrodes  210  and  230 . Voltage V is positive voltage that is sufficiently high to disassociate ion species  222  and  224  that were bound together in relatively immobile structures  226 . Voltage V further attracts mobile negative ions to the positive terminal (specifically electrode  210  in  FIG. 2B ) and attracts mobile positive ions to the negative terminal (or electrode  230  in  FIG. 2B ). In the illustrated embodiment, ions  222 , which are of the species that changes the functional character of device  200 , are positively charged and are concentrated near negative electrode  230  in  FIG. 2B , and ions  224  are negatively charged and are concentrated near positive electrode  210  in  FIG. 2B . More generally, the ion species that activates or changes the operational character (e.g., resistance or color) of device  200  can be either positively or negatively charged and therefore may move in directions opposite to those described for the exemplary embodiment. 
         [0014]    The time t S  required for device  200  to switch from the state of  FIG. 2A  to that of  FIG. 2B  in general depends on the magnitude of voltage V, the mobility of ions  222 , and the thickness of layer  220 . Layer  220  is preferably between a few nanometers to a couple hundred of nanometers thick to provide fast switching at applied voltages on the order of a couple of volts. The mobility of ions  222  depend on the particular material used for layer  220  and the impurity corresponding to ions  222 , but in typical ionic devices, switching times between 100 ns and a few milliseconds can be achieved. 
         [0015]    Voltage V can be turned off when device  200  reaches the state of  FIG. 2B , at which point mobile ions  222  and  224  can drift or diffuse from the positions of  FIG. 2B  toward respective equilibrium concentration profiles. The characteristic relaxation rate of each species  222  and  224  will generally differ depending on the mobility of ions  222  and  224  in layer  220 . Relaxation with no applied voltage will generally be due to diffusion, but can be altered if surface charge collects at the interfaces with electrodes  210  and  23 , for example, as a result of differences in electron bands in layer  220  and electrodes  210  and  230 , chemical gradients in layer  220 , or net charge in the bulk of layer  220 . Device  200  is preferably such that ions  224  are more mobile than ions  222  and/or present in higher concentrations in layer  220  than are ions  222 , so that ions  222  will diffuse at most a small distance before bonding with available ions  224  and being fixed in immobile structures  226 . 
         [0016]      FIG. 2C  shows the state of device  200  resulting after the positive voltage V that brings, about the state of  FIG. 2B  is turned off. In the illustrated configuration, ions  224  are much more mobile than ions  222  and diffuse much more rapidly across layer  220 . As a result, ions  222  are mostly bound in structures  226  near electrode  230 . The state of ionic device  200  in  FIG. 2C  may correspond to a high resistance state or a different color state when compared to the state of device  200  shown in  FIG. 2A . 
         [0017]    The use of interacting species  224  improves the ratio of retention time t R  to switching time t S  when compared to, conventional ionic devices. In particular, the ratio of retention time t R  to switching time t S  for device  200  proportional to the quantity given in Equation 1. In Equation 1, V is the applied voltage during switching in units k B Tle, where k B  is the Boltzmann constant, T is the temperature, and e is the magnitude of the electron charge. Values ΔU and Δ A  are energies in thermal units k B T in Equation 1. In particular, energy ΔU is the reaction energy for formation of molecule  226 . Energy U A  is the hopping activation energy of ions  224  in layer  220  and is less than the hopping activation energy U B  of ions  222  in layer  220  when ions  224  are more mobile than ions  222  in layer  220 . Concentration is the concentration of ions  222  and is much greater than the concentration n B  of active ions  222 . 
         [0000]        t   R   /t   s ∝V exp [Δ U−U   A   ]n   A   Equation 1
 
         [0018]    The ratio t R /t S  can generally be improved by increasing the background concentration n A  of species  224 , increasing the difference between hopping activation energies of species  222  and  224 , and/or increasing the difference between reaction energy ΔU and hopping activation energy U A  of ions  224 . The concentrations n A  and n B  are parameters of device  200  that can be adjusted provided that concentrations n A  or n B  within a range that provides the desired variation in the functional characteristics of device  200 . The mobility, hopping energy, and reaction energy are inherent to the materials used and are relevant to the selection of materials for use in device  200 . Ratio t R /t S  can also be improved by increasing the thickness of layer  220  but with the tradeoff of a slower switching time t S . 
         [0019]      FIGS. 3A ,  3 B, and  3 C shows different states of an ionic device  300  that employs an active ion species  222  and a binding species  324  that is either uncharged or immobile.  FIG. 3A  shows a state of ionic device  300  in which species  222  and  324  are uniformly distributed a layer of ionic conductor  222 . The number and concentration of binding species  324  is greater than the number and concentration of active ions  222 , and as a result of exothermic reactions with binding ions  224 , nearly all of the active ions  222  are bound to respective binding impurities  324  in molecular structures  326 . The state of  FIG. 3A  may correspond to an equilibrium state of device  300  and also correspond to a low resistance state of device  300  when device  300  is memristive. 
         [0020]      FIG. 3B  shows how a voltage V of sufficient magnitude applied between electrodes  210  and  230  can disassociate or free active ions  222  from binding impurities  324  and attract the freed active ions  222  to the opposite polarity electrode  230  in  FIG. 3B . Binding impurities  324  in this embodiment are either uncharged or immobile, and thus retain their fixed distribution if immobile or the equilibrium distribution resulting from diffusion in ionic conductor layer  220  when impurities are mobile but uncharged. 
         [0021]      FIG. 3C  illustrates how when the applied voltage is turned off, active ions  222  interact with binding impurities  326  to form structures  326  that are either immobile in ionic conductor  220  or less mobile in ionic conductor  220  than are free active ions  222 .  FIG. 3C  specifically shows a state of device  300  in which active ions  222  are bound in atomic structures  326  with a high concentration near electrode  230 , but more generally, once the applied voltage driving movement of active ions  222  is turned off, the reaction with binding species  324  distributed throughout layer  220  quickly binds active ions  222 . The binding reaction can increase the retention time of any distribution of active ions  222  that can be achieved through application of bias voltages, while switching times for the states can still be rapid, being nearly the same as switching times associated with the mobility of free active ions  222 . 
         [0022]    Devices  200  and  300  as described above can employ a variety of different material combinations that provide devices  220  and  300  with electrically switched operational characteristics that can be retained for extended times after applied voltages are off. Some examples of materials for ionic conductor layer  220  in device  200  or  330  include any solid state mixed ionic/electronic semiconductor material and/or porous semiconductor material based on organic and inorganic compounds. For example, layer  220  could be titanium dioxide, tungsten trioxide, zirconium dioxide doped with calcium oxide and yttrium oxide, silver sulfide, silver iodide, copper iodide, or rubidium silver iodide to name a few. Combinations of impurity species  222  and  224  for device  200  could be any combination of mobile charged species where binding species  224  has significantly greater mobility in layer  220 . Combinations of impurity species  222  and  324  for device  300  could be any combination of a mobile ionic species  222  and an uncharged or immobile binding species  324 . For example, some relatively fast diffusing impurity species include elements such as H, Li, Ag, Pl, Au, Na, Ti, Cu, Ca, and K, and relatively slower impurity species include elements O (or oxygen vacancies), C, N, Si, and I, which can form ions or not depending on the composition of layer  220 . 
         [0023]    Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. Various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.