Patent Publication Number: US-2022214298-A1

Title: Epitaxial wafer and method of fabricating the same, and electrochemical sensor

Description:
TECHNICAL FIELD 
     The present disclosure relates to the field of electrochemical sensors, and in particular to epitaxial wafer and a method of fabricating the same, and an electrochemical sensor. 
     BACKGROUND ART 
     Electrochemical sensors have wide application areas in medical diagnostics, environmental monitoring, food analysis, industrial process control and biodefense. A sensing electrode or transducer of an electrochemical sensor transforms a chemical signal such as a (bio-)chemical reaction or attachment of ions at the electrode surface into an electrical signal. This electrical signal is then amplified and read out on a corresponding detecting instrument to identify the concentration of an analyte of interest. 
     The electrode surface is often functionalized by a layer on top to turn the sensor selective for a certain (bio-) chemical reaction or a particular ion. In the case of bio-sensors, enzymes (for example glucose oxidase for the oxidation/detection of glucose for medical diagnostics) are often attached to the electrode surface. In the case of ion-sensors, ion-selective membranes are used for selective permeation of an ion of interest to specially detect a chemical reaction of the ion of interest. 
     So far, The reference electrode of an existing electrochemical sensor is usually a liquid-filled Ag/AgCl reference electrode or a simple but unstable solid electrode. There are no satisfactory solutions for stable all-solid-state reference electrodes to realize full all-solid-state amperometric or potentiometric electrochemical sensors. 
     SUMMARY 
     The objects of the present disclosure include providing a high-performance epitaxial wafer used for reference electrode which is capable of maintaining a constant electric potential in a solution, independent of the solution/analyte concentration so as to ensure accurate detection of a voltage between a working electrode and the reference electrode. 
     The objects of the present disclosure further include providing a reference electrode with an InGaN-based semiconductor heterostructure to replace the conventional liquid-filled Ag/AgCl electrode to implement a reference electrode with an all-solid-state structure. 
     In addition, the present disclosure also provides a method of fabricating an epitaxial wafer for a reference electrode of an electrochemical sensor. 
     In one aspect, the present disclosure provides an epitaxial wafer for a reference electrode of an electrochemical sensor, comprising: a substrate; an InGaN layer formed on a surface of the substrate and having an In content between 20% and 60% so as to ensure that a transition from negatively charged surface states to positively charged surface states occurs within a composition range; and an InN layer formed on a surface of the InGaN layer facing away from the substrate to act as a stabilization layer for a surface charge of the InGaN layer. 
     Optionally, the InN layer has an InN deposition amount between 0.5 and 1.5 monolayers to ensure a surface coverage. 
     Optionally, the InGaN layer is a homogeneous layer with uniform In content or a heterostructure layer with varying In content of any design and morphology. 
     Optionally, the InGaN layer is composed of In 0.40 Ga 0.60 N. 
     Optionally, the InGaN layer has a thickness between 100 nm and 500 nm. 
     Optionally, a material of the substrate is one selected from sapphire, Si, SiC, and GaN. 
     In another aspect, the present disclosure further discloses an electrochemical sensor, comprising: a working electrode and a reference electrode made of epitaxial wafer above; a voltage detecting device electrically connected to the working electrode and the reference electrode, respectively, for detecting a voltage value between the working electrode and the reference electrode; and a computing device electrically connected to the voltage detecting device to receive a voltage value detection signal generated by the voltage detecting-device, and configured to calculate and determine the concentration of the substance to be detected based on the voltage value 
     In a further aspect, the present disclosure provides a method of fabricating an epitaxial wafer above, comprising: providing a substrate; epitaxially growing an InGaN layer on a surface of the substrate; and epitaxially growing an InN layer on a surface of the InGaN layer facing away from the substrate. 
     Optionally, a manner of growing the InGaN and InN layers is one selected from: molecular beam epitaxy, metalorganic vapor phase epitaxy, and chemical vapor deposition. 
     Optionally, the InGaN and InN layers are grown by molecular beam epitaxy. 
     The reference electrode made of an InGaN-based semiconductor epitaxial wafer of the present disclosure comprises at least three layers of structures, which are a substrate, an InGaN layer, and an InN layer in sequence. The InGaN layer with an In content between 20% and 60% allows a transition from negatively charged surface states to positively charged surface states so as to generate an electrochemical response independent of the concentration of the solution to be detected. The InN layer with a high density of intrinsic, positively charged surface states acts as a stabilization layer for the surface charge of the InGaN layer to further improve the electrochemical stability of the reference electrode in the present embodiment, and the combination of the InGaN layer and the InN layer further enables the reference electrode of the present embodiment to have a very stable electrochemical response. 
     In addition, each layer of the reference electrode of the present disclosure is of a solid-state structure, the use of a liquid-filled Ag/AgCl or similar reference electrode in the prior art is avoided, and the structure is simpler and more compact and does not have the disadvantages of liquid leakage and periodic replacement of the liquid. 
     In the method of fabricating the epitaxial wafer of the present disclosure, an InGaN layer and an InN layer are formed sequentially on a substrate by molecular beam epitaxy, and the formed InGaN and InN layers have uniform and very thin thickness and have extremely excellent electrochemical stability. Moreover, the fabrication method involves simple operations, and the complex processes for fabrication of the liquid-filled reference electrode in the prior art are avoided. 
     At least one of the above and other objects, advantages and features of the present disclosure will become more apparent to those skilled in the art from the following detailed description of specific embodiments of the present disclosure with reference to accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Some specific embodiments of the present disclosure will be described in detail hereinafter with reference to accompanying drawings in an exemplary but non-limiting way. The same reference numerals refer to the same or similar parts or portions in the accompanying drawings. Those skilled in the art will understand that these accompanying drawings are not necessarily drawn to scale. In the accompanying drawings: 
         FIG. 1  is a schematic diagram showing the entire structure of an potentiometric sensor according to an embodiment of the present disclosure; 
         FIG. 2 a    is a schematic diagram of a substrate of a multilayered structure of a epitaxial wafer according to an embodiment of the present disclosure; 
         FIG. 2 b    is a schematic diagram of growing an InGaN layer on the substrate shown in  FIG. 2   a;    
         FIG. 2 c    is a schematic diagram of a multilayer-structured epitaxial wafer according to an embodiment of the present disclosure; 
         FIG. 3  is a graph of experimental data on a fabricated reference electrode; and 
         FIG. 4  is a flowchart of a method of fabricating a epitaxial wafer according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Firstly, the present embodiment provides an electrochemical sensor which is mainly used for detecting the concentration of a certain substance in a solution to be detected. Among the various operation principles of electrochemical sensors, amperometric and potentiometric readouts are the most common technologies for detection of the concentration of a solution. In amperometric sensing, a current generated by an external voltage between a sensing electrode (i.e., working electrode) and a counter electrode (commonly a Pt wire or mesh) versus the voltage measured between the sensing electrode and a reference electrode, as function of the analyte concentration, is used as a detection signal. A complete amperometric sensor device, therefore, comprises a sensing electrode, a reference electrode, and a counter electrode. In potentiometric sensing, the potential difference/voltage between a sensing electrode and a reference electrode, as function of the analyte concentration, is a detection signal. A complete potentiometric sensor device, therefore, comprises a sensing electrode and a reference electrode. In amperometric sensing, the current, ideally, depends linearly on the concentration of the analyte to be detected. In potentiometric sensing, the voltage, ideally, depends linearly on the logarithm of the concentration of the analyte to be detected. 
     The choice of the preferred sensor solution has been extensively discussed in the literatures. The current research and development mostly focus on finding materials for highly sensitive sensing electrodes (i.e., selection of materials for working electrodes). For example, most recently, graphene, a carbon nanotube, or a semiconductor nanowire is proven to be a good material for fabrication of a working electrode. 
     However, currently, much less attention is paid on the reference electrode. As mentioned, both amperometric sensing and potentiometric sensing require a reference electrode to complete the function of the entire sensor device. The reference electrode is capable of providing a constant, stable potential which is independent of the analyte concentration, and it can be said that the reference electrode is a component essential for precise measurement in an electrochemical sensor. 
     The most widely used reference electrode is the liquid-filled Ag/AgCl electrode. It typically consists of a chlorinated Ag wire immersed in a saturated KCl solution, which is in contact with the analyte solution through a porous, inert material, such as ceramic frit or cellulose. The Ag/AgCl reference electrode establishes a complete redox cell with constant potential. Such conventional liquid-filled reference electrodes commonly have the following problems. The electrodes are bulky and difficult to miniaturize. They are not compatible with planar device fabrication technology, have a limited lifetime and need cumbersome maintenance. Therefore, the conventional liquid-filled reference electrodes need to be replaced with new technologies for realizing compact and robust electrochemical sensors and sensor arrays. 
     There are some solid-state electrodes at present, for example, inert noble metal wires of Au, Ag, Pt or the like are simply used as pseudo-reference electrodes, but such all-solid-state reference electrodes generally do not result in a stable response. 
     Another approach to realize compact all-solid-state reference electrodes is the reduction of the sensitivity of the surface of all-solid-state sensing electrodes, such as SiO2, Al2O3 or HfO2, through passivation of active sites. For passivation of active sites, e.g., silanes, PVC or polypyrroles are used. However, it is difficult to significantly reduce the sensitivity to a predictable target below 10 mV/decade of analyte concentration with desired stability or acceptable drift below 1 mV/hour in a solution with fixed analyte concentration. The overall sensor sensitivity and stability are significantly reduced when such electrodes are used as a reference electrode in a differential setup with the sensing electrode. 
       FIG. 1  shows the potentiometric sensor of the present disclosure with voltmeter, the sensor generally comprises a working electrode  20  and a reference electrode  10 , a voltage detecting device  30 , and a computing device  40 . The working electrode  20  and the reference electrode  10  are immersed in the solution to be detected, an electromotive force is generated between the working electrode  20  and the reference electrode  10 , and the computing device  40  determines the concentration of the solution to be detected based on the value of the above-mentioned electromotive force. 
     The electrochemical reaction under study occurs on the working electrode  20 . The working electrode  20  may be either solid or liquid, and various electrically conductive solid materials can be used as the electrodes. The electrochemical reaction under study is not affected by a reaction occurring in the working electrode  20  itself, and can be measured in a large potential region. The working electrode  20  must not react with the solvent or electrolyte component and should not have an excessively large area, and preferably has a surface cleaned by a simple method. In the present embodiment, an InGaN-based semiconductor composite structure may be selected and used as the working electrode  20 . 
     The reference electrode  10  measures the relative potential of the working electrode  20 . It will be described in detail hereinafter. 
     The voltage detecting device  30  is electrically connected to the working electrode  20  and the reference electrode  10 , respectively, for detecting a voltage value between the working electrode  20  and the reference electrode  10 . The voltage detecting device  30  may be any device that measures a voltage, such as a voltmeter, a multimeter, or the like. In the present embodiment, a multimeter keithley 2400 is preferred. 
     The computing device  40  is electrically connected to the voltage detecting device  30  to receive a voltage value detection signal generated by the voltage detecting device  30 , and is configured to calculate and determine the concentration of the substance to be detected based on the voltage value. In the present embodiment, the above-mentioned computing device  40  may be a desktop computer, a laptop, or the like. In some other embodiments of the present disclosure, it may also be a small device such as a microprocessor so as to be integrated with the above-mentioned voltage detecting device  30  to form a single device. 
     The electrochemical sensor of the present embodiment operates based on the following principle: when the two electrodes are immersed in a solution to be detected, an electrochemical reaction for detecting the concentration of the solution occurs on the working electrode  20 . The electrochemical reaction causes a charge transfer between the solution and the working electrode. This charge transfer in turn changes the surface potential of the working electrode which generates—an electromotive force between the working electrode  20  and the reference electrode  10 , and the voltage detecting device  30  detects the magnitude of the above-mentioned electromotive force. In general, when the substance to be detected in the solution has a higher concentration, the electrochemical reaction is conducted more intensely, and hence a higher electromotive force is generated. In other words, the electromotive force generated between the working electrode  20  and the reference electrode  10  has a certain correspondence relationship with the concentration of the liquid to be detected, and the correspondence relationship can be expressed in the form of a function. Ideally, in voltage detection, the electromotive force depends linearly on the logarithm of the concentration of the analyte to be detected. The above function expression may be encoded into the computing device  40  in the form of a computational program, and the computing device  40  determines the value of the concentration of the substance to be detected in the solution from the magnitude of the electromotive force detected by the voltage detecting device  30 . 
     The electrochemical sensor described above is not limited to use as a solution sensor, it could also be used in other substance, such as soil. 
     The present embodiment further provides an epitaxial wafer used for a reference electrode  10 . The reference electrode  10  can be applied to the electrochemical sensor described above. It generally comprises three layers of structures, which, as shown in  FIGS. 2 a  to 2 c   , are a substrate  11 , an InGaN layer  12 , and an InN layer  13  in sequence. 
     Here, a material of the substrate  11  may be one selected from sapphire, Si, SiC, and GaN, and a Si substrate  11  is preferred in the present embodiment. The InGaN layer  12  is formed on a surface of the substrate  11 , with an In content between 20% and 60%, so as to ensure that the transition from negatively charged surface states to positively charged surface states occurs within the composition range, thereby generating an electrochemical response independent of the concentration of the solution to be detected. The inventors have found through experiments that, as shown in Table 1, when the In content of the InGaN layer  12  is between 20% and 60%, the transition from the most negatively charged surface state to the positively charged surface state can be achieved. Preferably, an optimum transition can be achieved at an In content of 45%, which means that the reference electrode thus produced performs best. Further, as shown in Table 2, the thickness of the InGaN layer may be between 10 nm and 1 μm, preferably between 100 nm and 500 nm, and within the above range, the surface charge stability of the reference electrode is optimum. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Capacity to change from a 
               
               
                 In content of InGaN 
                 negatively charged surface state 
               
               
                 layer 
                 to a positively charged surface state 
               
               
                   
               
             
            
               
                 10% 
                 worse 
               
               
                 20% 
                 better 
               
               
                 45% 
                 best 
               
               
                 60% 
                 better 
               
               
                 80% 
                 worse 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 the thickness of InGaN 
                 stability of surface charge 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 10 
                 nm 
                 less stable 
               
               
                 100 
                 nm 
                 better stable 
               
               
                 300 
                 nm 
                 best stable 
               
               
                 500 
                 nm 
                 better stable 
               
               
                 1 
                 μm 
                 less stable 
               
               
                   
               
            
           
         
       
     
     The InN layer  13  is formed on a surface of the InGaN layer  12  facing away from the substrate  11  to act as a stabilization layer for the surface charge of the InGaN layer  12 . The InN layer  13  readily establishes a high density of intrinsic, positively charged surface states, which is tested to be up to 2×10 13  cm −2 . This is the highest density of surface states among all III-V semiconductors, such that it can effectively act as a stabilization layer for the surface charge of the InGaN layer  12 . 
     It has been experimentally found that, as shown in Table 3, the thickness of the InN layer sheet can be between 0.1 and 2 monolayer molecules, preferably 0.5 to 1.5 monolayer molecules, and the surface charge stability of the reference electrode is best within the above range. When the thickness of the InN layer exceeds 1.5 monolayers (for example, 2 monolayers), although the surface charge is relatively stable, the fabricated reference electrode will have a high response to the solution concentration. This affects the stability performance of the reference electrode, therefore the thickness of the InN layer is preferably between 0.5 to 1.5 monolayer molecules. 
     
       
         
           
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 the thickness of InN 
                 stability of surface charge 
               
               
                   
               
             
            
               
                 0.1 monolayers 
                 less stable 
               
               
                 0.5 monolayers 
                 better stable 
               
               
                 1.5 monolayers 
                 best stable 
               
               
                   2 monolayers 
                 stable but high response to 
               
               
                   
                 concentration 
               
               
                   
               
            
           
         
       
     
     Used as the reference electrode  10 , the epitaxial wafer with a multilayered structure described above must have stable electrical properties. In the present embodiment, firstly, the InGaN layer  12  with an In content between 20% and 60% allows a transition from negatively charged surface states to positively charged surface states so as to generate an electrochemical response independent of the concentration of the solution to be detected; and in addition, the InN layer  13  with a high density of intrinsic, positively charged surface states further improves the electrochemical stability of the reference electrode  10  in the present embodiment, and the combination of the InGaN layer  12  and the InN layer  13  further enables the reference electrode  10  of the present embodiment to have a stable electrochemical response. 
     Each layer of the epitaxial wafer of the present embodiment is of a solid-state structure, the use of a liquid-filled Ag/AgCl or similar reference electrode  10  in the prior art is avoided, and the structure is simpler and more compact. 
     Inventors carried out a relevant experiment in order to further confirm the high performance of the reference electrode which is made of epitaxial wafer described above. The reference electrode  10  of the present embodiment was immersed in solution together with a conventional Ag/AgCl reference electrode. The voltage between the reference electrode and the Ag/AgCl electrode was measured. As the Ag/AgCl electrode is a perfect reference electrode, all voltage changes come from the reference electrode  10  of the present embodiment to be tested for stability. The reference electrode selected and used in the experiment had an InGaN layer with an In content in the central range of the above-mentioned range values (i.e., an In content of about 40%). The InGaN layer  12  was covered with a thin InN layer  13  having a thickness between 0.5 and 1.5 monolayers. Specifically, in this experiment, a 1.5-monolayers-InN/200-nm-In 0.40 Ga 0.60 N/Si(111) substrate epitaxial layer structure was selected and used as the reference electrode  10 . In this experiment, KCl aqueous solutions at different concentrations were selected and used as solutions to be detected. The inventors found that the above reference electrode, when being immersed in the KCl aqueous solutions, had a stable potential measured versus an Ag/AgCl conventional electrode, which substantially did not change with the concentration of KCl. 
     Specifically, referring to  FIG. 3 , the inventors further carried out an experiment where the reference electrode  10  was alternately immersed for 150 seconds in 0.1M KCl and 1M KCl aqueous solutions. 
     The measured voltage as a function of time exhibited an extremely slight change with the change of the concentration of the KCl solution. The response was less than 5 mV/decade of the KCl concentration change. Also the long-time drift of the voltage in a solution with fixed KCl concentration (not shown here) was below 1 mV/hour. This demonstrates the performance required for a reference electrode in electrochemical sensor devices. All these data are superior among all-solid-state electrodes attempted so far. 
     The present embodiment further provides a method of fabricating an InGaN-based electrochemical electrode epitaxial wafer. As shown in  FIG. 4 , it generally comprises several steps as follows: 
     In step S 402 , a substrate  11  is provided. As shown in  FIG. 2 a   , the material of the substrate  11  may be one selected from Si, sapphire, SiC, and GaN, and a Si substrate  11  is preferred in the present embodiment. Before an InGaN layer  12  is grown epitaxially, the substrate  11  might need to be subjected to pretreatment such as ultrasonic cleaning or high-temperature heating, of which specific steps are well-known to those skilled in the art, and will not be described in detail herein. 
     In step S 404 , an InGaN layer  12  is epitaxially grown on a surface of the substrate  11 . As shown in  FIG. 2 b   , the InGaN layer  12  shown in  FIG. 2 b    is epitaxially grown on the substrate  11 . Epitaxial growth means growing, on the monocrystalline substrate  11 , a monocrystalline layer meeting certain requirements and having a well defined crystallographic relationship with the substrate  11 , as expanding the original wafer outwards to a certain extent. Epitaxial growth is a technology of fabricating monocrystalline thin films, which is a method of growing thin films layer by layer on the appropriate substrate  11  under suitable conditions along a direction of crystallographic axis of the material of the substrate  11 . This technology has following advantages: a low temperature of the substrate  11  in use, easy and precise control over beam flux intensity, and capability of quickly adjusting components, composition and doping concentration of films and layers with variation of sources. With such technology, a monocrystalline thin film with a thickness of tens of atomic layers can be fabricated, and ultrathin layered materials with a quantum microstructure can be formed by alternately growing thin films having different components and doped with different methods. The above epitaxial growth method may be molecular beam epitaxy, metalorganic vapor phase epitaxy, or chemical vapor deposition method. The molecular beam epitaxy is an epitaxial film-making method, and is also a special vacuum coating process. The metalorganic vapor phase epitaxy and chemical vapor deposition rely on gas source transmission and pyrolysis reaction to be realized, with synthesis and decomposition taking place simultaneously. When hydrogen carries metalorganic compound vapors and non-metallic hydrides to be over the substrate  11  heated inside a reaction chamber, a series of chemical reactions take place and an epitaxial layer is generated on the substrate  11 . 
     The InGaN structure may be a homogeneous semiconductor with uniform In content or a heterostructure with varying In content of any design and morphology, e.g., planar layer, corrugated layer, nanowall network, or nanocolumns. As to the so-called semiconductor heterostructure, semiconductor thin films of different compositions and/or different materials are deposited in sequence on the single substrate  11 . Since the semiconductor heterostructure can restrict electrons and holes within an intermediate layer, its electronic properties can be tailored. In other words, in the present embodiment, the electronic properties of the InGaN layer located at an intermediate layer can be tailed so as to adjust the electrochemical response of the electrode. 
     As described above, the In content in InGaN is between 20% and 60% (i.e., covering the central composition range where the transition from negatively charged surface states to positively charged surface states occurs). The In content can be adjusted by a Ga-to-In flux/flow ratio. InGaN is grown at a temperature between 300 and 600° C. The InGaN layer  12  is grown at a rate between 0.1 and 1 micrometer per hour. The final InGaN layer has a thickness between 100 nm and 500 nm. 
     In step S 406 , an InN layer  13  is epitaxially grown on a surface of the InGaN layer  12  facing away from the substrate  11 . The InN layer  13  is grown in the same manner as the manner in which the InGaN layer  12  is grown. Growing the InN layer  13  follows the growth of the InGaN layer  12 , after closing the Ga supply. A growth interruption can be inserted to adjust the growth conditions for the InN layer when the InN layer  13  is to be grown under different conditions from those of the InGaN layer. InN is grown at a temperature between 200 and 500° C. The InN layer  12  is grown at a rate between 0.01 and 1 micrometer per hour. After this step, an electrochemical electrode epitaxial wafer of the reference electrode of the present disclosure is finalized. 
     Finally, it is necessary to fabricate an electrode from the above formed electrochemical electrode epitaxial wafer. A metal contact may be deposited on the InN layer, which is formed in a manner such as photolithography, metal deposition, or lift-off. The above metal may be Al, Au, or Ni, and preferably Ni or Au. Annealing is performed at 200 to 400° C. for 5 to 10 min for ohmic contact formation. The above layer structure is cleaved into dices of, e.g., 0.1×0.1 cm 2  to 1×1 cm 2 , preferably 0.4×0.4 cm 2  to 6×6 cm 2 . Metal wires are bonded to the metal contacts on the InN layer. The metal contact and wire are subsequently covered by insulating epoxy or insulating cement to fabricate a reference electrode for an electrochemical sensor. 
     So far, those skilled in the art will recognize that, although multiple exemplary embodiments of the present disclosure have been shown and described in a detailed way herein, many other alterations or modifications conforming to the principle of the present disclosure can be directly determined or inferred according to the contents disclosed in the present disclosure without departing from the spirit and scope of the present disclosure. Therefore, the scope of the present disclosure should be construed and deemed as covering all of these other alterations or modifications.