Abstract:
Disclosed is a photo detector. The photo detector includes: a conductive substrate; an insulating layer formed on the conductive substrate; a single-layer graphene formed at one part of an upper end of the insulating layer and formed in one layer; a multi-layer graphene formed at the other part of the upper end of the insulating layer and formed in multiple layers; a first electrode formed at an end of the single-layer graphene; and a second electrode formed at an end of the multi-layer graphene.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2015-0055369, filed on Apr. 20, 2015, in the Korean Intellectual Property Office, the entire contents of which are incorporated herein by reference in their entirety. 
     BACKGROUND 
     1. Field 
     The present disclosure relates to a photo detector, and more particularly, to a photo detector, which detects incident light by using a periodic arrangement of a single-layer graphene and a multi-layer graphene. 
     2. Description of the Related Art 
     There are photo detectors having a structure, in which heterogeneous metals are positioned at both ends of the photo detector. The photo detector has heterogeneous metal electrodes, so that a process, such as multi-stage photo lithography or lift-off, is additionally required. 
     There is a graphene photo detector using a flow of carriers generated when nanoparticles are inserted between the graphenes and light is radiated between graphene nanoparticles. The photo detector requires an additional process, in which a graphene phase needs to form the nanoparticles. Further, the photo detector is utilized as a limited photo detector for detecting only light of a specific wavelength responding to an interaction between the graphene nanoparticles. 
     Further, there is a photo detector, in which when electrodes are formed at both ends of a graphene and light is radiated to a boundary surface of the graphene and the electrodes in a vertical direction, a photo current flows between both electrodes. In this case, in the photo detector, a pair of electron and hole is generated in the graphene, which receives light, so that a current is generated. In the photo detector, light needs to be always radiated to the boundary surface of the graphene and the metal, and when light is radiated to both boundary surfaces of the graphene and the metal at the same time, currents having the same size flowing in opposite directions are generated at both sides, so that a sum of photo currents becomes zero. 
     In this respect, a photo detector, which is capable of avoiding forming heterogeneous electrodes and detecting an optical signal even in a situation where light is radiated to a boundary surface of a graphene and a metal, has been required. 
     SUMMARY OF THE INVENTION 
     The present disclosure has been made in an effort to solve the above-described problems associated with the prior art, and provides a photo detector, which is capable of avoiding forming heterogeneous electrodes and detecting an optical signal even in a situation where light is radiated to a boundary surface of a graphene and a metal, has been required. 
     An exemplary embodiment of the present disclosure provides a photo detector, including: a conductive substrate; an insulating layer formed on the conductive substrate; a single-layer graphene formed at one part of an upper end of the insulating layer and formed in one layer; a multi-layer graphene formed at the other part of the upper end of the insulating layer and formed in multiple layers; a first electrode formed at an end of the single-layer graphene; and a second electrode formed at an end of the multi-layer graphene. 
     The single-layer graphene and the multi-layer graphene may have an interdigitated structure. 
     Each of the single-layer graphene and the multi-layer graphene may be implemented in a form of quadrangular saw teeth. 
     The single-layer graphene and the multi-layer graphene may have a structure, in which the saw teeth portions formed based on a direction connecting the first electrode and the second electrode are sequentially and alternately and disposed. 
     The quantity of photo current may be increased by applying a voltage between the substrate and the electrodes. 
     A boundary surface of the single-layer graphene and the multi-layer graphene may be formed between the first electrode and the second electrode. 
     In the photo detector of the present disclosure, the single-layer graphene and the multi-layer graphene are formed at the upper end of the insulating layer, so that it is possible to avoid forming of heterogeneous metal electrodes and minimize damage to the graphene during a manufacturing process of the photo detector. Further, the photo detector according to the present disclosure uses electrodes formed of a single metal by using a photo-thermoelectric effect generated on the boundary surface of the single-layer graphene and the multi-layer graphene, thereby simplifying a structure of an element and not being dependent on a wavelength of incident light. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments will now be described more fully hereinafter with reference to the accompanying drawings; however, they may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the example embodiments to those skilled in the art. 
       In the drawing figures, dimensions may be exaggerated for clarity of illustration. It will be understood that when an element is referred to as being “between” two elements, it can be the only element between the two elements, or one or more intervening elements may also be present. Like reference numerals refer to like elements throughout. 
         FIG. 1  is a diagram illustrating a photo detector according to an exemplary embodiment of the present disclosure. 
         FIG. 2  is a diagram illustrating a photo detector according to another exemplary embodiment of the present disclosure. 
         FIGS. 3A to 3N  are diagrams illustrating a process of manufacturing the photo detector according to an exemplary embodiment of the present disclosure. 
         FIG. 4  is a diagram illustrating a method of manufacturing the photo detector according to an exemplary embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, exemplary embodiments according to the present disclosure will be described in detail with reference to the accompanying drawings. In the description below, it should be noted that only parts necessary for understanding operations according to various exemplary embodiments of the present disclosure will be described, and descriptions of other parts may be omitted so as to avoid unnecessarily obscuring the subject matter of the present disclosure. 
     The present disclosure provides a photo detector, which is capable of avoiding forming heterogeneous electrodes and detecting an optical signal even in a situation where light is radiated to a boundary surface of a graphene and a metal, has been required. 
       FIG. 1  is a diagram illustrating a photo detector according to an exemplary embodiment of the present disclosure. 
     Referring to  FIG. 1 , a photo detector  100  includes a conductive substrate  110 , an insulating layer  120 , graphenes  130  and  140 , and electrodes  151  and  152 . Here, the photo detector  100  includes a graphene photo detector. 
     The conductive substrate  110  is a substrate for forming the photo detector  100 , for example, a graphene photo detector. 
     The insulating layer  120  is formed on the conductive substrate  110 . 
     The graphenes  130  and  140  are formed on the insulating layer. In this case, the graphenes  130  and  140  include a single-layer graphene  130  and a multi-layer graphene  140 , and a boundary surface  10  of the graphenes  130  and  140  are formed according to the combination of the two types of graphenes  130  and  140 . 
     The graphene is a material in which carbon atoms are connected with each other to form a thin plane structure shaped like a beehive, and has an electrical characteristic. In this case, the carbon atoms may be connected with each other to form one carbon atom layer, and the graphene may be formed of a single-layer or multi-layer carbon atom layer. In this case, a thickness of the single-layer graphene  130  may be the same as that of one carbon atom. The carbon atom has a basic unit of a 6-membered ring, and may also be formed of a 5-membered ring or a 7-membered ring. 
     That is, the single-layer graphene  130  represents a graphene formed of one layer, and the multi-layer graphene  140  represents a graphene formed of two or more layers, that is, a plurality of layers. 
     The first electrode  151  is positioned at one end of the graphenes  130  and  140 , and the second electrode  152  is formed at the other end of the graphenes  130  and  140 . 
     An operation of the photo detector  100  will be described below. 
     When light is radiated to the boundary surface  10  of the single-layer graphene  130  and the multi-layer graphene  140 , a photo current flows by a photo-thermoelectric effect. When light is radiated to the boundary surface  10  of the single-layer graphene  130  and the multi-layer graphene  140 , a temperature of the boundary surface  10  becomes higher than a temperature of the boundary surface of the metal electrode, to which light is not radiated, and a photo current flows between the electrodes  151  and  152  positioned at both ends of the graphenes  130  and  140  by the photo-thermoelectric effect. In this case, when a voltage is applied between the electrodes  151  and  152  and the conductive substrate, the quantity of photo current may be increased. This is similar to an increase in a current according to an application of a gating voltage. 
       FIG. 2  is a diagram illustrating a photo detector according to another exemplary embodiment of the present disclosure. 
     Referring to  FIG. 2 , a photo detector  200  includes a conductive substrate  210 , an insulating layer  120 , graphenes  230  and  240 , and electrodes  251  and  252 . Here, f the photo detector  200  is different from the photo detector of  FIG. 1  in the structures of the graphenes  230  and  240 . 
     The single-layer graphene  230  and the multi-layer graphene  240  has a structure, in which the single-layer graphene  230  and the multi-layer graphene  240  are engaged with each other (that is, an interdigitated electrode structure), and a recess for inserting the multi-layer graphene  240  is formed in the single-layer graphene  230 , and a recess for inserting the single-layer graphene  230  is formed in the multi-layer graphene  240 . That is, the single-layer graphene  230  and the multi-layer graphene  240  have a form of quadrangular saw teeth, and are engaged with each other. The quadrangular saw teeth as the forms of the single-layer graphene  230  and the multi-layer graphene  240  are illustrative, and the single-layer graphene  230  and the multi-layer graphene  240  may be implemented in various patterns, such as a trapezoid shape and a polygonal shape. 
     Accordingly, the single-layer graphene  230  and the multi-layer graphene  240  has structures, in which the saw teeth portions formed based on a line connecting the first electrode  251  and the second electrode  252  are sequentially and alternately disposed. In this case, a form, a length, and a thickness of the saw teeth portion may be variously adjusted for performance of the photo detector. 
     The aforementioned structures of the graphenes  230  and  240  may improve photoelectric conversion efficiency. This may increase a physical length of the boundary surface  20  of the single-layer graphene  230  and the multi-layer graphene  240  to improve photoelectric conversion efficiency. Accordingly, it is possible to obtain a larger photo current with respect to the radiation of the same quantity of light. 
       FIGS. 3A to 3N  are diagrams illustrating a method of manufacturing the photo detector according to an exemplary embodiment of the present disclosure. 
     Referring to  FIGS. 3A to 3N , in  FIG. 3A , a substrate, in which the insulating layer  120  is formed on the conductive substrate  110 , may be used. For example, a substrate, in which a silica dioxide (SiO 2 ) layer is formed on a doped silicon substrate, may be used. Accordingly, the conductive substrate  110  may be a doped silicon substrate, and the insulating layer  120  may be a silica dioxide layer. 
     In  FIG. 3B , the single graphene  130  and a PMMA film  311 , which are combined by, for example, a Chemical Vapor Deposition (CVD) process, are simultaneously formed on the prepared substrate. In this case, a graphene-polymethyl methacrylate (PMMA) sample, in which the PMMA is coated on the single graphene combined through the CVD process, may be transferred to the substrate as it is. 
     In  FIG. 3C , a shadow mask  312  for forming a metal mask is aligned on the graphene-PMMA formed on the substrate. Then, an appropriate metal (copper (Cu) and gold (Au)) is deposited. 
     In  FIG. 3D , a metal mask  313  is formed by the shadow mask  312 . 
     In  FIG. 3E , an unnecessary PMMA layer  314  is removed by an O 2  ash process by using the metal mask  313 . 
     In  FIG. 3F , the metal mask  313  is removed. The metal mask is the portion  313  positioned at the topmost end in  FIG. 3E , and when the metal mask  313  is removed, the PMMA  315  is positioned at the topmost end. 
     In  FIG. 3G , when the PMMA  315  formed on the single-layer graphene is removed, a finally patterned single-layer graphene  316  is obtained. 
     In  FIG. 3H , the graphene-PMMA  317  sample the patterned single-layer graphene  316  used in  FIG. 3B  is transferred to the substrate. 
     In  FIG. 3I , a shadow mask  318  for forming a metal mask is aligned on the graphene-PMMA  317  formed on the substrate. In this case, the shadow mask  318  is formed in a pattern having a larger distance than that in  FIG. 3C  so that the single-layer graphene  130  and the multi-layer graphene  140  are simultaneously formed. Then, an appropriate metal (copper (Cu) and gold (Au)) is deposited. 
     In  FIG. 3J , a metal mask  319  is formed by the shadow mask  318 . 
     In  FIG. 3K , an unnecessary PMMA layer is removed by an O 2  ash process by using the metal mask  319 . 
     In  FIG. 3L , the metal mask  319  is removed. When the metal mask  319  is removed, the single-layer graphene  130  and the multi-layer graphene  140  may obtain the patterned graphenes. 
     In  FIG. 3M , a shadow mask  321  for forming the metal electrode for measuring a current of the graphene photo detector is aligned. 
     In  FIG. 3N , a final graphene photo detector, in which the appropriate metal electrodes are formed, may be manufactured. 
     The aforementioned process of manufacturing the graphene photo detector uses the graphene-PMMA sample, thereby minimizing a negative influence of a photoresist (artificial doping of the graphene) used in a semiconductor process. 
     Herein, the processes of  FIGS. 3A to 3N  may be sequentially performed. 
       FIG. 4  is a diagram illustrating a method of manufacturing the photo detector according to an exemplary embodiment of the present disclosure. 
     Referring to  FIG. 4 , a single-layer graphene and a PMMA film, which are combined, are formed on a prepared substrate (operation  411 ), and the method proceeds to operation  413 . Here, the prepared substrate is a substrate, on which an insulating layer is formed, and a substrate, in which a silica dioxide layer is formed on a doped silicon substrate, may be used. 
     A first shadow mask is aligned on the formed graphene PMMA (operation  413 ), and the method proceeds to operation  415 . 
     A first metal mask is formed by depositing a metal by using the aligned first shadow mask (operation  415 ), and the method proceeds to operation  417 . 
     An unnecessary graphene-PMMA layer is removed through an ash process using the first metal mask (operation  417 ). 
     The first metal mask is removed (operation  419 ), and the method proceeds to operation  421 . 
     The PMMA formed in the single-layer graphene is removed (operation  421 , and the method proceeds to operation  423 . In this case, the patterned single-layer graphene is formed. 
     The graphene-PMMA sample on the patterned single-layer graphene is transferred to the substrate (operation  423 ), and the method proceeds to operation  425 . 
     A second shadow mask is aligned on the formed graphene-PMMA (operation  425 ), and the method proceeds to operation  427 . 
     A second metal mask is formed by depositing a metal by using the aligned second shadow mask (operation  427 ), and the method proceeds to operation  429 . 
     An unnecessary graphene-PMMA layer is removed by using the second metal mask (operation  429 ), and the method proceeds to operation  431 . 
     The second metal mask and the PMMA are removed (operation  431 ), and the method proceeds to operation  433 . 
     A third shadow mask is aligned (operation  433 ), and the method proceeds to operation  435 . A metal electrode is formed by depositing an appropriate metal by using the third shadow mask, and the method proceeds to operation  435 . 
     A photo detector, that is, a graphene photo detector, is generated by forming the metal electrode (operation  435 ), and the method is terminated. 
     The photo detector suggested in the present disclosure may avoid the forming of heterogeneous metal electrodes and minimize damage to the graphene during the manufacturing process. Further, the photo detector suggested in the present disclosure uses electrodes formed of a single metal by using a photothermoelectric effect generated on the boundary surface of the single-layer graphene and the multi-layer graphene, thereby simplifying a structure of an element and not being dependent on a wavelength of incident light. The photo detector suggested in the present disclosure may be utilized as an image sensor operable in wavelength bands of ultraviolet rays (UV), infrared rays (IR), and visible ray. 
     In the detailed description of the present disclosure, the particular exemplary embodiment has been described, but various modifications are available without departing from the scope of the present disclosure. Therefore, the scope of the present disclosure is not limited to the exemplary embodiments described, but shall be defined by the claims to be described below and the equivalents to the claims.