Patent Publication Number: US-10312399-B2

Title: Photodiode with decreased dark current and method for manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of Korean Patent Application No. 10-2017-0028577 filed on Mar. 6, 2017, in the Korean Intellectual Property Office, the disclosure of which is hereby incorporated by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a photodiode with a decreased dark current and a method for manufacturing the same, and more particularly to a technology for reducing a dark current of a photodiode with a germanium (Ge) substrate. 
     2. Description of the Related Art 
     In an information society, semiconductors are essential elements for processing, storing, and converting information. A photodiode is a semiconductor device that converts an optical signal into an electrical signal. A photodiode may be disposed in an image sensor such as a Charge Coupled Device (CCD) or Complementary Metal Oxide Semiconductor (CMOS), and may convert received light into an electrical signal. 
     Meanwhile, removing a dark current increases performance of the photodiode. The dark current may be described as a signal or current measured in the absence of light energy on the photodiode, resulting in reduced accuracy of the photodiode. The dark current may cause noise in a pixel signal, such that performance of an image sensor having a photodiode with substantial dark current is deteriorated. Therefore, many developers and companies are conducting intensive research into technology for removing a dark current from the photodiode. 
     Although special chemical processing is performed on a substrate having a photodiode to remove a dark current, such chemical processing has difficulty in collecting photocharges. A technology for maintaining or improving performance of a photodiode while simultaneously reducing dark current of the photodiode, and a method for manufacturing the photodiode, will hereinafter be described. 
     SUMMARY 
     It is an object of the present disclosure to provide a technology for reducing a dark current in a photodiode. 
     It is an object of the present disclosure to provide a technology for reducing a dark current by inserting an interlayer into selected portions of an electrode layer of the photodiode. 
     Objects of the present disclosure are not limited to the above-described objects and other objects and advantages can be appreciated by those skilled in the art from the following descriptions. Further, it will be easily appreciated that the objects and advantages of the present disclosure can be practiced by means recited in the appended claims and a combination thereof. 
     In accordance with one aspect of the present disclosure, a photodiode having a reduced dark current includes a semiconductor layer, a first contact part, a second contact part, and an active region. The first contact part disposed in a first region of the semiconductor layer includes an interlayer and at least one metal layer. The second contact part disposed in a second region of the semiconductor layer includes at least one metal layer. The active region is disposed between the first contact part and the second contact part. The first contact part and the second contact part are arranged asymmetrical to each other. 
     In accordance with another aspect of the present disclosure, a method for manufacturing a photodiode having a reduced dark current includes depositing an interlayer dielectric film over a semiconductor layer etching a first region from among the interlayer dielectric film, depositing an interlayer over the etched first region and the interlayer dielectric film, exposing the interlayer dielectric film by etching the interlayer other than the first region, etching a second region separated from the first region, from among the interlayer dielectric film, and depositing a first metal layer over the interlayer of the first region, and depositing a second metal layer over the semiconductor layer of the second region. 
     According to an exemplary embodiment of the present disclosure, photodiode performance can be improved by reducing a dark current of the photodiode. 
     Further, according to an exemplary embodiment of the present disclosure, photodiode performance can be improved by reducing a dark current by inserting an interlayer into some parts of an electrode layer of the photodiode. 
     It should be noted that effects of the present disclosure are not limited to those described above and other effects will be apparent to those skilled in the art from the following descriptions. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view illustrating a Metal-Semiconductor-Metal (MSM) photodiode in which metal and semiconductor are arranged in an overlapping manner. 
         FIG. 2A  and  FIG. 2B  are band diagrams illustrating current generated in a photodiode. 
         FIG. 3  is a view illustrating a photodiode having a reduced dark current according to an embodiment of the present disclosure. 
         FIG. 4  is a view illustrating a photodiode having an interlayer dielectric film formed of an interlayer dielectric material according to an embodiment of the present disclosure. 
         FIG. 5  is a view illustrating an interdigitated photodiode according to an embodiment of the present disclosure. 
         FIG. 6  is a view illustrating a photodiode in which contact parts of the same type are arranged close to one another according to an embodiment of the present disclosure. 
         FIG. 7  is a view illustrating a Metal-Insulator-Semiconductor (MIS) photodiode according to an embodiment of the present disclosure. 
         FIG. 8  is a flowchart illustrating a process for manufacturing a photodiode having a reduced dark current according to an embodiment of the present disclosure. 
         FIGS. 9 to 13  are conceptual diagrams illustrating processes for creating a MIS photodiode according to an embodiment of the present disclosure. 
         FIG. 14  is a band diagram illustrating flow of photocharges in a MIS photodiode according to an embodiment of the present disclosure. 
         FIGS. 15 and 16  illustrate the magnitude of a dark current according to thickness of an interlayer according to an embodiment of the present disclosure. 
         FIGS. 17 to 20  illustrate differences in flow between a dark current and a photocurrent according to interlayer thickness. 
         FIGS. 21 and 22  illustrate electrical characteristics of photodiodes according to embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments are described in sufficient detail to enable those skilled in the art in the art to easily practice the technical idea of the present disclosure. Detailed descriptions of well known functions or configurations may be omitted so that the gist of the present disclosure is not obscured. Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Throughout the drawings, like reference numerals refer to like elements. 
     The terms used in the present application are merely used to describe specific embodiments and are not intended to limit the present disclosure. Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meanings as understood by those skilled in the art. 
     A singular expression may include a plural expression unless otherwise stated in the context unless specially described. Terms defined in a generally used dictionary may be analyzed to have the same meaning as the context of the relevant art and may not be analyzed to have ideal meaning or excessively formal meaning unless clearly defined in the present application. 
     The embodiments of the present disclosure will hereinafter be described centering upon the photodiode. A photodiode may be constructed in various ways and is not limited to the specific embodiments of the present disclosure. For convenience of description, photodiode having two contacts are described. However, it should be understood that embodiments of the present disclosure are described in limited detail, and that embodiments of the present disclosure are not limited to the features in the following description. 
       FIG. 1  is a view illustrating a Metal-Semiconductor-Metal (MSM) photodiode  10  in which metal and semiconductor are arranged in an overlapping manner. For example, semiconductor layer  11  overlaps with metal layers of the electrodes. Referring to  FIG. 1 , the MSM photodiode  10  includes a SiO 2  layer  11  arranged on a germanium (Ge) wafer  100 . The SiO 2  layer  11  is etched at electrode regions so that the electrodes are in direct contact with the underlying Ge substrate  100 . Specifically, each electrode comprises a hole Schottky barrier layer  12  that is disposed directly on the surface of the Ge wafer  100  in the electrode region. A titanium (Ti) layer  13  and a gold (Au) layer  14  may be arranged over the hole Schottky barrier  12 . 
     The germanium (Ge) MSM photodiode shown in  FIG. 1  has a hole dark current that directly relates to the hole Schottky barrier height. In particular, the hole Schottky barrier height (SBH) is inversely proportional to dark current. In order to reduce the dark current, a large bandgap material bay be inserted into the hole Schottky barrier  12  or doping can be applied to the barrier  12 , resulting in reduction of the hole dark current. However, inserting a large-bandgap material may cause resistance in collection of photocharges, resulting in a reduced photocurrent. 
     Referring to  FIG. 2A , a band diagram illustrates that a Schottky barrier height (SBH) of a hole is relatively low at 0.56 eV. As a result, a dark current (I dark, h ) of the hole is large as denoted by  205 . In contrast, the band diagram shown in  FIG. 2B  illustrates an example in which a Schottky barrier height (SBH) is relatively high as shown in  FIG. 1 . As a result, dark current in  FIG. 2B  is relatively low. 
       FIG. 2B  illustrates an increased hole SBH that may be caused, for example, by a large-bandgap material as shown in  FIG. 1 . As a result, dark current electron flows designated as (I dark, e ) and (I dark, h ) are reduced as denoted by  210  and  220 . In contrast, when the Schottky barrier height is increased, resistance is introduced into the photocurrent 1 , such that the photocurrent may decrease as shown in I photo, e, degraded    230  and I photo, h, degraded    240 . In other words, while increasing the SBH of electrodes in an MSM photodiode can reduce dark current, it also reduces photocurrent, which degrades performance of the photodiode. 
     In addition, technology for reducing a dark current using doping has disadvantages. Doping processes are typically expensive and difficult to apply and add unacceptable cost and complexity to a process for manufacturing a photodiode. 
     In order to address the above-mentioned issues, a photodiode with a reduced dark current and a method for manufacturing the same will be described with reference to the attached drawings. 
       FIG. 3  is a view illustrating a photodiode having a reduced dark current according to an embodiment of the present disclosure. Referring to  FIG. 3 , view  1001  is a cross-sectional view taken along the line A-A′ of view  1002 , and  1002  is a top view of the photodiode. The photodiode may be classified into first regions  1091   a  and  1091   b  having first contact parts, which are connected to a cathode on the semiconductor layer  1000 , and second regions  1092   a  and  1092   b  having second contact parts, which are connected to an anode. Accordingly, numbers  1091  and  1092  may refer to both first and second regions, and the respective first and second contact parts disposed within those regions. An active region  1093   a  may be disposed between the first region  1091   a  and the second region  1092   a , and an active region  1093   b  may be disposed between the first region  1091   b  and the second region  1092   b.    
     The active regions  1093   a  and  1093   b  may be defined by a doped portion of the substrate  1000 . In some embodiments, the entire substrate  1000  is doped, so that an entire upper surface portion of the substrate  1000  is effectively an active region. In other embodiments, limited portions of the substrate are doped, such as the regions  1093   a  and  1093   b  that are disposed in spaces between anode and cathode contact parts. 
     In the embodiment shown in  FIG. 3 , the anode and cathode electrodes are offset from one another such that spaces between anode-cathode region pairs  1091   a - 1092   a  and  1091   b - 1092   b  are less than a space between second region  1092   a  and first region  1091   b . The first contact parts and the second contact parts may have different structures from one another. 
     The first region  1091   a  may include an interlayer  1010   a  and a metal layer  1020   a . The second region  1091   b  may include an interlayer  1010   b  and a metal layer  1020   b . First region  1091   a  and second region  1091   b  may be electrically connected to each other. In other words, fingers of an interdigitated photodiode may be constructed as seen in view  1002 . 
     In accordance with one embodiment of the present disclosure, the cathode electrode includes an interlayer material that reduces a conduction band offset (CBO) between the semiconductor layer  1000  and each of the interlayers ( 1010   a ,  1010   b ). In an embodiment, the interlayer material causes the CBO to be negligible, or zero. In other embodiments, the CBO is equal to or lower than 1.0 eV, 0.5 eV, 0.3 eV, 0.1 eV, or 0.01 eV. For example, in an embodiment in which the semiconductor layer  1000  is formed of germanium (Ge), the interlayer  1010   a  or  1010   b  may include a material such as TiO 2 . In addition, the effect of reducing the dark current can be enhanced by adjusting thickness of the interlayers. 
     In  FIG. 3 , each of the metal layers ( 1020 ,  1030 ) may comprise one or more material layers. A material of each of the metal layers may be selected according to one or more material present in the interlayers ( 1010   a ,  1010   b ). When each of the interlayers ( 1010   a ,  1010   b ) is formed of TiO 2 , each of the metal layers ( 1020 ,  1030 ) may be formed of titanium (Ti). Alternatively, a lower metal layer of each of the metal layers may be formed of a first metal such as titanium (Ti), and an upper metal layer of each of the metal layers is formed of a different metal such as gold (Au), so that each of the metal layers ( 1020 ,  1030 ) includes two separate metal layers. 
     In various embodiments, each of the metal layers may be selected from a group that includes gold (Au), silver (Ag), aluminum (Al), cobalt (Co), chromium (Cr), copper (Cu), gadolinium (Gd), hafnium (Hf), indium (In), iridium (Ir), magnesium (Mg), manganese (Mn), molybdenum (Mo), nickel (Ni), lead (Pb), palladium (Pd), platinum (Pt), rhodium (Rh), tantalum (Ta), titanium (Ti), tungsten (W), and zinc (Zn). In addition, a metal layer may be formed of an alloy of one or more materials contained in the above-mentioned group. 
     Alternatively, first metal layers disposed in the first regions ( 1091   a ,  1091   b ), e.g. metal layers  1020 , and second metal layers disposed in the second regions ( 1092   a ,  1092   b ) e.g. metal layers  1030 , may be formed of different constituent elements, or constituent materials of the metal layers and the other metal layers may be implemented with different compositions. In accordance with another embodiment of the present disclosure, the metal layer  1020  disposed in each of the first regions ( 1091   a ,  1091   b ) may be formed of titanium (Ti), and the other metal layer  1030  disposed in each of the second regions ( 1092   a ,  1092   b ) may be formed of gold (Au). 
     In an embodiment, an interlayer dielectric film formed of an interlayer dielectric material may be disposed over the semiconductor layer  1000 , and the first regions ( 1091   a ,  1091   b ) and the second regions ( 1092   a ,  1092   b ) may then be etched. 
       FIG. 4  is a view illustrating a photodiode having an interlayer dielectric film  1040  formed of an interlayer dielectric material according to an embodiment of the present disclosure.  FIG. 4  illustrates the interlayer dielectric film  1040  arranged over the exposed upper surface of Ge layer  1000 . In one embodiment of the present disclosure, the interlayer dielectric film  1040  may be formed of SiO 2 . The interlayer dielectric film  1040  may have a greater height than each of the interlayers ( 1010   a ,  1010   b ) so that the dielectric film overlaps with and completely covers sidewalls of the interlayers. The upper surface of interlayer dielectric film  1040  may be disposed below upper surfaces of each of the metal layers ( 1030   a ,  1030   b ) in the second regions ( 1092   a ,  1092   b ), so that metal layers  1030  are exposed above the surface of dielectric layer  1040 . In addition, the interlayer dielectric film  1040  may be coated with an anti-reflective material. 
     In the embodiments shown in  FIGS. 3 and 4 , each of the first region  1091   a  equipped with the interlayer  1010   a  and the first region  1091   b  equipped with the interlayer  1010   b  may be referred to as a Metal-Insulator-Semiconductor (MIS) contact part. Each of the second regions ( 1092   a ,  1092   b ) may be referred to as a Metal-Semiconductor (MS) contact part. 
     Two contact parts may be have different constituent layers because the interlayer is disposed in only one of the contact parts. 
     In  FIGS. 3 and 4 , the CBO between the semiconductor layer  100  and each of the interlayers ( 1010   a ,  1010   b ) may be equal to or less than a predetermined value, and a Valence Band Offset (VBO) between the semiconductor layer  1000  and each of the interlayers ( 1010   a ,  1010   b ) may be equal to or higher than a predetermined value. For example, the VBO may be higher than about 2.9 eV, resulting in reduction of a dark current. The layers may be arranged so that the CBO is minimized. In embodiments, the CBO value is less than 1.0 eV, less than 0.5 eV, less than 0.3 eV, less than 0.1 eV, or less than 0.01 eV. Accordingly, in some embodiments, the CBO approaches zero. In an embodiment, when expressed with one significant digit, the CBO is 0.0, or zero. 
       FIG. 5  is a view illustrating an interdigitated photodiode that embodies features of the photodiode described with respect to  FIGS. 3 and 4 .  FIG. 5  illustrates an interlayer dielectric film  1040  surrounding an interdigitated photodiode. In one embodiment, the interlayer dielectric film  1040  may be formed of SiO 2 . A first connection electrode part  520  and a second connection electrode part  530  may be disposed in an etched region of the interlayer dielectric film  1040 . 
     The first connection electrode part  520  may be connected to a cathode, and may also be connected to one or more first contact parts ( 520   a ,  520   b ,  520   c ). The second connection electrode part  530  may be connected to an anode, and may also be connected to one or more second contact parts ( 530   a ,  530   b ,  530   c ). 
     An interlayer similar to interlayer  1010  discussed above may be disposed in each of the first contact parts ( 520   a ,  520   b ,  520   c ) used as the MIS contact parts. Each of the second contact parts ( 530   a ,  530   b ,  530   c ) acting as the MS contact parts may not include the interlayer. The overlap region  1093  of the first contact parts ( 520   a ,  520   b ,  520   c ) and the second contact parts ( 530   a ,  530   b ,  530   c ) may be an active region. The first contact parts ( 520   a ,  520   b ,  520   c ) and the second contact parts ( 530   a ,  530   b ,  530   c ) may be alternately arranged as shown in  FIG. 5 . In another embodiment, for convenience of fabrication, the first contact parts ( 520   a ,  520   b ,  520   c ) and the second contact parts ( 530   a ,  530   b ,  530   c ) may also be constructed as shown in  FIG. 6 . 
       FIG. 6  is a view illustrating a photodiode in which the same-type contact parts are arranged adjacent to one another according to an embodiment of the present disclosure. In other words, two first contact parts ( 520   b ,  520   c ) from among the first contact parts ( 520   a ,  520   b ,  520   c ) may be arranged adjacent to each other, and two second contact parts ( 530   a ,  530   b ) from among the second contact parts ( 530   a ,  530   b ,  530   c ) may be arranged adjacent to each other. As the number of interdigitated photodiodes increases, a spatial margin or process margin may be obtained in a process for depositing and etching one or more interlayers, when the same contact parts can be arranged adjacent to each other. 
     In  FIGS. 5 and 6 , the first connection electrode part  520  and at least one first contact part ( 520   a ,  520   b ,  520   c ) connected thereto may have different structures. In particular, the interlayer may not be disposed below the connection electrode part  520 , and the interlayer may be disposed only in each of the first contact parts ( 520   a ,  520   b ,  520   c ). 
     For example, the photodiodes shown in  FIGS. 5 and 6  may be constructed as follows. The photodiode may include a first connection electrode part  520  connected to the cathode on a semiconductor layer  1000  (as seen in  FIGS. 3 and 4 ) and at least one first contact part ( 520   a ,  520   b ,  520   c ) connected to the first connection electrode part  520 . The photodiode may further include a second connection electrode part  530  connected to the anode on the semiconductor layer  1000  and at least one second contact part ( 530   a ,  530   b ,  530   c ) connected to the second connection electrode part  530 . 
     Each of the first contact parts ( 520   a ,  520   b ,  520   c ) may include the interlayer and at least one metal layer, and each of the second contact parts ( 530   a ,  530   b ,  530   c ) may include at least one metal layer. The above-mentioned interlayer may not be disposed in the first connection electrode part  520 . 
     As described above with respect to  FIG. 3 , a photodiode may be classified into first regions, or first contact parts ( 1091   a ,  1091   b ) connected to a cathode on the semiconductor layer  1000 , and second regions, or second contact parts ( 1092   a ,  1092   b ), connected to an anode. The active region  1093   a  is disposed between the first region  1091   a  and the second region  1092   a , and the active region  1093   b  is disposed between the first region  1091   b  and the second region  1092   b . The first contact parts and the second contact parts may have different layer structures. 
       FIG. 7  is a view illustrating a Metal-Insulator-Semiconductor (MIS) photodiode  700  according to an embodiment of the present disclosure. The view of MIS photodiode  700  illustrated in  FIG. 7  is a cross-sectional view taken along line B-B′ of  FIGS. 5 and 6 . 
     In the MIS photodiode  700 , the semiconductor layer  1000   g  may be formed of germanium (Ge). A cathode of the MSM photodiode  700  may include a Metal-Insulator-Semiconductor (MIS) structure formed of TiO 2 . 
     In more detail, the SiO 2  layers ( 1040   a ,  1040   b ,  1040   c ), each of which is an interlayer dielectric film, may be disposed on the germanium (Ge) semiconductor layer  1000   g  which is a Ge wafer, and TiO 2  may be formed as an interlayer material  710   a  in the cathode region. When the TiO 2  interlayer is present as shown in  FIG. 7 , dark current can be reduced. 
     In an embodiment in which a conduction band offset (CBO) between a TiO 2  material of interlayer  710   a  and the germanium (Ge) material of substrate  1000   g  is zero, photocharges may be effectively collected without causing resistance. The hole Schottky barrier is effectively increased due to large-bandgap characteristics of the TiO 2  material, resulting in reduction of a dark current. In addition, fabrication simplicity can be maintained by not applying a doping process. A process for inserting the TiO 2  interlayer  710   a  shown in  FIG. 7  will hereinafter be described. 
       FIG. 8  is a flowchart illustrating a process for manufacturing a photodiode having a reduced dark current according to an embodiment of the present disclosure. 
     Referring to  FIG. 8 , an interlayer dielectric film may be deposited over a semiconductor layer (S 810 ). A first region the interlayer dielectric film may be etched (S 820 ). The first region may refer to a region in which the interlayer will be disposed and the above-mentioned first MIS contact parts are disposed. The etching process of S 820  may be a wet etching process. 
     An interlayer material such as TiO 2  may be deposited over the etched first region and the interlayer dielectric film (S 830 ) using, for example, Atomic Layer Deposition (ALD). Although embodiments of the present disclosure use the specific example of TiO 2  as the interlayer material, in other embodiments, other materials with a low CBO value for a substrate interface may be used. In such embodiments, substrate and interlayer materials may be selected to minimize the CBO value at the interface. As an example of the deposition material, a material for allowing the CBO of the semiconductor layer to be low or zero may be used as the deposition material. A deposition process may be performed in a manner that the above exemplary material constructs the interlayer. 
     The interlayer other than the first region may be removed to expose the interlayer dielectric film (S 840 ). In an embodiment, portions of the interlayer material that are deposited over the upper surface of dielectric film  1040  are removed by a polishing process such as a chemical mechanical polishing process (CMP). In another embodiment, the interlayer may be removed by a dry-etch process. 
     The second region of the interlayer dielectric film separated from the first region may be etched (S 850 ). In an embodiment, the second region may be wet-etched. The second region may refer to a region in which a metal material is deposited directly on the surface of the substrate material without any intervening interlayer material. 
     Thereafter, a first metal layer may be selectively deposited over the interlayer material in the first region (S 860 ), and a second metal layer may be deposited over the semiconductor layer exposed in the second region (S 870 ). When the same metal material is deposited over the first region and the second region, the steps S 860  and S 870  may be performed at the same time. In other words, in an embodiment in which different materials are used for anode and cathode electrodes, the different materials are deposited in separate processes. On the other hand, when both electrodes include the same material, it may be applied in a single deposition process. Persons of skill in the art will recognize that the metal layers may be formed using a variety of processes, including selective and bulk deposition and removal. 
       FIGS. 9 to 13  illustrate processes for forming an MIS photodiode according to an embodiment of the present disclosure. The fabrication process of  FIG. 8  will hereinafter be described with reference to  FIGS. 9 to 13 .  FIG. 9 , illustrates a SiO 2  layer  1040  deposited over a Ge wafer  1000   g , as disclosed in step S 810  of  FIG. 8 . Thickness of the SiO 2  layer according to one embodiment may be 100 nm, and the SiO 2  layer may be deposited, for example, using e-beam evaporator or sputtering process. 
     As shown in  FIG. 10 , a photolithography process may be performed on the deposited SiO 2  layer  1040  to etch the SiO 2  layer, thereby forming an cathode MIS contact region  901 , as explained with respect to step S 820  of  FIG. 8 . The etching may be a wet etching process performed using a 1:25 diluted HF (hydrogen fluoride) solution. After etching, a portion of the Ge wafer  1000   g  may be exposed as shown in region  901 . 
     As an example of step S 830  of  FIG. 8 , a TiO 2  layer  710  may be deposited over all exposed surfaces, as shown in  FIG. 11 . As an example of the deposition process, the TiO 2  layer may be deposited using atomic layer deposition (ALD). In an embodiment, the deposition process may be performed at a temperature of 250° C. 
     As seen in  FIG. 12 , portions of TiO 2  layer  710  are removed from upper surfaces of dielectric layer  1040   c  as explained with respect to S 840 . In addition, an etching process is performed to remove a portion of dielectric film  1040   c , thereby exposing an upper surface of substrate  1000   g  as described with respect to S 850  of  FIG. 8 . In an embodiment, SiO 2  and TiO 2  materials of respective dielectric film  1040   b  and interlayer material  710  may be plasma-etched to form MS contact region  902  acting as the anode region. In an embodiment, the TiO 2  layer  710  is removed from upper surfaces of film  1040   b , thereby exposing film  1040   c , while the TiO 2  layer  710   a  remains in the MIS contact region  901 , which is the cathode region. The TiO 2  layer remaining on dielectric film  1040   b  may be dry-etched using SF 6  (sulfur hexafluoride), or removed by CMP. Thereafter, for etching of the MS contact region  902  acting as the second region in the remaining SiO 2  layer, the wet etching process may be performed using the HF solution as described above. 
       FIG. 13  illustrates an example of S 860  and S 870  of  FIG. 8 . In  FIG. 13 , the MIS contact region  901  acting as the cathode region in  FIG. 12  and the MS contact region  902  acting as the anode region in  FIG. 12  are filled by depositing one or more electrode metal in the respective spaces. In an embodiment in which a plurality of electrode metal materials are present, each metal material may be deposited in a discrete layer. 
     For example, as seen in  FIG. 13 , each of a first layer  521   a  and a second layer  522   a  may be the cathode electrode, and each of a first layer  531   a  and a second layer  532   a  may be the anode electrode. In this case, the first layers ( 521   a ,  531   a ) of the respective electrodes may be formed of titanium (Ti), and the second layers ( 522   a ,  532   a ) of the respective electrodes may be formed of gold (Au). 
     If an interlayer such as a TiO 2  layer is disposed only in the cathode portion of the photodiode through the processes of  FIGS. 8 to 13 , holes or electrons generated by photons striking the photodiode may be collected without causing a substantial amount of resistance. 
       FIG. 14  is a band diagram illustrating flow of photocharges in an MIS photodiode according to an embodiment of the present disclosure. The concept of  FIG. 14  will hereinafter be described in comparison to the band diagrams of  FIG. 2 . As seen at  1301  of  FIG. 14 , the conduction band offset (CBO) of TiO 2  and germanium (Ge) may be zero due to the presence of the inserted interlayer, so that photocharges can be collected without causing tunneling resistance. That is, photocharges can be effectively collected without resistance losses. 
     In an embodiment that includes the MIS structure in which the TiO 2  material having a large bandgap is inserted as the interlayer, the hole Schottky barrier of the cathode is greatly improved, resulting in reduction of a hole dark current of a MSM photodiode with a germanium substrate. Finally, a doping process is not performed in the process described with respect to  FIGS. 8 to 13 , such that the technological advantage of fabrication simplicity of the MSM photodiode can be maintained. 
     In the process of  FIGS. 8 to 13 , the cathode region  901  may have a Metal-Insulator-Semiconductor (MIS) contact structure, and the anode region  902  may have a Metal-Semiconductor (MS) contact structure. 
     The region  1040   b  between two contacts may be an active region. In an embodiment, one or more of region  1040   b  and SiO 2  regions ( 1040   a ,  1040   c ) may be coated with an anti-reflective coating. In the active region  1040   b , a photocurrent caused by incident light may flow between the MIS-type contact and the MS-type contact The incident light may be, for example, infrared light. In one embodiment, the infrared light has a wavelength of λ=1.55 um. However, embodiments are not limited to this example—in other embodiments, the infrared light may be a wavelength in a communication band such as the C band, S band or L band, or another wavelength. 
     The extent of the reduction of the dark current may be changed according to the height of the interlayer  710   a  according to various embodiments. A detailed description thereof will hereinafter be described. 
       FIGS. 15 and 16  are graphs illustrating the magnitude of a dark current according to thickness of the interlayer according to an embodiment of the present disclosure.  FIGS. 15 and 16  illustrate graphs based on embodiments in which TiO 2  is included in the interlayer. 
       FIG. 15  illustrates I-V correlation graphs at 0 nm, 5 nm, 7 nm and 9 nm. Here, 0 nm denotes the absence of the interlayer formed of TiO 2 , 5 nm denotes that the interlayer has thickness of 5 nm, 7 nm denotes that the interlayer has thickness of 7 nm, and 9 nm denotes that the interlayer has thickness of 9 nm. If the TiO 2  material is disposed as the interlayer, it can be recognized that the dark current is reduced according to thickness of the TiO 2  interlayer. 
       FIG. 16  shows a reduction of the dark current at 1V. A detailed description thereof is as follows. 
     When the first structure having no TiO 2  (TiO 2  thickness=0 nm) is compared with the second structure with a 5 nm interlayer formed of TiO 2 , it can be recognized that the dark current decreases by 227 times compared to the first structure having no interlayer. 
     When the first structure having no TiO 2  (TiO 2  thickness=0 nm) is compared with the structure having TiO 2  thickness of 7 nm, it can be recognized that the dark current decreases by 7,900 times compared to the first structure having no interlayer. 
     Likewise, when the first structure having no TiO 2  (TiO 2  thickness=0 nm) is compared with the structure having TiO 2  thickness of 9 nm, it can be recognized that the dark current according to the embodiment decreases by 17,000 times compared to the first structure having no interlayer. 
     Therefore, an interlayer of an embodiment of the present disclosure may have a thickness of 5 nm to 9 nm. Of course, the interlayer may be formed to have various thicknesses in various embodiments in consideration of characteristics of the semiconductor layer, an objective function of the photodiode, and a difference in constituent materials of the interlayer. 
       FIGS. 17 to 20  are graphs illustrating a difference in flow between a dark current and a photocurrent according to the interlayer.  FIG. 17  shows electrical characteristics of emitted light emitted from a laser having a wavelength of 1.55 μm.  FIGS. 17 to 20  show graphs based on embodiments in which the interlayer is formed of TiO 2 . 
       FIG. 17  shows characteristics of the dark current and the photocurrent for a structure having no interlayer. 
       FIG. 18  shows characteristics of the dark current and the photocurrent for a structure in which the interlayer has thickness of 5 nm.  FIG. 19  shows characteristics of the dark current and the photocurrent for a structure in which the interlayer has thickness of 7 nm.  FIG. 20  shows characteristics of the dark current and the photocurrent for a structure in which the interlayer has thickness of 9 nm. 
     As seen in  FIG. 17 , when no interlayer material is present, the dark current is relatively high, resulting in an on-off ratio of 1.04.  FIG. 18  shows an embodiment in which a 5 nm thick layer of TiO 2  is present, resulting in a substantially reduced level of dark current. Although the photocurrent is also reduced at higher voltages compared to the embodiment of  FIG. 17 , the reduction in dark current is greater, so the on-off ratio is 8.67, which is improved compared to  FIG. 17 . Similarly, the embodiments of  FIG. 19  and  FIG. 20 , which have interlayer thicknesses of 7 nm and 9 nm, respectively, have dark current reduction amounts that exceed the photocurrent reduction amounts, resulting in on-off ratios of 262 and 419, respectively. Therefore, embodiments of the present application are substantial improvements to conventional photodiode technology. 
       FIGS. 21 and 22  are graphs showing electrical characteristics of photodiodes according to embodiments of the present disclosure. The graphs of  FIGS. 21 and 22  are obtained from embodiments in which the interlayer is formed of TiO 2 . In  FIG. 21 , graph  2001  shows normalized photo-to-dark current ratios (NPDR) for various thicknesses of the interlayer. The graph  2002  of  FIG. 22  shows NPDR based on interlayer thickness at 1V Graph  2002  shows that the NPDR of an embodiment in which a TiO 2  interlayer is present is about 6,600 times greater than the NPDR of an embodiment in which no interlayer material is present. 
     In accordance with one embodiment of the present disclosure, the electrode layers present in a cathode portion of the photodiode are different from the electrode layers present in an anode portion of the photodiode, thereby reducing dark current, resulting in increased performance of the photodiode. 
     A photodiode according to an embodiment of the present disclosure is an interdigitated photodetector that has different types of contact parts. When the contact parts protrude from the connection electrode part in an interdigitated structure, the interlayer may be disposed only in the contact parts, or fingers, of the cathode, and no interlayer may be disposed in the connection electrode portion of the cathode. 
     Embodiments of the present disclosure can reduce dark current by inserting an interlayer into portions of the electrode layer of the photodiode, preserving fabrication simplicity while improving photodiode performance. 
     As is apparent from the above description, a photodiode and the method for manufacturing the same according to embodiments of the present disclosure can reduce a dark current by asymmetrically constructing an electrode layer of a photodiode, resulting in increased photodiode performance. 
     A photodiode and the method for manufacturing the same according to an embodiment of the present disclosure can reduce a dark current by inserting an interlayer into some parts of an electrode layer of the photodiode, resulting in increased fabrication simplicity and increased performance of the photodiode. 
     The present disclosure described above may be variously substituted, altered, and modified by those skilled in the art to which the present invention pertains without departing from the scope and sprit of the present disclosure. Therefore, the present disclosure is not limited to the above-mentioned exemplary embodiments and the accompanying drawings.