Patent Publication Number: US-2023141520-A1

Title: Optical Receiver

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national phase entry of PCT Application No. PCT/JP2020/017455, filed on Apr. 23, 2020, which application is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a light-receiving device composed of a semiconductor. 
     BACKGROUND 
     A light-receiving element plays a role in converting an optical signal propagating in an optical fiber into an electrical signal in optical communications. With an increased communication capacity in data centers or the like in recent years, an increase in transmission capacity of an optical fiber communication system is awaited. This requires high-speed light-receiving elements to be used for optical fiber communication systems, and thus semiconductor light-receiving elements such as a photodiode (PD) are commonly used. This type of light-receiving element for optical communication generally uses a substrate composed of InP, on which a light-receiving layer composed of an InP-based compound semiconductor including a light-absorbing layer is formed. For example, InGaAs having a large light absorption coefficient in a communication wavelength band (1.55 μm or 1.3 μm) is used for the light-absorbing layer. 
     Because a light-receiving sensitivity of a common normal incidence PD as described in NPL 1 is determined by a light path length in a light-absorbing layer, high light-receiving sensitivity is generally achieved by increasing the thickness of the light-absorbing layer. On the other hand, a band of the PD is determined by a carrier transit time, an element capacitance, a resistance, and the like, and increasing the thickness of the light-absorbing layer increases the carrier transit time, causing a decrease in the band. As described above, a normal incidence type PD has a trade-off relationship between a light-receiving sensitivity and a band. 
     To solve this issue, an oblique incidence light-receiving device has been proposed, in which light is incident on a light-receiving layer from an oblique direction and propagated obliquely with respect to a lamination direction of a light-absorbing layer (NPL 2). 
     This oblique incidence light-receiving device will be described with reference to  FIG.  11   . The light-receiving device includes a light-receiving element in which a first contact layer  302  composed of InGaAsP or the like, a light-absorbing layer  303  composed of InGaAs, and a second contact layer  304  composed of InGaAsP or the like are laminated in this order on an InP substrate  301 . A first electrode  311  is connected to the first contact layer  302 , and a second electrode  312  is connected to the second contact layer  304 . 
     Further, the light-receiving device includes a facet surface  305  in a direction of (1, −1, −1) on a side surface of the InP substrate  301  by a method, such as wet etching. Light incident on the facet surface  305  from a lateral direction is incident on a side of a back surface (the first contact layer  302 ) of the light-receiving element at an incident angle of 65° and is incident on the light-absorbing layer  303  at an incident angle of 54°. As a result, a light path length is increased by 1.7 times as compared with that of a normal incidence light-receiving device, and thus a significant increase in sensitivity can be expected. 
     CITATION LIST 
     Non Patent Literature 
     NPL 1: M. Nada et al., “Inverted InAlAs/InGaAs Avalanche Photodiode with Low-High-Low Electric Field Profile”, Japanese Journal of Applied Physics, vol. 51, 02BG03, 2012. 
     NPL 2: Y. Hirota et al., “Reliable non-Zn-diffused InP/InGaAs avalanche photodiode with buried n-InP layer operated by electron injection mode”, Electronics Letters, vol. 40, no. 21, pp. 2004. 
     SUMMARY 
     Technical Problem 
     Because light propagates obliquely in the light-receiving element (light-absorbing layer) in the oblique incidence light-receiving device described above, the beam spot of the light propagating in the light-receiving element including the light-absorbing layer is expanded as compared with that of a normal incidence light-receiving device. To prevent a decrease in sensitivity due to incident light leakage caused by expansion of the beam spot, it is necessary to increase the area in a plan view of a light propagation path (light-absorbing layer) in the light-receiving element depending on the above-described beam spot. However, the increase in the area of the light-receiving element causes an increase in the operation area of the light-receiving element, thus decreasing the band due to an increase in an element capacitance. For example, an existing oblique incidence structure as illustrate in  FIG.  11    has the size in a plan view of the beam spot in the first contact layer  302 , in which the length in an incident direction (x direction) is 2.9 times larger than that in a y direction perpendicular to the incident direction. 
     It is a common technique to reduce the spot size of incident light by using an external lens and achieve an optical coupling with little loss for an element having a small light-receiving area when signal light is incident on a light-receiving element, and the spot size of incident light in a normal incidence structure can be reduced to about 10 μm. However, the beam spot cannot be further narrowed with an external lens having a long focal length, and the oblique incidence structure illustrated in  FIG.  11    allows the spot size of incident light to be reduced only to about 29 μm×10 μm. This makes it difficult to reduce the size of a light-receiving element. 
     As described above, the beam spot of light incident on the light-receiving element is expanded in the light-receiving device having an oblique incidence structure. This makes it difficult to reduce the size of the light-receiving element and achieve high speed operation. 
     The present disclosure has been made to solve the above-described issue, and an object thereof is to allow for further reducing the size of a light-receiving element without causing a decrease in band in a light-receiving device having an oblique incidence structure. 
     Means for Solving the Problem 
     A light-receiving device according to the present disclosure includes a light-receiving element formed on a main surface of a substrate, a light incidence surface formed on a side portion of the substrate at an acute angle or an obtuse angle with respect to a plane of the substrate and having an inclined surface forming one plane, and a lens for focusing light incident on the light-receiving element. The light-receiving element includes a back surface incidence photodiode including a first semiconductor layer formed on the substrate and composed of a first conductivity type semiconductor, a light absorbing layer formed on the first semiconductor layer and composed of a semiconductor, a second semiconductor layer formed on the light-absorbing layer and composed of a second conductivity type semiconductor, a first electrode connected to the second semiconductor layer, and a second electrode connected to the first semiconductor layer. Light incident from the light incidence surface is reflected on a side of a back surface of the substrate and is incident on the light-receiving element obliquely with respect to a plane of the light-absorbing layer. 
     Effects of Embodiments of the Invention 
     As described above, according to the present disclosure, since the lens is provided to focus the light which is incident from the light incidence surface formed on the side portion of the substrate at an acute angle or an obtuse angle with respect to the plane of the substrate on which the light-receiving element is formed and is incident on the light-receiving element, the size of the light-receiving element can be further reduced without causing a decrease in band in the light-receiving device having the oblique incidence structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a cross-sectional view illustrating a configuration of a light-receiving device according to a first embodiment of the present disclosure. 
         FIG.  2 A  is a perspective view illustrating part of a configuration of a light-receiving device according to the first embodiment of the present disclosure. 
         FIG.  2 B  is a plan view ( FIG.  2 B (a)) and side views ( FIGS.  2 B (b) and (c)) illustrating part of a configuration of a light-receiving device according to the first embodiment of the present disclosure. 
         FIG.  3    is a plan view ( FIG.  3 ( a ) ) and side views ( FIGS.  3 ( b ) and ( c ) ) illustrating part of a configuration of a light-receiving device according to a second embodiment of the present disclosure. 
         FIG.  4    is a perspective view illustrating a configuration of a light-receiving device according to a third embodiment of the present disclosure. 
         FIG.  5    is a perspective view illustrating a configuration of a light-receiving device according to a fourth embodiment of the present disclosure. 
         FIG.  6    is a cross-sectional view illustrating a configuration of a light-receiving device according to a fifth embodiment of the present disclosure. 
         FIG.  7    is a cross-sectional view illustrating a configuration of a light-receiving device according to a sixth embodiment of the present disclosure. 
         FIG.  8    is a cross-sectional view illustrating a configuration of a light-receiving device according to a seventh embodiment of the present disclosure. 
         FIG.  9    is a cross-sectional view illustrating a configuration of a light-receiving device according to the seventh embodiment of the present disclosure. 
         FIG.  10    is a cross-sectional view illustrating a configuration of a light-receiving device according to an eighth embodiment of the present disclosure. 
         FIG.  11    is a cross-sectional view illustrating a configuration of an oblique incidence light-receiving device. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Hereinafter, a light-receiving device according to an embodiment of the present disclosure will be described. 
     First Embodiment 
     First, a light-receiving device according to the first embodiment of the present disclosure will be described with reference to  FIGS.  1 ,  2 A and  2 B . The light-receiving device includes a light-receiving element  102  formed on a main surface of a substrate  101 , a light incidence surface  106  which is formed on a side portion of the substrate  101  at an acute angle or an obtuse angle with respect to a plane of the substrate  101  and is composed of an inclined surface forming one plane, and a lens  107  for focusing light incident on the light-receiving element  102 . 
     The substrate  101  is composed of, for example, InP. The light-receiving element  102  includes a first semiconductor layer  103  which is formed on the substrate  101  and is composed of a first conductivity type semiconductor, a light-absorbing layer  104  which is formed on the first semiconductor layer  103  and is composed of a semiconductor, and a second semiconductor layer  105  which is formed on the light-absorbing layer  104  and is composed of a second conductivity type semiconductor. The light-receiving element  102  is a so-called back surface incidence type photodiode. 
     The first semiconductor layer  103  is composed of, for example, a first conductivity type (n type, for example) InGaAsP. The light-absorbing layer  104  is composed of i-InGaAs. The second semiconductor layer  105  is composed of a second conductivity type (p type, for example) InGaAsP. 
     Further, the light-receiving element  102  includes a first electrode  121  connected to the second semiconductor layer  105  and a second electrode  122  connected to the first semiconductor layer  103 . These electrodes can be composed of a laminated metal structure made of Ti/Pt/Au, for example. 
     In the light-receiving device according to the first embodiment, the light incident from the light incidence surface  106  is reflected on a side of a back surface of the substrate  101  and is incident on the light-receiving element  102  obliquely with respect to a plane of the light-absorbing layer  104 . In the first embodiment, the angle between the main surface of the substrate  101  in the region where the light-receiving element  102  is formed and the light incidence surface  106  is an acute angle, and the light incident from the light incidence surface  106  is reflected on a side of the main surface of the substrate  101 , reflected on the side of the back surface of the substrate  101 , and then incident on the light-receiving element  102 . In the first embodiment, the lens  107  is disposed at a position where the light incident from the light incidence surface  106  is reflected on the side of the back surface of the substrate  101 . 
     Further, in the first embodiment, the lens  107  has a curvature in an incident direction of the light incident from the light incidence surface  106  (x direction), and has a shape similar to a so-called cylindrical lens. 
     In the light-receiving device according to the first embodiment having the structure described above, the light incident from the light incidence surface  106  in a state of being parallel to the plane of the substrate  101  is refracted by the light incidence surface  106  to change the traveling direction of the light on a plane (xz plane) parallel to a plane perpendicular to the plane of the substrate  101 , and is reflected by the main surface of the substrate  101  to change the traveling direction of the light again on the xz plane. Then, the light is reflected by the surface of the lens  107  to change the traveling direction of the light at the third time on the xz plane and is obliquely incident on the back surface of the light-receiving element  102 . Here, because the lens  107  has a curvature in the x direction and has a capability to focus light in the x direction, the light reflected by the surface of the lens  107  can form a spot of the light passing through the light-receiving element  102  into a perfect circle shape. 
     Specific curvature and focal length of the lens  107  are arbitrary design items because they also depend on an optical system on an incident side, but the lens  107  can be designed so that the focal length is equivalent to the thickness of the substrate  101 . In order to reduce the spot size in the x direction to one third or less, the thickness of the substrate  101  is preferably 150 μm or less. 
     According to the first embodiment, as described above, the expansion of the spot due to the use of the oblique incidence structure can be reduced, and high-speed operation with a downsized light-receiving element can be achieved. 
     First, the first semiconductor layer  103 , the light-absorbing layer  104 , and the second semiconductor layer  105  are caused to undergo crystal growth on the substrate  101  composed of InP. Each semiconductor layer may be grown using, for example, a well-known metal-organic chemical vapor deposition (MOCVD) method. Subsequently, the first semiconductor layer  103 , the light-absorbing layer  104 , and the second semiconductor layer  105  are processed into a mesa shape by the publicly known photolithographic technique and an etching technique. Then, the first electrode  121  and the second electrode  122  are formed by a manufacturing technique such as vapor deposition, lift-off, or the like. 
     Next, a protective film which covers the light-receiving element  102  and has an opening in a region of the substrate  101  where the light incidence surface  106  is to be formed is formed by the publicly known photolithographic technique, and the substrate  101  is selectively etched by wet etching using the formed protective film as a mask and hydrochloric acid as an etchant. For example, the main surface of the substrate  101  composed of InP is a (001) surface, and the side wall of the substrate  101  is a (−110) surface. By anisotropically etching the portion of the side surface of the substrate  101  where the protective film is opened with the above-described wet etching, the (1, −1, −1) surface of InP is exposed, that is, a facet surface in the (1, −1, −1) direction is formed, and the light incidence surface  106  is formed. 
     Next, the substrate  101  is thinned from the side of the back surface by a well-known polishing technique such as mechanical polishing, and then the lens  107  is formed in a predetermined position on the back surface of the substrate  101 . The lens  107  may be formed by using, for example, such a resist pattern transfer technique as described in the reference document. First, a so-called positive photoresist composed of, for example, a novolac resin is applied to the back surface of the substrate  101  to form a resist layer. Next, the formed resist layer is exposed and developed by the publicly known lithographic technique to form a cuboid resist pattern having a rectangular shape in a plan view. 
     Next, the formed resist pattern is reflowed by heating to, for example, 100° C. to 200° C. By this heat treatment, the resist pattern has a shape obtained by cutting out a part of a cylinder. Next, using the reflowed resist pattern as a mask, the back surface of the substrate  101  is etched by a dry etching technique having perpendicular anisotropy such as reactive ion etching under the processing conditions such that the resist pattern and the substrate  101  have the same etching rate. By this etching treatment, the shape of the reflowed resist pattern can be formed on the back surface of the substrate  101 , and the lens  107  similar to a cylindrical lens can be obtained. 
     Second Embodiment 
     Next, a light-receiving device according to the second embodiment of the present disclosure will be described with reference to  FIG.  3   . In the first embodiment described above, the lens  107  is a so-called cylindrical lens having a curvature in the incident direction of the light incident from the light incidence surface  106  (x direction), but the present disclosure is not limited thereto. For example, a lens  107   a  having a curvature in the incident direction of the light incident from the light incidence surface  106  (x direction) as well as in a direction perpendicular to the incident direction of the light incident from the light incidence surface  106  (y direction) in a plane parallel to the plane of the substrate  101  can be also disposed (formed) on the back surface of the substrate  101 . The curvature in the incident direction (x direction) and the curvature in the direction perpendicular to the incident direction (y direction) are different from each other. Other configurations are similar to the configurations of the first embodiment described above. 
     As described in the first embodiment, when the lens  107  similar to a cylindrical lens is used, it is difficult to focus the spot in the y direction on the light-receiving element  102  in a plan view to a size of 10 μm or less, and the same applies to the case where an external lens (external optical system) having a long focal length is used. Thus, there is a room for improvement in increasing speed by reducing element size. In contrast, by using the lens  107   a  according to the second embodiment, it is possible to reduce the spot size while maintaining the spot shape on the light-receiving element in a perfect circular shape. 
     Specific curvature and focal length of the lens  107   a  are arbitrary design items because they also depend on an external optical system on an incident side, but the lens  107   a  can be formed so that the focal length is equivalent to the thickness of the substrate  101 . In order to reduce the spot size in the x direction and the y direction to 10 μm or less, the thickness of the substrate  101  is preferably 150 μm or less. 
     According to the second embodiment, as described above, the expansion of the spot due to the use of the oblique incidence structure can be reduced, and high-speed operation with a downsized light-receiving element can be achieved. Although the second embodiment has been described with reference to the lens  107   a  having an elliptical shape in a plan view, the spot size can be reduced even when a lens having a perfect circular shape in a plan view is used. However, in that case, the spot shape of light in the plan view on the light-receiving element  102  is elliptical. Note that the lens  107   a  can be formed in the same manner as the lens  107  described above. The lens  107   a  may be formed by forming a resist pattern having an elliptical shape in a plan view and reflowing it. 
     Third Embodiment 
     Next, a light-receiving device according to the third embodiment of the present disclosure will be described with reference to  FIG.  4   . In the light-receiving device according to the third embodiment, a plurality of light-receiving elements  102   a,    102   b,    102   c  and  102   d  are formed on the main surface of the substrate  101 . Each of the light-receiving elements  102   a,    102   b,    102   c  and  102   d  is identical to the light-receiving element  102  according to the first embodiment described above. The light-receiving elements  102   a,    102   b,    102   c  and  102   d  are arranged on a straight line extending in the y direction perpendicular to the incident direction on the main surface of the substrate  101 . Incident light is incident on each of the light-receiving elements  102   a,    102   b,    102   c  and  102   d.    
     When the plurality of light-receiving elements  102   a,    102   b,    102   c  and  102   d  are provided on the main surface of the substrate  101 , the length of the lens  107  in they direction is longer than the arrangement length of the plurality of light-receiving elements  102   a,    102   b,    102   c  and  102   d.  With this configuration, incident light can be focused by one lens  107  to each of the plurality of light-receiving elements  102   a,    102   b,    102   c  and  102   d.  In this configuration, even with the plurality of light-receiving elements  102   a,    102   b,    102   c  and  102   d,  it is not necessary to form a plurality of lenses, and thus manufacturing is facilitated. In addition, light can be focused by the same lens  107 , and thus characteristics variations between the light-receiving elements  102   a,    102   b,    102   c  and  102   d  can be reduced. 
     In addition, according to the third embodiment, because an oblique incidence structure is used as with embodiments 1 to 3 described above, the expansion of the spot can be reduced, and high-speed operation with a downsized light-receiving element can be achieved. 
     Fourth Embodiment 
     Next, the fourth embodiment of the present disclosure will be described with reference to  FIG.  5   . In the light-receiving device according to the fourth embodiment, the plurality of light-receiving elements  102   a,    102   b,    102   c  and  102   d  are formed on the main surface of the substrate  101 . These configurations are similar to those of the third embodiment described above. In the fourth embodiment, the lens  107   a  having a curvature in the x direction as well as in the y direction is used. 
     According to the fourth embodiment, there is an advantage that intervals between light-receiving elements are not limited by the size of a lens, as compared with a configuration in which the same number of light-receiving elements and lenses are provided. 
     Fifth Embodiment 
     Next, the fifth embodiment of the present disclosure will be described with reference to  FIG.  6   . The light-receiving device according to the fifth embodiment includes a recessed portion  108  formed on the back surface of the substrate  101 . The recessed portion  108  is, for example, a groove extending in a direction (the y direction) perpendicular to the incident direction (the x direction). In the fifth embodiment, the lens  107  is formed on the bottom surface of the recessed portion  108 . Further, in the fifth embodiment, a metal layer  109  formed to cover the surface of the lens  107  is provided. The metal layer  109  functions as a mirror. Furthermore, in the fifth embodiment, a protective film no is provided to protect the lens  107  on which the metal layer  109  is formed. Other configurations are similar to the configurations of the first embodiment described above. 
     In mounting the light-receiving device on a module, there is a possibility that the lens  107  may be damaged to cause a reduction in reflectance by the contact between a package substrate or the like and the back surface of the substrate  101 . By forming the recessed portion  108  and providing the lens  107  on the bottom portion thereof, it is possible to prevent the lens from coming into contact with other parts in mounting. In addition, by forming the protective film  110 , an effect of preventing the surface of the lens  107  from being scratched or being contaminated with impurities can be expected. The protective film  110  can be composed of a resin, for example. The protective film  110  can also be composed of SiN, SiO 2 , or the like. 
     However, when the protective film  110  is formed, there is a possibility that a refractive index difference with respect to the lens  107  may decrease and the reflectance on the surface of the lens  107  may decrease. For example, at the interface between InP (refractive index: 3.2) and air (refractive index: 1.0), total reflection occurs at an incident angle of 18° or greater. On the other hand, at the interface between InP and SiN (refractive index: 2.0), total reflection occurs at an incident angle of 39° or greater. In this way, there is a concern that reflectance may decrease by forming the protective film  110 . In contrast, by forming the metal layer  109 , an effect of preventing a decrease in reflectance at the surface of the lens  107  is produced. 
     In the fifth embodiment, as in embodiments 1 to 4 described above, the expansion of the spot due to the use of an oblique incidence structure can be reduced, and high speed operation with a downsized light-receiving element can be achieved. Note that, the structure of the light-receiving device according to the fifth embodiment can be made by forming the recessed portion  108  before forming the lens  107  described in the first embodiment. The recessed portion  108  can be formed by the publicly known lithographic technique and a dry etching technique. Further, after the lens  107  is formed, the metal layer  109  can be formed by depositing Au or the like with a deposition technique such as vapor deposition. Furthermore, after the metal layer  109  is formed, the protective film no can be formed by depositing SiN, SiO 2 , or the like with a deposition technique such as chemical vapor deposition. 
     Sixth Embodiment 
     Next, the sixth embodiment of the present disclosure will be described with reference to  FIG.  7   . In the light-receiving device according to the seventh embodiment, the lens  107  is formed on the main surface of the substrate  101 . In the sixth embodiment, a recessed portion  111  is formed on the main surface of the substrate  101 , and the lens  107  is formed on the bottom surface of the recessed portion  108 . The lens  107  is disposed at a position where the light incident from the light incidence surface  106  is reflected on the main surface side of the substrate  101 . Other configurations are similar to the configurations of the first embodiment described above. 
     In the light-receiving device according to the sixth embodiment having the structure described above, the light incident from the light incidence surface  106  in a state of being parallel to the plane of the substrate  101  is refracted by the light incidence surface  106  to change the traveling direction of the light on a plane (xz plane) parallel to a plane perpendicular to the plane of the substrate  101 . Subsequently, the light is reflected by the surface of the lens  107  on the main surface side of the substrate  101  to change the traveling direction of the light again on the xz plane. Then, the light is reflected by the back surface of the lens  107  to change the traveling direction of the light at the third time on the xz plane and is obliquely incident on the back surface of the light-receiving element  102 . Here, because the lens  107  has a curvature in the x direction and has a capability to focus light in the x direction, the light reflected by the surface of the lens  107  can form a spot of the light passing through the light-receiving element  102  into a perfect circle shape. 
     According to the sixth embodiment, the recessed portion in and the lens  107  are formed on the front surface side of the substrate  101  on which the light-receiving element  102  is formed. Thus, for example, in the lithographic technique for forming the recessed portion  111  and the lens  107 , alignment of exposure on the side of the back surface of the substrate  101  is not necessary, and the light-receiving device can be manufactured without a complicated process. In addition, because thinning of the substrate  101  can be performed after the formation of the lens  107 , the lens  107  can be formed on the substrate  101  being thick and having a high mechanical strength. 
     In the sixth embodiment, as in embodiments 1 to 5 described above, the expansion of the spot due to the use of an oblique incidence structure can be reduced, and high speed operation with a downsized light-receiving element can be achieved. By the way, the substrate  101  can also be composed of a material different from the InP-based compound semiconductor constituting the light-receiving element. The substrate  101  can be composed of Si, for example. Si has a higher processability by dry etching than a material system such as InP, and thus makes it easier to form lenses. Thus, the light-receiving device can be made with a higher processing accuracy. 
     For example, after the light-receiving element  102  is formed on a growth substrate composed of InP or the like, the growth substrate is thinned by mechanical polishing or the like. Subsequently, a substrate made of silicon is attached to the thinned growth substrate to form the substrate  101 . Then, the recessed portion and the lens are formed as described above. In addition, the lens does not need to be composed of the same material as the substrate, that is, a lens formed of a material system such as glass can be attached onto a predetermined location. 
     Seventh Embodiment 
     Next, the seventh embodiment of the present disclosure will be described with reference to  FIG.  8   . In the light-receiving device according to the eighth embodiment, an angle between the main surface of a substrate  101  in a region where the light-receiving element  102  is formed and a light incidence surface  106   a  is an obtuse angle. In the seventh embodiment, the light incidence surface  106   a  is formed to face the front surface side of the substrate  101   a.  Thus, in the seventh embodiment, incident light is incident on the light incidence surface  106   a  from above the substrate  101   a.    
     In the seventh embodiment, the light incident from the light incidence surface  106   a  is reflected on a side of a back surface of the substrate  101   a  and is incident on the light-receiving element  102 , and the lens  107  is disposed at a position where the light incident from the light incidence surface  106   a  is reflected on the side of the back surface of the substrate  101   a.  Other configurations are similar to the configurations of the first embodiment described above. 
     In the seventh embodiment, when the lens  107  is not used, the spot size on the light-receiving element  102  can be reduced only to 23 μm×10 μm, and thus it is difficult to reduce the operating area of the light-receiving element  102 . On the other hand, by providing the lens  107 , an effect of reducing the expansion of the spot size can be expected. 
     In addition, in the seventh embodiment, it is not necessary to form a side surface of the substrate  101   a  by cleavage or the like at a predetermined distance from the location of the light-receiving element  102 . This is because the light incidence surface  106   a  faces the front surface side of the substrate  101   a.  Thus, light can be incident on the light-receiving element  102  without forming the side surface at a predetermined position by cleavage after the light-receiving element  102  is formed on the substrate  101   a.  Consequently, characteristics evaluation can be done in a wafer form. The light incidence surface  106   a  can be formed with an etching technique such as wet etching by using, for example, a facet surface of InP in a (1, 1, 1) direction. 
     Eighth Embodiment 
     Next, the eighth embodiment of the present disclosure will be described with reference to  FIG.  9   . In the light-receiving device according to the eighth embodiment, an angle between the main surface of the substrate  101   a  in a region where the light-receiving element  102  is formed and the light incidence surface  106   a  is an obtuse angle. In the eighth embodiment, the light incidence surface  106   a  is formed to face the front surface side of the substrate  101   a.  Thus, in the eighth embodiment, incident light is incident on the light incidence surface  106   a  from above the substrate  101   a.  These configurations are similar to those of the seventh embodiment described before. 
     In the eighth embodiment, the light incident from the light incidence surface  106  is reflected on a side of a back surface of the substrate  101  and is incident on the light-receiving element  102 , and the lens  107  is disposed on the incident side of the light incident from the light incidence surface  106   a.  In the eighth embodiment, a protective film  112  is provided to cover the light incidence surface  106   a,  and the lens  107  is disposed on the protective film  112 . The protective film  112  can be composed of a resin, or an insulation material such as SiN and SiO 2 , for example. In the present example, the surface of the protective film  112  on which the lens  107  is formed forms the same plane as the main surface of the substrate  101   a.    
     According to the eighth embodiment, because the light incidence surface  106   a  is protected by the protective film  112 , an effect of preventing the light incidence surface  106   a  from being scratched or being contaminated with impurities can be expected. In addition, in the eighth embodiment, because the oblique incidence structure is used as with embodiments 1 to 7 described above, the expansion of the spot can be reduced, and high-speed operation with a downsized light-receiving element can be achieved. 
     Ninth Embodiment 
     Next, a ninth embodiment of the present disclosure will be described with reference to  FIG.  10   . In the light-receiving device according to the ninth embodiment, an angle between the main surface of the substrate  101   a  in a region where the light-receiving element  102  is formed and the light incidence surface  106   a  is an obtuse angle. In the ninth embodiment, the light incidence surface  106   a  is formed to face the front surface side of the substrate  101   a.  Thus, in the ninth embodiment, incident light is incident on the light incidence surface  106   a  from above the substrate  101   a.  These configurations are similar to those of the seventh embodiment described before. 
     In the ninth embodiment, the light incident from the light incidence surface  106  is reflected on a side of a back surface of the substrate  101  and is incident on the light-receiving element  102 , and the lens  107  is disposed on the incident side of the light incident from the light incidence surface  106   a.  In the ninth embodiment, the lens  107  is disposed on the light incidence surface  106   a.  Other configurations are similar to the configurations of the ninth embodiment described above. The lens  107  is composed of, for example, a material such as Si, and attached to the light incidence surface  106   a.  In the ninth embodiment, because the oblique incidence structure is used as with the first to eighth embodiments described above, the expansion of the spot can be reduced, and high-speed operation with a downsized light-receiving element can be achieved. 
     Note that, although InP or Si is used as the substrate in the above description, the substrate is not limited thereto and can be composed of SiC, GaN, glass, or the like. In addition, although the case where the light-absorbing layer is composed of InGaAs has been described, the light-absorbing layer is not limited thereto and can be composed of other semiconductors such as Ge. 
     Light may be incident from the top surface or the back surface of the light-receiving element, or can be incident from a side surface, or can be incident from an oblique direction. 
     Besides, although the method using a facet surface for forming the light incidence surface has been described, the method is not limited thereto, and the light incidence surface can be formed by an arbitrary processing method such as dicing. Further, a spherical or an aspherical lens, or a Fresnel lens may be used as the lens. Furthermore, an anti-reflective layer may be formed on the light incidence surface. In addition, in the light-receiving element, it is within the scope of general design to provide a mirror on the upper side (on the second semiconductor layer) for increasing a light path length of the light passing through the light-receiving element. Moreover, although a so-called Pin-type photodiode has been described as an example in the above description, the light-receiving element may be composed of an avalanche photodiode. 
     As described above, according to the present disclosure, because the lens is provided to focus the light incident from the light incidence surface formed on the side portion of the substrate at an acute angle or an obtuse angle with respect to the plane of the substrate on which the light-receiving element is formed and incident on the light-receiving element, the size of the light-receiving element can be further reduced without causing a decrease in band in the light-receiving device having the oblique incidence structure. 
     Meanwhile, the present disclosure is not limited to the embodiments described above, and it will be obvious to those skilled in the art that various modifications and combinations can be implemented within the technical idea of the present disclosure. 
     [Reference] O. Wada, “Ion-Beam Etching of InP and Its Application to the Fabrication of High Radiance InGaAsP/InP Light Emitting Diodes”, J. Electrochem. Soc., vol. 131, no. 10, pp. 2373-2380, 1984. 
     REFERENCE SIGNS LIST 
       101  Substrate 
       102  Light-receiving element 
       103  First semiconductor layer 
       104  Light-absorbing layer 
       105  Second semiconductor layer 
       106  Light incidence surface 
       107  Lens 
       121  First electrode 
       122  Second electrode.