Patent Publication Number: US-2022223631-A1

Title: Light detector, light detection system, lidar device, mobile body, and vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-002071, filed on Jan. 8, 2021; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a light detector, a light detection system, a lidar device, a mobile body, and a vehicle. 
     BACKGROUND 
     There is a light detector that detects light incident on a semiconductor region. It is desirable to improve the performance of the light detector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic plan view illustrating a light detector according to a first embodiment; 
         FIGS. 2A and 2B  are schematic views illustrating a portion of the light detector according to the first embodiment; 
         FIGS. 3A to 3C  are schematic cross-sectional views illustrating calculation results of the optical path of the light detector; 
         FIG. 4  is a graph illustrating calculation results of the optical absorption efficiency of the light detector; 
         FIGS. 5A and 5B  are schematic plan views illustrating portions of other light detectors according to the first embodiment; 
         FIG. 6  is a cross-sectional view illustrating a portion of another light detector according to the first embodiment; 
         FIG. 7  is a plan view illustrating a light detector according to a second embodiment; 
         FIG. 8  is a schematic cross-sectional view illustrating a portion of the light detector according to the second embodiment; 
         FIGS. 9A to 9D  are schematic plan views illustrating portions of other light detectors according to the second embodiment; 
         FIGS. 10A and 10B  are schematic cross-sectional views illustrating portions of other light detectors according to the second embodiment; 
         FIG. 11  is a schematic plan view illustrating a portion of a light detector according to the second embodiment; 
         FIG. 12  is a schematic cross-sectional view illustrating a portion of the light detector according to the second embodiment; 
         FIG. 13  is a schematic plan view illustrating a portion of the light detector according to the second embodiment; 
         FIG. 14  is a plan view illustrating a portion of the light detector according to the second embodiment; 
         FIG. 15  is a plan view illustrating a portion of the light detector according to the second embodiment; 
         FIGS. 16A and 16B  are schematic cross-sectional views illustrating calculation results of the distribution range of the avalanche region of the light detector; 
         FIG. 17  is a graph illustrating calculation results of the distribution of the avalanche probability; 
         FIGS. 18A to 18C  are schematic cross-sectional views illustrating calculation results of the optical path of the light detector; 
         FIG. 19  is a schematic view illustrating a lidar (Laser Imaging Detection and Ranging (LIDAR)) device according to a third embodiment; 
         FIG. 20  is a drawing for describing the detection of the detection object of the lidar device; and 
         FIG. 21  is a schematic top view of a vehicle that includes the lidar device according to the third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to one embodiment, a light detector includes a plurality of elements. Each of the elements includes a first semiconductor region of a first conductivity type, a second semiconductor region of the first conductivity type, and a third semiconductor region of a second conductivity type. The second semiconductor region is located on the first semiconductor region and has a higher first-conductivity-type impurity concentration than the first semiconductor region. The third semiconductor region is located on the second semiconductor region. The elements are arranged at a first period in a second direction crossing a first direction. The first direction is from the first semiconductor region toward the second semiconductor region. A quenching part is electrically connected with the third semiconductor region. A plurality of lenses are located respectively on the elements. One of the lenses is positioned on one of the elements. A refracting layer is located between the elements and the lenses. The refracting layer has a first thickness. A ratio of the first thickness to the first period is not less than 0.16 and not more than 0.72. 
     According to one embodiment, a light detection system includes the light detector described above, and a distance measuring circuit calculating a time-of-flight of light from an output signal of the light detector. 
     According to one embodiment, a lidar device includes a light source irradiating light on an object, and the light detection system described above. The system detects light reflected by the object. 
     According to one embodiment, a mobile body includes the lidar device described above. 
     According to one embodiment, a vehicle includes a plurality of the lidar devices described above, and a vehicle body. The lidar devices are located at four corners of the vehicle body. 
     Various embodiments are described below with reference to the accompanying drawings. 
     The drawings are schematic and conceptual; and the relationships between the thickness and width of portions, the proportions of sizes among portions, etc., are not necessarily the same as the actual values. The dimensions and proportions may be illustrated differently among drawings, even for identical portions. 
     In the specification and drawings, components similar to those described previously or illustrated in an antecedent drawing are marked with like reference numerals, and a detailed description is omitted as appropriate. 
     First Embodiment 
       FIG. 1  is a schematic plan view illustrating a light detector according to a first embodiment. 
     As illustrated in  FIG. 1 , the light detector  100  according to the first embodiment includes multiple elements  10  (light-receiving elements), a refracting layer  30 , and multiple lenses  40 . The refracting layer  30  is located on the multiple elements  10 ; and the lenses  40  are located on the refracting layer  30 . 
     The multiple elements  10  are arranged in an array configuration or a lattice shape along the X-Y plane. For example, the multiple elements  10  are arranged periodically at a uniform pitch in an X-axis direction and a Y-axis direction. 
     In the description of embodiments, a direction that crosses the X-Y plane or a direction from a first semiconductor region  11  (referring to  FIG. 2A ) toward a second semiconductor region  12  (referring to  FIG. 2A ) described below are taken as a Z-axis direction. A direction perpendicular to the Z-axis direction is taken as the X-axis direction (a second direction). A direction perpendicular to the Z-axis direction and the X-axis direction is taken as the Y-axis direction (a third direction). In the description, the direction from the first semiconductor region  11  toward the second semiconductor region  12  is called “up”, and the opposite direction is called “down”. These directions are based on the relative positional relationship between the first semiconductor region  11  and the second semiconductor region  12  and are independent of the direction of gravity. “Up” corresponds to the side of the light detector at which the lenses  40  are mounted and the light is incident. 
     The multiple lenses  40  are respectively located on the multiple elements  10 . In other words, the multiple lenses  40  are arranged to match the period of the multiple elements  10 ; and one of the multiple lenses  40  is positioned on one of the multiple elements  10 . The multiple lenses  40  are, for example, a microlens array. The microlens array increases the light-receiving sensitivity of the light detector by refracting the optical path of the light that is irradiated on an area at which the light-receiving element is not provided to modify the optical path to irradiate the light into the light-receiving element. 
     The multiple elements  10  are arranged at a first period L 1  in the X-axis direction and are arranged at a second period L 2  in the Y-axis direction. In one element  10  of the example, the length in the X-axis direction and the length in the Y-axis direction are substantially the same. Also, in the example, when viewed from the Z-axis direction, the shapes of the multiple elements  10  are substantially the same, and the first period L 1  and the second period L 2  are substantially equal. 
     For example, the multiple elements  10  include a first element  10   a,  a second element  10   b,  and a third element  10   c.  The second element  10   b  is adjacent to the first element  10   a  in the X-axis direction. The third element  10   c  is adjacent to the first element  10   a  in the Y-axis direction. The period (the pitch) is the repeating unit length between positions. The first period L 1  corresponds to the length connecting the X-axis direction center of the first element  10   a  and the X-axis direction center of the second element  10   b.  The second period L 2  corresponds to the length connecting the Y-axis direction center of the first element  10   a  and the Y-axis direction center of the third element  10   c.  For example, among the other elements  10  that are adjacent to the first element  10   a,  the element  10  that minimizes the distance connecting the center of the first element  10   a  and the center of the other element is taken as the second element  10   b.    
       FIGS. 2A and 2B  are schematic views illustrating a portion of the light detector according to the first embodiment. 
       FIG. 2A  is a line A 1 -A 2  cross section shown in  FIG. 1 . Namely,  FIG. 2A  is a cross section of one part of the light detector  100  shown in  FIG. 1 , that is, one period of the periodic structure. As illustrated in  FIG. 2A , the light detector  100  includes an electrode  50 , a semiconductor layer  22  (a second semiconductor layer), the element  10  (the first semiconductor region  11 , the second semiconductor region  12 , and a third semiconductor region  13 ), the refracting layer  30 , the lens  40 , and a conductive part  61 .  FIG. 2B  is a circuit diagram corresponding to  FIG. 2A . As illustrated in  FIG. 2B , the light detector  100  further includes a quenching part  63  and a first interconnect  51 . The quenching part  63  and the first interconnect  51  are not illustrated in  FIG. 2A  for convenience. Interconnects, etc., also are not illustrated in  FIG. 1   
     The electrode  50  is, for example, a back electrode. The semiconductor layer  22  is located on the electrode  50  and is electrically connected with the electrode  50 . The semiconductor layer  22  is, for example, a semiconductor substrate of a first conductivity type. 
     The first semiconductor region  11  is located on the semiconductor layer  22  and contacts the semiconductor layer  22 . The first semiconductor region  11  is of the first conductivity type and is electrically connected with the semiconductor layer  22 . 
     The second semiconductor region  12  is located on the first semiconductor region  11  and contacts the first semiconductor region  11 . The second semiconductor region  12  is of the first conductivity type and is electrically connected with the first semiconductor region  11 . The first-conductivity-type impurity concentration of the second semiconductor region  12  is greater than the first-conductivity-type impurity concentration of the first semiconductor region  11 . 
     The third semiconductor region  13  is located on the second semiconductor region  12  and contacts the second semiconductor region  12 . The third semiconductor region  13  is of a second conductivity type and is electrically connected with the second semiconductor region  12 . 
     The first conductivity type is one of a p-type or an n-type. The second conductivity type is the other of the p-type or the n-type. Hereinbelow, the first conductivity type is described as the p-type, and the second conductivity type is described as the n-type. 
     The first semiconductor region  11 , the second semiconductor region  12 , and the third semiconductor region  13  are, for example, regions that are located in one semiconductor layer  21  (a first semiconductor layer). A p-n junction is formed at the interface between the second semiconductor region  12  and the third semiconductor region  13 . For example, the second semiconductor region  12  and the third semiconductor region  13  are diffusion layers located in the semiconductor layer  21 . A photodiode is formed of the second and third semiconductor regions  12  and  13 . The multiple elements  10  each include the first semiconductor region  11 , the second semiconductor region  12 , and the third semiconductor region  13 . 
     In the example, the width of the third semiconductor region  13  is greater than the width of the second semiconductor region  12 . The “width” is the length along a direction (e.g., the X-axis direction) perpendicular to the Z-axis direction. However, according to embodiments, the width of the third semiconductor region  13  may be equal to the width of the second semiconductor region  12 . The second semiconductor region  12  and the third semiconductor region  13  are arranged with portions of the first semiconductor region  11  in a direction perpendicular to the Z-axis direction. In other words, the second semiconductor region  12  and the third semiconductor region  13  are surrounded with the first semiconductor region  11  along the X-Y plane. A side surface  12   s  of the second semiconductor region  12  and a side surface  13   s  of the third semiconductor region  13  contact the first semiconductor region  11 . The center of the second semiconductor region  12  or the center of the third semiconductor region  13  corresponds to the center of the element  10  in a direction perpendicular to the Z-axis direction. 
     The semiconductor layer  21  (the first semiconductor region  11 , the second semiconductor region  12 , and the third semiconductor region  13 ) and the semiconductor layer  22  include at least one semiconductor material selected from the group consisting of silicon, silicon carbide, gallium arsenide, and gallium nitride. For example, the semiconductor layer  21  and the semiconductor layer  22  include silicon. For example, the first-conductivity-type impurity concentration of the semiconductor layer  22  is greater than the first-conductivity-type impurity concentration of the first semiconductor region  11 . The second semiconductor region  12  is obtained by implanting, for example, boron as a p-type impurity into silicon. The third semiconductor region  13  is obtained by implanting, for example, phosphorus, arsenic, or antimony as an n-type impurity into silicon. The semiconductor layer  21  is, for example, an epitaxial layer that is formed on a substrate. For example, the (100) plane of the single-crystal silicon that is included in the semiconductor layers  21  and  22  is perpendicular to the Z-axis direction. 
     The refracting layer  30  is located between the multiple elements  10  (the semiconductor layers  21 ) and the multiple lenses  40 . The refracting layer  30  contacts the light-receiving surfaces of the multiple elements  10 , i.e., upper surface  21 U of the semiconductor layer  21 . The refracting layer  30  contacts the multiple lenses  40 . It is favorable for the thickness of the refracting layer  30  to be not less than 1 μm (micrometers) and not more than 10 μm, and more favorably not less than 2 μm and not more than 8 μm. 
     The refracting layer  30  is, for example, an insulating layer. For example, a light-transmissive material is included in the refracting layer  30 . The refracting layer  30  includes silicon and one selected from the group consisting of oxygen and nitrogen. For example, the refracting layer  30  includes at least one of silicon oxide or silicon nitride. The refractive index of the refracting layer  30  is, for example, not less than 1.4 and not more than 2. In this specification, the refractive index is the absolute refractive index. 
     The thickness (the length along the Z-axis direction) of the refracting layer  30  is taken as a first thickness I. As described below, it is favorable for the ratio of the first thickness Ito the first period L 1  (first thickness/first period=I/L 1 ) to be, for example, not less than 0.16 and not more than 0.72, and more favorably not less than 0.24 and not more than 0.64. 
     The conductive part  61  is located on the third semiconductor region  13  and contacts the third semiconductor region  13 . The conductive part  61  is electrically connected with the third semiconductor region  13 . In the example, at least a portion of the conductive part  61  is located in the refracting layer  30 . At least a portion of the conductive part  61  is arranged with a portion of the refracting layer  30  in the X-axis direction and the Y-axis direction; that is, when viewed from the Z-axis direction, at least a portion of the conductive part  61  includes a region that does not overlap a portion of the refracting layer  30  and is surrounded with the refracting layer  30  along the X-Y plane. A short-circuit due to the conductive part  61  contacting the outside can be suppressed thereby. 
     The quenching part  63  is electrically connected with the conductive part  61 . Thereby, one end of the quenching part  63  is electrically connected with the third semiconductor region  13  via the conductive part  61 . Multiple quenching parts  63  are provided; and the multiple quenching parts  63  are electrically connected respectively with the multiple semiconductor regions  13 . Another end of the quenching part  63  is electrically connected with the first interconnect  51 . 
     For example, the quenching part  63  is a quenching resistance; and the electrical resistance of the quenching part  63  is greater than the electrical resistance of the conductive part  61 . The quenching part  63  includes, for example, polysilicon as a resistor. An n-type impurity or a p-type impurity may be added to the quenching part  63 . The resistance of the quenching part  63  is, for example, not less than 50 kΩ and not more than 2 MΩ. 
     The electrode  50 , the conductive part  61 , and the interconnects include, for example, at least one metal selected from the group consisting of titanium, tungsten, copper, gold, and aluminum. The electrode  50  and the conductive part  61  are, for example, aluminum (or an aluminum-including material), copper (or a copper-including material), gold (or a gold-including material), indium tin oxide (ITO), or a combination of another metal material and these materials. 
     The lens  40  is located on the refracting layer  30 . For example, one lens  40  is positioned on one element  10 . The upper surface of the lens  40  is upwardly convex. The lens  40  concentrates light toward the element  10 . In the example, the shape of the lens  40  is substantially circular when viewed from the Z-axis direction; and the adjacent lenses  40  are separated from each other (referring to  FIG. 1 ). It is favorable for the width of the lens  40  to be not less than 8 μm and not more than 30 μm, and more favorably not less than 10 μm and not more than 15 μm. The curvature radius of the upper surface of the lens  40  is, for example, not less than 4 μm and not more than 10 μm. 
     The lens  40  includes a light-transmissive resin. The resin includes, for example, an acrylic resin. The lens  40  can include, for example, an acrylic resin (TMR-C006) made by Tokyo Ohka Kogyo Co., Ltd. For example, the lenses  40  are formed by coating a transparent planarization film and a photosensitive microlens resin on the refracting layer  30 , by using photolithography technology to form a resin pattern having about the same period as the light-receiving elements, and by subsequently causing thermal deformation of the resin pattern by heat treatment. It is favorable for the refractive index of the microlens resin used to make the lens  40  to be not less than 1.4 and not more than 2. For example, the refractive index of the lens  40  is greater than the refractive index of the refracting layer  30 . The refractive index of the lens  40  may be less than the refractive index of the refracting layer  30 . 
     Operations of the light detector  100  will now be described. 
     The light that is incident on the light detector  100  from above passes through the lens  40  and is incident on the refracting layer  30 . The light that is incident on the refracting layer  30  passes through the refracting layer  30  and is incident on the element  10  (the semiconductor layer  21 ). A charge is generated in the semiconductor layer  21  when the light is incident on the element  10 . When the charge is generated, a current flows in the quenching part  63  and the first interconnect  51 . The incidence of the light on the element  10  can be detected by detecting the current that flows in the first interconnect  51 . 
     The conductive part  61  and the electrode  50  drive the light-receiving element by applying a voltage to the second and third semiconductor regions  12  and  13 . The electrical signal that is generated by the photoelectric conversion in the second semiconductor region  12  is output to a driver/reader (not illustrated in the drawings) via the quenching part  63 , the first interconnect  51 , etc. A voltage is applied between the second semiconductor region  12  and the third semiconductor region  13  by controlling the potential of the electrode  50 . For example, a reverse voltage is applied between the second semiconductor region  12  and the third semiconductor region  13 . The element  10  functions as an avalanche photodiode. A reverse voltage that is greater than the breakdown voltage may be applied between the second semiconductor region  12  and the third semiconductor region  13 . In other words, the element  10  may operate in a Geiger mode. By operating in a Geiger mode, a pulse signal that has a high gain and a short time constant is output. The light-receiving sensitivity of the light detector  100  can be increased thereby. 
     The quenching part  63  is provided to suppress the continuation of the avalanche breakdown that occurs when light is incident on the element  10 . A voltage drop that corresponds to the electrical resistance of the quenching part  63  occurs when avalanche breakdown occurs and a current flows in the quenching part  63 . The potential difference between the second semiconductor region  12  and the third semiconductor region  13  is reduced by the voltage drop; and the avalanche breakdown stops. The next light that is incident on the element  10  can be detected thereby. 
     As described above, a resistor that generates a large voltage drop may be provided as the quenching part  63 ; instead of a resistor, a control circuit that blocks the current may be provided as the quenching part  63 . For example, the control circuit includes a comparator, a control logic part, and two switching elements. An active quenching circuit is applicable to the control circuit. The active quenching circuit can have a known configuration. 
       FIGS. 3A to 3C  are schematic cross-sectional views illustrating calculation results of the optical path of the light detector. 
     In the example illustrated in  FIG. 3A , the thickness of the refracting layer is 1 μm, and the thickness of the refracting layer to the period of the elements (I/L 1 ) is 0.08. In the example illustrated in  FIG. 3B , the thickness of the refracting layer is 5 μm, and the thickness of the refracting layer to the period of the elements (I/L 1 ) is 0.4. In the example illustrated in  FIG. 3C , the thickness of the refracting layer is 15 μm, and the thickness of the refracting layer to the period of the elements (I/L 1 ) is 1.2. 
     The parameters other than the thickness of the refracting layer are common to the calculations of the optical paths of  FIGS. 3A to 3C . The width (the length along the X-axis direction) of the lens  40  is 12.5 μm; the height (the length along the Z-axis direction) of the lens  40  is 5.3 μm; the refractive index of the semiconductor layer  21  is 3.621; the refractive index of the refracting layer  30  is 1.452; and the refractive index of the lens  40  is 1.545. Otherwise, the elements illustrated in  FIGS. 3A to 3C  are similar to the description relating to  FIGS. 2A and 2B . 
     In the simulation, the wavelength of the light that is incident on the light detector is 900 nm. The lens  40  and the refracting layer  30  control the optical path of the light that is irradiated on the element  10 . A light Li (an incident light) that is irradiated on the lens  40  is refracted at the surface of the lens  40  toward the center of the element  10  and passes through the lens  40  and the refracting layer  30 . Subsequently, the light Li is re-refracted at the interface between the refracting layer  30  and the semiconductor layer  21  and travels straight through the semiconductor layer  21 . 
     As illustrated in  FIGS. 3A to 3C , the optical path of the incident light that passes through the semiconductor layer  21  is changed by modifying the thickness of the refracting layer  30  to the first period L 1  (I/L 1 ) even without modifying the lens  40 . The characteristics of the element  10  can be improved by the thickness of the refracting layer  30  to the first period L 1  (I/L 1 ) because the optical path of the light that passes through the semiconductor layer  21  is changed by the thickness of the refracting layer  30  to the first period L 1  (I/L 1 ). 
       FIG. 4  is a graph illustrating calculation results of the optical absorption efficiency of the light detector. 
       FIG. 4  illustrates the calculation results of the optical absorption efficiency when I/L 1  is changed by the thickness of the refracting layer in a light detector similar to those of  FIGS. 3A to 3C . Here, absorption refers to a phenomenon in which the incident light generates electrons due to photoelectric conversion in the light detector. When evaluating the optical absorption efficiency, first, the region (the avalanche region) that has a likelihood (an avalanche probability) of avalanche multiplication of the carriers generated in the semiconductor layer  21  is calculated. The optical absorption efficiency is evaluated by the ratio of a light intensity Ia absorbed in the avalanche region to a light intensity Ii incident on the light detector (absorbed light intensity/incident light intensity=Ia/Ii). Accordingly, the probability of light being detected in the light detection element increases as the value of Ia/Ii increases. The avalanche region of  FIG. 16B  that is described below is used as the avalanche region used to calculate the optical absorption efficiency of  FIG. 4 . 
     As illustrated in  FIG. 4 , a high optical absorption efficiency (light detection efficiency) is obtained when the ratio of the first thickness Ito the first period L 1  (I/L 1 ) is not less than 0.16 and not more than 0.72. In other words, the sensitivity of the light detector  100  can be improved. 
     By setting the ratio I/L 1  to be not less than 0.16 and not more than 0.72, it is considered that the optical path of the light that is incident on the light detector  100  is easily concentrated at the element center. For example, a distribution range R 1   b  of the light that is incident on the semiconductor layer  21  illustrated in  FIG. 3B  is narrower than a distribution range R 1   a  of the light that is incident on the semiconductor layer  21  illustrated in  FIG. 3A . That is, the light that is incident on the semiconductor layer  21  is more concentrated in the element center in  FIG. 3B  than in  FIG. 3A . 
     For example, the detection probability of the incident light (e.g., the likelihood of avalanche multiplication of the generated carriers) has a distribution in the semiconductor layer  21  of the element  10 . In the light detector  100  in which the multiple elements  10  are arranged in the light-receiving surface, the detection probability is high at the center of the element  10  compared to the end portion of the element  10  (the outer perimeter portion when viewed from the Z-axis direction). For example, there are also cases where a separation structure that electrically or optically separates adjacent elements is located at the end portion of the element  10 . For example, a trench structure or an impurity concentration distribution of the semiconductor layer  21  are examples of the separation structure. It is considered that a high light detection efficiency is obtained by concentrating the incident light at the element center. 
     According to the embodiment, it is favorable for the ratio I/L 1  to be not less than 0.16. Thereby, for example, the optical path can easily pass through the center of the element  10  at which the detection probability of the light is high. For example, the refracting layer  30  may be thicker than the lens  40 . The optical path length of a portion of the incident light in the refracting layer  30  may be greater than the optical path length of the portion of the incident light in the lens  40 . 
     On the other hand, the light detection efficiency decreases as the ratio I/L 1  exceeds 0.72. When the ratio I/L 1  is large, the light that is incident on the light detector  100  easily converges in the refracting layer  30  before being incident on the semiconductor layer  21 . For example, a distribution range R 2   c  of the light in the refracting layer  30  illustrated in  FIG. 3C  is narrower than a distribution range R 2   b  of the light in the refracting layer  30  illustrated in  FIG. 3B . When the light converges in the refracting layer  30 , the light that is in the semiconductor layer  21  travels toward the outer side of the element  10 . For example, a distribution range R 3   c  of the light in the semiconductor layer  21  illustrated in  FIG. 3C  is wider than a distribution range R 3   b  of the light in the semiconductor layer  21  illustrated in  FIG. 3B . When the light that is in the semiconductor layer  21  travels toward the outer side of the element  10 , the light does not easily pass through the element center at which the detection probability is high in the semiconductor layer  21 . 
     According to the embodiment, it is favorable for the ratio I/L 1  to be not more than 0.72. Thereby, for example, the optical path can easily pass through the center of the element  10  at which the detection probability of the light is high. From the perspective of the material cost and the strain due to the stress of the refracting layer  30  as well, it is favorable for the refracting layer  30  not to be too thick. For example, it is favorable for the first thickness I of the refracting layer  30  to be not less than 2 μm and not more than 8 μm. 
     The thickness (the length along the Z-axis direction) of the semiconductor layer  21  (the first semiconductor region  11 ) is taken as a second thickness T (referring to  FIG. 2A ). It is favorable for the ratio of the second thickness T to the first period L 1  (second thickness/first period=T/L 1 ) to be not less than 0.4. For example, the ratio T/L 1  is set to be not less than 0.4 by reducing the first period L 1 . The resolution of the light detector  100  can be improved by reducing the first period L 1 . Also, for example, the ratio T/L 1  may be set to be not less than 0.4 by increasing the second thickness T. By making the semiconductor layer  21  thick, the optical path length in the semiconductor layer  21  can be ensured, and the detection probability of the light can be increased. 
     On the other hand, when the first period L 1  is too short, there is a risk that the proportion of the element end portion (outer perimeter portion) in the light-receiving surface of the element  10  may become large, and the light that passes through the element center at which the light detection probability is high may decrease. Conversely, it is favorable for the ratio T/L 1  of the second thickness T to the first period L 1  to be not more than 1.2. In such a case, the first period L 1  can be prevented from being too short, and the reduction of the light passing through the element center can be suppressed. For example, it is favorable for the first period L 1  to be not less than 10 μm and not more than 15 μm. 
     When the first period L 1  is relatively short, it is more favorable for the ratio I/L 1  of the first thickness I to the first period L 1  to be not less than 0.16 and not more than 0.72. Thereby, even when the first period L 1  is relatively short and the proportion of the element end portion is relatively large, the light can easily pass through the element center; and the reduction of the light that passes through the element center can be suppressed. 
     According to embodiments as described above, for example, both a smaller surface area and high sensitivity of the light-receiving element are realized. 
       FIGS. 5A and 5B  are schematic plan views illustrating portions of other light detectors according to the first embodiment. 
       FIG. 5A  illustrates a portion of a light detector  101  according to a modification of the first embodiment. Thus, among the multiple lenses  40 , the adjacent lenses  40  may contact each other. Also, portions of the adjacent lenses  40  may overlap each other. 
       FIG. 5B  illustrates a portion of a light detector  102  according to a modification of the first embodiment. In the example, the shape of the lens  40  is a rectangle in which the corners have curvature when viewed from the Z-axis direction. When viewed from the Z-axis direction, the shape of the lens  40  may not be circular, and may be substantially a polygon or a rounded polygon. 
       FIG. 6  is a cross-sectional view illustrating a portion of another light detector according to the first embodiment. 
     Similarly to  FIG. 2A ,  FIG. 6  illustrates a portion of a light detector  103  according to a modification of the first embodiment. In the light detector  103 , the refracting layer  30  has a stacked structure that includes multiple layers. Otherwise, a description similar to that of the light detector  100  is applicable to the light detector  103 . The conductive part  61 , the quenching part  63 , and the first interconnect  51  are not illustrated in  FIG. 6 . 
     In the example, the refracting layer  30  includes a first layer  31 , a second layer  32 , and a third layer  33 . The first layer  31  is located between the multiple lenses  40  and the multiple elements  10 . The second layer  32  is located between the first layer  31  and the multiple lenses  40 . The third layer  33  is located between the second layer  32  and the multiple lenses  40 . The stacked structure is not always limited to three layers and may be two layers, four layers, or more. 
     The thicknesses of the first to third layers  31  to  33  may be the same or different from each other. The refractive indexes of the first to third layers  31  to  33  may be the same or different from each other. The refractive indexes of the first to third layers  31  to  33  each are, for example, not less than 1.4 and not more than 2. For example, the first to third layers  31  to  33  each include silicon and one selected from the group consisting of oxygen and nitrogen. For example, the first to third layers  31  to  33  each include at least one of silicon oxide, silicon nitride, or a resin. 
     The refracting layer  30  may perform a role other than the optical path control of the incident light described above. For example, the refracting layer  30  may include the role of a stopper in a manufacturing process (e.g., etching polishing, etc.) of the light detector, the role of protecting electrodes (e.g., the conductive part  61 ) from contacting the outside and short-circuiting, etc. In such a case, the refracting layer  30  may be made by stacking insulating layers according to the roles. For example, the first layer  31  may be a silicon oxide film that protects the surface of the semiconductor layer  21 ; the second layer  32  may be a silicon nitride film that is a stopper layer in a process; and the third layer  33  may be a silicon oxide film that protects an electrode. 
     Second Embodiment 
       FIG. 7  is a plan view illustrating a light detector according to a second embodiment. 
     As illustrated in  FIG. 7 , the light detector  120  according to the second embodiment includes multiple structure bodies  70 . For example, the structure body  70  is a trench part that is located in the semiconductor layer  21 . Otherwise, a description similar to that of the light detector  100  according to the first embodiment is applicable to the light detector  120 . The refracting layer  30 , the multiple lenses  40 , the conductive part  61 , the quenching part  63 , etc., are not illustrated in  FIG. 7 . 
     In the example, the structure body  70  is substantially octagonal when viewed along the Z-axis direction. The multiple structure bodies  70  respectively surround the multiple elements  10  along the X-Y plane. Therefore, the multiple structure bodies  70  are arranged to match the period of the multiple elements  10 . For example, the multiple structure bodies  70  include a first structure body  70   a  and a second structure body  70   b.  The first structure body  70   a  surrounds the first element  10   a.  The second structure body  70   b  surrounds the second element  10   b.    
     Herein, “surround” includes not only the case where an unbroken component continuously surrounds another component, but also the case where multiple components are separated from each other and arranged around the other component. For example, the other component can be considered to be surrounded with the multiple components when the other component is positioned inside a path obtained by tracing along the multiple components. 
     The first structure body  70   a  and the second structure body  70   b  each are octagonal when viewed from the Z-axis direction. Here, an octagon may be a rounded octagon. In one structure body  70  of the example, the length in the X-axis direction and the length in the Y-axis direction are substantially the same. In the example, when viewed from the Z-axis direction, the shapes of the multiple structure bodies  70  are substantially the same. The period of the arrangement of the multiple structure bodies  70  may be substantially equal to the period of the arrangement of the multiple elements  10  (the first period L 1  and the second period L 2 ). 
       FIG. 8  is a schematic cross-sectional view illustrating a portion of the light detector according to the second embodiment. 
       FIG. 8  shows a line A 3 -A 4  cross section shown in  FIG. 7 . As illustrated in  FIG. 8 , at least a portion of the structure body  70  is located in the semiconductor layer  21 . 
     In the example, the level of the upper end of the structure body  70  is substantially equal to the level of the upper surface of the semiconductor layer  21  (the light-receiving surface). The upper surface of the structure body  70  may contact the refracting layer  30 . In the example, the structure body  70  does not reach the semiconductor layer  22 ; and the lower end of the structure body  70  is higher than the lower surface of the semiconductor layer  21 . At least a portion of the structure body  70  is arranged with the first to third semiconductor regions  13  in a direction perpendicular to the Z-axis direction. 
     The second semiconductor region  12  and the third semiconductor region  13  do not contact the structure body  70 . A portion of the first semiconductor region  11  is positioned between the second semiconductor region  12  and the structure body  70  and contacts the second semiconductor region  12  and the structure body  70 . Another portion of the first semiconductor region  11  is positioned between the third semiconductor region  13  and the structure body  70  and contacts the third semiconductor region  13  and the structure body  70 . However, the second semiconductor region  12  and the third semiconductor region  13  may contact the structure body  70 . 
     The refractive index of the structure body  70  is different from the refractive index of the semiconductor layer  21 . The refractive index of the structure body  70  is different from the refractive indexes of the first to third semiconductor regions  13 . The structure body  70  is insulative. The structure body  70  is provided to suppress the conduction and optical interference between the elements  10 . The movement of secondary photons and carriers between the elements  10  is suppressed by the structure body  70 . When secondary photons are generated by light being incident on the element  10 , the secondary photons that travel toward the adjacent elements  10  are reflected and refracted at the interface of the structure body  70 . By providing the structure body  70 , crosstalk noise can be reduced. 
     For example, at least a portion of the first structure body  70   a  is located between the first semiconductor region  11  of the first element  10   a  and the first semiconductor region  11  of the second element  10   b.  The movement of the secondary photons and the carriers between the first element  10   a  and the second element  10   b  can be suppressed thereby. 
     The multiple structure bodies  70  are provided independently for each element. In other words, the multiple structure bodies  70  are separated and do not physically contact each other. For example, at least a portion of the second structure body  70   b  is located between the first semiconductor region  11  of the second element  10   b  and at least a portion of the first structure body  70   a.  The first structure body  70   a  and the second structure body  70   b  are separated from each other. Thereby, for example, compared to when one separation structure is located between the adjacent elements  10 , the number of interfaces of the structure body  70  between the adjacent elements  10  is increased. By increasing the number of interfaces, when secondary photons are generated in the element  10 , the secondary photons that travel toward the adjacent elements  10  are more easily reflected. Crosstalk noise can be further reduced thereby. A semiconductor region  14  (e.g., a portion of the semiconductor layer  21 ) may be located between the first structure body  70   a  and the second structure body  70   b.  The semiconductor region  14  contacts the first and second structure bodies  70   a  and  70   b.  For example, the semiconductor region  14  extends in the Y-axis direction between the structure bodies  70  that are adjacent in the X-axis direction. The semiconductor region  14  extends in the X-axis direction between the structure bodies  70  that are adjacent in the Y-axis direction. 
     The structure body  70  includes an insulating material. For example, the structure body  70  includes silicon and one selected from the group consisting of oxygen and nitrogen. For example, the structure body  70  includes silicon oxide or silicon nitride. The structure body  70  may have a stacked structure. 
     When the structure body  70  is provided, for example, there are cases where defects that are caused by a thermal expansion coefficient difference occur at the interface between the element  10  and the structure body  70 . For example, crystal defects occur in the semiconductor layer  21  at the end portion (the outer perimeter portion) of the element  10 . There are cases where such defects cause noise. For example, there is a possibility that carriers that are generated in the defects may cause avalanche multiplication in the semiconductor layer  21 . 
     In the light detector  120  as well, for example, the ratio of the first thickness Ito the first period L 1  is not less than 0.16 and not more than 0.72. Thereby, similarly to the first embodiment, the optical path more easily passes through the element center at which the detection probability of the light is high. The regions through which much of the light passes can be separated from the element end portion at which crystal defects easily occur. For example, by reducing the avalanche probability at the element end portion, a high light detection efficiency can be obtained while suppressing the noise. 
       FIGS. 9A to 9D  are schematic plan views illustrating portions of other light detectors according to the second embodiment. 
       FIG. 9A ,  FIG. 9B ,  FIG. 9C , and  FIG. 9D  illustrate portions of light detectors  121 ,  122 ,  123 , and  124  according to modifications of the second embodiment. Compared to the light detector  120  illustrated in  FIG. 7 , the shapes of the structure bodies  70  of the light detectors  121 ,  122 ,  123 , and  124  are different. Otherwise, a description similar to that of the light detector  120  is applicable to the light detectors  121 ,  122 ,  123 , and  124 . 
     As in the light detector  121  illustrated in  FIG. 9A , the structure body  70  may be rectangular when viewed from the Z-axis direction. The shape of the structure body  70  may be a polygon or a rounded polygon when viewed from the Z-axis direction. 
     As illustrated in  FIGS. 9B to 9D , the structure body  70  discontinuously surrounds the element  10  when viewed from the Z-axis direction. In other words, the structure body  70  may not have a perfectly ring-shaped structure when viewed from the Z-axis direction, and may have a shape in which a portion is open. Interference between the conductive part  61  and the structure body  70  can be suppressed thereby. For example, a portion of the conductive part  61  may be located at the discontinuous section of the structure body  70 . By changing the shape of the structure body  70 , the arrangement of the conductive part  61 , the quenching part  63 , the first interconnect  51 , etc., is easy. 
     In the light detector  122  illustrated in  FIG. 9B , a substantially octagonal structure body  70  surrounds the element  10  when viewed from the Z-axis direction. The planar shape of the structure body  70  includes one opening  70   i  (a section where the ring shape is discontinuous) that is located in one side of the octagon. 
     In the light detector  123  illustrated in  FIG. 9C , a substantially rectangular structure body  70  surrounds the element  10  when viewed from the Z-axis direction. The planar shape of the structure body  70  includes a total of four openings  70   i  located at the corners of the rectangle. 
     In the light detector  124  illustrated in  FIG. 9D , a substantially rectangular structure body  70  surrounds the element  10  when viewed from the Z-axis direction. The planar shape of the structure body  70  includes a total of two openings  70   i  located at opposite corners of the rectangle. 
     As illustrated in  FIGS. 9B to 9D , the semiconductor layer  21  includes a semiconductor region  15  that is positioned in the opening  70   i  when viewed from the Z-axis direction. The element inside the structure body  70  is continuous with the semiconductor region  14  outside the structure body  70  via the semiconductor region  15 . 
       FIGS. 10A and 10B  are schematic cross-sectional views illustrating portions of other light detectors according to the second embodiment. 
     Similarly to  FIG. 8 ,  FIGS. 10A and 10B  illustrate portions of light detectors  125  and  126  according to modifications of the second embodiment. Compared to the light detector  120  illustrated in  FIG. 8 , the arrangements of the structure bodies  70  of the light detectors  125  and  126  are different. Otherwise, a description similar to that of the light detector  120  is applicable to the light detectors  125  and  126 . 
     For example, the depth of the structure body  70  can be modified according to the manufacturing processes and/or the characteristics of the element  10 . For example, as illustrated in  FIG. 10A , the structure body  70  may extend through the semiconductor layer  21  and extend to the semiconductor layer  22 . In other words, the lower end of the structure body  70  may be lower than the lower surface of the semiconductor layer  21 . A lower portion  70   p  of the structure body  70  may be surrounded with the semiconductor layer  22  in the X-Y plane. The lower portion  70   p  of the structure body  70  may physically contact the semiconductor layer  22 . 
     As illustrated in  FIG. 10B , the structure body  70  may extend into the refracting layer  30 . In other words, the upper end of the structure body  70  may be higher than the upper surface of the semiconductor layer  21 . An upper portion  70   q  of the structure body  70  may be surrounded with the refracting layer  30  in the X-Y plane. 
     Because the structure body  70  extends into the refracting layer  30  or into the semiconductor layer  21 , the crosstalk between elements can be further suppressed. 
       FIG. 11  is a schematic plan view illustrating a portion of a light detector according to the second embodiment. 
       FIG. 12  is a schematic cross-sectional view illustrating a portion of the light detector according to the second embodiment. 
     These drawings illustrate the light detector  127  according to a modification of the second embodiment. The lens  40  and the refracting layer  30  are not illustrated in  FIG. 11 .  FIG. 12  shows an A 4 -A 5  cross section of  FIG. 11 . 
     As illustrated in  FIG. 12 , the structure body  70  may include a first insulating layer IL 1  and a second insulating layer IL 2 . The second insulating layer IL 2  is located between the first insulating layer IL 1  and the element  10  and between the first insulating layer IL 1  and the semiconductor layer  22 . For example, the first insulating layer IL 1  and the second insulating layer IL 2  include silicon oxide; and the second insulating layer IL 2  has a dense structure compared to the first insulating layer IL 1 . 
     As illustrated in  FIG. 12 , a p-type semiconductor region  23  may be located between the semiconductor layer  22  and the structure body  70  in the Z-axis direction. The p-type impurity concentration in the semiconductor region  23  is greater than the p-type impurity concentration in the first semiconductor region  11 . 
     In the example, a quenching resistance is electrically connected to each element  10  as the quenching part  63 . For example, when viewed from the Z-axis direction, the quenching part  63  exists at a different position from a photodiode PD. For example, the quenching part  63  is arranged with the structure body  70  or the semiconductor region  14  in the Z-axis direction. Another end of the quenching part  63  is electrically connected with the first interconnect  51 . 
     The multiple conductive parts  61  are connected respectively to the multiple elements  10 . The multiple conductive parts  61  each include a contact  64  and a connection interconnect  65 . The quenching part  63  is electrically connected with the third semiconductor region  13  via the contact  64  and the connection interconnect  65  and is electrically connected with the first interconnect  51  via a contact  66 . 
     The contacts  64  and  66  include a metal material. For example, the contacts  64  and  66  include at least one selected from the group consisting of titanium, tungsten, copper, and aluminum. The contacts  64  and  66  may include a conductor that is made of a silicon compound or a nitride of at least one selected from the group consisting of titanium, tungsten, copper, and aluminum. As shown in  FIG. 12 , the contact  64  may include a metal layer  64   a  and a metal layer  64   b.  The contact  66  may include a metal layer  66   a  and a metal layer  66   b.  The metal layer  64   b  is located between the metal layer  64   a  and the first layer  31 , between the metal layer  64   a  and the second layer  32 , and between the metal layer  64   a  and the third layer  33 . The metal layer  66   b  is located between the metal layer  66   a  and the second layer  32  and between the metal layer  66   a  and the third layer  33 . For example, the metal layers  64   a  and  66   a  include tungsten. The metal layers  64   b  and  66   b  include titanium. The metal layer  64   b  may include a titanium layer and a titanium nitride layer that is located between the titanium layer and the metal layer  64   a.  The metal layer  66   b  may include a titanium layer and a titanium nitride layer that is located between the titanium layer and the metal layer  66   a.  The connection interconnect  65  includes at least one selected from the group consisting of copper and aluminum. 
     For example, the position in the Z-axis direction of the quenching part  63  is between the position in the Z-axis direction of the third semiconductor region  13  and the position in the Z-axis direction of the first interconnect  51 . One first interconnect  51  is electrically connected with multiple photodiodes PD arranged in the Y-axis direction. 
     The electrical resistance of the quenching part  63  is greater than the electrical resistances of the contact  64 , the contact  66 , and the connection interconnect  65 . The quenching resistance includes polysilicon as a semiconductor material. An n-type impurity or a p-type impurity may be added to the quenching resistance. 
     For example, the refracting layer  30  includes the first to fourth layers  31  to  34 . The first to third layers  31  to  33  are located between the fourth layer  34  and the multiple elements  10  in the Z-axis direction. The first layer  31  and the second layer  32  are located between the third layer  33  and the multiple elements  10  in the Z-axis direction. The first layer  31  is located between the second layer  32  and the multiple elements  10  in the Z-axis direction. 
     The contacts  64  and  66  are surrounded with the first to third layers  31  to  33  along the X-Y plane. A portion of the first layer  31  is located between the semiconductor region  14  and the quenching part  63  in the Z-axis direction. The first interconnect  51  and the connection interconnect  65  are surrounded with the fourth layer  34 . 
     The p-type impurity concentration in the first semiconductor region  11  is, for example, not less than 1.0×10 13  atoms/cm 3  and not more than 1.0×10 16  atoms/cm 3 . By setting this concentration range, the depletion layer can sufficiently spread in the first semiconductor region  11 ; and the photon detection efficiency or the light-receiving sensitivity can be increased. 
     The p-type impurity concentration in the second semiconductor region  12  is, for example, not less than 1.0×10 16  atoms/cm 3  and not more than 1.0×10 18  atoms/cm 3 . By setting this concentration range, the second semiconductor region  12  can have a p-n junction with the third semiconductor region  13 ; and a depletion layer can easily spread in the second semiconductor region  12 . 
     The n-type impurity concentration in the third semiconductor region  13  is, for example, not less than 1.0×10 18  atoms/cm 3  and not more than 1.0×10 21  atoms/cm 3 . By setting this concentration range, the electrical resistance in the third semiconductor region  13  can be reduced, and the loss of the carriers in the second semiconductor region  12  can be reduced. 
     The p-type impurity concentration in the semiconductor layer  22  is not less than 1.0×10 17  atoms/cm 3  and not more than 1.0×10 21  atoms/cm 3 . A conductive layer that includes a metal may be used instead of the semiconductor layer  22 . For example, a conductive layer that includes at least one selected from the group consisting of aluminum, copper, titanium, gold, and nickel is used. 
       FIG. 13  is a schematic plan view illustrating a portion of the light detector according to the second embodiment. 
     As shown in  FIG. 13 , the light detector  127  further includes a common interconnect  54  and a pad  55 .  FIG. 11  shows portion P shown in  FIG. 13 . One first interconnect  51  is electrically connected to multiple elements  10  arranged in the Y-axis direction. Multiple first interconnects  51  that are arranged in the X-axis direction are electrically connected with the common interconnect  54 . The common interconnect  54  is electrically connected with not less than one pad  55 . An interconnect of an external device is electrically connected to the pad  55 . 
       FIGS. 14 and 15  are plan views illustrating portions of the light detector according to the second embodiment. 
     As shown in  FIG. 14 , the structure body  70  is a five-or-higher-sided polygon when viewed from the Z-axis direction. In the example of  FIG. 14 , the structure body  70  is octagonal when viewed from the Z-axis direction. For example, the structure body  70  may include a pair of first extension portions  71 - 1  that extend in the Y-axis direction, a pair of second extension portions  71 - 2  that extend in the X-axis direction, and multiple couplers  71 C. The first to third semiconductor regions  11  to  13  (the photodiode PD) are located between the pair of first extension portions  71 - 1  in the X-axis direction. The first to third semiconductor regions  11  to  13  are located between the pair of second extension portions  71 - 2  in the Y-axis direction. Each coupler  71 C links one end of the first extension portion  71 - 1  and one end of the second extension portion  71 - 2 . 
     According to such a structure, the angles of the corners of the structure body  70  can be increased, and the stress can be relaxed at the corners of the structure body  70 . For example, by relaxing the stress, the occurrence of cracks in the semiconductor region  14  can be suppressed, and operation errors that are caused by the occurrence of cracks can be suppressed. Also, when a crack occurs in a semiconductor layer when forming the structure body  70 , there is a possibility that the resist may enter the crack in a subsequent photolithography process. When the resist enters the crack, residue of the resist may remain in the crack when the resist is stripped away. The residue of the resist causes organic contamination of an oxidation furnace in a subsequent heating process such as oxidization. These problems can be solved by relaxing the stress at the corners of the structure body  70 . 
     The length in the Y-axis direction of the first extension portion  71 - 1  is greater than the length in the Y-axis direction of the coupler  71 C. The length in the X-axis direction of the second extension portion  71 - 2  is greater than the length in the X-axis direction of the coupler  71 C. For example, when viewed from the Z-axis direction, the coupler  71 C has a straight-line shape; and the structure body  70  is octagonal. When viewed from the Z-axis direction, it is favorable for the shape of the structure body  70  to have eight or more sides. In other words, it is favorable for an angle θ 1  between the first extension portion  71 - 1  and the coupler  71 C to be not less than 135 degrees. It is favorable for an angle θ 2  between the second extension portion  71 - 2  and the coupler  71 C to be not less than 135 degrees. The exterior angle between the first extension portion  71 - 1  and the coupler  71 C and the exterior angle between the second extension portion  71 - 2  and the coupler  71 C can be increased thereby. By increasing these exterior angles, the stress can be relaxed at the corners of the structure body  70 . 
     It is favorable for a length Ln 1  in the X-axis direction of the coupler  71 C and a length Ln 2  in the Y-axis direction of the coupler  71 C each to be not less than 1 μm. The stress that is generated at the coupler  71 C can be further relaxed thereby. 
     Or, as shown in  FIG. 15 , the structure body  70  may be a polygon in which the corners are curved when viewed from the Z-axis direction. In other words, the coupler  71 C may be curved when viewed from the Z-axis direction. In the example of FIG.  15 , the structure body  70  is a rounded rectangle when viewed from the Z-axis direction. The stress at the corners of the structure body  70  can be relaxed by such a structure. 
     For example, one end of the coupler  71 C that is linked to the first extension portion  71 - 1  is along the Y-axis direction. The other end of the coupler  71 C that is linked to the second extension portion  71 - 2  is along the X-axis direction. Thereby, the coupler  71 C is smoothly linked to the first and second extension portions  71 - 1  and  71 - 2 . Similarly, the coupler  71 C may be curved when viewed from the Z-axis direction. By curving the coupler  71 C, the concentration of the stress at the structure body  70  can be further relaxed. 
     The structure of the light detector is not limited to the illustrated examples; and various modifications are possible. For example, when viewed from the Z-axis direction, the shape of the structure body  70  (a first structure body) that surrounds one of the multiple photodiodes PD (a first photodiode) may be different from the shape of another structure body  70  (a second structure body) that surrounds another one of the multiple photodiodes PD (a second photodiode). When viewed along the Z-direction, the shape of the first photodiode may be different from the shape of the second photodiode. When viewed along the Z-direction, the surface area of the first photodiode may be different from the surface area of the second photodiode. The electrical resistance of the quenching part  63  that is electrically connected with the first photodiode and the electrical resistance of the quenching part  63  that is electrically connected with the second photodiode may be different from each other. The operating voltages that are applied to the first interconnect  51  that is electrically connected with the first photodiode and the first interconnect  51  that is electrically connected with the second photodiode may be different from each other. The signal of the first interconnect  51  that is electrically connected with the first photodiode and the signal of the first interconnect  51  that is electrically connected with the second photodiode may be separately read. 
     Simulations based on theoretical calculations for the light detectors of the first and second embodiments will now be described. In the following simulations, the semiconductor layer  22  is a high-concentration p-type substrate having a boron concentration of 4.5×10 18 /cm 3 ; the semiconductor layer  21  is a semiconductor layer having a boron concentration of 1.0×10 15 /cm 3 ; and the first period L 1  of the element  10  (the light-receiving cell) is 12.5 μm. 
       FIGS. 16A and 16B  are schematic cross-sectional views illustrating calculation results of the distribution range of the avalanche region of the light detector. 
     First, the distribution of the region (the avalanche region) in which there is a likelihood (an avalanche probability) of avalanche multiplication of the carriers generated in the semiconductor layer  21  was confirmed by a theoretical calculation. The shading of the colors in the figures illustrates the level of the avalanche probability. The avalanche probability is lower as the color is darker (black); and the avalanche probability is higher as the color is lighter (white). For the light detector to obtain a high photon detection efficiency, it is desirable for the overlap between the avalanche region and the optical path of the incident light in the light-receiving element to be large. 
     The distribution of the avalanche region is affected by the impurity concentration profile in the semiconductor layer  21 .  FIGS. 16A and 16B  show the avalanche region distribution of the light-receiving cell in which the boron (B) implantation conditions for making the second semiconductor region  12  are modified between two conditions. 
       FIG. 16A  is the result of the calculation of the avalanche region distribution when the boron conditions include an acceleration voltage of 400 keV and a dose of 3.3×10 12 /cm 2 .  FIG. 16B  is the result of the calculation of the avalanche region distribution when the boron implantation conditions include an acceleration voltage of 400 keV and a dose of 2.6×10 12 /cm 2 . The voltage applied to the light-receiving cell was set to a voltage that was 5 V greater than the breakdown voltage. 
     The peak of the impurity concentration profile of  FIG. 16A  is higher than the peak of the impurity concentration profile of  FIG. 16B . In  FIG. 16A , the avalanche region spreads in the X-axis direction (and the Y-axis direction) and has little spreading in the Z-axis direction. For example, in  FIG. 16A , the length along the Z-axis direction of a distribution range RA of the avalanche region is less than the length along the X-axis direction of the distribution range RA of the avalanche region. On the other hand, in  FIG. 16B , the avalanche region has little spreading in the X-axis direction (and the Y-axis direction), but spreads in the Z-axis direction. For example, in  FIG. 16B , the length along the Z-axis direction of the distribution range RA of the avalanche region is greater than the length along the X-axis direction of the distribution range RA of the avalanche region. 
     For example, the light detector according to the embodiment is used to detect near-infrared light for LiDAR (e.g., a wavelength that is not less than 0.7 μm and not more than 2.5 μm). Penetration length of near-infrared light into the semiconductor layer  21  is long. In such a case, it is desirable for the overlap between the avalanche region and the optical path of the incident light to be large to increase the light detection efficiency of the light detector. For example, a light-receiving element that includes an avalanche region such as that of  FIG. 16B  is desirable. 
     As described above in the second embodiment, when a trench structure such as the structure body  70  is used, for example, defects that are caused by the thermal expansion coefficient difference occur at the interface vicinity between the trench structure and the semiconductor layer  21 . The avalanche multiplication due to thermions generated at the defects causes noise output of the light detector. Therefore, to reduce the noise of the light detector, a shape of the avalanche region such as that of  FIG. 16B  is more desirable than a shape of the avalanche region such as that of  FIG. 16A . For example, when operating the element  10 , the length along the Z-axis direction of the depletion layer that is formed by the second and third semiconductor regions  12  and  13  may be greater than the length along the X-axis direction of the depletion layer. 
       FIG. 16B  corresponds to the calculation results performed for the light-receiving elements shown in  FIGS. 3A to 3C . 
       FIG. 17  is a graph illustrating calculation results of the distribution of the avalanche probability.  FIG. 17  shows calculation results of the distribution of an avalanche probability Pa 1  in a range p-p′ of the avalanche region shown in  FIG. 16B . The avalanche probability increases proximate to the element center and decreases proximate to the element end portion that is separated from the element center. Therefore, the light detection efficiency increases as the optical path of the incident light passes proximate to the light-receiving element center. 
       FIGS. 18A to 18C  are schematic cross-sectional views illustrating calculation results of the optical path of the light detector. 
       FIGS. 18A to 18C  illustrate the distribution range RA of the avalanche region shown in  FIG. 16B  overlaid on  FIGS. 3A to 3C . 
     As illustrated in  FIG. 18A , when the thickness of the refracting layer  30  to the first period L 1  (I/L 1 ) is 0.08, much of the incident light passes through the distribution range RA of the avalanche region, but the light also passes through an avalanche region edge of low avalanche probability (the end portion of the distribution range RA). 
     As illustrated in  FIG. 18B , when the thickness of the refracting layer  30  to the first period L 1  (I/L 1 ) is 0.4, the optical path concentrates more in the distribution range RA of the avalanche region, and much of the incident light passes through the element center at which the avalanche probability is high. 
     As illustrated in  FIG. 18C , when the thickness of the refracting layer  30  to the first period L 1  (I/L 1 ) is 1.2, the convergence of the optical path is too early; therefore, the optical path starts to re-diverge where the light reaches the avalanche region. Therefore, a portion of the incident light passes through an avalanche region edge of low avalanche probability. 
     Third Embodiment 
       FIG. 19  is a schematic view illustrating a lidar (Laser Imaging Detection and Ranging (LIDAR)) device according to a third embodiment. 
     The embodiment is applicable to a long-distance subject detection system (LIDAR) or the like including a line light source and a lens. The lidar device  5001  includes a light-projecting unit U that projects laser light toward an object  411 , and a light-receiving unit R (also called a light detection system) that receives the laser light from the object  411 , measures the time of the round trip of the laser light to and from the object  411 , and converts the time into a distance. 
     In the light-projecting unit U, a laser light oscillator (also called a light source)  404  produces laser light. A drive circuit  403  drives the laser light oscillator  404 . An optical system  405  extracts a portion of the laser light as reference light, and irradiates the rest of the laser light on the object  411  via a mirror  406 . A mirror controller  402  projects the laser light onto the object  411  by controlling the mirror  406 . Herein, “project” means to cause the light to strike. 
     In the light-receiving unit R, a reference light detector  409  detects the reference light extracted by the optical system  405 . A light detector  410  receives the reflected light from the object  411 . A distance measuring circuit  408  measures the distance to the object  411  based on the reference light detected by the reference light detector  409  and the reflected light detected by the light detector  410 . An image recognition system  407  recognizes the object  411  based on the results measured by the distance measuring circuit  408 . 
     The lidar device  5001  employs ToF in which the time (the time-of-flight) of the round trip of the laser light to and from the object  411  is measured and converted into a distance. The lidar device  5001  is applied to an automotive drive-assist system, remote sensing, etc. Good sensitivity is obtained particularly in the near-infrared region when the light detectors of the embodiments described above are used as the light detector  410 . Therefore, the lidar device  5001  is applicable to a light source of a wavelength band invisible to humans. For example, the lidar device  5001  can be used for obstacle detection in a vehicle. 
       FIG. 20  is a drawing for describing the detection of the detection object of the lidar device. 
     A light source  3000  emits light  412  toward an object  600  that is the detection object. A light detector  3001  detects light  413  that passes through the object  600 , is reflected by the object  600 , or is diffused by the object  600 . 
     For example, the light detector  3001  realizes highly-sensitive detection when the light detector according to the embodiment described above is used. It is favorable to provide multiple sets of the light detector  410  and the light source and to preset the arrangement relationship in the software (which is replaceable with a circuit). For example, it is favorable for the arrangement relationship of the sets of the light detector  410  and the light source to be at uniform spacing. Thereby, an accurate three-dimensional image can be generated by the output signals of each light detector  410  complementing each other. 
       FIG. 21  is a schematic top view of a vehicle that includes the lidar device according to the third embodiment. 
     For example, the lidar device  5001  is mounted in a mobile body such as a vehicle, a drone, a robot, etc. In the example of  FIG. 21 , a vehicle  700  is used as the mobile body. The vehicle  700  according to the embodiment includes the lidar devices  5001  at four corners of a vehicle body  710 . Because the vehicle according to the embodiment includes the lidar devices at the four corners of the vehicle body, the environment in all directions of the vehicle can be detected by the lidar devices. 
     In each of the embodiments described above, the relative levels of the impurity concentrations between the semiconductor regions can be confirmed using, for example, a SCM (scanning capacitance microscope). The carrier concentration in each semiconductor region can be considered to be equal to the activated impurity concentration in each semiconductor region. Accordingly, the relative levels of the carrier concentrations between the semiconductor regions also can be confirmed using SCM. The impurity concentration in each semiconductor region can be measured by, for example, SIMS (secondary ion mass spectrometry). 
     According to embodiments, a light detector, a light detection system, a lidar device, a mobile body, and a vehicle can be provided in which the performance can be improved. 
     In the specification of the application, “perpendicular” refer to not only strictly perpendicular but also include, for example, the fluctuation due to manufacturing processes, etc. It is sufficient to be substantially perpendicular. 
     Hereinabove, exemplary embodiments of the invention are described with reference to specific examples. However, the embodiments of the invention are not limited to these specific examples. For example, one skilled in the art may similarly practice the invention by appropriately selecting specific configurations of components included in light detectors such as multiple elements, quenching parts, multiple lenses, refracting layers, etc., from known art. Such practice is included in the scope of the invention to the extent that similar effects thereto are obtained. 
     Further, any two or more components of the specific examples may be combined within the extent of technical feasibility and are included in the scope of the invention to the extent that the purport of the invention is included. 
     Moreover, all light detectors, light detection systems, lidar devices, mobile bodies, and vehicles practicable by an appropriate design modification by one skilled in the art based on the light detectors, the light detection systems, the lidar devices, the mobile bodies, and the vehicles described above as embodiments of the invention also are within the scope of the invention to the extent that the purport of the invention is included. 
     Various other variations and modifications can be conceived by those skilled in the art within the spirit of the invention, and it is understood that such variations and modifications are also encompassed within the scope of the invention. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.