Patent Publication Number: US-2017363722-A1

Title: Photo detector, photo detection device, and lidar device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2016-119865, filed on Jun. 16, 2016, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a photo detector, a photo detection device, and a LIDAR (Laser Imaging Detection and Ranging) device. 
     BACKGROUND 
     A photo detector using an avalanche photo diode (APD) detects weak light, and amplifies a signal to be outputted. When an APD is made of silicon (Si), light sensitivity characteristic of the photo detector largely depends on absorption characteristic of silicon. The APD made of silicon most absorbs light with a wavelength of 400-600 nm. The APD hardly has sensitivity to light of a near infra-red wavelength band. In order to improve the sensitivity of a photo detector using silicon, a device is known in which a depletion layer is made very thick, such as several ten μm, to have sensitivity to light of a near infra-red wavelength band. However, a drive voltage of the photo detector might, become very high, such as several hundred volts. 
     Accordingly, in a photo detector using silicon, a structure to confine light inside the photo detector has been considered, in order to enhance detection efficiency of light of a near infra-red wavelength band. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a diagram showing a photo detector of a first embodiment. 
         FIG. 1B  is a GG′ sectional view of the photo detector of the first embodiment. 
         FIG. 1C  is an SS′ sectional view of the photo detector of the first embodiment. 
         FIG. 2A  is a diagram showing the photo detector of the first embodiment. 
         FIG. 2B  is a circuit diagram of the photo detector of the first embodiment. 
         FIG. 2C  is a DD′ sectional view of the photo detector of the first embodiment. 
         FIG. 3A  is a diagram showing a modification of the first embodiment. 
         FIG. 3B  is a GG′ sectional view of the modification of the first embodiment, 
         FIG. 3C  is an SS′ sectional view of the modification of the first embodiment. 
         FIG. 4A  is a diagram shewing a photo detector of a second embodiment. 
         FIG. 4B  is a diagram showing a photo detector of the second embodiment. 
         FIG. 4C  is a diagram showing characteristics of the photo detector of the second embodiment. 
         FIG. 4D  is a diagram showing characteristics of the photo detector of the second embodiment. 
         FIG. 5A  is a diagram showing a photo detector of a third embodiment. 
         FIG. 5B  is a diagram showing characteristics of the photo detector of the third embodiment. 
         FIG. 6A  is a diagram showing a photo detector of a fourth embodiment. 
         FIG. 6B  is a diagram showing characteristics of the photo detector of the fourth embodiment. 
         FIG. 7  is a diagram showing a photo detector of a fifth embodiment. 
         FIG. 8A  is a diagram showing a photo detector of a sixth embodiment. 
         FIG. 8B  is a GG′ sectional view of the photo detector of the sixth embodiment. 
         FIG. 8C  is an SS′ sectional view of the photo detector of the sixth embodiment. 
         FIG. 9A  is a diagram showing a photo detector of a seventh embodiment. 
         FIG. 9B  is a GG′ sectional view of the photo detector of the seventh embodiment. 
         FIG. 9C  is an SS′ sectional view of the photo detector of the seventh embodiment. 
         FIG. 10A  is a diagram showing photo detectors of the seventh embodiment. 
         FIG. 10B  is a diagram showing characteristics of the photo detectors of the seventh embodiment. 
         FIG. 11A  is a diagram showing a photo detector of an eighth seventh embodiment. 
         FIG. 11B  is a GG′ sectional view of the photo detector of the eighth embodiment. 
         FIG. 11C  is an SS′ sectional view of the photo detector of the eighth embodiment. 
         FIG. 12A  is a diagram showing a photo detection device of a ninth seventh embodiment. 
         FIG. 12B  is a diagram showing a photo detection device of the ninth embodiment. 
         FIG. 12C  is a diagram showing a photo detection device of the ninth embodiment. 
         FIG. 13  is a diagram showing a photo detection device of a tenth embodiment. 
         FIG. 11A  is a diagram showing a photo detector of an eleventh embodiment. 
         FIG. 14B  is a GG′ sectional vies of the photo detector of the eleventh embodiment. 
         FIG. 14C  is as SS′ sectional view of the photo detector of the eleventh embodiment. 
         FIG. 15A  is a diagram showing the photo detector of the eleventh embodiment. 
         FIG. 15B  is a diagram shoeing characteristics of the photo detector of the eleventh embodiment. 
         FIG. 15C  is a diagram showing characteristics of the photo detector of the eleventh embodiment. 
         FIG. 16A  is a diagram showing a modification of the eleventh embodiment. 
         FIG. 16B  is a GG′ sectional view of the modification of the eleventh embodiment. 
         FIG. 16C  is an SS′ sectional view of the modification of the eleventh embodiment. 
         FIG. 17A  is a diagram showing a photo detection device of a twelfth embodiment. 
         FIG. 17B  is a diagram showing the photo detection device of the twelfth embodiment. 
         FIG. 18A  is a diagram showing a manufacturing method of a photo detector. 
         FIG. 18B  is a diagram showing the manufacturing method of a photo detector. 
         FIG. 18C  is a diagram showing the manufacturing method of a photo detector. 
         FIG. 18D  is a diagram stowing the manufacturing method of a photo detector. 
         FIG. 18E  is a diagram showing the manufacturing method of a photo detector. 
         FIG. 18F  is a diagram showing the manufacturing method of a photo detector. 
         FIG. 19A  is a diagram showing a measuring system of a thirteenth embodiment. 
         FIG. 19B  is a diagram showing a measuring system of the thirteenth embodiment. 
         FIG. 19C  is a diagram showing a measuring system of the thirteenth embodiment. 
         FIG. 20  is a diagram showing a LIDAR device of a fourteenth embodiment 
     
    
    
     DETAILED DESCRIPTION 
     According to one embodiment, a photo detector is provided with a semiconductor layer having a first light receiving surface and a second light receiving surface opposite to the first light receiving surface, and a diffraction grating which is provided on the first light receiving surface side of the semiconductor layer and has convex portions. The convex portions are arranged in one direction at. a predetermined cycle. 
     Hereinafter, further embodiments will be described with reference to the drawings. Ones with the same symbols show the similar ones. In addition, the drawings are schematic or conceptual, and accordingly, the relation between a thickness and a width in each portion, and a ratio coefficient of sizes between portions are not necessarily identical to those of the actual ones. In addition, even when the same portions are shown the dimensions and the ratio coefficients thereof may be shown differently depending on the drawings. 
     First Embodiment 
       FIG. 1A  is a diagram showing a photo detector  1003 ,  FIG. 1B  is a GG′ sectional view of the photo detector  1003 , and  FIG. 1C  is an SS′ sectional view of the photo detector  1003 . 
     In  FIG. 1A , the photo detector  1003  is composed of a substrate  90 , a one-dimensional diffraction grating (a diffraction grating)  801 , a p +  type semiconductor layer  32 , a p −  type semiconductor layer  30 , a p +  type semiconductor layer  31 , an n type semiconductor layer  40 , a reflective material  21 , a first electrode not shown, and an insulating layer not shown. 
     The p + type semiconductor layer  32 , the p −  type semiconductor layer  30 , the p +  type semiconductor layer  31 , and the n type semiconductor layer  40  are collectively called a semiconductor layer  5 . In the drawings described later, the description of the p +  type semiconductor layer  32 , the p −  type semiconductor layer  30 , the p +  type semiconductor layer  31 , and the n type semiconductor layer  40  is omitted, and they will be described simply as the semiconductor layer  5 . 
     The semiconductor layer  5  has a first light receiving surface and a second light receiving surface opposite to the first light receiving surface. For example, when the p +  type semiconductor layer  32  side is decided as the first light receiving surface, the n type semiconductor layer  40  side at a side opposite to the p +  type semiconductor layer  32  side becomes the second light receiving side. 
     The semiconductor layer  5  is composed of the p type semiconductor layer and the n type semiconductor layer in this order from the first light receiving surface toward the second light receiving surface. 
     That is, in the present embodiment, the semiconductor layer  5  is composed of the p +  type semiconductor layer  32 , the p −  type semiconductor layer  30 , the p +  type semiconductor layer  31 , and the n type semiconductor layer  40  in this order, from the first light receiving surface toward the second light receiving surface. 
     In addition, the semiconductor layer  5  may not be provided with the p +  type semiconductor layers  31 ,  32 , and may be a laminated structure of a p type semiconductor layer and an n type semiconductor layer. The semiconductor layer  5  may be composed of an n type semiconductor layer and a p type semiconductor layer in this order from the first light receiving surface toward the second light receiving surface. In addition, the semiconductor layer  5  may be composed of an n +  type semiconductor layer, an n −  type semiconductor layer, an n +  type semiconductor layer, and a p type semiconductor layer in this order, from the first light receiving surface toward the second light receiving surface. In addition, the laminated structure of the p +  type semiconductor layer  32 , the p −  type semiconductor layer  30 , the p +  type semiconductor layer  31 , and the n type semiconductor layer  40 , or the laminated structure of the n +  type semiconductor layer, the n −  type semiconductor layer, the n +  type semiconductor layer, and the p type semiconductor may be configured from the second light receiving surface toward the first light receiving surface. 
     In the photo detector  1003 , the semiconductor layer  5  is composed of Si (silicon), for example. It is more preferable to select Si as the material of the semiconductor layer  5 , because the manufacturing cost thereof is not expensive. 
     The one-dimensional diffraction grating  801  and the substrate  90  are provided at the first light receiving side of the semiconductor layer  5 . The one-dimensional diffraction grating  801  is provided between the semiconductor layer  5  and the substrate  90 . The one-dimensional diffraction grating  801  is arranged in one direction. The one-dimensional diffraction grating  801  has convex portions and concave portions. The convex portion and the concave portion are alternately arranged at a predetermined cycle. The convex portion and the concave portion are linear and in parallel with each other. An enlarged view of the one-dimensional diffraction grating  801  surrounded by a round frame in  FIG. 1A  is shown in each of  FIGS. 4A, 4B  described later. 
     The substrate  90  is provided on the first light receiving side of the semiconductor layer  5 . The substrate  90  is provided on the one-dimensional diffraction grating  801  at a side opposite to the semiconductor layer  5 . The substrate  90  transmits light. The substrate  90  supports the semiconductor layer  5 . It is possible that the substrate  90  is not provided. 
     As shown also in  FIG. 1C , a plurality of depletion layers  71  are provided one-dimensionally and separately from each other inside the semiconductor layer  5 . 
     The reflective material  21  is provided on the second light receiving surface side of the semiconductor layer  5 . The reflective material  21  reflects light incident into the semiconductor layer  5 . The reflective material  21  may be provided with a function of an electrode as well. Because a refractive index of the semi conductor layer  5  is different from that of the outside of the semiconductor layer  5 , light incident into the semiconductor layer  5  is reflected by an interface of the semiconductor layer  5  and the outside of the semiconductor layer  5 . For the reason, it is possible that the reflective material  21  is not provided. 
     It is supposed that the light incident into the p +  type semiconductor layer  32  serving as the light receiving surface is near infrared light with a wavelength of not less than 750 nm and not more than 1000 nm. 
     A length of the semiconductor layer  5  in a direction from the light receiving surface toward the reflective material  21  is not less than 1 μm and not more than 15 μm. 
     The substrate  90  may be adhered to the semiconductor layer  5  via an adhesive layer not shown, for example. 
     Here, a light  400  is incident from the p +  type semiconductor layer  32  serving as the light receiving surface of the photo detector  1003 . The incident light  400  is absorbed by the depletion layer  71  formed by the p +  type semiconductor layer  31  and the p −  type semiconductor layer  30 . The incident light  400  is converted into electron-hole pairs in the depletion layer  71 . 
     When a voltage serving as a reverse bias is applied between the pn junction of the p −  type semi conductor layer  30  and the n type semiconductor layer  40 , electrons of the electron-hole pairs flow in the direction of the n type semiconductor layer  40 . Holes of the electron-hole pairs flow in the direction of the p +  type semiconductor layer  32 . At this time, if the voltage is increased, the flowing speeds of the electrons and the holes are accelerated in the depletion layer  71 . Particularly, in the p +  type semiconductor layer  31 , electrons come in collision with atoms in the p −  type semiconductor layer  30 , to generate new electron-hole pairs. This phenomenon is called avalanche amplification. The avalanche amplification is a reaction which occurs in chains. The avalanche amplification is generated, and thereby the photo detector  1003  can detect weak light. 
     A thickness d of the semiconductor layer  5  between the first electrode and the reflective material  21  is 1-15 for example. If this thickness d is smaller than 1 μm, a region of the depletion layer  71  becomes small. Accordingly, a detection efficiency and an amplification factor of light of the photo detector  1003  become low. If the thickness d is larger than 15 μm, it becomes necessary to apply a high voltage when electrodes are respectively provided on the both ends of the semi conductor layer  5 . In addition, the increase of light absorption outside the depletion layer  71 , occurs, and causes reduction of the light detection efficiency. 
     In the photo detector  1003 , a dead time when light cannot be detected is generated after the avalanche amplification has occurred. The dead time of the photo detector  1003  is made short, and thereby the photo detector  1003  can detect light efficiently. In order to make the dead time of the photo detector  1003  short, it is necessary to promptly take out the electrons and holes existing inside the photo detector  1003  to the outside. At this time, a speed at which the electrons and holes are taken out to the outside of the photo detector  1003  is determined by an capacitance C of the photo detector  1003 . The capacitance C depends on an area S of the p +  type semiconductor layer  32  serving as the light receiving surface. The smaller the area S of the p +  type semiconductor layer  32  serving as the light receiving surface is, the smaller the capacitance C of the photo detector  1003  becomes. The smaller the area S of the p +  type semiconductor layer  32  serving as the light receiving surface is, the more promptly the electrons and holes existing inside the photo detector  1003  can be taken out to the outside. 
     Accordingly, it is preferable that the area S of the p +  type semiconductor layer  32  serving as the light receiving surface is not more than 100 μm×100 μm. On the other hand, when the area S of the p +  type semiconductor layer  32  serving as the light receiving surface is too small, the detection sensibility of the photo detector  1003  is decreased. In order to make the reduction of the dead time compatible with the detection sensibility of light, it is preferable that the area S of the p +  type semiconductor layer  32  serving as the light receiving surface is 25 μm×25 μm, for example. 
     In the GG′ sectional view of  FIG. 1B , this incident light  400  is diffracted by the one-dimensional diffraction grating  801 , and proceeds in a direction in which the one-dimensional diffraction grating  801  is arranged. 
     In the SS′ sectional view of  FIG. 1C , the depletion layers  71  are arranged in the same direction as the one-dimensional diffraction grating  801 . Accordingly, the direction in which the light  400  proceeds by the one-dimensional diffraction grating  801  and the direction in which a plurality of the depletion layers  71  are arranged are the same. The light  400  is diffracted by the one-dimensional diffraction grating  801 , and is absorbed by the plurality of depletion layers  71 . 
     A plurality of the depletion layers  71  are provided as in the case of the photo detector  1003 , even though an area of each of the depletion layers  71  is not made large, a detection area of light of the depletion layer  71  is maintained, and thereby a high speed response is enabled. Because light can be diffracted only in a specific direction by the one-dimensional diffraction grating  801 , it is possible to realize a photo detector with higher space-saving property and higher efficiency, than a photo detector  1004  using a two-dimensional diffraction grating which will be described later. 
       FIG. 2A  is a diagram showing a photo detector  1003 ′ that is the photo detector  1003  seen from the diffraction grating  801  side,  FIG. 2B  is a circuit diagram of the photo detector  1003 ′, and  FIG. 2C  is a DD′ sectional view of the photo detector  1003 ′. 
     In the photo detector  1003 ′ of  FIG. 2A , quench resistors  200   a ,  200   b,    200   c  are provided outside the region of the p +  type semiconductor layer  32  serving as the light receiving surface. 
     An insulating layer  50  is provided between the quench resistors  200   a,    200   b,    200   c  and the semiconductor layer  5 . The quench resistors  200   a,    200   b,    200   c  are connected to photo detection regions  1003 ′ a ,  1003 ′ b ,  1003 ′ c ; via first electrodes  10   a , respectively. Each of the photo detection regions  1003 ′ a ,  1003 ′ b ,  1003 ′ c  is the p +  type semiconductor layer  32  serving as the light receiving surface. 
     When the quench resistors  200   a,    200   b,    200   c  and the first electrodes  10   a  are respectively provided corresponding to the photo detection regions  1003 ′ a ,  1003 ′ b ,  1003 ′ c , it is possible to make the depletion layers corresponding to the respective photo detection regions  1003 ′ a ,  1003 ′ b ,  1003 ′ c  inside the semiconductor layer  5 . 
     Wires  11  are provided between the quench resistors  200   a,    200   b ,  200   c  and the insulating layer  50 , respectively. The wires  11  connect among the quench resistor  200   a,  the quench resistor  200   a  and the quench resistor  200   c.    
     In  FIGS. 2B, 2C , the quench resistor  200   a  is connected to the photo detection region  1003 ′ a . The quench resistor  200   b  is connected to the photo detection region  1003 ′ b . The quench resistor  200   c  is connected to the photo detection region  1003 ′ c.    
     The photo detection regions  1003 ′ a ,  1003 ′ b ,  1003 ′ c  are connected in parallel with each other via the quench resistors  200   a,    200   b,    200   c,  respectively. 
     The photo detector  1003 ′ is composed of the photo detection regions  1003 ′ a ,  1003 ′ b ,  1003 ′ c , but the outputs of them are subjected to signal processing as one output. 
     Modification 1 of First Embodiment 
     Hereinafter, a modification of the first embodiment for showing an effect of the above-described photo detector  1003  is shown. 
       FIG. 3A  is a diagram showing a photo detector  1004 ,  FIG. 5B  is a GG′ sectional view of the photo detector  1004 , and  FIG. 3C  is an SS′ sectional view of the photo detector  1004 . 
     The same symbols are given to the same portions as in  FIG. 1A , and the description thereof will be omitted. 
     In  FIG. 3A , the photo detector  1004  is provided with a two-dimensional diffraction grating  821 . The two-dimensional diffraction grating  821  is provided on the light receiving surface side of the semiconductor layer  5 . The two-dimensional diffraction grating  821  is provided between the semiconductor layer  5  and the substrate  90 . The two-dimensional diffraction grating  821  is two-dimensionally arranged within the whole surface of the first light receiving surface. The two-dimensional diffraction grating  821  has convex portions and a concave portion. In the two-dimensional diffraction grating  821 , the convex portions are arranged two-dimensionally. The convex portions are arranged at a predetermined cycle. 
     In the photo detector  1004 , the incident light  400  is diffracted by the two-dimensional diffraction grating  821 . The light  400  is detected by a plurality of the depletion layers  71 . 
     In  FIG. 3B , the incident light  400  is diffracted within the whole surface of the first light receiving surface by the two-dimensional diffraction grating  821 . 
     In  FIG. 3C , the plurality of depletion layers  71  are two-dimensionally arranged within the whole surface inside the semiconductor layer  5 . The plurality of depletion layers  71  absorb the light  400  diffracted by the two-dimensional diffraction grating  821 . 
     Accordingly, in a case of detecting a large portion of the diffracted light  400 , the depletion layers  71  have to be provided two-dimensionally and broadly. On the other hand, in the photo detector  1003 , the light  400  is diffracted in a specified direction by the one-dimensional diffraction grating  801 , and accordingly, the light  400  is not diffused within the surface thereof. In the photo detector  1003 , it is only necessary to provide a smaller number of the depletion layers  71 , compared with the photo detector  1004 . 
     Second Embodiment 
       FIG. 4A  is a diagram showing a photo detector  1005 ″,  FIG. 4B  is a diagram showing a photo detector  1005 ,  FIG. 4C  is a diagram showing the relation between a height d of the photo detector  1005  and a light absorption efficiency, and  FIG. 4D  is a diagram showing light absorption efficiencies of the photo detectors  1005 ″,  1005 . 
     The same symbols are given to the same portions as in  FIG. 1A , and the description thereof will be omitted. 
     The photo detector  1005 ″ of  FIG. 4A  is an enlarged view of the convex portion of the one-dimensional diffraction grating  801  surrounded by the round frame in  FIG. 1A . 
     In  FIG. 4A , the photo detector  1005 ″ is obtained by replacing the one-dimensional diffraction grating  801  of the photo detector  1003  by a stepwise one-dimensional diffraction grating  802 ′.  FIG. 4A  shows one cycle portion of the stepwise one-dimensional diffraction grating  802 ′. 
     The stepwise one-dimensional diffraction grating  802 ′ has a step and is stepwise. A height of the stepwise one-dimensional diffraction grating  802 ′ per step is decided as d. A length (width) of the stepwise one-dimensional diffraction grating  802 ′ in the horizontal direction per step is decided as w. The stepwise one-dimensional diffraction grating  802 ′ is composed of the same material as the semiconductor layer  5 , for example. 
     The photo detector  1005  of  FIG. 4B  is an enlarged view of the convex portion of the one-dimensional diffraction grating  801  surrounded by the round frame in  FIG. 1A , and the photo detector  1005  is obtained by replacing the convex portion of the one-dimensional diffraction grating  801  by a convex portion of a stepwise one-dimensional diffraction grating  802 . The photo detector  1005  is provided with the stepwise one-dimensional diffraction grating  802 , as a modification of the one-dimensional diffraction grating  801  of the photo detector  1003 .  FIG. 4B  shows one cycle portion of the stepwise one-dimensional diffraction grating  802 . 
     The stepwise one-dimensional diffraction grating  802  has a stepwise shape with a plurality of steps. The stepwise one-dimensional diffraction grating  802  has more steps than the above-described stepwise one-dimensional diffraction grating  802 ′. A height of the stepwise one-dimensional diffraction grating  802  per step is decided as d. A length (width) of the stepwise one-dimensional diffraction grating  802  in the horizontal direction per step is decided as w. The stepwise one-dimensional diffraction grating  802  is composed of the same material as the semiconductor layer  5 , for example. 
       FIG. 4C  shows the relation between a value of the height d per step of the stepwise one-dimensional diffraction grating  802  of the photo detector  1005  and a light absorption efficiency. 
       FIG. 4C  is calculated by simulation. The condition of simulation was that the substrate  90  is made of glass, the semiconductor layer  5  is made of silicon with a thickness of 8 μm, the reflective material  21  is made of aluminum with a thickness of 200 nm. In addition, a length (width) of the depletion layer  71  in the horizontal direction is 2 μm, and the light is a randomly polarized light with a wavelength of 900 nm. The length (width) w of the stepwise one-dimensional diffraction grating  802  per step in the horizontal direction is 400 nm. In addition, this simulation was calculated with a finite difference time domain method, and a periodic boundary condition was used for the x direction, and a completely absorption boundary condition was used for the y direction. The cyclic boundary condition was used for the x direction, and thereby the depletion layers  71  serving as detector regions are arranged in the x direction. 
     It is found from  FIG. 4C  that a light absorption efficiency of the photo detector  1005  is more improved, by providing the stepwise one-dimensional diffraction grating  802  (d≠0), than a case in which the stepwise one-dimensional diffraction grating  802  is not provided (d=0). The stepwise one-dimensional diffraction grating  802  is provided, and thereby it becomes easy to confine the light inside the semiconductor layer  5 . 
       FIG. 4D  is calculated by simulation. The condition of simulation was that the lengths (widths) w of the stepwise one-dimensional diffraction gratings  802 ′,  802  per step are 400 nm, and the heights d are 250 nm, respectively. Each of the stepwise one-dimensional diffraction gratings  802 ′,  802  is made of silicon. The other conditions are the same as in  FIG. 4C . 
     S 1 ″ shows a light absorption efficiency of the photo detector  1005 ″, and S 1  shows a light absorption efficiency of the photo detector  1005 . In addition, in  FIG. 4D , a light absorption efficiency REF 1  of a photo detector of a comparative example 1 is also shown for reference. REF 1  of the comparative example 1 is a light absorption efficiency of the photo detector  1003  in which the one-dimensional diffraction grating  801  is not provided. 
     Further, in  FIG. 4D , a light absorption efficiency S′ 1  of the photo detector  1005  is also shown for reference, in a case in which not a periodic boundary condition but a finite region is used for the x direction in the photo detector  1005 . S′ 1  was calculated by simulation in which a width of the depletion layer  71  in the x direction is 20 μm. 
     The wavelength dependencies of light of S 1 ″ and S 1  are more suppressed than that of REF 1 , and S 1 ″ and S 1  realize high absorption efficiencies of light. S 1  has lower wavelength dependency of a light absorption efficiency than S 1 ″. Accordingly, the photo detector  1008  realizes a more stable light absorption efficiency than the photo detector  1005 ″. The more the number of steps of the stepwise one-dimensional diffraction grating  802  is made, the more the wavelength dependency of the light absorption efficiency of the photo detector  1005  is suppressed. Accordingly, the more the number of steps of the stepwise one-dimensional diffraction grating  802  is made, the more stable light absorption efficiency the photo detector  1005  realizes. 
     Third Embodiment 
       FIG. 5A  is a diagram showing a photo detector  1007 ,  FIG. 5B  is a diagram showing a light absorption efficiency of the photo detector  1007 . 
     In  FIG. 5A , the photo detector  1007  is further provided with a path separation layer (a spacer layer)  59  in the photo detector  1005 . The same symbols are given to the same portions as in  FIG. 1A , and the description thereof will be omitted. 
     The path separation layer  59  is provided between the semiconductor layer  5  and the reflective material  21 . A refractive index of the path separation layer  59  is lower than a refractive index of the semiconductor layer  5 . The path separation layer  59  is composed of a film of an oxide such as SiO 2 , a film of a nitride such as SiN, for example, 
     The light  400  diffracted by the stepwise one-dimensional diffraction grating  802  is totally reflected by an interface of the semiconductor layer  5  and the path separation layer  59 . The light  400  is confined within the semiconductor layer  5 . 
     The light  400  which has not been totally reflected by the interface of the semiconductor layer  5  and the path separation layer  59  is reflected by an interface of the path separation layer  50  and the reflective material  21 , and is incident into the semiconductor layer  5 . 
     The photo detector  1007  reduces reflection loss of light in the reflective material  21  by the path separation layer  59 . 
       FIG. 5B  shows wavelength dependency of a light absorption efficiency (S 3 ) of the photo detector  1007 . 
       FIG. 5B  is calculated by simulation. The condition of simulation was that the substrata  90  is made of glass, the semiconductor layer  5  is made of silicon with a thickness of 8 μm, the reflective material  21  is made of aluminum with a thickness of 200 nm. In addition, a width of the depletion layer  71  is 2 μm. The light  400  is a randomly polarized light. The width w of the stepwise one-dimensional diffraction grating  802  per step is 400 nm, the height d is 250 nm. The stepwise one-dimensional diffraction grating  802  is composed of silicon. A thickness of the path separation layer  50  is 1.1 μm. A refractive index of the path separation layer  59  is about 1.5. 
     A light absorption efficiency (S 1 ) of the above-described photo detector  1005  is also shown in  FIG. 5B . As shown in  FIG. 5B , S 3  realizes a higher light absorption efficiency than S 1 . 
     Fourth Embodiment 
       FIG. 6A  is a diagram showing a photo detector  1006 , and  FIG. 6B  is a diagram showing a light absorption efficiency of the photo detector  1006 . 
     In  FIG. 6A , the photo detector  1006  is not provided with the reflective material  21  in the photo detector  1005 . The same symbols are given to the same portions as in  FIG. 1A , and the description thereof will be omitted. 
     The light  400  incident into the photo detector  1006  is diffracted by the stepwise one-dimensional diffraction grating  802 . The diffracted light is incident into the semiconductor layer  5 , and is totally reflected by an interface of the semiconductor layer  5  at a side opposite to the first light receiving surface of the semiconductor layer  5  and the outside. A reflectance when the light  400  is totally reflected by the interface of the semiconductor layer  5  and the outside is higher than a reflectance when the light is reflected by the reflective material  21  of the photo detector  1005 . For the reason, the photo detector  1006  realizes a higher light absorption efficiency than the photo detector  1005 . 
       FIG. 6B  shows wavelength dependency of a light absorption efficiency (S 2 ) of the photo detector  1006 . 
       FIG. 6B  is calculated by simulation. The condition of simulation was that the substrate  90  is made of glass, the semiconductor layer  5  is made of silicon with a thickness of 8 μm. In addition, a length (width) of the depletion layer  71  in the horizontal direction is 2 μm. The light  400  is a randomly polarized light. The length (width) w of the stepwise one-dimensional diffraction grating  802  per step is 400 nm, and the height d thereof is 250 nm. The stepwise one-dimensional diffraction grating  802  is composed of silicon. 
     In  FIG. 6B , the light absorption efficiency (S 1 ) of the above-described photo detector  1005  is shown. S 2  realizes a higher light absorption efficiency than S 1 . 
     Fifth Embodiment 
       FIG. 7  is a diagram showing a photo detector  1008 . 
     The same symbols are given to the same portions as in  FIG. 1A , and the description thereof will be omitted. 
     The photo detector  1008  is provided with a one-dimensional diffraction grating (diffraction grating)  803 , as the one-dimensional diffraction grating (diffraction grating) in the photo detector  1003 . 
     The one-dimensional diffraction grating  803  is a blazed (saw-tooth) phase diffraction grating. In the one-dimensional diffraction grating  803 , two kinds of blazed phase diffraction gratings with different blaze directions face to each other. The one-dimensional diffraction grating  803  may be made a stepwise diffraction grating. It is known that the one-dimensional diffraction grating  803  can be designed so as to make a diffraction efficiency to a specific diffraction order high. Accordingly, the blasé directions are faced so that the lights are diffracted to the center of the photo detector  1008 , the lights hardly escape to the outside of the photo detector  1008 . For the reason, in the photo detector  1008 , a high detection efficiency of light is realized. 
     In addition, a position of the contact point of the two blazed phase diffraction gratings of the one-dimensional diffraction grating  803  is not necessarily the center of the photo detector  1008 . 
     Sixth Embodiment 
       FIG. 8A  is a diagram showing a photo detector  1009 ,  FIG. 8B  is a GG′ sectional view of the photo detector  1008 , and  FIG. 8C  is an SS′ sectional view of the photo detector  1009 . 
     The same symbols are given to the same portions as in  FIG. 1A , and the description thereof will be omitted. 
     In the photo detector  1009 , the stepwise one-dimensional diffraction grating  802  is provided on the second light receiving surface side of the semiconductor layer  5 . The substrate  90  is provided on the first light receiving surface side of the semiconductor layer  5 . The reflective material  21  is provided on the stepwise one-dimensional diffraction grating  802  at a side opposite to the semiconductor layer  5  side. 
     The stepwise one-dimensional diffraction grating  802  diffracts the light  400  which has passed through the semiconductor layer  5 . The diffracted light  400  is reflected by the reflective material  21  toward the depletion layer  71  of the semiconductor layer  5 . 
     As shown in  FIG. 8B  and  FIG. 8C , the stepwise one-dimensional diffraction grating  802  is arranged in accordance with the arrangement direction of the depletion layers  71 . 
     Seventh Embodiment 
       FIG. 9A  is a diagram showing a photo detector  1010 ,  FIG. 9B  is a GG′ sectional view of the photo detector  1010 , and  FIG. 8C  is an SS′ sectional view of the photo detector  1010 . 
     The same symbols are given to the same portions as in  FIG. 8A , and the description thereof will be omitted. 
     In the photo detector  1010 , the reflective material  21  is provided between the substrate  90  and the semiconductor layer  5 . The light incident from the first light receiving surface of the semiconductor layer  5  is diffracted by the stepwise one-dimensional diffraction grating  802 . The diffracted light is absorbed by the depletion layer  71 . The light which has once passed through the depletion layer  71  out of the diffracted light is reflected by the reflective material  21  and is absorbed by the depletion layer  71 . 
     As shown in  FIG. 9B  and  FIG. 9C , the stepwise one-dimensional diffraction grating  802  is arranged in accordance with the arrangement direction of the depletion layers  71 . 
       FIG. 10A  is a diagram showing photo detectors  1010 ,  1011 ,  1018 , and  FIG. 10B  is a diagram showing wavelength dependency of a light absorption efficiency in the photo detectors  1010 ,  1011 ,  1018 . 
     The same symbols are given to the same portions as in  FIG. 9A , and the description thereof will be omitted. 
       FIG. 10A  is a diagram in which the convex portion of the stepwise one-dimensional diffraction grating  802  of the photo detector  1010  of  FIG. 9A  is enlarged. 
     Further, as shown in  FIG. 10A , the photo detector  1011  is not provided with the reflective material  21  in the photo detector  1010 . The photo detector  1018  is provided with the path separation layer (spacer layer)  58  between the semiconductor layer  5  and the substrate  90  in the photo detector  1010 . 
     In  FIG. 10B , S′- 1  shows a light absorption efficiency of the photo detector  1010 , S′- 2  shows a light absorption efficiency of the photo detector  1011 , and S′- 3  shows a light absorption efficiency of the photo detector  1018 . 
       FIG. 10B  is calculated by simulation. The condition of simulation was that the substrate  90  is made of glass, the semiconductor layer  5  is made of silicon with a thickness of 8 μm, the reflective material  21  is made of aluminum with a thickness of 200 nm. A width of the depletion layer  71  is 2 μm. The light  400  is a randomly polarized light. The width w of the stepwise one-dimensional diffraction grating  802  per step is 400 nm, and the height d is 250 nm. The stepwise one-dimensional diffraction grating  302  is composed of silicon. A thickness of the path separation layer  59  of the photo detector  1018  is 1.1 μm, and a refractive index thereof is about 1.5. 
     For reference, a light absorption efficiency REF 1 ′ is also shown in a case in which the stepwise one-dimensional diffraction grating  802  is not provided in the photo detector  1010 . 
     As shown in  FIG. 10B , each of S′- 1 , S′- 2 , and S′- 3  realizes a higher light absorption efficiency than REF 1 ′. Particularly, S′- 3  realizes the highest light absorption efficiency among them. 
     Eighth Embodiment 
       FIG. 11A  is a diagram showing a photo detector  1012 ,  FIG. 11B  is a GG′ sectional view of the photo detector  1012 , and  FIG. 11  is an SS′ sectional view of the photo detector  1012 . 
     The same symbols are given to the same portions as in  FIG. 1A  and  FIG. 8A , and the description thereof will be omitted. 
     The substrate  90  is provided on the reflective material  21  at a side opposite to the stepwise one-dimensional diffraction grating  802  side. The light incident from the first light receiving surface side is absorbed by the depletion layer  71 . The light which has once passed through the depletion layer  71  out of the light incident from the first light receiving surface side is diffracted by the stepwise one-dimensional diffraction grating  802 , and further reflected by the reflective material  21  and is absorbed by the depletion layer  71 . 
     As shown in  FIG. 11B  and  FIG. 11C , the stepwise one-dimensional diffraction grating  802  is arranged in accordance with the arrangement direction of the depletion layers  71 . 
     Ninth Embodiment 
       FIG. 12A  is a diagram showing a photo detection device  1013 ,  FIG. 12B  is a diagram showing a photo detection device  1014 , and  FIG. 12C  is a diagram showing a photo detection device  1013 ′. 
     In  FIG. 12A , the photo detection device  1013  is composed of a plurality of photo detectors  1013   a.  Units  1 - 4  in  FIG. 12A  are each the photo detector  1013   a.  In the photo detection device  1013 , the plurality of photo detectors  1013   a  are arranged one-dimensionally. Each of the plurality of photo detectors  1013   a  is the photo detector according to any of the above-described first to eighth embodiments. The light receiving surfaces of the plurality of photo detectors  1013   a  are arranged two-dimensionally. The photo detection device  1013  can obtain one-dimensional position information of the detected light, and so on. 
     In  FIG. 12B , the photo detection device  1014  is composed of a plurality of photo detectors  1014   a.  The units  1 - 2  in  FIG. 12B  are each the photo detector  1014   a.  In the photo detection device  1014 , the photo detectors  1014   a  are arranged one-dimensionally. Each of the plurality of photo detectors  1014   a  is the photo detector according to any of the above-described first to eighth embodiments. The light receiving surfaces of the plurality of photo detectors  1014   a  are arranged two-dimensionally. The photo detector  1014   a  outputs one output signal as the photo detector  1003 ′ shown in  FIG. 2A . The photo detection device  1014  can obtain one-dimensional position information of the detected light. 
     In  FIG. 12C , the photo detection device  1013 ′ is composed of a plurality of photo detectors  1013 ′ a . Units  11 - 24  in  FIG. 12C  are each the photo detector  1013 ′ a . In the photo detection device  1013 ′, the photo detectors  1013 ′ a  are arranged two-dimensionally. Each of the photo detector  1013 ′ a  is the photo detector of any of the first to eighth embodiments. The light receiving surfaces of the plurality of photo detectors  1013 ′ a  are arranged two-dimensionally. The photo detection device  1013 ′ can obtain two-dimensional position information of the detected light, and so on. 
     Tenth Embodiment 
       FIG. 13  is a diagram showing a photo detection device  1015 . 
     The photo detection device  1015  is further provided with reflection walls  29  in the photo detection device  1013  of  FIG. 12A . For example, each of the reflection walls  29  is provided between the photo detector  1013   a  and the photo detector  1013   a.  When the light  400  is not sufficiently diffracted by the one-dimensional diffraction grating of the photo detector  1013   a,  the reflection wall  29  plays a role to return the light  400  which has gone outside the detection region to the detection region again. When the cycle direction of the one-dimensional diffraction grating and the arrangement direction of the detection regions within the photo detector do not sufficiently coincide with each other, the reflection wall  29  is effective. 
     Eleventh Embodiment 
       FIG. 14A  is a diagram showing a photo detector  1016 .  FIG. 14B  is a GG′ sectional view of the photo detector  1016 , and  FIG. 14C  is an SS′ sectional view of the photo detector  1016 . 
     The same symbols are given to the same portions as in  FIG. 1A , and the description thereof will be emitted. 
     In the photo detector  1016  of  FIG. 14A , the cycle direction of the stepwise one-dimensional diffraction grating  802  is different from the arrangement direction of the depletion layers  71 . In addition, in the photo detector  1016 , a groove (void portion)  600  is provided in at least a part of the periphery of the first light receiving surface. 
     In the GG′ sectional view of  FIG. 14B , the cycle direction of the stepwise one-dimensional diffraction grating  802  is inclined to the arrangement direction of the depletion layers  71  by 45 degrees, for example. Dotted lines shown in  FIG. 14B  show the positions of the depletion layers  71  inside the semiconductor layer  5 . 
     In the SS′ sectional view of  FIG. 14C , the grooves  600  are provided so as to surround the whole or a part of the periphery of the depletion layer  71  region. 
     In the photo detector  1016 , when the light  400  is diffracted by the stepwise one-dimensional diffraction grating  802 , it is incident into the groove  600 . Since the cycle direction of the stepwise one-dimensional diffraction grating  802  and the arrangement direction of the depletion layers  71  are different, the light  400  which has been diffracted by the stepwise one-dimensional diffraction grating  802  has a specific incident angle to the groove  600 . Since the groove  600  is filled with air, for example, the light  400  is totally reflected by the interface of the semiconductor layer  5  and the groove  600 . Since the totally reflected light  400  is also totally reflected by the other groove  600  in the same manner, the light  400  is confined within the detection region surface. 
       FIG. 15A  is a diagram showing a condition to confine the light within the detection region.  FIG. 15B  is a diagram showing the relation between an angle α and angles θ 1 , θ 2 , and  FIG. 15C  is a diagram showing the relation between an angle θ and a reflectance of the groove  600 . 
       FIG. 15A  is a diagram of the photo detector  1016  when seen from the first light receiving surface side. Here, it is assumed that the semiconductor layer  5  is silicon, and the groove  600  is filled with air. When an angle formed by the linearly arranged convex portions or concave portions of the stepwise one-dimensional diffraction grating  802  and the groove  600  is α, the light  400  is incident to the interface between air and the semiconductor layer  5  in the groove  600  at an incident angle θ 1 . 
       FIG. 15B  shows the relation between the angle α and the angle θ 1 . 
     The horizontal axis shows the angle α, and the vertical axis shows an angle θ 1 . And the vertical axis also shows an angle θ 2  described later. 
     The condition in which the light  400  is totally reflected by the interface in the groove  600  is that θ 1  is not less than 15.8 (deg). Accordingly, the angle α is also decided as 15.8 (deg). Further, when the totally reflected light  400  has been incident into another interface of the groove  600  at the incident angle θ 2 , it is necessary to make the incident angle θ 2  15.8 (deg), so as to make the light  400  to be totally reflected. At this time, since the angle α is expressed by 90−θ 2  (deg), the angle α becomes 90−15.8 (deg). Accordingly, when regarding the angle α formed by the linearly arranged convex portions or concave portions and the groove  600 , 15.8 (deg)≦α≦90−15.8 (deg), it is possible to completely confine the light  400  within the photo detection region. 
       FIG. 15C  shows the relation between the length (width) x of the groove  600  in the horizontal direction and a reflectance of light of the interface in the groove  600 . 
     The horizontal axis shows the incident angle θ 1  (θ 2 ), and the vertical axis shows the reflectance. 
       FIG. 15C  is calculated by simulation. The semiconductor layer  5  is composed of silicon. It is assumed that the inside of the groove  600  is filled with air. A wavelength of the light was decided as 905 nm, and the width of the groove  600  was decided as x. If the width x of the groove  600  is a length that is not less than at least the wavelength of the light, when the incident angle θ to the groove  600  becomes not less than a critical angle, it is possible to obtain the same effect as the effect when the width x is made an infinite value. However, if the width x is made too large, the photo detection region of the photo detector  1016  might be decreased. For the reason, the width x becomes not more than 1 mm at a maximum, for example. 
     Modification of Eleventh Embodiment 
       FIG. 16A  is a diagram showing a photo detector  1017 ,  FIG. 16B  is a GG′ sectional view of the photo detector  1017 , and  FIG. 16C  is an SS′ sectional view of the photo detector  1017 . 
     The same symbols are given to the same portions as in  FIG. 14A , and the description thereof will be omitted. 
     In  FIG. 16A  and  FIG. 16B , the photo detector  1017  is provided with the two-dimensional diffraction grating  821 . In  FIG. 16B , dotted lines show the positions of the depletion layers  71  inside the semiconductor layer  5 . 
     In the photo detector  1017  of  FIG. 18C , the incident light  400  is diffracted by the two-dimensional diffraction grating  821 . The light  400  diffracted by the two-dimensional diffraction grating  821  spreads in all directions and reaches the groove  600 . A part of the diffracted light  400  is totally reflected by the interface of the semiconductor layer  5  and the groove  600 . But the remainder of the diffracted light  400  might pass from the semiconductor layer  5  to the groove  600 . 
     On the other hand, in the photo detector  1016  of  FIG. 14A , the whole of the light  400  is totally reflected by the interface of the semiconductor layer  5  and the groove  600 . For the reason, the photo detector  1016  is easy to realize a higher detection efficiency than the photo detector  1017 . 
     Twelfth Embodiment 
       FIG. 17A  is a diagram showing a photo detection device  1008 , and  FIG. 17B  is a diagram of the photo detection device  1008  seen from an as plane or a yz plane. 
     In the photo detection device  1008 , a plurality of the photo detectors  1008   a  are arranged. The photo detector  1008   a  is the photo detector  1016  or the photo detector  1017  which is described above. In the photo detection device  1008 , the plurality of photo detectors  1008   a  are arranged, and thereby two-dimensional information can be obtained. 
     (Manufacturing Method) 
       FIGS. 18A to 18F  are diagrams for describing a manufacturing method of the photo detector  1003 . Here, an example of a case to use Si as the semiconductor material will be shown. 
     To begin with, in  FIG. 18A , an SOI (Silicon On Insulator) substrate is prepared. The SOI substrate has a structure in which a silicon substrate  91 , a BOX (buried oxide layer)  52 , an active layer (n type semiconductor layer)  40  are laminated in this order. The p −  type semiconductor layer  30  is formed on the n type semiconductor layer  40  by epitaxial growth. 
     Next, in  FIG. 19B , impurities (boron, for example) are implanted into the p −  type semiconductor layer  30  so that a part of the region of the p −  type semiconductor layer  30  becomes the p +  type semiconductor layer  31 . By this means, the p +  type semiconductor layer  31  composing a photo detection element is formed on a portion of the active layer  40  of the SOI substrate. In addition, a first mask not shown is formed on the p −  type semiconductor layer  30 , and p type impurities are implanted into the p −  type semiconductor layer  30  using this first mask, to form the p +  type semiconductor layer  32  on the p −  type semiconductor layer  30  serving as a photo detection region. 
     In  FIG. 18C , after the above-described first mask is removed, the one-dimensional diffraction grating  801  is formed in the x direction on the upper portion of the p −  type semiconductor layer  30 , by dry etching or wet etching, for example. 
     In  FIG. 18D , the insulating layer  50  is formed. The first electrode  10  is formed so as to cover the insulating layer  50  and a peripheral portion of the p +  type semiconductor layer  32 . For example, metal such as Ag, Al, Au, Cu or an alloy thereof is used for the first electrode  10 . 
     In  FIG. 18E , a passivation layer  82  is formed so as to cover the one-dimensional diffraction grating  801  and the first electrode  10 . A support substrate  92  is provided on the passivation layer  82 . The support substrate  92  may be directly adhered to the passivation layer  82 , or the support substrate  92  and the passivation layer  82  may be adhered to each other using an adhesive layer not shown. After the support substrate  92  is provided, the silicon substrate  91  is subjected to dry etching. In this dry etching, a reaction gas such as SF 6  can be used, for example. When a reaction gas having etch selectivity of the silicon substrate  91  and the BOX  52  is used in this dry etching, the BOX  52  can be used as an etching stop film. In addition, when the silicon substrate  91  is sufficiently thick, a polishing process such as back grinding and CMP (Chemical Mechanical Polishing), or wet etching may be used together. When wet etching is used, KOH or TMAH (Tetra-Methyl-Ammonium Hydroxide) can be used as etchant. When the silicon substrate  91  is etched by means of this, the BOX  52  is exposed. 
     In  FIG. 18F , the exposed BOX  52  is removed by etching, and thereby the n type semiconductor layer  40  is exposed. As this etching, wet etching with hydrofluoric acid or the like can be used. Wet etching like this is used, and thereby etch selectivity of the BOX  52  and silicon can be sufficiently ensured, and the exposed BOX  52  can be selectively removed. After the n type semiconductor layer  40  is exposed, the reflective material  21  is formed on the n type semiconductor layer  40  so as to cover at least the photo detection region in which the p +  type semiconductor layers  31 ,  32  are provided. 
     Thirteenth Embodiment 
       FIG. 19A  is a diagram showing a measuring system, and  FIGS. 19B, 19C  are diagrams each showing a specific example of the measuring system. 
     The measuring system is composed of at least a photo detection device  1010  and a light source  3000 . 
     In the measuring system, the light source  3000  emits a light  410  to a measuring object  500 . The photo detection device  1019  detects a light  411  which has passed through the measuring object  500  or has reflected or diffused from the measuring object  500 . The measuring system may be configured such that the light source  3000  and the photo detection device  1019  are respectively housed in separate chassis, for example, as shown in  FIG. 19B . Or the light source  3000  and the photo detection device  1010  may be housed in the same chassis, as shown in  FIG. 19C . Any of the photo detectors or the photo detection devices of the above-described embodiments is used as the photo detection device  1019 , and thereby it is possible to realize a measuring system with high sensitivity, particularly in the near infra-red region. 
     Fourteenth Embodiment 
       FIG. 20  is a diagram showing a LIDAR (Laser Imaging Detection and Ranging) device  5001 . 
     The LIDAR device  5001  is provided with a light projecting unit and a light receiving unit. 
     The light projecting unit is composed of a light oscillator  304 , a drive circuit  303 , an optical system  305 , a scan mirror  306 , and a scan mirror controller  302 . The light receiving unit is composed of a reference light detector  309 , a photo detection device  310 , a distance measuring circuit  308 , and an image recognition system  307 . 
     In the light projecting unit, the laser light oscillator  304  emits laser light. The drive circuit  303  drives the laser light oscillator  304 . The optical system  305  extracts a part of the laser light as reference light, and irradiates an object  501  with the other laser light via the mirror  306 . The scan mirror controller  302  controls the scan mirror  306 , to irradiate an object  501  with the laser light. 
     In the light receiving unit, the reference light detection device  309  detects the reference light, extracted by the optical system  305 . The photo detection device  310  receives the reflected light from the object  501 . The distance measuring circuit  308  measures a distance to the object  501 , based on the reference light detected by the reference light photo detection device  309  and the reflected light detected by the photo detection device  310 . The image recognition system  307  recognizes the object  501  based on the result measured by the distance measuring circuit  308 . 
     The LIDAR device  5001  is a distance image sensing system employing a light flight time ranging method (Time of Flight) which measures a time required for a laser light to reciprocate to a target, and converts the time into a distance. The LIDAR device  5001  is applied to an on-vehicle drive-assist system, remote sensing, and so on. If any of the photo detectors or the photo detection devices of the above-described embodiments is used as the photo detection device  310 , the LIDAR device  5001  expresses good sensitivity, particularly in a near infra-red region. For this reason, it becomes possible to apply the LIDAR device  5001  to a light source in a human-invisible wavelength band. The LIDAR device  5001  can be used for obstacle detection for vehicle, for example. 
     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 embodiments described herein may be embodied in a variety of other forms; furthermore, various emissions, 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 inventions.