Abstract:
A semiconductor light detecting element includes: a semiconductor substrate; and a distributed Bragg reflector layer of a first conductivity type, an optical absorption layer, and a semiconductor layer of a second conductivity type, sequentially laminated on the semiconductor substrate. The distributed Bragg reflector layer includes first and second alternately laminated semiconductor layers with different band-gap wavelengths, sandwiching the wavelength of detected incident light. The sum of thicknesses a first and a second semiconductor layer is approximately one-half the wavelength of the incident light detected.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor light receiving element including a distributed Bragg reflector layer, and in particular to a semiconduct or light receiving element having a high light-receiving sensitivity to incident light in the vicinity of the 1.3 mm band. 
     2. Background Art 
     A photodiode having a distributed Bragg reflector (DBR) layer between an optical absorption layer and a semiconductor substrate has been proposed. Light that has not been absorbed in and has passed through the optical absorption layer is reflected by the DBR layer and absorbed again in the optical absorption layer. Thereby, high quantum efficiency can be obtained in the photodiode having the DBR layer. 
     SUMMARY OF THE INVENTION 
     In a DBR layer having a plurality of pairs wherein InGaAsP layers and InP layers are alternately laminated, the reflectance R when the band-gap wavelength of the InGaAsP layers is made to be 1.2 μm was obtained by calculation. In order to suppress the reflection from layers other than the DBR layers, AR coating was carried out on the surface of a device. The calculation was carried out referring to Chapter 1 of “Principles of Optics 1”, written by Max Born and Emil Wolf, issued by Tokai University Press. 
     As a result of calculation, it was found that the number of pairs of InGaAsP layers and InP layers is required to be 17 or more, and the total layer thickness of DBR layers is required to be 3.3 μm or more in order to obtain, for example, 70% or more reflectance. Therefore, problems wherein the uniform control of the thickness and the material composition of the DBR layers on the wafer surface is difficult, the obtaining of reflectance as designed is difficult, and reproducibility is poor, were caused. 
     To realize high reflectance with a small number of pairs, a method for increasing difference in refractive indices of materials used in the multilayer reflective film layer has been generally known. For example, there has been proposed a DBR layer using InP as the material having a low refractive index, and InGaAsP (band-gap wavelength: 1615 nm) having the As-P composition ratio of 9:1 as the material having a high refractive index (e.g., refer to Japanese Patent Laid-Open No. 05-304279). For the DBR layer, when a material composition wherein difference in refractive indices becomes largest was selected, the reflectance for incident light in the vicinity of the 1.3 μm band was calculated. Although reflectance was sharply elevated with up to about 10 pairs, the reflectance was saturated at around 15 pairs, and 40% or higher reflectance could not be obtained. Consequently, the light-receiving sensitivity to incident light in the vicinity of the 1.3 μm band was low. 
     In view of the above-described problems, an object of the present invention is to provide a semiconductor light receiving element having a high light-receiving sensitivity to incident light in the vicinity of the 1.3 mm band. 
     According to the present invention, a semiconductor light receiving element comprises: a semiconductor substrate; and a distributed Bragg reflector layer of a first conductivity type, an optical absorption layer, and a semiconductor layer of a second conductivity type which are sequentially laminated on the semiconductor substrate, wherein the distributed Bragg reflector layer includes first semiconductor layers and second semiconductor layers which are alternately laminated, each first semiconductor layer has a band-gap wavelength which is larger than a wavelength of an incident light, each second semiconductor layer has a band-gap wavelength which is smaller than the wavelength of the incident light, a reflectance peak wavelength of the distributed Bragg reflector layer is 1.20 μm to 1.35 μm, a sum of an optical layer thickness of one of the first semiconductor layers and an optical layer thickness of one of the second semiconductor layers is approximately half of the wavelength of the incident light, and the band-gap wavelength of each first semiconductor layer is 1.30 μm to 1.55 μm. 
     The present invention makes it possible to provide a semiconductor light receiving element having a high light-receiving sensitivity to incident light in the vicinity of the 1.3 mm band. 
     Other and further objects, features and advantages of the invention will appear more fully from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view showing a semiconductor light receiving element according to a first embodiment. 
         FIG. 2  is a sectional view showing a semiconductor light receiving element according to the first comparative example. 
         FIG. 3  is a graph showing a result of calculating the sensitivity of a semiconductor light receiving element according to the first embodiment by changing the band-gap wavelength of an InGaAsP layer. 
         FIG. 4  is a sectional view showing a semiconductor light receiving element according to a third embodiment. 
         FIG. 5  is a sectional view showing a semiconductor light receiving element according to a fourth embodiment. 
         FIG. 6  is a sectional view showing a semiconductor light receiving element according to a fifth embodiment. 
         FIG. 7  is a sectional view showing a semiconductor light receiving element according to a sixth embodiment. 
         FIG. 8  is a sectional view showing a semiconductor light receiving element according to a seventh embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereafter, the embodiments of the present invention will be described referring to the drawings. The like components are denoted by the same reference numerals, and the descriptions thereof will be omitted. 
     First Embodiment 
       FIG. 1  is a sectional view showing a semiconductor light receiving element according to a first embodiment. The semiconductor light receiving element is a photodiode having DBR layers. 
     On an n-type InP substrate  10  (semiconductor substrate), an n-type DBR layer  12  (DBR layer of first conductivity type), an i-InGaAs optical absorption layer  14  having a low carrier concentration (optical absorption layer), and a p-type InP window layer  16  (semiconductor layer of second conductivity type) are sequentially formed. On the p-type InP window layer  16 , an insulating film  18  composed of SiN or the like also functioning as both a reflection preventing film and a surface protecting film, and an anode (p-type electrode)  20 , are formed. On the back side of the n-type InP substrate  10 , a cathode (n-type electrode)  22  is formed. 
     The n-type DBR layer  12  is formed by alternately laminating 15 pairs of n-type InP layers  12   a  (second semiconductor layers) having a low refractive index and n-type InGaAsP layers  12   b  (first semiconductor layers) having a high refractive index. The band-gap wavelength of the n-type InP layers  12   a  is smaller than the wavelength λ, of the incident light. On the other hand, the band-gap wavelength of the n-type InGaAsP layers  12   b  is larger than the wavelength λ of the incident light. 
     The wavelength λ of the incident light is in the vicinity of the 1.3 μm band. The reflectance peak wavelength of the n-type DBR layer  12  is 1.20 μm to 1.35 μm. The band-gap wavelength of the n-type InGaAsP layer  12   b  is 1.30 μm to 1.55 μm. 
     The thickness of the i-InGaAs optical absorption layer  14  is 1 μm. The optical layer thickness of one of the n-type InP layers  12   a  and the optical layer thickness of one of the n-type InGaAsP layers  12   b  is about ¼ the wavelength λ of the incident light, respectively. For example, when λ is 1.30 μm, if the refractive index of InP is 3.2, the thickness of the n-type InP layer  12   a  is 0.099 μm, and if the refractive index of InGaAsP is 3.38, the thickness of the n-type InGaAsP layer  12   b  is 0.094 μm. 
     However, the present invention is not limited to the above values, but the sum (d1×n1+d2×n2) of the optical layer thickness of one of the n-type InP layers  12   a  (layer thickness: d1, refractive index: n1) (=layer thickness×refractive index) and the optical layer thickness of one of the n-type InGaAsP layers  12   b  (layer thickness: d2, refractive index: n2) can be approximately half of wavelength λ of the incident light (=λ/2), that is, approximately half of the wavelength at the reflectance peak. According to the present embodiment, the sum becomes 0.60 μm to 0.675 μm. Thereby, the n-type DBR layer  12  operates as a reflection layer for the incident light at high efficiency. 
     The operation of the semiconductor light receiving element according to the present embodiment will be described. A reverse bias of 0.5 to 3 V is applied so that the potential of the anode  20  becomes lower than the potential of the cathode  22 . The incident light is introduced from the upper side of the drawing into the i-InGaAs optical absorption layer  14  through the insulating film  18  and the p-type InP window layer  16 . Then, the incident light is absorbed in the i-InGaAs optical absorption layer  14 . 
     When the thickness of the i-InGaAs optical absorption layer  14  is t, and the absorption coefficient of the i-InGaAs optical absorption layer  14  to the incident light is a, the proportion of the incident light absorbed in the i-InGaAs optical absorption layer  14  (=quantum efficiency) is represented by the following equation (1):
 
1−exp(−α·t)  (1)
 
     The light that has not been absorbed in and has passed through the i-InGaAs optical absorption layer  14  is reflected by the n-type DBR layer  12 , and is absorbed again in the i-InGaAs optical absorption layer  14 . When the reflectance of the light by the n-type DBR layer  12  is R, the quantum efficiency when the return light by the n-type DBR layer  12  is considered is represented by the following equation (2):
 
1−exp(−α·t)+R·exp(−α·t)·(1−exp(−α·t))  (2)
 
     The difference between Equation (1) and Equation (2) is the increment of the quantum efficiency by the n-type DBR layer  12 . The i-InGaAs optical absorption layer  14  is depleted by the reverse bias. The depletion layer is subjected to an electric field, and the electrons and holes flow to the sides of the cathode  22  and the anode  20 , respectively, and are taken out as electric currents. 
     The effect of the semiconductor light receiving element according to the present embodiment will be described comparing with comparative examples.  FIG. 2  is a sectional view showing a semiconductor light receiving element according to the first comparative example. The n-type DBR layer  100  is formed by alternately laminating 15 pairs of n-type InP layers  100   a  and n-type InGaAsP layers  100   b  having different refractive indices. Both the n-type InP layer  100   a  and the n-type InGaAsP layer  100   b  have large band-gaps, and do not absorb the incident light. The band-gap wavelength of the n-type InGaAsP layer  100   b  is 1.2 μm. The second comparative example uses InGaAs layers in place of the n-type InGaAsP layers  100   b . The optical layer thickness of each layer in the n-type DBR layer  100  is ¼ the wavelength of the incident light, 1.3 μm. Components other than the DBR layer are identical to those in the semiconductor light receiving element according to the present embodiment. 
     The sensitivities of the semiconductor light receiving element according to the present embodiment wherein the band-gap wavelength of the n-type InGaAsP layer  12   b  is made to be 1.35 μm, and the semiconductor light receiving elements according to the first and second comparative examples were measured, respectively. As a result, the sensitivity of the present embodiment was 0.9 A/W or higher, and was the highest. The reason is that high reflectance can be obtained because a large difference in refractive indices between the n-type InP layer  12   a  and the n-type InGaAsP layer  12   b  in the n-type DBR layer  12  can be sufficiently taken in the present embodiment, and the effect of optical absorption in the n-type DBR layer  12  is small. On the other hand, in the first comparative example, sufficient reflectance of the n-type DBR layer cannot be obtained with about 15 pairs of InGaAsP layers and InP layers. In the second comparative example, the optical absorption in the n-type DBR layer becomes large, and sufficient reflectance cannot be obtained. 
       FIG. 3  is a graph showing a result of calculating the sensitivity of a semiconductor light receiving element according to the first embodiment by changing the band-gap wavelength of an InGaAsP layer. The wavelength of the incident light was 1.3 μm, the thickness of the i-InGaAs optical absorption layer  14  was 1 μm, and the number of pairs in the n-type DBR layer  12  was 15. It is found that the sensitivity becomes maximal when the band-gap wavelength of the n-type InGaAsP layer  12   b  is between 1.30 μm and 1.55 μm, and the sensitivity is lowered when the band-gap wavelength is longer or shorter than this range. 
     It is also preferable that the n-type DBR layer  12  includes 20 or less pairs of an n-type InP layer  12   a  and an n-type InGaAsP layer  12   b . Thereby, the thickness and the material composition of the n-type DBR layer  12  can be uniformly controlled on the wafer surface, reflectance as designed can be obtained, and the reproducibility is excellent. In addition, with the present embodiment, the reflectance of the n-type DBR layer  12  for the incident light in the vicinity of 1.3 μm band can be elevated even with 20 pairs or less. 
     In place of the n-type InP layer  12   a , an InGaAsP layer, an AlGaInAs layer, a GaInNAs layer, or the like, whose band-gap wavelength is smaller than the wavelength of the incident light, may also be used. In place of the n-type InGaAsP layer  12   b , an AlGaInAs layer, whose band-gap wavelength is larger than the wavelength of the incident light, may also be used. 
     Second Embodiment 
     In a second embodiment, the band-gap wavelengths of a plurality of n-type InGaAsP layers  12   b  in the n-type DBR layer  12  become smaller closer to the i-InGaAs optical absorption layer  14  and become larger further from the i-InGaAs optical absorption layer  14 . The average band-gap wavelength of the plurality of n-type InGaAsP layers  12   b  is 1.35 μm to 1.55 μm. The sum of the optical layer thickness of one layer of n-type InP layers  12   a  and the optical layer thickness of one layer of n-type InGaAsP layers  12   b  is approximately half the wavelength λ of the incident light, that is, approximately half the reflectance peak wavelength. Other components are identical to the components of the first embodiment. Also by this configuration, the same effect as in the first embodiment can be obtained. 
     Third Embodiment 
       FIG. 4  is a sectional view showing a semiconductor light receiving element according to a third embodiment. The semiconductor light receiving element is a planar-type pin-photodiode wherein the p-type region is formed by selective diffusion. 
     An n-type InP layer  24  having a low carrier concentration is formed on an i-InGaAs optical absorption layer  14  and a p-type InP layer  26  (semiconductor layer of the second conductivity type) is formed in a part of the n-type InP layer  24  by selective diffusion or the like. The configuration of the n-type DBR layer  12  is the same as in the first embodiment or the second embodiment. Thereby, the same effect as in the first embodiment can be obtained. 
     Fourth Embodiment 
       FIG. 5  is a sectional view showing a semiconductor light receiving element according to a fourth embodiment. The semiconductor light receiving element is a planar-type InP avalanche photodiode. 
     An n-type InP multiplication layer  28  (carrier multiplication layer) is formed on an i-InGaAs optical absorption layer  14 , and a p-type InP layer  26  (semiconductor layer of the second conductivity type) is formed in a part of the n-type InP multiplication layer  28  by selective diffusion or the like. A guard ring  30  is formed in the periphery of the p-type InP layer  26  by the implantation of Be ions. The n-type InP multiplication layer  28  performs the avalanche multiplication of photocarriers generated in the i-InGaAs optical absorption layer  14 . The configuration of the n-type DBR layer  12  is the same as in the first embodiment or the second embodiment. Thereby, the same effect as in the first embodiment can be obtained. 
     Fifth Embodiment 
       FIG. 6  is a sectional view showing a semiconductor light receiving element according to a fifth embodiment. The semiconductor light receiving element is a planar-type AlInAs avalanche photodiode. 
     An n-type AlInAs multiplication layer  32  (carrier multiplication layer) and a field relaxation layer  34  are formed between the n-type DBR layer  12  and the i-InGaAs optical absorption layer  14 . The n-type AlInAs multiplication layer  32  performs the avalanche multiplication of photocarriers generated in the i-InGaAs optical absorption layer  14 . Other components are identical to the components of the third embodiment. 
     Since the n-type InP layer  12   a  having a low heat resistance is present in the vicinity of the n-type AlInAs multiplication layer  32  to become a heat generating source, highly efficient heat dissipation can be performed. If an AlInAs layer same as the n-type AlInAs multiplication layer  32  is used in place of the n-type InP layer  12   a  as a layer having a low refractive index for the n-type DBR layer  12 , crystals can be stably grown because of the same material. 
     Sixth Embodiment 
       FIG. 7  is a sectional view showing a semiconductor light receiving element according to a sixth embodiment. The semiconductor light receiving element is a planar-type AlInAs avalanche photodiode as in the fifth embodiment. 
     An n-type AlInAs layer  36  having a high carrier concentration is inserted between the n-type InP layer  12   a  in the n-type DBR layer  12  and the n-type AlInAs multiplication layer  32 . Other constitutions are identical to the constitutions of the fifth embodiment. Thereby, since the electric field of the n-type AlInAs multiplication layer  32  is not applied to the n-type InP layer  12   a , the multiplication of the holes in the n-type InP layer  12   a  is suppressed, and low-noise avalanche photodiode can be realized. 
     Seventh Embodiment 
       FIG. 8  is a sectional view showing a semiconductor light receiving element according to a seventh embodiment. The semiconductor light receiving element is a backside incident resonance photodiode wherein light is incident from the substrate side. 
     An i-InGaAs optical absorption layer  14  having a low carrier concentration (optical absorption layer) and a p-type DBR layer  38  (DBR layer of the second conductivity type) are sequentially formed on an n-type InP substrate  10  (semiconductor substrate of the first conductivity type). An insulating film  18  composed of SiN or the like, which is used as both a reflection preventing film and a surface protecting film, and an anode (p-type electrode)  20  are formed on the p-type DBR layer  38 . A cathode (n-type electrode)  22  and a reflection preventing film  40  are formed on the backside of the n-type InP substrate  10 . The incident light enters from the backside of the n-type InP substrate  10 . 
     The p-type DER layer  38  is formed by alternately laminating 15 pairs of p-type InP layers  38   a  (second semiconductor layers) having a low refractive index and p-type InGaAsP layers  38   b  (first semiconductor layers) having a high refractive index. The band-gap wavelength of the p-type InP layer  38   a  is smaller than the wavelength λ of the incident light. On the other hand, the band-gap wavelength of the p-type InGaAsP layer  38   b  is larger than the wavelength λ of the incident light. 
     The wavelength λ of the incident light is in the vicinity of the 1.3 μm band. The reflectance peak wavelength of the p-type DBR layer  38  is 1.20 μm to 1.35 μm. The band-gap wavelength of the p-type InGaAsP layer  38   b  is 1.30 μm to 1.55 μm. 
     The thickness of the i-InGaAs optical absorption layer  14  is 1 μm. The optical layer thickness of one of the p-type InP layers  38   a  and the optical layer thickness of one of the p-type InGaAsP layers  38   b  is about ¼ the wavelength λ of the incident light, respectively. However, the present invention is not limited to the above values, but the sum of the optical layer thickness of one of the p-type InP layers  38   a  and the optical layer thickness of one of the p-type InGaAsP layers  38   b  can be about half of wavelength λ of the incident light, that is, about half of the wavelength at the reflectance peak. Thereby, the p-type DBR layer  38  operates as a reflection layer for the incident light at high efficiency. 
     Using the above-described configuration, the effect same as in the first embodiment can be obtained. Furthermore, since the absorption coefficient of the p-type InGaAsP layers  38   b  having a high refractive index is small, the loss of the incident light is reduced. In addition, since the anode  20  operates as a high reflectance mirror, the light that has passed through the p-type DBR layer  38  is further reflected and can contribute to absorption, and a higher sensitivity can be anticipated. 
     Eighth Embodiment 
     In an eighth embodiment, the band-gap wavelength of a plurality of p-type InGaAsP layers  38   b  in the p-type DBR layer  38  becomes smaller as approaching the i-InGaAs optical absorption layer  14  and becomes larger as separating from the i-InGaAs optical absorption layer  14 . The average band-gap wavelength of a plurality of the p-type InGaAsP layers  38   b  is 1.35 μm to 1.55 μm. Other constitutions are identical to the constitutions of the seventh embodiment. Also by these constitutions, the same effect as in the seventh embodiment can be obtained. 
     Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. 
     The entire disclosure of a Japanese Patent Application No. 2009-220018, filed on Sep. 25, 2009 including specification, claims, drawings and summary, on which the Convention priority of the present application is based, are incorporated herein by reference in its entirety.