Abstract:
In a mesa type PIN-PD formed using a heavily doped semiconductor material, a high frequency response is degraded as slow carriers occur in a heavily doped layer when light incident into a light receiving section transmits through an absorbing layer and reaches the heavily doped layer on a side near the substrate. In a p-i-n multilayer structure, a portion corresponding to a light receiving section of a heavily doped layer on a side near a substrate is previously made thinner than the periphery of the light receiving section by an etching or selective growth technique, over which an absorbing layer and another heavily doped layer are grown to form the light receiving section of mesa structure. This makes it possible to form a good ohmic contact and to realize a PIN-PD with excellent high frequency response characteristics.

Description:
CLAIM OF PRIORITY 
   The present application claims priority from Japanese patent application serial no. 2006-184303, filed on Jul. 4, 2006, the content of which is hereby incorporated by reference into this application. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a surface illuminated photodiode and an optical receiver module on which the surface illuminated photodiode is mounted. More particularly, the invention relates to a surface illuminated photodiode appropriate for use in a module for optical communication and to an optical receiver module appropriate for use in an optical communication system. 
   There has been put into practice a p-i-n type photodiode (PIN-PD) as a device on the receiving side for optical transmission using an optical fiber. The PIN-PD is excellent in high frequency response as electrons and holes, which are generated due to incident light, move by strong drift field in an absorbing layer. 
   The basic structure of the PIN-PD is that an absorbing layer (I layer) is provided between a p-type cap layer and p-type contact layer, and an n-type contact layer. The principal of operation of the PIN-PD is that light incident into a reverse biased PD is converted to carriers (electrons and holes) in a depleted absorbing layer in which the holes spread and run to a p-electrode side and the electrons spread and run to an n-electrode side, and that the carrier is taken out as a current. 
   In the PIN-PD described in JP-A No. 32097/1996 and JP-A No. 82827/1993, a semiconductor material having an energy band gap larger than the absorbing layer is used for a p-type or n-type heavily doped layer. 
   In the technology described in JP-A No. 32097/1996 or JP-A No. 82827/1993, an accumulation (pile up) of carriers may occur due to heterojunction in which the energy band gap of the p-type or n-type semiconductor layer is larger than the absorbing layer (I layer). It is disadvantageous that high frequency response is degraded when the pile up occurs. To overcome this problem, there is also developed a mesa type PIN-PD having a p-type or n-type semiconductor layer formed using a heavily doped semiconductor material in which the energy band gap is equal to that of the absorbing layer or the difference therebetween is very small for the purpose of preventing the accumulation of carriers. However, such a PIN-PD has a problem that the high frequency response is degraded as slow carriers occur in the heavily doped layer when light incident into a light receiving section transmits through the absorbing layer and reaches the heavily doped layer on the side near the substrate. Although there may be a method of thinning the heavily doped layer to reduce the occurrence of such slow carriers, it is not easy to realize from the point of view of the etching accuracy and controllability of thickness in the formation of a mesa structure. 
   SUMMARY OF THE INVENTION 
   In a p-i-n multilayer structure, a portion corresponding to a light receiving section of a heavily doped layer on a substrate side is previously made thinner than the periphery of the light receiving section by an etching or selective growth technique, over which an absorbing layer and another heavily doped layer are grown to form the light receiving section of mesa structure. The heavily doped layer corresponding to the light receiving section is thin, so that the optical absorption of incident light transmitted through the absorbing layer is reduced in the heavily doped layer. The controllability of the thickness of the heavily doped layer is also ensured around the light receiving section during mesa formation. This makes it possible to form a good ohmic contact. As a result, it is possible to obtain a photodiode with excellent high frequency response characteristics. 
   An optical receiver module having excellent high frequency response characteristics can be obtained by applying this photodiode to the optical receiver module. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will now be described in conjunction with the accompanying drawings, in which: 
       FIGS. 1A to 1D  are cross-sectional views each illustrating a photodiode manufacturing process; 
       FIG. 2  is a cross-sectional view of a photodiode; 
       FIG. 3  is a view illustrating the characteristics of high frequency response of the photodiode; 
       FIG. 4  is a block diagram of an optical receiver module; 
       FIG. 5  is a cross-sectional view of a photodiode. 
       FIG. 6  is a cross-sectional view illustrating a photodiode manufacturing process; and 
       FIGS. 7A and 7B  are cross-sectional views each illustrating a photodiode manufacturing process. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter modes for carrying out the present invention will be described using the embodiments with reference to the accompanying drawings. Substantially like parts are denoted by like reference numerals and the description thereof will not be repeated. Further, hatching is not daringly applied to the cross sections in the cross-sectional views. This is to avoid complexity of illustration. 
   Embodiment 1 
   A photodiode according of the embodiment 1 will be described with reference to  FIGS. 1A-1D  to  FIG. 4 . Here  FIGS. 1A to 1D  are cross-sectional views each illustrating a photodiode manufacturing process.  FIG. 2  is a cross-sectional view of a photodiode.  FIG. 3  is a view illustrating the characteristics of high frequency response of the photodiode.  FIG. 4  is a block diagram of an optical receiver module. 
   The photodiode is obtained, as shown in  FIG. 1A , by forming an InP buffer layer (undope, film thickness of 0. 2 μm)  102  and an n-type InGaAsP contact layer (n type, impurity concentration: 1×10^18 cm^−3 (1×10 18  cm −3 ), film thickness of 0.4 μm, Eg=0.8 eV)  103   a  over a semi-insulating InP substrate  101  by the Metal Organic Chemical Vapor Deposition method, followed by etching the InGaAsP contact layer  103   a  only in a light receiving section to the InP buffer layer  102  by use of a mixed acid of sulfuric, hydrogen peroxide, and water (first multilayer growth). 
   Subsequently as shown in  FIG. 1B , an n-type InGaAsP contact layer (n type, impurity concentration: 1×10^18 cm^−3, film thickness of 0.3 μm, Eg=0.8 eV)  103   b , an Images absorbing layer (undope, impurity concentration: 5×10^14 cm^−3, film thickness of 1.5 μm, Eg=0.75 eV)  104 , an InGaAsP cap layer (p type, impurity concentration: 1×10^18 cm^−3, film thickness of 0.1 μm, Eg=0.8 eV)  105 , and an InGaAs contact layer (p type, impurity concentration: 1×10^19 cm^−3, film thickness of 0.1 μm, Eg=0.75 eV)  106  are regrown by the Metal Organic Chemical Vapor Deposition method (second multilayer growth). 
   As shown in  FIG. 1C , wet etching is performed from the InGaAs contact layer  106  of the periphery of a circular light receiving section to the InP buffer layer  102  by use of a mixed acid of phosphoric acid and sulfide (first mesa formation). 
   Next as shown in  FIG. 1D , the light receiving section is formed by further wet etching the n-type InGaAsP contact layer  103   a  and the InP buffer layer  102  roundly in the periphery of the light receiving section in order to reduce capacitance, leaving an n-type contact portion  110  of a surface of the n-type InGaAsP contact layer  103   a  by use of a phosphoric acid etchant (second mesa formation). 
   Referring to  FIG. 2 , an SiN film (film thickness of 0.2 μm)  114  and an SiO2 film (film thickness of 0.8 μm)  115  are formed as protective films by a spatter or CVD method. Subsequently the following processes are performed: a partial etching process of the protective films (SiN/SiO2) for contact with the heavily doped semiconductor layer; a partial etching process of the SiO2 film  115  to make the SiN film of the light receiving section a reflection protective film; a formation process of a p-electrode  121  and an n-electrode  122  formed by an electron beam evaporation technique; a grinding process of the bottom of the substrate; and a formation process of a bottom electrode  123 . In this way a p-i-n type photodiode  100 A was manufactured. 
   The film thickness of a heavily doped contact layer  103  of the light receiving section on the side near the substrate of a surface illuminated photodiode portion shown in the center of  FIG. 2  is formed thinner than the film thickness of the periphery of the light receiving section. 
   In  FIG. 3 , the abscissa is the frequency and the ordinate is the S 21  characteristics. Characteristics (a) of  FIG. 3  were obtained from an estimation of the frequency response characteristics of the photodiode  100 A. Characteristics (b) of  FIG. 3  are the characteristics of a photodiode with no difference in the film thickness of the n-type contact layer between the light receiving section and the periphery thereof. The characteristics (b) of  FIG. 3  show that gain reduction occurs in a low frequency area in which slow carriers are generated in the n-type contact layer by the transmitted light that is not completely absorbed in the absorbing layer as the film thickness of the n-type contact layer is uniform. 
   On the other hand, it is seen from the characteristics (a) of  FIG. 3  that the amount of gain degradation can be reduced in the low frequency area and that the frequency characteristics are improved also in a high frequency area. With respect to the n-type contact portion of the periphery of the light receiving section, there is a margin in the etching amount in the mesa formation with a 0.7-μm thick n+layer, so that a heavily doped layer with a thickness of 0.5 μm or more can be left after etching, providing a sufficient low contact resistance of 1 ohm or less with good reproducibility. 
   In the embodiment 1, preferably the film thickness of the n-type heavily doped layer of the photodiode portion is in a range of 0.1 μm to 0.4 μm. Preferably the film thickness of the n-type heavily doped layer of the electrode portion is thicker than the film thickness of the photodiode portion by 0.1 μm or more. Further preferably the difference between the energy band gap of the n-type heavily doped layer and the energy band gap of the absorbing layer is within 0.2 eV. 
   In the embodiment  1 , there are used InGaAsP as the n-type contact layer, InGaAs as the absorbing layer material, InGaAsP as the p-type cap layer, and InGaAs as the p-type contact layer. However it goes without saying that the same effect as the embodiment 1 is obtained in the case where any of the material systems, namely, InGaAs, InGaAsP, InGaAlAs, GaInNAs, or GaInNAsSb having an absorption end including a light-receiving wavelength range, is used for the p-i-n structure. Although the thicknesses and impurity concentrations of the growth layers are specifically described in the embodiment 1, it is also possible to employ a structure with another combination of thicknesses and concentrations depending on the applications as the photodiode, such as of high large, wavelength range, and large diameter. Also in this case the same effect can be expected. 
   Having described the top illuminated photodiode in the embodiment 1, a bottom illuminated photodiode can also be realized with the main part configuration equivalent to that of the top illuminated photodiode. This can be easily understood by those skilled in the art. The top illuminated photodiode and the bottom illuminated photodiode are both the surface illuminated photodiode. 
     FIG. 4  is an optical receiver module in which the photodiode of the embodiment 1 is mounted. An optical receiver module  130  shown in  FIG. 4  is an optical receiver module in which a photodiode  100  is implemented together with a TIA (Trans Impedance Amplifier) with limiting amplifier  132 . Here the TIA with limiting amplifier  132  includes a preamplifier  1321 , a feedback resister  1322 , and a limiting amplifier  1323 , serving as a negative feedback amplifier for converting current input to voltage output. The optical receiver module  130  receives an optical signal indicated by the arrow, and outputs as an electric signal from an OUT 1  terminal  133  which is the positive phase output of the TIA with limiting amplifier  132  and from an OUT 2  terminal  134  which is the negative phase output thereof. The optical receiver module  130  has excellent high frequency response characteristics with less band degradation even in the case of low voltage and large input, due to a little or no difference in the energy band gap of the heterojunction between each of the layers in the multilayer structure. 
   Embodiment 2 
   A photodiode of the embodiment 2 will be described with reference to  FIG. 5 . Here  FIG. 5  is a cross-sectional view of a photodiode. Incidentally the details of the formation process of this photodiode are the same as those in the embodiment 1 and will be omitted. 
   In  FIG. 5 , a light receiving section of a heavily doped contact layer  306  is provided on the side near a substrate of a surface illuminated photodiode portion in a center of a photodiode  300 . The light receiving section of the heavily doped contact layer  306  is formed thinner than the periphery of the light receiving section. An InP buffer layer (undope, film thickness of 0.2 μm)  302  and a p-type InGaAs contact layer (p type, impurity concentration: 1×10^19 cm^−3, film thickness of 0.4 μm, Eg=0.75 eV)  306  are formed over a semi-insulating InP substrate  301  by the Metal Organic Chemical Vapor Deposition method. Then the p-type InGaAs contact layer  306  is etched only in the light receiving section to the InP buffer layer  302 . 
   Subsequently, using the Metal Organic Chemical Vapor Deposition method, the p-type InGaAs contact layer  306  is regrown followed by the regrowth of an InGaAlAs cap layer (p type, impurity concentration: 3×10^18 cm^−3, film thickness of 0.1 μm, Eg=0.8 eV)  305 , InGaAs absorbing layer (undope, impurity concentration: 5×10^14 cm^−3, film thickness of 1.7 μm, Eg=0.75 eV)  304 , and InGaAs contact layer (n type, impurity concentration: 7×10^18 cm^−3, film thickness of 0.1 μm, Eg=0.75 eV)  303 . 
   In order to form the light receiving section from the multilayer structure described above, the InGaAlAs cap layer  305  is etched from the InGaAs contact layer  303  by wet etching. Further the p-type InGaAs contact layer  306  and the InP buffer layer  302  are etched in the periphery of the light receiving section. At this time, a surface of the p-type InGaAs contact layer  306  becomes a p-type contact portion  311 . The etching of the p-type InGaAs contact layer  306  and InP buffer layer  302  is to reduce capacitance by providing a wire and a pad portion over the semi-insulating substrate through protective films. 
   Subsequently the following processes are performed: a formation process of an SiN film (film thickness of 0.2 μm)  314  and SiO2 film (film thickness of 0.8 μm)  315  as protective films by the spatter or CVD method; a through hole etching process of the protective films for contact with the heavily doped semiconductor layer; a partial etching process of the SiO2 film  315  to make the SiN film  314  of the light receiving section a reflection protective film; a formation process of a p-electrode  321  and n-electrode  322  by the electron beam evaporation technique; a substrate grinding process; and a formation process of a bottom electrode  323 . In this way a p-i-n type photodiode  300  was manufactured. 
   In the embodiment 2, the frequency response characteristics were estimated and the same characteristics as in  FIG. 3  were obtained. In other words, the amount of gain degradation was able to be reduced in the low frequency area. With respect to the p-type contact portion of the periphery of the light receiving section, there is a margin in the etching amount in the mesa formation with a 0.5-μm thick p+layer, so that a heavily doped layer with a thickness of 0.3 μm or more can be left after etching, providing a sufficient low contact resistance of 1 ohm or less with good reproducibility. 
   In the embodiment 2, preferably the film thickness of the p-type heavily doped layer of the photodiode portion is in a range of 0.1 μm to 0.4 μm. Preferably the film thickness of the p-type heavily doped layer of the electrode portion is thicker than the film thickness thereof in the photodiode portion by 0.1 μm or more. Further preferably the difference between the energy band gap of the p-type heavily doped layer and the energy band gap of the absorbing layer is within 0.2 eV. 
   In the embodiment 2, there are used InGaAsP as the n-type contact layer, InGaAs as the absorbing layer material, InGaAsP as the p-type cap layer, and InGaAs as the p-type contact layer. However it goes without saying that the same effect as the embodiment 2 is obtained in the case where any of the material systems, namely, InGaAs, InGaAsP, InGaAlAs, GaInNAs, or GaInNAsSb having an absorption end including a light receiving wavelength range, is used for the p-i-n structure. Although the thicknesses and concentrations of the growth layers are specifically described in the embodiment 2, it is also possible to employ a structure with another combination of thicknesses and concentrations depending on the applications as the photodiode, such as of high large, wavelength range, and large diameter. Also in this case the same effect can be expected. 
   Similarly to the embodiment 1, in the case where the photodiode of the embodiment 2 is implemented together with the TIA as the optical receiver module shown in  FIG. 4 , it is possible to realize an optical receiver module having excellent high frequency response characteristics with less band degradation even in the case of low voltage and large input, due to a little or no difference in the energy band gap of the heterojunction between each of the layers in the multilayer structure. 
   Embodiment 3 
   In the embodiments 1 and 2, the light receiving section of the heavily doped contact layer on the side near the substrate of the surface illuminated photodiode is formed thinner than the periphery of the light reception section by etching the heavily doped contact layers  103 ,  306  after the growth, followed by regrowing them. However, a similar structure can be formed by selective growth through the Metal Organic Chemical Vapor Deposition method using an oxide film or a nitride film. This will be described as the embodiment 3 with reference to  FIG. 6 . Here  FIG. 6  is a cross-sectional view illustrating a photodiode manufacturing process. 
   During the process to a photodiode  100 B of  FIG. 6 , a mask  124  of acid film having a round foramen is formed on the semi-insulation InP substrate  101  over which the InP buffer layer (undope, film thickness of 0.2 μm)  102 , n-type InGaAsP contact layer (n type, impurity concentration: 1×10^18 cm^−3, film thickness of 0.3 μm, Eg=0.8 eV)  103 , InGaAs absorbing layer (undope, impurity concentration: 5×10^14 cm^−3, film thickness of 1.5 μm, Eg=0.75 eV)  104 , InGaAsP cap layer (p type, impurity concentration: 1×10^18 cm^−3, film thickness of 0.1 μm, Eg=0.8 eV)  105 , and InGaAs contact layer (p type, impurity concentration: 1×10^19 cm^−3, film thickness of 0.1 μm, Eg=0.75 eV)  106  are formed by the Metal Organic Chemical Vapor Deposition method. 
   Subsequently, the photodiode is formed by a wafer process similar to the embodiments 1, 2 by use of this multilayer structure. 
   The embodiment 3 takes advantage of the fact that the film thickness is increased only in the periphery of the oxide film step portion in crystal growth by the Metal Organic Chemical Vapor Deposition method. It is grown within the mask  124  having the round foramen. As a result, the film thickness of the heavily doped contact layer in a central portion of the round foramen is thinner than the film thickness of the heavily doped contact layer in the portion of the round foramen near the mask. When the light receiving section of the surface illuminated photodiode is provided in the central portion of the round foramen, similarly to the embodiments 1 and 2, it is possible to reduce the amount of gain degradation in the low frequency area. It is also possible to obtain sufficient low contact resistance with good reproducibility. 
   Incidentally although the oxide film is used as the mask material having the round foramen in the embodiment 3, the nitride film may be used as well. 
   In the embodiment 3, the frequency response characteristics were estimated and the same characteristics as in  FIG. 3  were obtained. In other words, the amount of gain degradation was able to be reduced in the low frequency area. With respect to the n-type contact portion of the periphery of the light receiving section, there is a margin in the etching amount in the mesa formation with a 0.5-μm thick n+layer, so that a heavily doped layer with a thickness of 0.3 μm or more can be left after etching, providing a sufficient low contact resistance of 1 ohm or less with good reproducibility. 
   Similarly to the embodiment 1, in the case where the photodiode of the embodiment 3 is implemented together with the TIA as the optical receiver module shown in  FIG. 4 , it is possible to realize an optical receiver module having excellent high frequency response characteristics with less band degradation even in the case of low voltage and large input, due to a little or no difference in the energy band gap of the heterojunction between each of the layers in the multilayer structure. 
   Embodiment 4 
   Referring to  FIGS. 7A and 7B , there will be described another embodiment 4 in which a light receiving section of a heavily doped contact layer on the side near the substrate of the surface illuminated photodiode portion is formed thinner than the periphery of the light receiving section. Here  FIGS. 7A and 7B  are cross-sectional views each illustrating a photodiode manufacturing process. 
   During the process to a photodiode  100 C of  FIG. 7A , an InP buffer layer (undope, film thickness of 0.2 μm)  102   a , an n-type InGaAsP contact layer (n type, impurity concentration: 1×10^18 cm^−3, film thickness of 0.3 μm, Eg=0.8 eV)  103   c , the InGaAs absorbing layer (undope, impurity concentration: 5×10^14 cm^−3, film thickness of 1.5 μm, Eg=0.75 eV)  104 , InGaAsP cap layer (p type, impurity concentration: 1×10^18 cm^−3, film thickness of 0.1 μm, Eg=0.8 eV)  105 , and InGaAs contact layer (p type, impurity concentration: 1×10^19 cm^−3, film thickness of 0.1 μm, Eg=0.75 eV)  106  are formed over the semi-insulating substrate InP  101 . Then a mesa shape of the light receiving section is formed by wet etching. 
   Subsequently, as shown in  FIG. 7B , an InP buffer layer (undope, film thickness of 0.2 μm)  102   b  and an n-type InGaAsP contact layer (n type, impurity concentration: 1×10^18 cm^−3, film thickness of 0.7 μm, Eg=0.8 eV)  103   d  are regrown in an area other than the light receiving section by the Metal Organic Chemical Vapor Deposition method. Thus there is formed a structure in which the heavily doped contact layer is thinner in the light receiving section than in the periphery of the light receiving section. Then the light receiving section is formed by the wafer process similar to the embodiments 1, 2. 
   In the embodiment 4, the frequency response characteristics were estimated and the same characteristics as in  FIG. 3  were obtained. In other words, the amount of gain degradation was able to be reduced in the low frequency area. With respect to the n-type contact portion of the periphery of the light receiving section, there is a margin in the etching amount in the mesa formation with a 0.5-μm thick n+layer, so that a heavily doped layer with a thickness of 0.3 μm or more can be left after etching, providing a sufficient low contact resistance of 1 ohm or less with good reproducibility. 
   Similarly to the embodiment 1, in the case where the photodiode of the embodiment 4 is implemented together with the TIA as the optical receiver module shown in  FIG. 4 , it is possible to realize an optical receiver module having excellent high frequency response characteristics with less band degradation even in the case of low voltage and large input, due to a little or no difference in the energy band gap of the heterojunction between each of the layers in the multilayer structure. 
   According to the present invention, it is possible to realize a photodiode and optical receiver module with excellent high frequency response characteristics for large input.