Abstract:
A photodiode array for near infrared rays that includes photodiodes having a uniform size and a uniform shape, has high selectivity for the wavelength of received light between the photodiodes, and has high sensitivity with the aid of a high-quality semiconducting crystal containing a large amount of nitrogen, a method for manufacturing the photodiode array, and an optical measurement system are provided. The steps of forming a mask layer  2  having a plurality of openings on a first-conductive-type or semi-insulating semiconductor substrate  1 , the openings being arranged in one dimension or two dimensions, and selectively growing a plurality of semiconductor layers  3   a ,  3   b , and  3   c  including an absorption layer  3   b  in the openings are included.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of (and claims the benefit of priority of) U.S. application Ser. No. 11/741,226, filed on Apr. 27, 2007, which claims the benefit of Japanese patent application serial number 2006-128326, filed on May 2, 2006 and serial number 2007-108695, filed on Apr. 17, 2007, the disclosures of which are hereby incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a photodiode array for near infrared rays that includes photodiodes that can be easily and appropriately spaced apart during manufacture thereof and that has high selectivity for the wavelength of received light, a method for manufacturing the photodiode array, and an optical measurement system. 
     2. Description of the Related Art 
     Existing photodiode arrays are manufactured by forming semiconductor layers including an absorption layer on a semiconductor substrate, for example, by epitaxial growth and then separating photodiodes constituting the photodiode array by etching the semiconductor layers between the photodiodes. 
     For example, the semiconductor layers between the photodiodes are etched by wet etching using an etchant (see, for example, Japanese Unexamined Patent Application Publication No. 2001-144278). Furthermore, it is also known that trench grooves are formed by inductively coupled plasma etching, which is one type of dry etching, for device isolation (see, for example, Japanese Unexamined Patent Application Publication No. 2001-352094). 
     Furthermore, a group III-V element containing nitrogen (N) is used as an absorption layer that can receive near infrared rays having a wavelength in the range of 1.7 μm. to 5 μm as the longest wavelength to be detected (see, for example, Japanese Unexamined Patent Application Publication No. 9-219563). 
     However, the device isolation by wet etching has the following problems: (1) etching proceeds not only in the depth direction, but also in the transverse direction of the semiconductor layers, making it difficult to form photodiodes having the same size in the photodiode array, and (2) for example, in a laminating structure of semiconductors composed of different materials, such as InP/InGaAs/InP, a difference in etching rate between InP and InGaAs results in a warped semiconductor layer after etching. 
     On the other hand, device isolation by dry etching may cause damage, which prevents dark current from being reduced. 
     In these photodiode arrays, when high selectivity for the wavelength of received light is desired between the photodiodes, a wavelength filter may be placed in front of the photodiodes to select the wavelength. However, an apparatus including such a photodiode array may become larger. 
     Furthermore, when GaInNAs is used as the material of the absorption layer for near infrared rays, it is believed that an increase in nitrogen content increases a band gap wavelength, thus providing a photodiode sensitive to a wavelength of about 5 μm. However, a semiconducting crystal is hardly doped with nitrogen in a thermal equilibrium state. It is therefore difficult to prepare a semiconducting crystal containing nitrogen as much as several percent. Hence, it is not easy to prepare a photodiode sensitive to a wavelength of about 5 μm. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a first object of the present invention to provide a photodiode array that includes appropriately spaced photodiodes having a uniform shape and a uniform edge face condition, a method for manufacturing the photodiode array, and an optical measurement system. 
     Furthermore, it is a second object of the present invention to provide a photodiode array having high selectivity for the wavelength of received light between the photodiodes, a method for manufacturing the photodiode array, and an optical measurement system. 
     Furthermore, it is a third object of the present invention to provide a photodiode array sensitive particularly in a near infrared region with the aid of a semiconducting crystal containing a large amount of nitrogen, a method for manufacturing the photodiode array, and an optical measurement system. 
     A photodiode array according to the present invention includes a first-conductive-type or semi-insulating semiconductor substrate, an insulating mask layer disposed on the semiconductor substrate and having a plurality of openings, and semiconductor layers disposed in each of the openings, the semiconductor layers including an absorption layer. The absorption layer includes a pn junction. This prevents the semiconductor layers of the photodiodes constituting the photodiode array from becoming warped, thus providing a sensor having a reduced dark current. 
     A photodiode array according to the present invention includes an insulating film, which is used as a mask layer, disposed on the semiconductor substrate. Furthermore, since the semiconductor layers are formed only in the mask openings, the semiconductor layers have a trapezoidal pyramid shape. Thus, the areas of upper layers of the semiconductor layers are smaller than the area of the corresponding opening. 
     Furthermore, the photodiode array may further include a high-concentration semiconductor layer of a first-conductive-type between the semiconductor substrate, the mask layer, and the absorption layers. This structure allows p-electrodes and n-electrodes to be formed on the same side, thus eliminating the need for wire bonding, for example, by using flip-chip packaging. 
     Furthermore, in the photodiode array, the semiconductor substrate may be of a first-conductive-type. P-electrodes may be formed at the top of the semiconductor layers including the absorption layer, and n-electrodes may be formed on a surface of the semiconductor substrate opposite a surface on which the mask layer and the absorption layers are disposed. This structure permits the use of a conventional packaging technology. 
     The shape of the semiconductor layers including the absorption layer may vary from one opening to another. This structure allows the absorption edge wavelength of the semiconductor layers to be changed in a manner that depends on the opening size. 
     At least the absorption layer of the semiconductor layers including the absorption layer may be composed of a group III-V compound semiconductor containing nitrogen. This allows the photodiode array to receive near infrared rays having a wavelength of 2 μm or more while the lattice constant of the absorption layer matches the lattice constant of the substrate. 
     At least the absorption layer of the semiconductor layers including the absorption layer may be composed of a group III-V compound semiconductor containing antimony. This improves the surface smoothness of the crystal. 
     The absorption layer may have a multiple quantum well structure of InGaAs and GaAsSb. This structure allows the absorption edge wavelength to be changed in a manner that depends on the combination of thicknesses of (InGaAs layer/GaAsSb layer) in a type II multiple quantum well structure. Because the shape of the semiconductor layers varies from one opening to another, the combination of thicknesses in the multiple quantum well structure varies with the size of the shape of the semiconductor layers. Thus, absorption layers different in absorption edge wavelength are formed in a manner that depends on the shape and the size of the openings. This provides a photodiode array that has spontaneously distributed absorption edge wavelengths and that is used in a wavelength region of near infrared rays. Both the InGaAs and the GaAsSb have a lattice constant that matches the lattice constant of the semiconductor substrate. 
     The absorption layer may have a multiple quantum well structure of GaAs 1-31 x-31 y Sb y N x  (0≦x≦0.02) and a group III-V compound semiconductor containing nitrogen. This structure can further increase the absorption edge wavelength with the aid of nitrogen. Besides, the combination of thicknesses in the multiple quantum well structure changes in a manner that depends on the shape and the size of the openings. Thus, such a photodiode array can have a variety of absorption edge wavelengths. Both the group III-V compound semiconductor containing nitrogen and the GaAs 1-31 x-31 y Sb y N x  (0≦x≦0.02) have a lattice constant that matches the lattice constant of an InP substrate. 
     The absorption layer may have a multiple quantum well structure of InGaAs and GaAsSb. The InGaAs may have a lattice constant smaller than that of the semiconductor substrate. The GaAsSb may have a lattice constant larger than that of the semiconductor substrate. The average lattice constant of the multiple quantum well structure may be matched with that of the semiconductor substrate within 0.2%. In such a structure, the InGaAs contains a smaller amount of indium than a lattice-matched InGaAs, and the GaAsSb contains a larger amount of antimony than a lattice-matched GaAsSb. As a result of the combination of lattice-mismatched quantum well layers, the absorption edge wavelength of the absorption layer of the multiple quantum well structure can easily be increased further. Furthermore, on average, the lattice constant of the multiple quantum well structure matches with that of the semiconductor substrate within 0.2%. Thus, the dark current can be reduced. 
     An optical measurement system according to the present invention may include a plurality of photodetectors disposed on a common semiconductor substrate and having different absorption edge wavelengths. The optical measurement system analyzes the characteristics of signal light by detecting a difference in sensitivity of the photodetectors resulting from different absorption edges. This eliminates the need for a filter even when high wavelength selectivity is desired, thus decreasing the size of the system. 
     A photodiode array according to the present invention includes the steps of forming a mask layer having a plurality of openings arranged in one dimension or two dimensions on a semiconductor substrate, and selectively growing semiconductor layers including an absorption layer in each of the openings In other words, the present inventors found that in epitaxial growth of a semiconductor layer on a semiconductor substrate, the formation of an insulating film on the semiconductor substrate, the formation of a pattern having one-dimensional or two-dimensional openings in the insulating film, and selective growth of the semiconductor layers under conditions where no semiconductor layer is formed on the insulating film can provide a photodiode array that includes photodiodes easily spaced apart and having a uniform size, a uniform shape, and a uniform end face shape. Furthermore, in a photodiode array manufactured by a method of the present invention, a photodiode including an absorption layer can have a different absorption edge wavelength from another by changing the size of the corresponding opening. Thus, a photodiode array having high selectivity for the wavelength of received light between the photodiodes can be provided. Furthermore, the supply of nitrogen in a method according to the present invention can provide a group III-V compound semiconductor layer of a high nitrogen content. 
     According to a method of the present invention, the semiconductor layers are formed only in the openings of the insulating film pattern formed on the semiconductor substrate. Hence, the photodiodes can be easily and appropriately spaced apart. This eliminates the need for etching for device isolation. Thus, the photodiodes can have a uniform size in the array without difficulty. Furthermore, the semiconductor layers have a uniform shape and a uniform end face shape. In addition, the semiconductor layers suffer no damage from dry etching. The dark current can therefore be reduced. 
     The plurality of openings may have the same shape or different shapes. When the openings have the same shape, the crystal compositions of the absorption layers may be the same to provide an array of photodiodes having the same band gap. When the openings have different shapes, the crystal compositions of the absorption layers may be different to provide an array of photodiodes having different band gaps. Thus, only the shape of the openings, particularly the area of the openings can be changed for the selection of wavelength. 
     Preferably, the mask layer is composed of silicon nitride. In the mask layer composed of silicon nitride, no semiconductor layer is formed on the mask layer. In addition, semiconductor raw materials smoothly diffuse on the mask layer, thus forming a high-quality semiconductor layer. 
     The semiconductor layers may be formed by selective growth while being doped with an impurity to form a pn junction, which receives light. 
     Alternatively, the pn junction may be formed by doping the absorption layer and its overlying layer(s) with an impurity after the selective growth of the semiconductor layers. 
     The pn junction for receiving light may be formed on the substrate by sequential epitaxial growth of the absorption layer and its overlying semiconductor layer(s) containing an impurity. Furthermore, the absorption layer and its overlying layer(s) of the plurality of semiconductor layers may be doped with an impurity to form an impurity region. 
     The present invention can provide a photodiode array that includes appropriately spaced photodiodes having a uniform shape and a uniform edge face condition, has high selectivity for the wavelength of received light between the photodiodes, and has high sensitivity particularly in a near infrared ray long wavelength region with the aid of a semiconducting crystal containing a large amount of nitrogen, a method for manufacturing the photodiode array, and an optical measurement system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of an InP/InGaNAs/InP photodiode array manufactured by a method according to the present invention; 
         FIG. 2  is a schematic cross-sectional view of a photodiode array according to Embodiment 1 of the present invention; 
         FIG. 3A  is a process drawing illustrating a patterned insulating film in a manufacturing process of the photodiode array according to Embodiment 1 of the present invention; 
         FIG. 3B  is a process drawing illustrating semiconductor layers formed by selective metal organic vapor phase epitaxy (MOVPE) in the manufacturing process of the photodiode array according to Embodiment 1 of the present invention; 
         FIG. 3C  is a process drawing illustrating an insulating polyimide film formed on both sides of photodiodes in the manufacturing process of the photodiode array according to Embodiment 1 of the present invention; 
         FIG. 3D  is a process drawing illustrating electrodes formed on photodiodes in the manufacturing process of the photodiode array according to Embodiment 1 of the present invention; 
         FIG. 4  is a schematic cross-sectional view of a photodiode array according to Embodiment 2 of the present invention; 
         FIG. 5A  is a process drawing illustrating a patterned insulating film in a manufacturing process of the photodiode array according to Embodiment 2 of the present invention; 
         FIG. 5B  is a process drawing illustrating semiconductor layers formed by selective metal organic vapor phase epitaxy (MOVPE) in the manufacturing process of the photodiode array according to Embodiment 2 of the present invention; 
         FIG. 5C  is a process drawing illustrating a diffusion mask in the manufacturing process of the photodiode array according to Embodiment 2 of the present invention; 
         FIG. 5D  is a process drawing illustrating p-type regions formed by the gas phase diffusion of zinc through window layers in the manufacturing process of the photodiode array according to Embodiment 2 of the present invention; 
         FIG. 5E  is a process drawing illustrating p-electrodes in the manufacturing process of the photodiode array according to Embodiment 2 of the present invention; 
         FIG. 5F  is a process drawing illustrating an n-electrode in the manufacturing process of the photodiode array according to Embodiment 2 of the present invention; 
         FIG. 6  is a schematic cross-sectional view of a multiple quantum well structure in a photodiode array according to Embodiment 3 of the present invention; 
         FIG. 7  is a graph illustrating the effect of the combination of thicknesses of layers in a multiple quantum well structure on the photoluminescence intensity; 
         FIG. 8  is a schematic cross-sectional view of a multiple quantum well structure in a photodiode array according to Embodiment 4 of the present invention; and 
         FIG. 9  is a graph illustrating the photoluminescence intensity of a multiple quantum well structure. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (The Features of the Present Invention) 
       FIG. 1  is a schematic cross-sectional view of an InP/InGaNAs/InP photodiode array manufactured by a method according to the present invention. In  FIG. 1 , reference numeral  1  represents an InP substrate, and reference numeral  2  represents a SiN mask layer (mask layer formed of an insulating film) In the present invention, as illustrated in  FIG. 1 , a layered InP/InGaNAs/InP structure composed of an InP layer  3   a , an InGaNAs layer (absorption layer)  3   b , and an InP layer  3   c  is selectively grown by metal organic vapor phase epitaxy (MOVPE) in the mask layer  2 , which is an insulating film (SiN) having an opening  21  formed on the InP substrate  1 , thus forming trapezoidal pyramid crystalline layers. Thus, the present invention can easily provide a photodiode array including appropriately spaced photodiodes having an excellent edge face condition without using etching. 
     Furthermore, an increase in the area of the opening in the insulating film pattern  2  increases the growth rate (film thickness) because of the inflow of raw materials from the top of the insulating film pattern  2 , and increases the nitrogen content in the InGaNAs layer  3   b . Utilized these phenomena, a change only in the shape, for example, the area of the opening in the mask  2  can provide laminating structures having different band gap wavelengths in the common substrate. 
     Embodiment 1 
     Embodiments of a photodiode array according to the present invention, a method for manufacturing the photodiode array, and an optical measurement system will be described below with reference to the drawings.  FIG. 2  is a schematic cross-sectional view of a photodiode array according to the present embodiment.  FIGS. 3A to 3D  are process drawings illustrating a process for manufacturing the photodiode array according to the present embodiment. In  FIG. 2  and  FIGS. 3A to 3D , reference numeral  11  represents an InP substrate. 
     In the photodiode array illustrated in  FIG. 2 , a SiN pattern  12  having openings  21   a  and  21   b  are disposed on the InP substrate  11 . An n-type InP layer  13 , an n-type InGaNAs layer (absorption layer)  14 , and a p-type InP layer (window layer or cap layer)  15  are sequentially stacked in the openings  21   a  and  21   b . These semiconductor layers have different areas, as illustrated in  FIGS. 3A to 3D . Furthermore, n-electrodes  22  and an anti-reflection film  23  are formed on a surface of the InP substrate  11  opposite a surface on which the openings  21   a  and  21   b  are disposed. 
     The terms “shape” and “area” of semiconductor layers and openings used herein refer to the shape and the area seen in the lamination direction relative to the photodiode array in  FIGS. 3A to 3D , that is, the shape and the area of a surface parallel to the substrate surface. 
     Furthermore, the n-electrodes  22  are formed on the bottom of the n-type InP layer  11 . P-electrodes  18  are formed on p-type regions in the window layers  15 . Furthermore, the entire photodiode array is covered with an insulating layer  19  such as a polyimide resin to prevent a leakage of electricity at end faces. 
     [Manufacturing method] 
     A photodiode array according to the present invention is manufactured as described below. A SiN insulating film having a thickness of 50 nm is formed on the n-InP substrate  11 . The SiN film is then etched to form the insulating film pattern  12  having the openings  21   a  and  21   b . The area of the openings in the SiN mask was 30×30 μm 2  ( FIG. 3A ). Semiconductor layers are formed on the patterned substrate  11  by selective MOVPE. The growth conditions include the growth temperature of 520° C. and the growth pressure of 10.13 kPa. As illustrated in  FIG. 3B , a laminating structure formed by selective MOVPE is composed of the n-type InP layer  13 , the n-type InGaNAsP layer  14 , and the p-type InP layer  15 . The growth of the n-type InGaNAsP layer  14 , followed by the growth of the p-type InP layer  15 , leads to the formation of the pn junction at the interface. 
     As illustrated in  FIG. 3C , the insulating polyimide film  19  is formed on both sides of photodiodes to cover the entire photodiode array. As illustrated in  FIG. 3D , the p-electrodes  18  containing AuZn are formed on the InP layers  15 . The n-electrodes  22  containing AuGeNi are formed on the bottom of the InP substrate  11 . The p-electrodes  18  or the n-electrodes  22  may be composed of another material. Furthermore, an anti-reflection film  23  containing SiON or SIO 2  is formed on the bottom of the InP substrate  11 . 
     Embodiment 2 
     In Embodiment 2, after the formation of semiconductor layers, zinc diffuses from a window layer  13  to an absorption layer  12 . 
     In a photodiode array illustrated in  FIG. 4 , a high-concentration n-type InP layer  24  is formed on a semi-insulating InP substrate  11 . A SiN mask (mask layer)  12  is formed on the high-concentration n-type InP layer  24 . An n-type InP layer  13 , an i-type or low-concentration n-type InGaNAs layer (absorption layer)  14 , and an n-type InP layer (window layer or cap layer)  15  are sequentially stacked in mask openings  21   a  and  21   b . These semiconductor layers have different areas, as illustrated in  FIG. 4 . A diffusion mask (insulating film)  20  having openings through which a p-type impurity zinc diffuses is formed on the sides of the absorption layers  14 , the sides of the window layers  15 , and the top end of the window layers  15 . P-type regions  16  are selectively formed in the openings, which are not covered with the insulating film  20 , by the diffusion of zinc. 
     Furthermore, n-electrodes  22  are formed on the high-concentration n-type InP layer  24 , and p-electrodes  18  are formed on the window layers  15 . An anti-reflection film  23  is formed on the bottom of the semi-insulating InP substrate  11 . 
     [Manufacturing method] 
     A photodiode array according to the present invention is manufactured as described below. A high-concentration n-type InP substrate  24  having a thickness of 2 μm and doped with silicon (carrier density: 3×10 18  cm −3 ) is formed on a semi-insulating InP substrate  11 . A SiN insulating film  20  having a thickness of 50 nm is then formed by plasma chemical vapor deposition (CVD). The SiN film is then etched by photolithography to pattern a mask layer  12  having openings. The areas of the openings in the SiN patterned layer were 20×20 μm 2  and 50×50 μm 2  ( FIG. 5A ). Semiconductor layers are formed on the substrate  11  having the patterned mask layer by selective MOVPE. The growth conditions include the growth temperature of 520° C. and the growth pressure of 10.13 kPa. As illustrated in  FIG. 5B , a laminating structure formed by selective MOVPE is composed of an n-type InP layer  13  having a thickness of 2 μm and doped with silicon (carrier density: 3×10 15  cm −3 ), a low-concentration n-type GaInNAsSb layer  14  having a thickness of 2.5 μm (carrier density: 8×10 15  cm −3 ), and an n-type InP layer  16  having a thickness of 1.5 μm and doped with silicon (carrier density: 3×10 15  cm −3 ). As illustrated in  FIG. 5C , the SiN diffusion mask (insulating film)  20  having a thickness of 50 nm and having openings over the semiconductor layers is then formed. As illustrated in  FIG. 5D , zinc is then diffused from the window layers  15  to the absorption layers  14  by gas phase diffusion to form p-type regions  16 . 
     As illustrated in  FIG. 5E , the p-electrodes  18  containing TiPt are then formed on the p-type regions  16 . As illustrated in  FIG. 5F , the n-electrodes  22  containing AuGeNi are formed on the n-type InP layer  24  exposed by etching. The p-electrodes  18  or the n-electrodes  22  may be composed of another material. Furthermore, the anti-reflection film  23  is formed on the bottom of the semi-insulating InP substrate  11 . 
     Embodiment 3 
     A feature of the present embodiment is that an absorption layer has a multiple quantum well structure. The multiple quantum well structure includes semiconductor layers formed in each of openings According to the mechanism described above, the combination of the thicknesses of the semiconductor layers changes in a manner that depends on the opening size. Thus, the resulting photodiode array has different absorption edge wavelengths. The mechanism will now be described. 
     The semiconductor substrate was an InP substrate. An insulating mask pattern having openings was formed in the same way as in Embodiment 1. The absorption layer was a multiple quantum well structure of InGaAs and GaAsSb. The lattice constants of the InGaAs and the GaAsSb match the lattice constant of the InP substrate. As illustrated in  FIG. 6 , 250 pairs of InGaAs layer and GaAsSb layer are alternately stacked. When the thicknesses of an InGaAs layer and a GaAsSb layer are denoted by a and b, respectively, the photoluminescence (PL) wavelength changes as described below (see  FIG. 7 ). The PL wavelength corresponds to the absorption edge wavelength.
 
( a, b )=(3 nm, 3 nm): PL wavelength 1.95 μm
 
( a, b )=(5 nm, 5 nm): PL wavelength 2.25 μm
 
( a, b )=(7 nm, 7 nm): PL wavelength 2.40 μm
 
     A change in the opening size of the mask pattern causes a change in the growth rate of epitaxial growth. Thus, the thicknesses of the InGaAs layer and the GaAsSb layer change in a manner that depends on the opening size. More specifically, a smaller opening size results in a larger growth rate. This is because a smaller opening size results in higher effective concentrations of raw materials supplied to the semiconductor substrate in the opening through the surface of the mask. Conversely, a larger opening size results in a smaller growth rate. 
     With reference to the PL wavelength in the combination of the thicknesses of the InGaAs layer and the GaAsSb layer as described above (as shown in  FIG. 7 ), the number of combinations of the thicknesses in each opening is roughly equal to the number of opening sizes. Thus, a photodiode array that has absorption edge wavelengths distributed according to the openings of the mask pattern can be provided by appropriately specifying the number of sizes of the openings of the mask pattern and their two-dimensional distribution. 
     Embodiment 4 
     The fundamental concept of the present embodiment is the same as that of Embodiment 3. A feature of the present embodiment is to utilize a multiple quantum well structure of GaAsSb and a group III-V semiconductor containing nitrogen, InGaAsN (N=1%). As illustrated in  FIG. 8 , when 150 pairs of (InGaAsN layer/GaAsSb layer)=(5 nm/5 nm) are stacked on an InP substrate, the PL wavelength was 3 μm (see  FIG. 9 ). As compared with (InGaAs layer/GaAsSb layer)=(5 nm, 5 nm) described above where the PL wavelength was 2.25 μm, use of a group III-V semiconductor containing nitrogen, InGaAsN (N=1%) in one well layer can achieve a significant increase in absorption edge wavelength. 
     The PL wavelength illustrated in  FIG. 9  shows that also in the present embodiment, a combination of (InGaAsN layer/GaAsSb layer)=(3 nm/3 nm) or (7 nm/7 nm) produces another PL wavelength, which is reasonably longer than the PL wavelength described above (as shown in  FIG. 7 ). With reference to the PL wavelength in the combination of the thicknesses of the nitrogen-containing InGaAsN layer and the GaAsSb layer, the number of combinations of the thicknesses in each opening is equal to the number of opening sizes. Thus, a photodiode array that has absorption edge wavelengths distributed according to the openings of the mask pattern can be provided by appropriately specifying the number of sizes of the openings of the mask pattern and their two-dimensional distribution. 
     Embodiment 5 
     In the present embodiment, the PL wavelength was estimated by computer experiment, instead of measuring the PL wavelength for an actual multiple quantum well structure. In an alternately laminating structure of InGaAs (5 nm) having a lattice mismatch of minus (−) 0.3% to an InP substrate and GaAsSb (5 nm) having a lattice mismatch of plus (+) 0.3% to the InP substrate, the PL wavelength is about 2.6 μm, as estimated by theoretical calculation. As compared with (InGaAs layer/GaAsSb layer)=(5 nm, 5 nm) in Embodiment 3 where the PL wavelength was 2.25 μm, the lamination of (InGaAs layer/GaAsSb layer) having minus and plus lattice mismatches can achieve a significant increase in absorption edge wavelength. Thus, also in the present embodiment, a combination of (InGaAs layer/GaAsSb layer)=(3 nm/3 nm) or (7 nm/7 nm) produces another PL wavelength, which is reasonably longer than the PL wavelength described above (as shown in  FIG. 7 ). With reference to the PL wavelength in the combination of the thicknesses of InGaAs having a lattice mismatch of minus (−) 0.3% to the InP substrate and GaAsSb having a lattice mismatch of plus (+) 0.3% to the InP substrate, the number of combinations of the thicknesses in each opening is equal to the number of opening sizes. Thus, a photodiode array that has absorption edge wavelengths distributed according to the openings of the mask pattern can be provided by appropriately specifying the number of sizes of the openings of the mask pattern and their two-dimensional distribution. 
     While the embodiments of the present invention are described above, these embodiments are presented only for the purposes of illustration and do not limit the scope of the present invention. The scope of the present invention is defined by the appended claims and embraces all changes that fall within the scope of the claims and the equivalence thereof.