Photodiode

Abstract: This invention provides a photodiode having a GaInAs light-absorbing layer on an InP substrate which can reduce temperature dependence of the spectral responsivity as well as enable the optical power measurement with less dependence on the incident light spectral width by using a material with smaller composition ratio of Ga for the light-absorbing layer to extend the absorption edge toward the longer wavelength side.

Description: This invention is applicable to photodiodes using hetero junction of III-V semiconductor compound crystals. This invention relates more particularly to an improvement in spectral responsivity. When a ternary or quaternary alloy semiconductor is expressed in this specification, component elements of the semiconductor are described by a series of elements like [GaInAs]. The composition of a semiconductor is expressed like [Ga x In 1-x1 As]. To receive light having a relatively long wavelength, there has been used a photodiode made of such materials as germanium Ge and gallium.indium.arsenic phosphide GaInAsP except for special cases. Ga x In 1-x As y P 1-x (x or y indicates composition) is particularly preferable as band gap energy thereof may be changed while lattice constant thereof is made to be matched with that of Inp crystal by selecting the composition properly. This makes it possible to epitaxially grow on an InP substrate a light-absorbing layer having any desired spectral range for available sensitivity within the wavelength of about 1 - 1.7 µm. Such light-absorbing layer of Ga x In 1-x As y P 1-y may be used as an avalanche photodiode by forming a pn junction within the light-absorbing layer, a pn junction between the layer and the substrate or a pn junction within the InP cap layer formed on the light-absorbing layer. It is also known to use such light-absorbing layer as a PIN photodiode by forming an InP cap layer thereon. Recently there has been manufactured a GaInAsP photodiode which is superior to Ge photodiode in such electric characteristics as dark current, frequency response, multiplication noise as well as the spectral responsivity in the wavelength longer than ca. 1.5 µm. Details are given in the reference material (1). Among Ga x In 1-x As y P 1-y crystals, those having x=0.47, y=1 or Ga 0.47 In 0.53 As have the smallest band gap energy which is 0.75 eV at room temperature of all those having the lattice constant matched with that of InP. When such crystals are used as the light-absorbing layer, the photodiode will have spectral responsivity of ca. 0.9 - 1.7 µm. The drop in the spectral responsivity in the range below 1 µm is attributable to light absorption by the InP cap layer provided on the incident side of the light-absorbing layer. The drop on the longer wavelength side is attributable to the fact that as the absorption edge of the light-absorbing layer of Ga 0.47 In 0.53 As exists around 1.65 µm, and that the light having the wavelength longer than that can not be absorbed. FIG. 1 shows an example of measurement of the spectral responsivity as a function of the wavelength for a conventional photodiode. As shown in the reference material (2), a commercial Ga 0.47 In 0.53 As/ InP-PIN photodiode is measured at different temperatures of -15.9°C, -4°C, 19.9°C and 39.7°C. The spectral responsivity is expressed in terms of the output electric current [A] per incident optical power [W], and the incident optical power was measured by a highly sensitive calorimeter. The spectral responsivity such as above may be calculated in simulation at a considerably high precision by.using the external quantum efficiency next = [spectral responsivity/absolute spectral responsivity] of the PIN photodiode structure obtained by experiment with consideration of the structure of the anti-reflection coating film against the incident light. The method of simulation is indicated in the reference material (3). The reference material (4) shows the fact that errors in optical power measurement, using a photodiode, are related to the spectral responsivity of the photodiode. Reference material (1): S. Sakai et al., Collection of papers of Telecommunications Association, Vol. J62-C, No.10, 1979/10. Reference material (2): H. Yamagishi, Y. Suzuki and A. Hiraida, "Precise Measurement of Photodiode Spectral Responses Using the Calorimetric Method", IEEE Trans. Instrum. Meas. 38 (1989) 578. Reference material (3): M. Wada, M. Seko, K.Sakakobara and Y. Sekiguchi, "GaInAs/InP PIN Photodiode Fabricated by MOVPE and a New Zn Diffusion Technique", Jpn. J. Appl. Phys. 29 (1990) L401. Reference material (4): B. Wendland and P. Wendland, "Optical Power Measurement Errors Arising From Source Spectral Width", Primary Standard Symposium, NBS Boulder, Sept. 1986. However, in the photodiode having the light-absorbing layer of Ga 0.47 In 0.53 As, the spectral responsivity starts to deteriorate from the wavelength close to 1.55 µm, which is an extremely important wavelength in optical communications, optical measurement and other applications as the wavelength is the minimum propagation loss region of silica glass optical fibers as well as the emission wavelength of GaInAsP lasers. This entails the following problems: (1) The spectral responsivity fluctuates widely with changes in temperature for wavelengths longer than 1.5 µm. (2) Optical power measurement errors are dependent on the spectral width of the incident light. The first problem of temperature dependence of spectral responsivity will be described below. FIG. 2 shows temperature dependent changes in spectral responsivity. The change to the spectral responsivity R T (λ) [A/W] at the temperature of T°C from the spectral responsivity R 20 (λ)[A/W] at 20°C is expressed in percentage, as calculated in accordance with the equation below: ΔR = R T λ - R 20 λ R 20 λ x 100 % As shown in FIG. 2, ΔR increases remarkably from the wavelength of 1.5 µm in the vicinity of the absorption-edge wavelength of the Ga 0.47 In 0.53 As light-absorbing layer. This is attributable to the temperature change ΔEg/T[eV/K] of the energy gap Eg (the band gap at the point Γ or where k=0 in the band structure) for direct transition of Ga 0.47 In 0.53 As and related to spectral responsivity characteristics such as the internal quantum efficiency and wavelength dependence of anti-reflection coating film. In order to analyze the above fact further, a photodiode having a section as shown in FIG. 3 is assumed. The photodiode is a PIN photodiode of InP/GaInAs/InP hetero junction structure wherein an n + type InP buffer layer 22, an n type GaInAs light-absorbing layer 23 and an n type InP cap layer 24 are epitaxially grown on an n + type InP substrate 21. An SiO 2 film 25 is provided around the light receiving area on the surface of the InP cap layer 24. In the light receiving area, a Zn-diffused region 26 is formed to extend from the cap layer 24 to the light-absorbing layer 23. An Au-Zn/Au electrode 27 is formed on the surface of the Zn diffused region 26 while an Au-Sn/Au electrode 28 is provided on the rear surface of the substrate 21. The cap layer 24 which includes the Zn diffused region 26 is coated with an Si 3 N 4 film 29 and an SiO 2 film 30 as an anti-reflection coating film. The impurity concentration and thickness of each layer which will affect the spectral responsivity are determined as below: buffer layer 22 1 - 3 x 10 18 cm -3 0.3 µm light-absorbing layer 23 1 - 2 x 10 15 cm -3 2.8 µm Si 3 N 4 film 29 0.117 µm SiO 2 film 30 0.12 µm For simplification, it is assumed that no recombination of photo-generated carrier will occur on the surface of the cap layer 24 and the interface of InP/GaInAs, and the effect of multiple reflection of light on the cap layer 24 will be overlooked as the effect thereof is negligible in the vicinity of the wavelength of 1.5 µm. In order to obtain the temperature dependent effect of the band gap energy, the band gap energy is calculated in accordance with the empirical formula of V.P Varshni; E g = E 0 - (αT 2 )/(T + β) InP and GaInAs have respectively the values of E 0 , α and β as shown in the table below. InP GaInAs E 0 [eV] 1.412 0.814 α(x 10 -4 ) [eV/K] -3.63 -4.906 β [K] 162 301 Under the above assumed conditions, the spectral responsivity was calculated at the temperatures of -16°C, -4°C, 20°C and 40°C in accordance with the cited reference (3). The results are indicated in FIGS. 5 and 6. FIG. 4 shows the calculations of the reflectance of the anti-reflection film, FIG. 5 those of the spectral responsivity, and FIG. 6 the temperature dependent changes ΔR of the spectral responsivity, respectively. In FIG. 6, calculations of ΔR are shown in solid lines, and measurements given in FIG. 2 are plotted in dashed and dotted lines and broken lines. Although there is slight deviation from the measured values, it is obvious that the temperature dependent changes ΔR of the spectral responsivity can be described by the calculation method mentioned above. The slight deviation from the measured values may be attributable to the fact that the absorption-edge wavelength of GaInAs (band gap energy) taken from the reference material for the calculation was slightly too large or on the side of the shorter wavelength. FIG. 7 is a modification of FIG. 6 wherein the changes ΔR in the spectral responsivity at the wavelengths of 1.3 µm and 1.55 µm are shown both in measured values and calculated values, noting the significance of these wavelengths in applications such as optical communications and optical measurement. The graph clearly shows that ΔR is larger at 1.55 µm than at 1.3 µm. Description will now be given to the second problem of the optical power measurement errors which depend on the incident light spectrum. FIG. 8 shows the spectral responsivity of a GaInAs/InP-PIN photodiode and errors made when the optical power meter is calibrated with the lights of the spectral width W = 10, 20 and 40 nm, respectively. FIG. 9 is a graph to show the dependence of the errors on the incident light spectrum at the wavelength of 1.55 µm. Errors in optical power measurement occur, as indicated in the reference material (4), because the spectral responsivity is not flat. This is not limited to wavelengths in the vicinity of 1.55 µm, but applicable to shorter wavelengths. Therefore, even if an optical power is calibrated at a specific wavelength by using a light source having the spectral responsivity fixed at a level, errors will occur depending on the spectral width of the light source when optical power is actually measured. A photodiode with a long-wavelength cutoff extanding to 2.4 µm is disclosed in Electronics Letters, Vol. 22, No. 22, October 1986 (pages 1206, 1207). The photodiode disclosed therein includes an InP substrate separated by a graded buffer zone from a light absorbing layer of composition Ga 0.28 In 0.72 As. The present invention aims to solve the aforementioned problems encountered in the prior art and to provide a photodiode which has a spectral responsivity less dependent on the temperature at around 1.55 µm, and which is capable of optical power measurement less dependent on the incident light spectral width. The invention provides a photodiode including an InP substrate supporting a light-absorbing layer which generates carriers when it absorbs incident light, the light-absorbing layer having the composition Ga x In 1-x AS, where x is less than 0.47, and wherein the photodiode includes, between the light-absorbing layer and the InP substrate, a buffer zone which includes a GaInAs layer of substantially the same composition as the light-absorbing layer and a strained multilayer formation, the GaInAs layer lying between the strained multilayer formation and the InP substrate. The strained multilayer may comprise plural GaInAs layers with lattice mismatch against InP larger than the materials of the light-absorbing layer and of a plurality of GaInAs and InP layers in alternation. For the buffer zone, the InP layer may be replaced with GaInAs, or with GaInAs/GaInAs which is a combination of ternary semiconductor alloys having lattice mismatch. A cap layer mainly composed of InP and an electrode contacting the surface of the cap layer may be further provided on the incident side of the light-absorbing layer. It is desirable that at least the light receiving region of the cap layer be made thinner than the region on which the electrode is provided. As the cap layer, InP may be replaced by Al p In 1-p As q P 1-q of the compositions of p and q having substantially the same lattice constant as the light-absorbing layer, especially with Al p In 1-p As or InAs q P 1-q . In this case, the cap layer or at least the region for receiving light thereon is desirably formed thinner than the region where the electrode is provided. Ga x In 1-x AS having x which is less than 0.47 has the absorption-edge wavelength which is longer than that of Ga 0.47 In 0.53 As, and the spectral responsivity in the range of 1.5 - 1.6 µm becomes flat. However, in the range where x < 0.47, lattice matching with InP cannot be obtained. Therefore, a buffer layer such as a strained multilayer is inserted between the InP substrate and the light-absorbing layer. This can decrease defects in crystal within the light-absorbing layer to prevent deterioration in the spectral responsivity. A photodiode provided with a buffer layer formed of a strained superlattice consisting of InGaAsP films having different lattice constants inserted between the InP substrate and the InGaAs absorbing layer is known from JP-A-62 291184. When a cap layer is formed, defects which will otherwise be caused by lattice mismatch with the light-absorbing layer may be prevented by forming the cap layer thinner. By forming the cap layer thinner to the extent that carrier surface recombination would not become large enough to decrease spectral responsivity, the light absorption on the side of shorter wavelength may be decreased as well as the deterioration of spectral responsivity can be prevented in the wavelength less than ca. 1 µm. Even when the cap layer is made thin, the region where electrode is formed should be formed thick. This can prevent the electrode metals from diffusing and affecting the junctions of photodiode and secure reliability in electrode formation. Lattice matching with the light-absorbing layer may be achieved by using as the material of the cap layer Al p In 1-p As or InAs q P 1-q instead of InP. In this case, the cap layer may be formed thin, too. FIG. 1 is a view to show an example of spectral responsivity of a conventional photodiode. FIG. 2 is a view to show temperature dependent changes in spectral responsivity. FIG. 3 is a view to show a device model for analysis. FIG. 4 is a view to show calculations of the reflectance of anti-reflection film. FIG. 5 is a graph to show calculations of spectral responsivity. FIG. 6 is a graph to show temperature dependent changes in spectral responsivity both in calculated and measured values. FIG. 7 is a graph to show changes in spectral responsivity at the wavelengths of 1.3 µm and 1.55 µm both in calculated and measured values. FIG. 8 is a graph to show the spectral responsivity of GaInAs/InP-PIN photodiode and the errors thereof. FIG. 9 is a graph to show the incident light dependence of errors at the wavelength of 1.55 µm. FIG. 10 is a section of an embodiment of the photodiode according to this invention. FIG. 11 is a partial enlarged view to show the embodiment of the photodiode. FIG. 12 is an SEM (secondary electron microscope) image of a structure where a buffer layer, a GaInAs layer, a strained multilayer, a light-absorbing layer and a cap layer are epitaxially grown on a substrate. FIG. 13 is a graph to show the result of X-ray diffraction measurement on the structure. FIG. 14 is a graph to show temperature dependence of photoluminescence peak energy. FIG. 15 is a graph to show the spectral responsivity of a trial photodiode. FIG. 16 is a graph to show spectral responsivity of various photodiodes. FIG. 17 is a graph to show changes in band gap energy and in lattice constant of Ga x In 1-x As and of Al p In 1-p As. FIG. 18 is a graph to show changes in band gap energy and in lattice constant of Ga 1-x In x As and of InAs q P 1-q . FIG. 10 is a sectional view to show the structure of an embodiment of the photodiode according to this invention, and FIG. 11 a partial and enlarged section thereof. This embodiment is an application of this invention to a PIN photodiode. This photodiode has a light-absorbing layer 3 which absorbs incident light and generates electric current carrier on an InP substrate 1. It is also provided with an InP cap layer 4 on the incident side of the layer 3, and the surface of the cap layer 4 is coated with an SiN x layer 5 around the light receiving region. A Zn-diffused region 6 is formed on the light-receiving region extending from the cap layer 4 toward the light-absorbing layer 3. An Au-Zn/Au elecrode 7 is provided on the surface of the Zn-diffused region 6, and an Au-Sn/Au electrode 8 is provided on the rear surface of the substrate 1. An SiN x film 9 and an SiO 2 film 10 are formed on the surface of the cap layer 4 including the Zn-diffused region 6 and the side surfaces of the device as anti-reflection film. This embodiment of the invention is characterized in that the light-absorbing layer 3 is composed mainly of Ga x In 1-x As with the composition x being less than 0.47, and a GaInAs layer 11 and a strained multilayer 12 are provided between the light-absorbing layer 3 and the substrate 1 as the buffer zone which decreases the lattice mismatch effect therebetween. The manufacture method of this device will now be described. All the epitaxial layers of InP and GaInAs are grown by MOVPE (metalorganic vapor phase epitaxy ). The substrate 1 used is an n + type InP crystal doped with S, on which is grown S-doped n + type InP for 50 - 70 nm at 400°C to be used as a buffer layer 2. Then, an S-doped n + type GaInAs having 0.5% lattice mismatch (Δa/a) as against InP is grown for 1.3 µm thickness as a GaInAs layer 11. The growth is temporarily suspended here. The temperature is repeatedly changed between 700°C and 200°C for several times to decrease crystal defects and internal strain within the layer 2. Then, S-doped n+ type GaInAs layers having 0.9% lattice mismatch and S-doped n+ type InP layers are alternately grown to be used as a strained multilayer 12. A GaInAs layer having lattice mismatch of 0.5% and n type carrier concentration of 1 - 3 x 10 15 cm -3 is epitaxially grown for about 1.3 µm thickness to be used as the light-absorbing layer 3. Moreover, an n type InP layer is thinnly (e.g. 20 nm thick) grown to be used as the cap layer 4. The structure obtained by this step is shown in FIG. 11. Then, using an SiN x film 5 as a mask, Zn is diffused to be used as the p + type Zn-diffused region 6. This forms a p + -n - junction within the light-absorbing layer 3. An SiN x film 9 and an SiO 2 film 10 are formed on the light,receiving region as the anti-reflection film, and ohmic electrodes 7, 8 are respectively formed on the surface and the reverse surface of the Zn-diffused region 6. FIG. 12 is an SEM image which shows a section of the structure epitaxially grown with the buffer layer 2, GaInAs layer 11, strained multilayer 12, light-absorbing layer 3 and cap layer 4 on the substrate 1. FIG. 13 shows the result of X-ray diffraction measurement of the same structure. Referring now to FIG. 13, it is understood that the peak (fullwidth half maximum ca. 100 sec) of GaInAs by the light-absorbing layer 3 is separated from the peak of InP by the substrate 1, the lattice mismatch becomes about 0.5%, and that the composition x < 0.47. FIG. 14 shows temperature dependence of photoluminescence peak energy respectively for the GaInAs with lattice mismatch of 0.5% and of the strained multilayer with lattice mismatch of 0.88%. The peak energy substantially coincides with the band gap energy. Photoluminescence peak energy of this GaInAs at room temperature (300 K) is 0.71 eV, and the absorption-edge wavelength corresponding thereto is 1.75 µm. Therefore, the spectral responsivity can be extended on the longer wavelength side. FIG. 15 shows the result of measurement of the spectral responsivity of a trial photodiode having aforementioned structure at 20°C. The absorption edge of the light-absorbing layer 3 was extended toward the longer wavelength side, and flat responsivity was obtained at the wavelength of 1.5 - 1.65 µm. As the thickness of the light-absorbing layer 3 was set for ca. 1 µm in this embodiment, the spectral responsivity remained 50 - 60 [A/W] (external quantum efficiency of about 50%), but if the thickness of the layer 3 is set at 2 µm or higher, the spectral responsivity could be 0.8[A/W] or higher. When the spectral responsivity was measured at 20°C and 0°C, temperature dependency of the spectral responsivity at the wavelength of 1.55 µm was larger only by 0.2 - 0.3% than that at 1.3 µm. Therefore, by extending the absorption-edge wavelength of GaInAs toward the longer wavelength side as in this case, a photodiode having flat spectral responsivity at the range near 1.5 µm can be realized. This minimizes temperature dependence of the spectral responsivity in the range, and reduces measured calibration errors when the incident light spectral width is large, which otherwise causes a critical problem in optical power measurement. By forming the cap layer 4 thinner, not only defects which would otherwise be caused by lattice mismatch can be prevented but also the spectral responsivity on the shorter wavelength side can be enhanced to achieve flat spectral responsivity characteristics. FIG. 16 shows spectral responsivity of various devices wherein : Embodiment 1: The sample is a trial photodiode made in accordance with the above embodiment and has an approximately 1 µm thick light-absorbing layer, and a 0.02 µm thick InP cap layer. Embodiment 2 : The sample is similar to the above Embodiment 1, but has the thickness of 2 µm in the light-absorbing layer. They are calculated values. Comparative Example 1: The sample is a photodiode having the conventional structure with the thickness of 1.4 µm in the light-absorbing layer and 0.07 µm in the InP cap layer. Comparative Example 2: The sample has the structure substantially similar to Comparative Example 1; the thickness of the light-absorbing layer is ca. 1.4 µm but the thickness of the InP cap layer is 0.07 µm. Comparative Example 3: The sample is a commercial GaInAs-PIN photodiode. The thickness of the light-absorbing layer is 2 µm or more. Ge: A commercial Ge-PIN photodiode. Except for Example 2, all the values are measured values. In Embodiment 1, the spectral responsivity is generally low as the light-absorbing layer is made thin, but in the longer wavelength side its responsivity is higher than conventional devices except for Ge and has flat characteristics. On the shorter wavelength side, it does not show a drop in responsivity as seen in the commercial device of Comparative Example 3, and has flat characteristics. This is attributable to the fact that the thickness of InP cap layer is reduced. The difference achieved is further shown when compared with Comparative Examples 1 and 2. The difference between Comparative Examples 1 and 2 is only the thickness of the InP cap layer. With only such a trivial difference, the spectral responsivity on the shorter wavelength side is enhanced, the effect of multiple reflection within the InP cap layer is reduced and the spectral responsivity is made flatter. Although the above statements refer to the cases where the material for the cap layer is InP, other materials that can achieve lattice match with Ga x In 1-x As of the light-absorbing layer may be used, for example, AlInAsP. FIG. 17 shows changes in the band gap energy and lattice constant in Ga x In 1-x As and Al p In 1-p As of respective compositions. When the absorption-edge wavelength of Ga x In 1-x As comprising the light-absorbing layer is extended on the longer wavelength side, lattice match therebetween can be obtained by changing the composition p of Al p In 1-p As correspondingly to the changes in lattice constant. In order to make the cap layer transparent against the incident light, the band gap energy of Al p In 1-p As is selected to be larger than the band gap energy of Ga x In 1-x As. FIG. 18 shows the band gap energy and lattice constant of Ga 1-x In x As and of InAs q P 1-q at respective compositions. However, in this figure, the axis of the composition against GaInAs is reversed from that in FIG. 17. In this case, when the absorption egde of Ga x In 1-x As constituting the light-absorbing layer is extended on the longer wavelength side, lattice match would be achieved if the composition q of InAs q P 1-q is modified correspondingly to the changes in the lattice constant. When materials other than InP are used for the cap layer, the light-receiving region should be made thinner than the region where electrode is formed. This makes the spectral responsivity flatter, improves the spectral responsivity on the shorter wavelength side (or the wavelength range where the cap layer absorbs light), and minimizes the effect of diffusion of electrode metal to the junctions of the photodiode to enhance the reliability of electrode formation. As described in the foregoing, the photodiode according to this invention is fabricated by changing the composition of GaInAs light-absorbing layer in a manner to make the absorption edge extend toward the longer wavelength side, and reducing the defects caused by entailed lattice mismatch with InP by means of a buffer layer. This can realize the spectral responsivity on the longer wavelength side which could not heretofore be obtained by the conventional photodiode using GaInAs which is matched in lattice with InP, and can realize flat spectral responsivity characteristcs in the wavelength range close to 1.5 µm. Errors which might be caused by the effect of temperature at the time of measurement of optical power or the effect of spectral width of the incident light can be remarkably reduced by eliminating the irregularity in spectral responsivity (quantum efficiency). The light around the wavelength of 1.55 µm is the band of emission wavelength for GaInAsP lasers which is the wavelength used for optical communications and optical measurement devices which mainly measure the optical power. Therefore, this invention photodiode is highly practical as it can measure such wavelength range with smaller errors. Further, if the light receiving region of the cap layer is reduced in thickness compared to the region where electrode is formed, the spectral responsivity in the shorter wavelength side can be improved without damaging the reliability of electrode formation. This can realize photodiodes having high spectral responsivity in the band of unprecedentedly wide wavelengths.

Claims: A photodiode including an InP substrate (1) supporting a light-absorbing layer (3) which generates carriers when it absorbs incident light, the light-absorbing layer (3) having the composition Ga x In 1-x As, where x is less than 0.47, and wherein the photodiode includes, between the light-absorbing layer (3) and the InP substrate (1), a buffer zone (11,12) which includes a GaInAs layer (11) of substantially the same composition as the light-absorbing layer (3) and a strained multilayer formation (12), the GaInAs layer (11) lying between the strained multilayer formation (12) and the InP substrate (1). A photodiode as claimed in claim 1, wherein the strained multilayer formation includes alternate layers of GaInAs and another layer, in which the GaInAs of the strained multilayer formation has a lattice mismatch against InP larger than the lattice mismatch between the light-absorbing layer and the InP substrate. A photodiode as claimed in claim 2, wherein the material of the other layers of the strained multilayer formation is InP. A photodiode as claimed in claim 2, wherein the material of the other layers of the strained multilayer formation is GaInAs having substantially no lattice mismatch with the InP substrate. A photodiode as claimed in claim 4, wherein the material of the other layers of the strained multilayer formation is a combination of two GaInAs compounds which have mismatch lattices. A photodiode as claimed in any one of the preceding claims, including a cap layer, through which light can reach the light-absorbing layer, on that surface of the light-absorbing layer which is remote from the buffer zone, and an electrode which contacts a part of the cap layer and leaves free the remainder of the cap layer for light to pass through to the light-absorbing layer, the part of the cap layer in contact with the electrode being thicker than said remainder of the cap layer. A photodiode as claimed in claim 6, in which the material of the cap layer is InP. A photodiode as claimed in any one of claims 1 to 5, including a cap layer, through which light can pass to the light-absorbing layer, on that surface of the light-absorbing layer which is remote from the buffer zone, and an electrode which contacts a part of the cap layer and leaves free the remainder of the cap layer for light to pass through to the light-absorbing layer, the cap layer having a lattice constant substantially the same as that of the light-absorbing layer. A photodiode as claimed in claim 8, in which the material of the cap layer is Al p In 1-p As q P 1-q , where p and q are comprised between 0 and 1, both limits included. A photodiode as claimed in claim 8 or claim 9, wherein the part of the cap layer which is in contact with the electrode is thicker than the part of the cap layer for light to pass through to the light-absorbing layer.

Inventors: IWAOKA HIDETO, SEKIGUCHI YOUICHI, SEKO MASAHITO, WADA MORIO, IWAOKA, HIDETO, SEKIGUCHI, YOUICHI, SEKO, MASAHITO, WADA, MORIO
Applicants: OPTICAL MEASUREMENT TECHNOLOGY, OPTICAL MEASUREMENT TECHNOLOGY DEVELOPMENT CO. LTD
Application year: 1991
Main classification: H01L31
IPC codes: H01L31