Light-receiving device

Abstract: There is disclosed a light-receiving device comprising an absorption layer (26) formed of a semiconductor material which is capable of generating electric charges in response to an incident light, a plurality of light-­ detecting regions (1) formed in the absorption layer (2b), each outputting an electric signal in response to an incident light signal, and a depletion region (5) formed between the adjacent light-detecting regions (1) for collecting electric charges generated in a portion of the absorption layer (26) between the adjacent light-detecting regions (1).

Description: BACKGROUND OF THE INVENTION: (Field of the Invention) The present invention relates to a light-receiving device and to an optical connector containing such light-receiving device. US-A-3 858 233 discloses a light-receiving device corresponding to the preamble of claim 1 comprising an absorption layer formed of a semiconductor material which is capable of generating electric charges in response to incident light, a plurality of light-detecting regions formed in said absorption layer, each forming a photo-diode outputting an electric signal in response to an incident light signal, and an impurity doped in a portion of said absorption layer to thereby form a depletion region formed in said absorption layer between said adjacent light-detecting regions to deplete said portion of said absorption layer between said adjacent light-detecting regions for collecting electric charges generated in a portion of said absorption layer between said adjacent light-detecting regions. The prior art light-receiving device has a surface with a stepped structure. The depletion regions are formed as isolated regions, and each isolated region is covered by a film which does not transmit light substantially. For preventing that residual electron-hole pairs contribute to the photoelectric current in the light-detecting regions, either the film is made of a conductive material and is larger than the isolated region, or the isolated region is electrically connected to the adjacent isolated regions by a lead wire which extends externally of the light-receiving device. The Journal of Lightwave Technology, vol. LT-3, no. 6, December 1985, pp. 1248-1263 discloses a planar PIN photodetector array bonded to a silicon V-groove fiber array which in turn is connected to a fiber ribbon pigtail for providing automatic optical alignment of all photodetectors in the array. A package having a plurality of terminals is provided for housing the photodetector array and fiber array arrangements. Examples for such a light-receiving device, is a PD (photo diode) array having a plurality of light-detecting regions formed on a semiconductor material. In the field of optical communication, parallel signal transmission using a plurality of parallel optical fibers has been attempted to realize a large-capacity communication system using conventional PDs and LDs. From this point of view, development of integrated PDs and LDs used in parallel signal transmission has been expected. As an integrated optical device of this type, PD arrays having sectional structures shown in Figs. 9A, 9B, and 9C have been proposed. In a PD array shown in Fig. 9A, semiconductor crystals 2a, 2b, and 2c of a first conductivity type are grown on a substrate 4, and an impurity is selectively diffused in semiconductor crystals 2b and 2c to form regions 1 of a second conductivity type, thus forming PIN-PD structures serving as light-detecting regions (e.g., Society of Electronic Information Communications, national spring meeting, 1988, C-352). In the device having the structure shown in Fig. 9A, however, light incident on a portion between adjacent light-detecting regions is absorbed by the absorption layer 2b, and an electric charge is generated in this region. The electric charge generated in the absorption layer 2b is diffused in a lateral direction, and undesirably flows into the region 1 of the second conductivity type. Accordingly, a current flows into the PIN-PD structure from the absorption layer, thus posing the following problems: (1) an electric charge generated outside a light-detecting region is superposed on a signal current and causes crosstalk; and (2) a response time of an array is prolonged by a diffusion current having a low response speed. In a PD array having a structure shown in Fig. 9B, semiconductor crystals 2a, 2b, and 2c of a first conductivity type are grown on a semiinsulating substrate 3, and light-detecting regions are then formed in the same manner as in Fig. 9A. Subsequently, a portion of the semiconductor crystals 2a, 2b, and 2c between adjacent light-detecting regions is removed until the semiinsulating substrate 3 is exposed (e.g., Society of Electronic Information Communications, national fall meeting, 1989, C-225). In the PD array having the structure shown in Fig. 9B, because of partial etching of the semiconductor crystals 2a, 2b, and 2c, lateral diffusion of an electric charge can be prevented, and the above-mentioned problems can be solved. However, since steps are inevitably formed on the surface of the PD array, such structure causes errors occurring due to a non-flat surface during a fabrication process (e.g., photolithographic errors), or a structure of a device protective film may become incomplete. In a PD array having a structure shown in Fig. 9C, after light-detecting regions 1 of a second conductivity type are formed in semiconductor crystals 2a, 2b, and 2c of a first conductivity type in the same manner as in Fig. 9A, a metal film 12 is formed on the surface of the semiconductor crystal 2a between the adjacent light-detecting regions. In the PD array having the structure shown in Fig. 9C, since the metal film 12 is formed on the surface of the semiconductor crystal 2c to shield incidence of stray light on a portion outside the light-detecting regions, crosstalk caused by lateral diffusion of an electric charge can be prevented. However, incident light is multiple-reflected between the surface of the metal film 12 and a surface of an optical fiber connector 13, and stray light is undesirably transmitted to an adjacent light-detecting region. Thus, the stray light incident on the adjacent light-detecting region often causes crosstalk. SUMMARY OF THE INVENTION: It is an object of the present invention to provide a light-receiving device which has a structure allowing easy working of semiconductor materials, and can prevent crosstalk between adjacent light-detecting regions. It is still another object of the present invention to provide an optical connector which has a structure allowing easy working, and can prevent crosstalk between adjacent light-detecting regions formed in a light-receiving device. The present invention provides a light-receiving device according to claim 1 and a corresponding optical connector according to claim 6. Preferred embodiments of the invention are indicated in dependent claims 2-5 and 7-9. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention. Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only. BRIEF DESCRIPTION OF THE DRAWINGS: Fig. 1A is a plan view of a PIN-PD array according to the first embodiment of the present invention; Fig. 1B is a sectional view taken along a line I - I of the PIN-PD array shown in Fig. 1A; Fig. 2 is a chart showing a distribution of a measured photo sensitivity in the surface of the device of the PIN-PD array shown in Figs. 1A and lB; Fig. 3 is a perspective view of an embodiment of an optical connector using the PIN-PD array of the first embodiment; Fig. 4 is a sectional view of a PIN-PD array according to the second embodiment of the present invention; Fig. 5 is a sectional view of a PIN-PD array according to the third embodiment of the present invention; Fig. 6 is a sectional view of a Schottky diode array which is not part of the present invention; Fig. 7 is a plan view of a PIN-PD array according to the fourth embodiment of the present invention, and shows an array in which an arrangement of light-detecting regions is changed; Fig. 8 is a plan view of a PIN-PD array which is not part of the present invention, and shows an array in which an electronic circuit is arranged adjacent to light-detecting regions; and Figs. 9A, 9B, and 9C are sectional views of conventional PD arrays. DESCRIPTION OF THE PREFERRED EMBODIMENTS: Figs. 1A and 1B are respectively a plan view showing a PIN-PD array according to the first embodiment, and a sectional view taken along a line I - I in Fig. 1A. In Figs. 1A and 1B, reference numeral 4 denotes an n + -type (first conductivity type) InP substrate. A nondoped InP buffer layer 2a (n = 2 x 10 15 cm -3 , thickness = 2 µm), a nondoped InGaAs absorption layer (or photoelectric conversion layer) 2b (n = 2 x 10 15 cm -3 , thickness = 3.5 µm), and a nondoped InP window layer 2c (n = 2 x 10 15 cm -3 , thickness = 2 µm) are sequentially grown on the substrate 4. Subsequently, an impurity Zn is selectively diffused in the layers 2b and 2c by the sealed ampoule method (impurity doping method using a sealed silica tube including a semiconductor wafer and an impurity material), thereby forming regions 1 and 5 of a second conductivity type. In this embodiment, regions 1 of the second conductivity type correspond to PIN-PDs or light-detecting regions, and a region 5 of the second conductivity type corresponds to a depleted region surrounding PIN-PDs. The diameter of each region 1 of the second conductivity type is 100 µm, and five light-detecting regions are formed in line. Since a distance between the centers of the adjacent regions 1 of the second conductivity type is 250 µm, an interval therebetween is 150 µm. The 90-µm wide depletion region 5 of the second conductivity type is formed at almost the center of the 150-µm interval, and is formed to surround the light-detecting regions, as shown in Fig. 1A. P- and n-side electrodes 8 and 9 of the PIN-PD array apply a bias voltage to the regions 1 of the second conductivity type formed in the absorption layer 2b, and detect signal charges therefrom. In this case, noise charges collected in the region 5 of the second conductivity type are extracted via the extracting electrode 6 formed on the region 5. An anti-reflection film 7 is formed on the nondoped InP layer 2c. Fig. 2 shows a measurement result of a distribution of a photosensitivity (or responsivity) of the PIN-PD array of the first embodiment. As can be seen from Fig. 2, no crosstalk caused by stray light incident on the region surrounding the light-detecting region is observed. In the PIN-PD array shown in Figs. lA and 1B, the p-type region 5 formed in the absorption layer is utilized to deplete a part of the region 5 of the second conductivity type and a portion around it between the adjacent light-detecting regions. The depleted portion of the absorption layer serves to prevent crosstalk. More specifically, when light directed to the light-detecting device becomes incident on the absorption layer between the adjacent light-detecting regions, an electric charge generated in the region is collected in the depleted portion, and can be prevented from flowing into the light-detecting regions. Therefore, crosstalk caused by light incident on a portion other than the light-detecting regions can be prevented. Since a current due to lateral diffusion, i.e., which flows due to a carrier density gradient has a low response speed, the response speed of the PIN-PD array may be decreased by this current. However, since the PIN-PD array of the present embodiment can exclude the current caused by lateral diffusion, a decrease in response speed of the PIN-PD array can be prevented. Furthermore, since the anti-reflection film 7 is formed on the regions 1 and 5 of the second conductivity type, stray light can be reduced. More specifically, when no anti-reflection film 7 is formed, light which is not incident on the light-detecting regions is multiple-reflected between the surface of the semiconductor crystal 2 and the surface of, e.g., an optical fiber connector and may reach adjacent light-detecting regions. According to the present invention, since the anti-reflection film 7 is formed, crosstalk caused by multiple reflection can also be eliminated. The anti-reflection film is formed of, e.g., SiN X , SiO 2 , or the like, and is formed by, e.g., plasma CVD or thermal CVD. Fig. 3 shows a fiber ribbon connector 50 as an embodiment of an optical connector which incorporates the light-receiving device shown in Figs. 1A and 1B. A PIN-PD array 55 as the light-receiving device is fixed to a package 51 of the fiber ribbon connector 50. Six bonding wires extending from p-side electrodes 8 and the extracting electrode 6 formed on the PIN-PD array 55, are connected to leads or terminals 53. A fiber ribbon 54 consisting of five optical fibers is fixed to a plug 52 of the fiber ribbon connector 50. The distal end of the fiber ribbon 54 is fixed to a steel plate 57. When the plug 52 is entirely inserted in the package 51, the distal end of the plug 52 is stopped just before the front surface of the PIN-PD array 55. The light-detecting regions of the PIN-PD array 55 face to the optical fibers at the distal end of the fiber ribbon 54, so that each one of the five light-detecting regions is optically coupled to a corresponding one of the five optical fibers. Two spring plates 56 in the package 51 press the metal plug 52 against the two rectangular metal surfaces inside the package 51 to keep the plug 52 in tight contact with the package 51. As a result, a large dimensional tolerance can be obtained in optical coupling between the light-detecting regions and the optical fibers. Since a depletion layer is formed in the absorption layer between the light-detecting regions in the PIN-PD array shown in Fig. 3, the dimensional tolerance of, e.g., the plug, the package, and the like for optical coupling is very large. Alternatively, the degree of integration of the light-detecting regions can be increased while maintaining the dimensional precision of, e.g., the plug, the package, and the like. Fig. 4 is a longitudinal sectional view of the PIN-PD array according to the second embodiment. A nondoped InGaAs absorption layer 2 (n = 1 x 10 15 cm -3 , thickness = 5 µm) is formed on a n + -type InP substrate 4. P + -type regions 1 and 5 as regions of a second conductivity type are formed by selective diffusion of Zn. A p-side electrode 8 is formed on each p + -type region 1, and an n-side electrode 9 is formed on the lower surface of the substrate 4. An anti-reflection film 7 is formed on the upper surface of the absorption layer 2. Optical fibers 14 are aligned with light-detecting regions corresponding to the p + -type regions 1 and their adjacent portions. In order to fix the optical fibers 14, an optical fiber connector 13 comprising two Si substrates on the bottom and upper surfaces of which V grooves are formed is used. The optical fiber connector 13 is fixed to an appropriate plug which is similar to the plug 52 shown in Fig. 3. On the other hand, the PIN-PD array is fixed to an appropriate package which is similar to the package 51 shown in Fig. 3. The plug and the package of these types have been disclosed by S. R. Forrest (JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. LT-3, NO. 6, DEC, 1985, pp. 1248 - 1263). In this embodiment, each p + -type region 1 forms a PIN-PD structure serving as a light-detecting region. Each p + -type region 5 has a crosstalk prevention function. Electric charges generated by light incident on a portion between the adjacent light-detecting regions are collected in the p + -type region 5 and do not reach the p + -type regions 1, thereby preventing crosstalk. Fig. 5 is a longitudinal sectional view of a PIN-PD array according to the third embodiment. The PIN-PD array of the third embodiment is a modification of the second embodiment. In this case, as a method of depleting an absorption layer, a region of a second conductivity type is formed at a position other than the absorption layer, and a depleted region around it extends into the absorption layer. A nondoped InP buffer layer (n = 2 x 10 15 cm -3 , thickness = 2 µm) 2a, a nondoped InGaAs (n = 2 x 10 15 cm -3 , thickness = 3.5 µm) 2b, and a nondoped InP window layer (n = 2 x 10 15 cm -3 , thickness = 2 µm) 2c are formed on an n + -type InP substrate (n = 2 x 10 18 cm -3 ) 4. P + -type regions 1 and 5 of a second conductivity type are formed by selective diffusion of Zn. A p-side electrode 8 is formed on each p + -type region 1, and an n-side electrode 9 is formed on the lower surface of the substrate 4. In this embodiment, electric charges generated by stray light are collected in the p + -type region 5 to prevent crosstalk. In addition, since the InP window layer 2c is stacked, a photosensitivity of the light-receiving device can be increased. An increase in photosensitivity involves a risk of crosstalk caused by stray light. However, stray light reaching the adjacent light-detecting regions due to multiple reflection can be shielded by the anti-reflection film 7 formed on the surface of the device, and at the same time, electric charges generated by the stray light can be collected in the p + -type region 5. Therefore, crosstalk can be minimized while maintaining a high photosensitivity of minimized while maintaining a high photosensitivity of each light-detecting region. Fig. 6 is a longitudinal sectional view of a Schottky diode array which is not part of the invention. The Schottky diode array is a modification of the fourth embodiment. In this case, as a method of depleting an absorption layer, a Schottky electrode is formed on the absorption layer between the light-detecting regions. After a nondoped Si absorption layer (n = 2 x 10 14 cm -3 , thickness = 50 µm) 2 is grown on an n + -type Si substrate (n = 2 x 10 18 cm -3 ) 4, Schottky electrodes 10 are deposited on the absorption layer 2 to form Schottky diode structures which serve as light-detecting regions. Other Schottky electrodes 11 are deposited between adjacent light-detecting regions to deplete the absorption layer. An n-side electrode 9 is formed on the lower surface of the n + -type Si substrate. An anti-reflection film 7 is formed on the surfaces of the absorption layer 2 and the Schottky electrodes, thereby preventing a decrease in photosensitivity due to reflection of incident light on the Schottky electrode surfaces. Since the anti-reflection film 7 is formed on the surfaces of the absorption layer 2 and the Schottky electrodes, propagation of stray light due to multiple reflection can be prevented. Thus, crosstalk caused upon incidence of stray light onto the adjacent light-detecting regions can be prevented. Electric charges generated by stray light incident on portions between the adjacent light-detecting regions are collected in the Schottky electrodes 11, thus preventing crosstalk. Semiconductor materials and their thicknesses in each of the above embodiments are not limited to those described there. More specifically, they can be arbitrarily selected according to, e.g., an objective wavelength. As for semiconductor materials, compound semiconductors such as GaAs, AlGaAs, CdTe, HgCdTe, InSb, and the like, or Si, Ge, and the like may be used. As an impurity, Be, Cd, or the like may be used in place of Zn. Fig. 7 is a plan view of a PIN-PD array according to the fourth embodiment. In this embodiment, a plurality of light-detecting regions are two-dimensionally formed on the array. A absorption layer 2 of a first conductivity type is grown on a semiconductor substrate, and an impurity is selectively diffused in a layer 2 to form regions 1 and 5 of second conductivity type. As shown in Fig. 7, the region 5 is formed to surround regions 1 which correspond to the plurality of light-detecting regions. As a result, the absorption layer 2 around and between adjacent light-detecting regions is depleted. Therefore, the plurality of light-detecting regions can be formed in an arbitrary two-dimensional pattern. Fig. 8 is a plan view of a PIN-PD array which is not part of the invention. Two light-detecting regions 1 of a second conductivity type are formed on a single semiconductor substrate, and regions 5 of the second conductivity type are formed between the regions 1 of the second conductivity type to form depletion regions for preventing crosstalk. In the PIN-PD array shown in Fig. 8, electronic circuits 15 and 16, e.g., FETs and the like for amplifying signals are formed on the single semiconductor substrate adjacent to the light-detecting regions 1. Since the region 5 of the second conductivity type is arranged between the region 1 of the second conductivity type and the electronic circuits 15 and 16, a portion therebetween is depleted, so that electric charges formed by stray light can be prevented from flowing into the electronic circuits 15 and 16 such as FETs.

Claims: A light-receiving device comprising: an absorption layer (2b) formed of a semiconductor material which is capable of generating electric charges in response to incident light, a plurality of light-detecting regions (1) formed in said absorption layer (2b), each forming a photo-diode outputting an electric signal in response to an incident light signal, and an impurity doped in a portion of said absorption layer (2b) to thereby form a depletion region (5) formed in said absorption layer (2b) between said adjacent light-detecting regions (1) to deplete said portion of said absorption layer (2b) between said adjacent light-detecting regions (1) for collecting electric charges generated in a portion of said absorption layer (2b) between said adjacent light-detecting regions (1), characterized by said depletion region (5) surrounding each of said light-detection regions (1), and said depletion region (5) forming one continuous, electrically connected region. A light-receiving device according to claim 1, characterized in that an anti-reflection film (7) is formed over a portion of said absorption layer (2b) between said adjacent light-detecting regions (1). A light-receiving device according to claim 1, characterized in that a single electrode (6) is provided connected to said depletion region (5) for extracting electric charges collected by said depletion region (5) between said adjacent light-detecting regions (1). A light-receiving device according to claim 1, characterized in that said light-detecting regions (1) are arranged in a one-dimensional array, and output electrodes (8) of said light-detecting regions (1) are alternately arranged at opposite sides of said array. A light-receiving device according to claim 1, characterized in that said light-detecting regions (1) are arranged in a two-dimensional array. An optical connector (50) containing a light-receiving device (55) according to one of the preceding claims, characterized by a package (51) for storing said light-receiving device; and a plurality of terminals (53) fixed to said package (51), each electrically connected to each light-detecting region. An optical connector according to claim 6, characterized by optical fiber holding means (52) for fixing a plurality of optical fibers (54) and optically coupling each optical fiber (54) to each light-detecting region of said light-receiving device (55). An optical connector according to claim 6, characterized in that said light-receiving device (55) comprises an electrode (6) for extracting electric charges generated in said depletion region (5), and said package (51) comprises an extracting terminal (53) electrically connected with said extracting electrode (6). An optical connector according to claim 7, characterized in that said optical fiber holding means (52) directs each distal end of said optical fibers (54) toward each light-detecting region of said light-receiving device (55).

Inventors: TONAI ICHIRO, TONAI, ICHIRO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1990
Main classification: H01L27
IPC codes: G02B6, H01L27, H01L31