Abstract:
Disclosed is a photodiode array which includes a plurality of p-i-n photodiodes arrayed on a semi-insulative semiconductor substrate, each photodiode including an n-type semiconductor layer grown on the substrate, an i-type semiconductor layer grown on the n-type semiconductor layer, a p-type semiconductor layer grown on the i-type semiconductor layer, an n-type electrode provided on the n-type semiconductor layer in a region exposed by partially removing the p-type semiconductor layer and the i-type semiconductor layer, and a p-type electrode provided on the p-type semiconductor layer. A trench is provided between the two adjacent photodiodes by partially removing the p-type semiconductor layer, the i-type semiconductor layer, and the n-type semiconductor layer. Consequently, the size and pitch of the photodiodes can be decreased and crosstalk between the photodiodes can be reduced. Also disclosed is an optical receiver device including the photodiode array.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
   This application is a divisional of U.S. Ser. No. 10/368,246, filed Feb. 18, 2003, now U.S. Pat. No. 6,949,770 which in turn claims the benefit of Japanese Application No. 2002-050531, filed Feb. 26, 2002. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a photodiode array and an optical receiver device including the photodiode array. More particularly, the invention relates to a photodiode array in which the size and pitch of photodiodes can be decreased and crosstalk between the photodiodes can be reduced. 
   2. Description of the Related Art 
   An example of a photodiode array used for an optical module is shown in  FIGS. 9(   a ) and  9 ( b ) and  FIG. 10 . The optical module is used for measuring the intensity of light passing through a multi channel optical fiber array or optical waveguide. The photodiode array used for the module includes a semi-insulative semiconductor substrate  10 , and an n-type InP layer  11 , an InGaAs layer (light receiving layer)  12 , and an InP layer (window layer or cap layer)  13  grown in that order on the substrate. In the InGaAs layer  12  and the InP layer  13 , p-type areas  14  are selectively formed by the diffusion of Zn. Light receiving areas (i-type areas between p-type areas and n-type areas) are arrayed at a predetermined distance. A p-type electrode  16  is placed above the p-type area  14 , and an n-type electrode  15  of each photodiode is placed on the bottom of the common substrate  10 . 
   However, in the photodiode array described above, crosstalk occurs, resulting in a malfunction. The reason for this is that since the individual photodiodes are formed on the common n-type semiconductor substrate, for example, photo carriers generated in photodiodes Ch 1  and Ch 3  enter the signal of the adjacent photodiode Ch 2 , resulting in a malfunction in the photodiode Ch 2 . 
   In order to reduce crosstalk, a photodiode array disclosed in Japanese Unexamined Patent Application Publication No. 2001-144278, for example, employs a mesa structure in which the individual photodiodes are isolated by etching. In the photodiode array, by employing the mesa structure, carrier diffusion in the transverse direction between the photodiodes is prevented and crosstalk between the adjacent photodiodes is reduced. 
   However, in the photodiode array described above, since the signal circuits by photo carriers use a common n-type electrode provided on a common n-type substrate, mixing of signals occurs through the common electrode, and therefore it is difficult to suppress crosstalk sufficiently. 
   SUMMARY OF THE INVENTION 
   It is a principal object of the present invention to provide a photodiode array capable of reducing crosstalk between photodiodes and an optical receiver device including the photodiode array. 
   In the photodiode array according to the present invention, in order to achieve the object mentioned above, a p-type semiconductor layer, an i-type semiconductor layer, and an n-type semiconductor layer are partially removed in each photodiode, and an independent electrode structure is adopted. 
   That is, in one aspect of the present invention, a photodiode array includes a plurality of p-i-n photodiodes arrayed on a semi-insulative semiconductor substrate, each photodiode including an n-type semiconductor layer grown on the substrate, an i-type semiconductor layer grown on the n-type semiconductor layer, a p-type semiconductor layer grown on the i-type semiconductor layer. A trench is provided between the two adjacent photodiodes by partially removing the p-type semiconductor layer, the i-type semiconductor layer, and the n-type semiconductor layer. An n-type electrode is provided on the n-type semiconductor layer in a region exposed by partially removing the p-type semiconductor layer and the i-type semiconductor layer, and a p-type electrode is provided on the p-type semiconductor layer. 
   Alternatively, each photodiode may include a p-type semiconductor layer, an i-type semiconductor layer, and an n-type semiconductor layer grown in that order on the substrate. A trench is provided between the two adjacent photodiodes by partially removing the n-type semiconductor layer and the i-type semiconductor layer. In such a case, a p-type electrode is provided on the p-type semiconductor layer in a region exposed by partially removing the n-type semiconductor layer and the i-type semiconductor layer, and an n-type electrode is provided on the n-type semiconductor layer. 
   In the conventional photodiode array, since the n-type electrode of each photodiode is provided on the common semiconductor substrate, it is not possible to reduce crosstalk. In the present invention, a semi-insulative semiconductor substrate is used, and the n-type layer of each photodiode is isolated from that of an adjacent photodiode, and an n-type or p-type electrode is formed on an n-type or p-type semiconductor layer, respectively, for each photodiode. Thereby, a signal receiving circuit is completely isolated for each photodiode, and crosstalk through the n-type electrode can be reduced. 
   The trench may reach to the semi-insulative semiconductor substrate. A leakage current between the n-type electrodes is influenced by the surface resistance of the semi-insulative InP substrate, and in particular, by a minimal amount of contaminants and residual water, etc., on the surface. Therefore, as the distance between the two adjacent photodiodes is increased, the photodiodes are less likely to be influenced by such substances, and accordingly crosstalk can be prevented. 
   The semi-insulative semiconductor substrate may be composed of Fe-doped indium phosphide (InP). 
   The trench may be filled with a resin which does not transmit receiving light. By such a construction, it is possible to intercept leakage light, scattered light, or stray light from the adjacent channels. Specific examples of the resin which does not transmit light include carbon-containing epoxy resins. 
   The photodiode array of the present invention may have a structure in which light to be incident on the p-i-n photodiodes enters from the semiconductor substrate side. For example, an antireflective coating may be provided on the rear surface (the surface not provided with the n-type semiconductor layer, i-type semiconductor layer, and p-type semiconductor layer) of the substrate, and a rear electrode provided with a window for incident light may be formed on the antireflective coating. Alternatively, a structure in which light enters from the side of the front surface of the substrate may be adopted. 
   In another aspect of the present invention, an optical receiver device may have a structure in which the photodiode array and an optical transmission line such as an optical fiber or optical waveguide are disposed on a Si bench such that signals from the optical fiber are transmitted into the photodiode array. 
   In such a case, an amplifier for the signals from the photodiodes may be placed behind the photodiode array. By using the amplifier, the signals from the photodiodes are amplified and sensitivity is improved. 
   As described above, in the photodiode array of the present invention, the individual photodiodes are isolated from each other by the trenches, and the n-type or p-type electrode is independently provided, instead of being provided on a common substrate, which results in reduction of crosstalk. Therefore, the photodiode array is suitable for use in optical receiver devices, etc. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1(   a ) is a plan view of a photodiode array of the present invention, and  FIG. 1(   b ) is a cross-sectional view of the photodiode array shown in  FIG. 1(   a ); 
       FIG. 2  is an equivalent circuit diagram of the photodiode array shown in  FIGS. 1(   a ) and  1 ( b ); 
       FIG. 3  is an enlarged view of the photodiode array shown in  FIGS. 1(   a ) and  1 ( b ); 
       FIG. 4  is an enlarged view which shows a variation of the photodiode array shown in  FIG. 3 ; 
       FIG. 5(   a ) is a plan view of a photodiode array of the present invention in which trenches extending into a substrate are provided, and  FIG. 5(   b ) is a cross-sectional view of the photodiode array shown in  FIG. 5(   a ); 
       FIG. 6(   a ) is a plan view of a photodiode array of the present invention in which trenches extending into a substrate are provided and the trenches are filled with a resin, and  FIG. 6(   b ) is a cross-sectional view of the photodiode array shown in  FIG. 6(   a ); 
       FIG. 7(   a ) is a plan view of an optical receiver device of the present invention, and  FIG. 7(   b ) is a side view of the optical receiver device shown in  FIG. 7(   a ); 
       FIG. 8  is an exterior view of a photodiode array of the present invention; 
       FIG. 9(   a ) is a plan view of a conventional photodiode array, and  FIG. 9(   b ) is a cross-sectional view of the photodiode array shown in  FIG. 9(   a ); and 
       FIG. 10  is an equivalent circuit diagram of the photodiode shown in  FIGS. 9(   a ) and  9 ( b ). 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The embodiments of the present invention will be described with reference to the drawings. In the drawings, the same elements are represented by the same reference numerals, and duplication of description is avoided. It is to be understood that the dimensional proportions of the individual elements in the drawing do not necessarily correspond to the actual proportions. 
   (Construction) 
     FIG. 1(   a ) is a plan view of a photodiode array of the present invention, and  FIG. 1(   b ) is a cross-sectional view of the photodiode array shown in  FIG. 1(   a ).  FIG. 2  is an equivalent circuit diagram of the photodiode array shown in  FIGS. 1(   a ) and  1 ( b ).  FIG. 3  is an enlarged view of the photodiode array shown in  FIGS. 1(   a ) and  1 ( b ), and  FIG. 4  is an enlarged view which shows a variation of the photodiode array shown in  FIG. 3 . On a Fe-doped semi-insulative InP substrate  10 , an n-type InP layer  11 , an InGaAs layer (light receiving layer)  12 , and an InP layer (window layer or cap layer)  13  are grown in that order from the bottom. All of the n-type InP layer  11 , the InGaAs layer  12 , and the InP layer  13  are circular, and the InGaAs layer  12  and the InP layer  13  have a smaller diameter than that of the n-type InP layer  11 . A p-type area  14  is selectively formed in the InGaAs layer  12  and the InP layer  13  by the diffusion of Zn. 
   An annular n-type electrode  15  is provided on the n-type InP layer  11  in a region exposed by partially removing the InP layer  13  and the InGaAs layer  12  by etching. A circular p-type electrode  16  is formed on the InP layer  13 . 
   The individual photodiodes are isolated by trenches  17  which are formed by removing the n-type InP layer  11 , the InGaAs layer  12 , and the InP layer  13 . In this embodiment, the trenches  17  extend to the surface of the substrate  10 . Although the photodiode array is designed to allow light to be incident from the substrate  10  side, a structure in which incident light enters from the front side may be adopted. 
   (Fabrication Method) 
   Such a photodiode array is fabricated by a method described below. On a semi-insulative InP substrate  10 , an n-type InP layer  11 , an InGaAs layer  12 , and an InP layer  13  are continuously formed by an organometallic vapor-phase epitaxial method or chloride vapor-phase epitaxial method. The second layer of InGaAs layer  12  is a non-doped high-purity layer, and the third layer of InP layer  13  is a non-doped high-purity layer or an n-type layer. Next, Zn is diffused into the InGaAs layer  12  and the InP layer  13  by a vapor-phase diffusion process to form a p-type area  14 . 
   The peripheries of the InP layer  13  and the InGaAs layer  12  are partially removed by selective etching to expose the n-type InP layer  11  such that a region for an n-type electrode is thereby formed. In the etching process, the surface of the n-type InP layer  11  is exposed by using hydrobromic acid and phosphoric acid in sequence. A reactive ion etching method may be used instead. In such a case, in order to improve efficiency, the InP layer  13  and the InGaAs layer  12  may also be etched so that the n-type InP layer is exposed  11  in the regions between the photodiodes. 
   Trenches  17  are formed by removing the remaining n-type InP layer  11  between the photodiodes. For that purpose, wet etching using hydrobromic acid and phosphoric acid, or the like, or reactive ion etching may be employed. Alternatively, the trenches  17  may be formed by mechanical processing using a dicing saw or the like. 
   As shown in  FIG. 3 , an insulating film  21  composed of SiO x N y  is formed on the surface of the photodiodes. At this stage, contact holes are formed in the insulating film  21  on the p-type area  14  and on the n-type InP layer  11  exposed by etching. An AuZn-containing p-type electrode  16  is formed on the p-type area  14 , and then an AuGeNi-containing n-type electrode  15  is formed on the n-type InP layer  11  exposed by etching. Other material may be used for the p-type electrode  16  or the n-type electrode  15 . 
   An antireflective coating  22  composed of SiO x N y  is formed on the rear surface of the substrate  10 , and a rear electrode  23  composed of TiAu is then formed on the antireflective coating  22 . Windows are formed in the rear electrode  23  so that receiving light enters the light receiving layer  12  from the substrate  10  side. Although the antireflective coating  22  is formed on the entire rear surface of the substrate  10  in the embodiment shown in  FIG. 3 , an antireflective coating  22  may be partially formed on the rear surface of the substrate and rear electrodes  23  may be provided in regions other than the antireflective coating  22  on the rear surface of the substrate as shown in  FIG. 4 . 
   In the construction shown in  FIGS. 1(   a ) and  1 ( b ), a leakage current between the n-type electrodes of the individual channels is influenced by the surface resistance of the semi-insulative InP substrate  10 , and in particular, by a small amount of contaminants and residual water, etc., on the surface. Therefore, as the separation distance between the two adjacent photodiodes is increased, the photodiodes are less likely to be influenced. 
     FIG. 5(   a ) is a plan view of a photodiode array which is designed so that the separation distance between the two adjacent photodiodes is increased, and  FIG. 5(   b ) is a cross-sectional view of the photodiode array shown in  FIG. 5(   a ). For example, if the acceptance surface diameter is 100 μm, the photodiode pitch is 250 μm, and the outer diameter of the n-type electrode  15  is 200 μm, a separation distance Δx between the two adjacent photodiodes is assumed to be 50 μm. Consequently, if a trench  17  is formed so as to enter into the semi-insulative InP substrate  10  at a depth of 50 μm, the separation distance between the n-type electrodes  15  is 150 μm, and thereby the photodiodes are further less likely to be influenced by the surroundings. The trench  17  may be formed in an annular shape corresponding to the shape of the electrode. Alternatively, as shown in  FIGS. 5(   a ) and  5 ( b ), the trench  17  may be formed in a linear shape by dicing. 
   As shown in  FIGS. 6(   a ) and  6 ( b ), if a trench (slit)  17  that extends into the substrate  10  is formed, for example, at a depth of 100 μm, and the trench  17  is filled with a resin (e.g., carbon-containing epoxy resin)  18  which does not transmit receiving light, it is possible to intercept leakage light, scattered light, or stray light from the adjacent channels. 
   (Optical Receiver Module) 
     FIG. 7(   a ) is a plan view of a 4-channel integrated optical receiver device which includes a 4-channel photodiode array fabricated by the method described above, and  FIG. 7(   b ) is a side view of the optical receiver device shown in  FIG. 7(   a ). A 4-channel photodiode array  20  including channels with an acceptance surface diameter of 100 μm is soldered to electrodes formed on a Si bench  30 . The Si bench  30  has a step and includes a lower stand  32  and an upper stand  33  with an inclined plane  31  therebetween. On the lower stand  32 , a multi-channel connector  40  is fixed and V-grooves  34  for placing optical fibers  41  are provided. 
     FIG. 8  is an exterior view showing the multi channel connector  40  and an optical fiber ribbon connector  45 . In the multi channel connector  40 , the optical fibers  41  protrude from one end face of a block-shaped packaging  42 , and guide pins  43  protrude from the other end face. On one end of the optical fiber ribbon connector  45 , fitting holes  46  are formed and optical fibers  47  are also arranged. By inserting the guide pins  43  into the respective fitting holes  46 , the multi channel connector  40  and the optical fiber ribbon connector  45  are connected to each other. 
   The photodiode array  20  is mounted above the inclined plane  31  as shown in  FIG. 7(   b ) such that a part of the photodiode array  20  overhangs. Received light is reflected at the inclined plane  31  having a mirror surface such that the light enters from the substrate side of the photodiode array  20 . In order to improve sensitivity, a pre-amplifier  50  is placed behind the photodiode array (at a place opposite to the optical fibers with respect to the photodiode array). The Si bench  30  is mounted on a lead frame  60  so as to be electrically connected, and an optical module is produced. 
   As a result of evaluation of the receiving sensitivity of the optical module thus obtained, each channel had a sensitivity of −35 dBm at a transmission rate of 156 Mbps. Such an effect is obtained not only when the optical module is used as a receiver but also when it is used as a transceiver device in which a receiver device and a transmitter device are combined.