Abstract:
A method of fabricating an avalanche photodiode is disclosed. The method includes the steps of growing a plurality of semiconductor layers sequentially on a semiconductor substrate; growing diffusion layer patterns having diffusion coefficients different from that of an amplifying layer on a portion on which a peripheral portion of a diffusion area is to be formed, on the semiconductor layers; and forming the diffusion area such that the depth of the peripheral portion thereof is different from that of the central portion thereof by diffusing impurities through the diffusion patterns.

Description:
CLAIM of PRIORITY  
       [0001]     This application claims the benefit of the earlier filing date, pursuant to 35 USC 119(a) to that patent application entitled “METHOD OF FABRICATING AVALANCHE PHOTODIODE” filed in the Korean Industrial Property Office on Feb. 23, 2005 and assigned Serial No. 2005-15166, the contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to fabrication of an optical device, and more particularly to a method for fabricating an avalanche photodiode including a diffusion area.  
         [0004]     2. Description of the Related Art  
         [0005]     A photodiode is a type of photoelectric conversion device, that converts a received light to electric signals and subsequently outputs the converted electrical signals. Among photodiodes, an avalanche photodiode is a type of photoelectric conversion device which can convert and amplify the converted electric signals and subsequently output the amplified signals.  
         [0006]     The avalanche photodiode must be formed accurately so that an included amplifying layer has an intended structure to realize the amplifying characteristics of the converted optical signals, and should overcome a yield phenomenon generated at the edges of a Zn diffusion area.  
         [0007]      FIG. 1  is a cross-sectional view of a conventional avalanche photodiode. Referring to  FIG. 1 , the conventional avalanche photodiode  100  includes an absorption layer  120 , a grading layer  130 , an electric field buffer layer  140 , and an amplifying layer  190 , which are sequentially grown on the semiconductor substrate  110 . Upper electrodes  181  and  182  are formed on the upper portion of the amplifying layer  190 , and a lower electrode  162  is grown on the lower portion of the semiconductor substrate  110 . A surface protection layer  161  is grown over the upper layers to protect the internal layers The semiconductor substrate  110  can be formed of a semiconductor material of N + —InP, (N + -doped Indium Phosphate) and the absorption layer  120  can be formed of N—InGaA (N-doped Indium Gallum Arsenic). In addition, the grading layer  130  can be formed of N—InGaAsP (N-doped Indium Gallum Arsenic Phosphate), and the electric field buffer layer  140  and the amplifying layer  190  can be formed of N—InP. The surface protection layer  162  can be formed of a material selected from the group consisting of SiNx, e.g., Silicon Nitride, Silicon Nitrate.  
         [0008]     A diffusion area  150  and guard ring areas  171  and  172  are formed at predetermined portions of the upper end of the amplifying layer  190 . The diffusion area  150  includes a center portion  152  and peripheral portions  151  and  153 . The height Wm and A of the central portion  152  from the electric field buffer layer  140  is lower than the heights B of the peripheral portions  151  and  153  from the electric field buffer layer  140 . That is, central area  152  is formed deeper into amplifying layer  190  than regions  151 ,  153 .  
         [0009]     The diffusion area  150  is conventionally formed by diffusion of impurities and a drive-in process, after a portion of the amplifying layer  190  is recess-etched.  
         [0010]     The light inputted into the avalanche photodiode  100  excites the absorption layer  120 , which generates an electron and a hole. The electron and the hole, are referred to as an electron-hole pair (hereinafter, EHP). Since inverse voltage is applied to the avalanche photodiode  100 , in the generated EHP the electron is discharged through an N type lower electrode  162  and the hole passes through the grading layer  130  and the buffer layer  140 , sequentially, and is inputted to the amplifying  190 . After the hole is amplified by amplifying layer  190 , it is outputted through the upper electrode  181 , which is of P type material.  
         [0011]     Since the photodiode  100  amplifies the electric signals converted from the light internally, it can output electric signals of relatively low noise and large output as compared with an amplifying device of another type.  
         [0012]     The avalanche photodiode needs an additional operation time to amplify the electric signals therein, and the operation time of the avalanche photodiode increases in proportion to the thickness of the amplifying layer. However, the increase in the operation time deteriorates the bandwidth characteristics of the avalanche photodiode. For reference, the amplifying layer of the avalanche photodiode up to a maximum thickness of 0.5 μm is able to obtain the operation characteristics of 2.5 Gbps. While, an amplifying layer of a maximum thickness of 0.2 μm is able to obtain operation characteristics of 10 Gbps.  
         [0013]     However, creation of the diffusion area requires great care as there is a possibility of an edge yield phenomenon if the width of the amplifying layer is too small. The edge yield phenomenon can be overcome by using the diffusion area and the guard rings ( 171 ,  172 ) to prevent the electric field from being concentrated on the edge.  
         [0014]     In addition, as a portion of the amplifying layer may be etched by wet or dry etching processes, the etched diffusion area has a problem of having a large allowable error in the range of more than ±100 Å.  
         [0015]     Therefore, the conventional method of creating the diffusion area has a problem in that the manufacturing processes are complicated in order to minimize the allowable error range and the yield rate of the product lowers.  
       SUMMARY OF THE INVENTION  
       [0016]     Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art and provides additional advantages, by providing a method of fabricating an avalanche photodiode by which a diffusion area can be easily formed and generation of errors can be minimized when forming the diffusion area.  
         [0017]     In one embodiment, a method of fabricating an avalanche photodiode includes the process of growing a plurality of semiconductor layers sequentially on a semiconductor substrate; growing diffusion layer patterns having diffusion coefficients different from that of an amplifying layer on a portion on which a peripheral portion of a diffusion area is to be formed and forming the diffusion area such that the depth of a peripheral portion thereof is different from that of a central portion by diffusing impurities through diffusion patterns.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The above features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0019]      FIG. 1  is a cross-sectional view for showing a conventional avalanche photodiode; and  
         [0020]      FIGS. 2A  to  2 F are views for showing the steps of manufacturing an avalanche photodiode according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0021]     Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. For the purposes of clarity and simplicity, a detailed description of known functions and configurations incorporated herein will be omitted as it may make the subject matter of the present invention unclear.  
         [0022]      FIGS. 2A  to  2 F are views showing steps of manufacturing an avalanche photodiode according to the present invention.  
         [0023]      FIG. 2A  shows a state in which semiconductor layers are sequentially grown on a semiconductor substrate. Referring to  FIG. 2A , the avalanche photodiode includes a buffer layer  220 , an absorption layer  230 , a grading layer  240 , an electric field buffing layer  250 , and an amplifying layer  260 , which are sequentially grown on the semiconductor substrates.  
         [0024]     The semiconductor substrate  210  can be formed of N + —InP, and the buffer layer  220  is grown on the semiconductor substrate  210 . The buffer layer  220  also can be formed of N + —InP or other similar N + -doped semiconductor material.  
         [0025]     The absorption layer  230  is grown on buffer layer  220 , and the absorption layer  230  can be formed of N—InGaAs or other similar N-doped semiconductor material. The absorption layer  230 , as discussed previously, is excited by an absorbed light and forms an electron-hole pair.  
         [0026]     The grading layer  240  includes a plurality of layers having a band gap between InP and InGaAs, wherein a hole, among the electron-hole pair generated in the absorption layer  230 , is injected into the amplifying layer  260 . The grading layer  240  can be formed of N − —InGaAsP.  
         [0027]     The electric field buffing layer  250  has a density and a thickness which are well regulated, and is called a charge sheet layer. The electric field buffing layer  250  can be formed of N—InP.  
         [0028]     The amplifying layer  260  is grown on the charge absorbing layer  250 , and can be formed of N—InP, for example.  FIG. 2B  is a view for showing a state in which diffusion patterns  271  and  272  are formed at corresponding positions for forming peripheral portions  281  and  282  on amplifying layer  260 .  FIGS. 2C  to  2 E are views for showing processes in which impurities of Zn or Cd or other materials having similar properties, are doped in the amplifying layer  260  on which the diffusion patterns  271  and  272  are formed.  
         [0029]     After the diffusion area  280  is formed, the diffusion patterns  271  and  272  are formed on the amplifying layer  260 . The diffusion patterns  271  and  272  are made of a material having a diffusion coefficient different from that of the amplifying layer  260 . More specifically, impurities of Zn or Cd, or other materials having similar properties, are diffused and driven-in on a portion of the amplifying layer  260  to form an impurity profile as shown in  FIG. 2F .  
         [0030]     That is, as shown in  FIG. 2C , an impurity layer  201  for doping the amplifying layer  260  is formed on the amplifying layer  260  on which the diffusion patterns  271  and  272  are formed, and current blocking layers  202  are formed at a position adjacent to the diffusion patterns  271  and  272 . A diffusion area  280  in which impurities of Zn or Cd, etc., is doped by diffusion and a drive-in process is formed in the amplifying layer  260 .  
         [0031]     Thereafter, as shown in  FIGS. 2D and 2E , a capping layer  203 , for preventing the impurity layer  201  from being diffused into the air during the doping process, is deposited. As mentioned above, the diffusion area  280 , doped by the diffusion of the impurity layer  201  and the drive-in process, is formed in the amplifying layer  260 .  
         [0032]     Since the present invention uses the diffusion patterns  271  and  272 , having a diffusion coefficient different from that of the amplifying layer  260 , the amplifying layer  260  need not be etched to form the diffusion area. That is, the diffusion patterns enables the thicknesses of the center portion  283  and the peripheral portions  281  and  282  of the diffusion area  280  to be regulated without any recess-etching process.  
         [0033]     The capping layer is deposited on the current blocking layer to prevent the impurities not doped in the diffusion area from being scattered into the air during the doping process. Therefore, the capping layer is removed after the doping process as shown in  FIG. 2F .  
         [0034]     Referring to  FIG. 2F , the avalanche photodiode further includes upper electrodes  204 , formed on the amplifying layer  260 , a lower electrode  205 , formed on the lower portion of the semiconductor substrate  210 , and current blocking layers  202 .  
         [0035]     As the diffusion area  280  according to the present invention can be formed by a diffusion process without etching the amplifying layer differently from the prior art, the depth thereof is easily controlled and the allowable error is significantly improved.  
         [0036]     In addition, the sizes and depths of the center portion  283  and the peripheral portions  281  and  282  of diffusion area  280  can be regulated according to the sizes, depths, and positions of the diffusion patterns  271  and  272  for forming the diffusion area  280 .  
         [0037]     The diffusion patterns  271  and  272  are formed at positions for forming the peripheral portions  281  and  282  of the diffusion area  280  on the amplifying layer  260 , and can be formed of one of or a combination of InGaAs, InGsAsP, for example, which have diffusion coefficients different from that of the amplifying layer with respect to Zn or Cd.  
         [0038]     The current blocking layers  202  can be formed of a material selected from a group consisting of dielectric materials of SiNx. Since the upper electrodes  204  are formed as a P-type ohmic electrode on the diffusion patterns of InGaAs or InGaAsP, they can have a contact resistance lower than those of the conventional electrodes by five to ten times and form a stable ohmic contact.  
         [0039]     While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.