Abstract:
In one aspect the invention relates to a high bandwidth shallow mesa semiconductor photodiode responsive to incident electromagnetic radiation. The photodiode includes an absorption narrow bandgap layer, a wide bandgap layer disposed substantially adjacent to the absorption layer, a first doped layer having a first conductivity type disposed substantially adjacent to the wide bandgap layer, and a passivation region disposed substantially adjacent to the wide bandgap layer and the first doped layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to U.S. Provisional Patent Application Ser. No. 60/245,902, filed on Nov. 3, 2000. Additionally, this application is a divisional of, and claims priority to U.S. patent application Ser. No. 10/002,909, filed on Nov. 2, 2001, now U.S. Pat. No. 6,756,613, the entire disclosure of which is incorporated by reference herein. 

   FIELD OF THE INVENTION 
   This invention relates generally to semiconductor devices, and more specifically to the structure of PIN photodiodes and APDs. 
   BACKGROUND OF THE INVENTION 
   For high-bit-rate, long-haul fiber-optic communications p-doped/intrinsic/n-doped (PIN) photodiodes and avalanche photodiodes (APDs) are frequently used as photodetectors due to their high sensitivity and bandwidth. Planar and mesa structures are two commonly used configurations for PIN Photodiodes and APDs. Mesa structure PIN photodiodes and APDs are sometimes grown by molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD). These fabrication techniques allow the thickness of the layers and the wafer to be accurately controlled. 
   Referring to  FIG. 1 , a mesa structure PIN  2  known in the prior art is shown. The structure includes a top metal contact  8 , two bottoms metal contacts  12 , a p-doped Indium Gallium Arsenide (InGaAs) ohmic contact layer  64  lattice matched to Indium Phosphide (InP), a p-doped InP layer  68 , an intrinsic narrow bandgap InGaAs absorption layer  76  lattice matched to InP, a n-doped InP layer  80 , and passivation regions  32 . 
   In fabrication after the layers  80 ,  76 ,  68 ,  64  are sequentially deposited, the mesa structure  84  is formed by chemical etching through the p-doped layers  64 ,  68  and the intrinsic absorption layer  76 . Next, the exposed sidewalls of the p-doped layers  64 ,  68  and the intrinsic absorption layer  76  that define the mesa structure  84  are passivated with dielectric materials, such as SiO 2  or SiN x . As part of this process, defects are inevitably introduced into the p-doped layers  64 ,  68  and the intrinsic absorption layer  76 . The intrinsic InGaAs absorption layer  76  has a low bandgap and the mesa etching introduced defects create extra intraband energy levels. These in turn lead to a high dark current. The dark current in InGaAs PIN photodiodes and APDs fabricated according to the above method is one factor in the generally low reliability of these devices. The low reliability of these devices includes low sensitivity and high noise. These disadvantages significantly restrict the use of InGaAs PIN photodiodes and APDs in optical communications systems. 
   Referring to  FIG. 2 , a planar structure PIN photodiode  4  known in the prior art is shown. The structure  4  includes a top metal contact  8 , two bottom metal contacts  12 , an intrinsic InGaAs layer  16 , an intrinsic InP layer  20 , an intrinsic absorption InGaAs layer  76 , a n-doped InP layer  28 , passivation regions  32 , a p-doped InGaAs diffusion region  36 , and a p-doped InP diffusion region  40 . 
   During fabrication of the planar structure PIN photodiode  4 , the n-doped InP layer  28 , the intrinsic InGaAs layer  76 , the intrinsic layer InP  20 , and the intrinsic InGaAs layer  16  are sequentially deposited. The p-doped regions  36  and  40  are then formed by diffusing, for example, Zinc (Zn) or Cadmium into the top central region of the device  4 . After the diffusion step, the top metal contact  8  and the passivation regions  32  are added. 
   Although avoiding the introduction of defects into the intrinsic InGaAs layer  76  during passivation, planar structure PIN photodiodes  4  have disadvantages in device performance and design flexibility. The introduction of the p-dopant by diffusion is not a precise process, and, therefore, the thickness of the p-doped regions  36  and  40  cannot be accurately controlled. In some instances the p-dopant diffuses into the intrinsic InGaAs layer  76 . In other instances the p-dopant does not diffuse completely through the intrinsic InP layer  20 , or even through the intrinsic InGaAs layer  76 . Another disadvantage of planar structure PIN photodiodes  4  is their higher parasitic capacitance. The parasitic capacitance exists between the conductive substrate and device pad. Mesa structure devices can avoid this problem, however, by employing a semi-insulating substrate. 
   An additional disadvantage of planar structure PIN photodiodes  4  is that their fabrication process is complex. In particular, the diffusion process requires that the surface of the layer to be doped be carefully prepared. A further disadvantage of planar structure PIN photodiodes  4  is the control of hazardous materials as part of the dopant diffusion. For example, in Zn diffusion, As, P, Zn 3 P 2 , and Zn 3 As 2 , are heated to approximately 550C. At this temperature, small evaporated and inhaled doses are lethal. 
   What is needed are PIN photodiodes and APDs that overcome the disadvantages of current PIN photodiodes and APDs. 
   SUMMARY OF THE INVENTION 
   In one aspect the invention relates to a high bandwidth shallow mesa semiconductor photodiode responsive to incident electromagnetic radiation. The photodiode includes an absorption narrow bandgap layer, a wide bandgap layer disposed substantially adjacent to the absorption layer, a first doped layer having a first conductivity type disposed substantially adjacent to the wide bandgap layer, and a passivation region disposed substantially adjacent to the wide bandgap layer and the first doped layer. 
   In one embodiment, the photodiode also includes a second doped layer disposed substantially adjacent to the absorption narrow bandgap layer. In another embodiment the photodiode also includes a third doped layer disposed substantially adjacent to the first doped layer and adapted to form an ohmic contact with a substantially adjacent metalization layer. In an additional embodiment, the photodiode also includes a second doped layer and an impact layer disposed substantially adjacent to the second doped layer and the absorption narrow bandgap layer. The ratio of the ionization coefficient for electrons relative to the ionization coefficient for holes for the impact layer is larger than the corresponding ratio for the absorption narrow bandgap layer, the wide bandgap layer, the first doped layer, and the second doped layer. 
   In a further embodiment, the first doped layer includes indium phosphide. In yet another embodiment, the absorption layer comprises indium gallium arsenide. In yet an additional embodiment, the wide bandgap layer varies in thickness from a deposition thickness t 1  to an etching thickness t 2 . 
   In another aspect the invention relates a method for fabricating high bandwidth shallow mesa semiconductor photodiode responsive to incident electromagnetic radiation. The method includes generating an absorption narrow bandgap layer, generating a wide bandgap layer disposed substantially adjacent to the absorption narrow bandgap layer, generating a first doped layer disposed substantially adjacent to the wide bandgap layer. The first doped layer has a first conductivity type. The method also includes etching a region of the first doped layer, etching a region of the intrinsic wide bandgap layer, and generating a passivation layer disposed substantially adjacent to the first doped layer and the intrinsic wide bandgap layer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and further advantages of the invention may be better understood by referring to the following description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a mesa PIN photodiode as known to the prior art; 
       FIG. 2  is a planar PIN photodiode as known to the prior art; 
       FIG. 3  is an embodiment of a PIN photodiode according to the invention; 
       FIG. 4  is an embodiment of an avalanche photodiode according to the invention; 
       FIG. 5  is a flowchart representation of a method for fabricating a shallow-mesa PIN photodiode according to the invention; and 
       FIG. 6  is a flowchart representation of a method for fabricating a shallow-mesa APD according to the invention. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 3 , one embodiment of a shallow mesa planar PIN photodiode  60  according to the invention is shown. The PIN photodiode  60  includes a top metal contact  8 , two bottoms metal contacts  12 , a p-doped In 0.53 Ga 0.47 As ohmic contact layer  64  latticed matched to InP, a p-doped InP layer  68 , a wide bandgap intrinsic InP layer  72 , an intrinsic narrow bandgap In 0.53 Ga 0.47 As absorption layer  76  lattice matched to InP, a n-doped InP layer  80 , and passivation regions  32 . 
   In an alternative embodiment, the wide bandgap intrinsic InP layer  72  is replaced with Indium/Aluminum/Arsenide (In x Al 1−x As) lattice matched to InP. In one embodiment, the shallow mesa planar PIN photodiode  60  operates according to the principles of back illumination. In this embodiment, the photons in an incident beam pass through the n-doped InP layer  80  and into the intrinsic In 0.53 Ga 0.47 As absorption layer  76 . In various other embodiments, the thickness of the layers and the dopant concentrations are selected according to Table 1. 
   
     
       
             
             
             
           
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               Layer 
               Thickness 
               Dopant Concentration 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               p-In x Ga 1−x As layer 64 
               50 
               nm 
               1 × 10 18 –5 × 10 18  cm −3   
             
             
               p-InP layer 68 
               0.2–0.4 
               um 
               1 × 10 18 –5 × 10 18  cm −3   
             
             
               i-InP layer 72 
               0.2–0.4 
               um 
               Not Applicable 
             
             
               i-In 0.53 Ga 0.47 As layer 76 
               1–3 
               um 
               Not Applicab1e 
             
             
               n-InP layer 80 
               0.5–1 
               um 
               1 × 10 18 –5 × 10 18  cm 3   
             
             
                 
             
           
        
       
     
   
   As part of fabrication the p-doped In 0.53 Ga 0.47 As layer  64 , the p-doped InP layer  68 , the intrinsic InP layer  72 , the intrinsic In 0.53 Ga 0.47 As layer  76 , and the n-doped InP layer  80  are deposited by MBE or MOCVD techniques. This means that the thickness of the layers  64 ,  68 ,  72 ,  76 ,  80  can be accurately controlled. Once all the layers  64 ,  68 ,  72 ,  76 ,  80  have been deposited, the mesa  84 ′ is formed by etching through the p-doped In 0.53 Ga 0.47 As layer  64  and the p-doped InP layer  68 , and into the intrinsic InP layer  72 . The etching process is controlled so that after completion the intrinsic InP layer  72  has a thickness t 1  in the range of 0.1–0.3 um. The lower bound on this range ensures that the intrinsic In 0.53 Ga 0.47 As layer  76  is adequately protected from the introduction of defects from the passivation process. The deposition thickness t 2  of the intrinsic InP layer  72  is chosen so as to minimize the carrier-transit time increase introduced by the additional layer  72 . 
   In general the introduction of defects into the intrinsic InP layer  72  during the passivation process does not lead to significant surface leakage current. This is due in part to the wideband gap of InP. In addition, potential dark current from this layer  72  is minimized by closely monitoring the etching process so that little of the sidewall of the intrinsic InP layer  72  is exposed. The passivated region of the InP layer  72  away from the mesa does not produce significant dark current because the electric field is relatively weak in this region. Similarly, the dark current from the p-doped In 0.53 Ga 0.47 As layer  64  and the p-doped InP layer  68  is not significant because the electric field in these regions is low. 
   Due to the confined lateral extent W of the mesa  84 ′, the electric field  92  is confined below and within the mesa  84 ′. This design feature defines the photosensitive region of the intrinsic In 0.53 Ga 0.47 As layer  76 , that is, the area of the intrinsic In 0.53 Ga 0.47 As layer  76  containing the electric field  92 . 
   Referring to  FIG. 4 , one embodiment of a shallow mesa planar APD  120  according to the invention is shown. The structure and fabrication of the upper portion of the APD  120  is similar to the shallow mesa planar PIN photodiode  60  discussed for  FIG. 3 . In particular, the upper portion includes a top metal contact  8 , post-etching passivation regions  32 , a p-doped In 0.53 Ga 0.47 As layer  64 , a p-doped InP layer  68 , an intrinsic InP layer  72 , and an intrinsic In 0.53 Ga 0.47 As absorption layer  76 . 
   The lower portion of the APD photodiode  120  includes an intrinsic InAlGaAs layer  124 , a p-doped InAlAs layer  128 , an intrinsic InAlAs layer  132  latticed matched to InP, a n-doped InP layer  80 , and two bottom metal contacts  12 . The intrinsic InAlGaAs layer  124  is present for bandgap matching purposes. The p-doped InAlAs layer  128  is present to assist in the modulation of the electric field. The intrinsic InAlAs layer  132  provides a region of large electric field to drive the electron impact ionization avalanche process. This is achieved in the APD  120  because the ratio of the ionization coefficient for electrons relative to the ionization coefficient for holes for the intrinsic InAlAs layer  132  is large with respect to the ratios of the other layers  64 ,  68 ,  72 ,  76 ,  124 ,  128 ,  80 . 
   In alternative embodiments of the shallow mesa planar APD  120 , the thickness of the layers  64 ,  68 ,  72 ,  76  is varied as described above in Table 1. In an alternative embodiment, the wide bandgap intrinsic InP layer  72  is replaced with Indium/Aluminum/Arsenide (In x Al 1−x As) lattice matched to InP. In one embodiment, the shallow mesa planar APD  120  operates according to the principles of back illumination. In this embodiment, the photons in an incident optical beam pass through the n-doped InP layer  80  and into the intrinsic In 0.53 Ga 0.47 As absorption layer  76 . In various embodiments, the thickness of the layers  124 ,  128 ,  80  and their dopant concentrations are selected according to Table 2. 
   
     
       
             
             
             
           
             
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
               Layer 
               Thickness 
               Dopant Concentration 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               i-InAlGaAs layer 124 
               0.25 
               um 
               Not Applicable 
             
             
               InAlAs layer 128 
               0.2–0.5 
               um 
               1 × 10 18 –5 × 10 18  cm −3   
             
             
               i-InAlAs layer 132 
               0.2–0.5 
               um 
               Not Applicable 
             
             
               n-InP layer 80 
               0.5–1 
               um 
               1 × 10 18 –5 × 10 18  cm −3   
             
             
                 
             
           
        
       
     
   
   Referring to  FIG. 5  a flowchart representation of a method  145  for fabricating a shallow-mesa PIN photodiode according to the invention is shown. The method  145  includes generating an absorption narrow bandgap layer (step  150 ), for example in one embodiment intrinsic In 0.53 Ga 0.47 As, and generating a wide bandgap layer (step  155 ), for example in one embodiment intrinsic InP, substantially adjacent to the narrow bandgap layer. The method  145  also includes generating a first doped layer (step  160 ), for example in one embodiment p-doped InP, substantially adjacent to the wide bandgap layer. The first doped layer has a first conductivity type. The thickness of the first doped layer is determined in part according to the etching accuracy and is generally small compared to the other layers in order to minimize the carrier transit time increase introduced by its presence. The method  145  additionally includes etching a region of the first doped layer (step  165 ) and etching a region of the intrinsic wide bandgap layer (step  170 ). The method  145  further includes generating a passivation region (step  175 ) disposed substantially adjacent to the first doped layer and the intrinsic wide bandgap layer. In etching the intrinsic wide bandgap layer, the processes of step  170  are designed to ensure that an adequate thickness of the first doped layer remains to protect the absorption narrow bandgap layer from defects introduced during the passivation step  175 . 
   In one embodiment, the method  145  also includes generating a second doped layer (step  180 ) disposed substantially adjacent to the absorption narrow bandgap layer. In another embodiment, the method  145  also includes generating a third doped layer (step  185 ) disposed substantially adjacent to the first doped layer and adapted to form an ohmic contact with a substantially adjacent metalization layer. 
   Referring to  FIG. 6  a flowchart representation of a method  190  for fabricating a shallow-mesa APD according to the invention is shown. The operation of the steps  150  through  175  is as described above with respect to  FIG. 5 . The method shown in  FIG. 6  also includes generating a second doped layer (step  195 ) and generating an impact layer (step  200 ) disposed substantially adjacent to the second doped layer and the absorption narrow bandgap layer. The impact layer is chosen so that the ratio of the ionization coefficient for electrons relative to the ionization coefficient for holes for the impact layer is larger than the corresponding ratio for the absorption narrow bandgap layer, the wide bandgap layer, the first doped layer, and the second doped layer. In one embodiment, the method  190  also includes generating a third doped layer (step  205 ) disposed substantially adjacent to the first doped layer and adapted to form an ohmic contact with a substantially adjacent metalization layer. 
   Those skilled in the art will recognize that the PIN and APD structures in  FIGS. 3 and 4 , respectively, each represent only a single PIN and APD embodiment and that the principles of the invention can equally well be applied to alternative PIN and APD structures known in the art. 
   Having described and shown the preferred embodiments of the invention, it will now become apparent to one of skill in the art that other embodiments incorporating the concepts may be used and that many variations are possible which will still be within the scope and spirit of the claimed invention. These embodiments should not be limited to disclosed embodiments but rather should be limited only by the spirit and scope of the following claims.