Starved source diffusion for avalanche photodiode

Abstract

Starved source diffusion methods for forming avalanche photodiodes (APDs) are provided for controlling the edge effect. The edge effect is controlled by reducing edge gain near the edges of an APD active region. This is accomplished by creating a sloped diffusion front near the edges of the active region. The sloped diffusion front is advantageously formed in a single doping step by using a patterned mask during doping. The patterned mask reduces the depth to which dopants diffuse in areas where it only partly covers the underlying layer. By covering more of the underlying layer nearer the edge and progressively less towards the center, the sloped diffusion front is formed. The shallower diffusion depth near the edge reduces the edge gain, and therefore the edge effect. As a result, an APD to fiber misalignment is less likely, and possibility of edge breakdown is greatly reduced.

1 . A method for manufacturing an avalanche photodiode, comprising: 
 forming at least an absorber layer and an avalanche layer over a substrate;    forming a mask over a surface of the avalanche layer the mask comprising a pattern; and    depositing a dopant over the mask and the avalanche layer, wherein the dopant is blocked by the mask but diffuses into the avalanche layer in areas wherein the mask is not present, wherein the mask pattern gradually reduces the depth to which the dopant diffuses into the avalanche layer such that a sloped diffusion front is formed.    
     
     
         2 . A method as defined in  claim 1 , wherein the pattern comprises a series of concentric rings leaving grips between adjacent rings.  
     
     
         3 . A method as defined in  claim 2 , wherein the concentric rings and/or the gaps therebetween have varying widths such that the gaps cover a greater surface area closer to the active region and a lesser surface area closer to the edge.  
     
     
         4 . A method as defined in  claim 2 , wherein the space between each concentric ring comprises a width of from about 0.5 μm to about 10 μm.  
     
     
         5 . A method as defined in  claim 2 , wherein the number of concentric rings comprises at least 3.  
     
     
         6 . A method as defined in  claim 2 , wherein the number of concentric rings comprises at least 5.  
     
     
         7 . A method as defined in  claim 1 , wherein the absorber layer comprises InGaAs and the avalanche layer comprises InP.  
     
     
         8 . A method as defined in  claim 7 , wherein the dopant comprises zinc.  
     
     
         9 . A receiver optical subassembly comprising an avalanche photodiode formed by a method as defined in  claim 1 , wherein the avalanche photodiode further includes at least one guard ring defined about the sloped diffusion front.  
     
     
         10 . An optical transceiver comprising an avalanche photodiode formed by a method as defined in  claim 1 , wherein the avalanche further includes at least one guard ring defined about the sloped diffusion front.  
     
     
         11 . A method for manufacturing an avalanche photodiode comprising: 
 forming an absorber layer, a charge layer, and an avalanche layer over a substrate;    forming a mask over a surface of the avalanche layer such that the surface of the avalanche layer comprises: 
 a center region wherein no mask is present;  
 an outer region covered entirely by the mask; and  
 a transition region extending from the outer region to the center region such that the mask covers more of the surface area of the avalanche region closer to the outer region and less of the surface area or the avalanche region closer to the center region; and  
   depositing a dopant over the mask and the avalanche layer, wherein the dopant is blocked in the outer region, not blocked in the center region, and partially blocked in the transition region such that the dopant diffuses deeper in the portions of the transition region where less mask is present such that the deposited dopant forms a sloped diffusion front in the avalanche layer.    
     
     
         12 . A method as defined in  claim 11 , wherein the pattern comprises a series of concentric rings having gaps between adjacent rings.  
     
     
         13 . A method as defined in  claim 13 , wherein the concentric rings and/or the gaps therebetween have varying widths such that the gaps cover a greater surface area closer to the active region and a lesser surface area closer to the edge of the avalanche photodiode.  
     
     
         14 . A method is defined in  claim 12 , wherein the space between each concentric ring comprises a width of from about 0.5 μm to about 10 μm.  
     
     
         15 . A method as defined in  claim 12 , wherein the number of rings over the active region comprises at least 3.  
     
     
         16 . A method as defined in  claim 12 , wherein the number of rings over the active region comprises at least 5.  
     
     
         17 . A method as defined in  claim 11 , wherein the absorber layer comprises InGaAs and the avalanche layer comprises InP.  
     
     
         18 . A method as defined in  claim 17 , wherein the dopant comprises zinc.  
     
     
         19 . A receiver optical subassembly comprising an avalanche photodiode formed by the method defined in  claim 11 , wherein the avalanche photodiode further includes at least one guard ring defined about the sloped diffusion front.  
     
     
         20 . An avalanche photodiode, comprising: 
 a multilayer structure comprising a substrate, an absorber layer, a charge layer, and an avalanche layer, wherein an active region is defined in the multilayer structure;    a diffusion layer formed within the avalanche layer, the diffusion layer comprising a sloped diffusion front within the active region such that edge gain is reduced; and    at least one guard ring defined about the diffusion layer.    
     
     
         21 . An avalanche photodiode as defined in  claim 20 , wherein the absorber layer comprises InGaAs, the avalanche layer comprises InP, the charge layer comprises InP, and the dopant comprises zinc.  
     
     
         22 . An avalanche photodiode comprising: 
 an avalanche layer positioned over a substrate;    a diffusion region defined at least partially in the avalanche layer, the diffusion region defining a sloped diffusion front; and    at least one guard ring defined about the diffusion region.    
     
     
         23 . An avalanche photodiode, comprising: 
 an avalanche layer positioned over a substrate;    a diffusion region defining a diffusion front that is substantially sloped with respect to a top surface of the avalanche photodiode; and    at least one guard ring defined about the diffusion region.    
     
     
         24 . An avalanche photodiode as defined in  claim 23 , wherein the diffusion front slopes from a central portion to a perimeter of the diffusion region.