Polarization doping in nitride based diodes

Abstract

A light emitting device comprising a three-dimensional polarization-graded (3DPG) structure that improves lateral current spreading within the device without introducing additional dopant impurities in the epitaxial structures. The 3DPG structure can include a repeatable stack unit that may be repeated several times within the 3DPG. The stack unit includes a compositionally graded layer and a silicon (Si) delta-doped layer. The graded layer is compositionally graded over a distance from a first material to a second material, introducing a polarization-induced bulk charge into the structure. The Si delta-doped layer compensates for back-depletion of the electron gas at the interface of the graded layers and adjacent layers. The 3DPG facilitates lateral current spreading so that current is injected into the entire active region, increasing the number of radiative recombination events in the active region and improving the external quantum efficiency and the wall-plug efficiency of the device.

I claim: 
     
         1 . A light emitting device comprising:
 a plurality of semiconductor layers;   an active region interposed between two of said semiconductor layers; and   a three-dimensional polarization-graded (3DPG) structure disposed on a first of said semiconductor layers and opposite said active region;   wherein said 3DPG structure comprises a graded layer compositionally graded from a first material to a second material over a grading distance.   
     
     
         2 . The light emitting device of  claim 1 , wherein said 3DPG structure comprises at least one repeatable stack unit and a terminal layer disposed such that said at least one stack unit is interposed between said first semiconductor layer and said terminal layer. 
     
     
         3 . The light emitting device of  claim 2 , said repeatable stack unit comprising:
 a graded layer that is compositionally graded from a first material to a second material over a grading distance, said graded layer disposed proximate to said first semiconductor layer such that said first material is closest to said first semiconductor layer; and   a delta-doped layer interposed between said graded layer and said first semiconductor layer.   
     
     
         4 . The light emitting device of  claim 3 , wherein said graded layer has a linear compositional grading. 
     
     
         5 . The light emitting device of  claim 3 , wherein said graded layer has a nonlinear compositional grading. 
     
     
         6 . The light emitting device of  claim 3 , said repeatable stack unit further comprising an n-type spacer layer disposed adjacent to said graded layer opposite said delta-doped layer. 
     
     
         7 . The light emitting device of  claim 6 , wherein said n-type spacer layer is uniformly doped. 
     
     
         8 . The light emitting device of  claim 3 , wherein said first material comprises indium gallium nitride of a first composition (In x Ga 1-x N) and said second material comprises indium gallium nitride of a second composition (In y Ga 1-y N), where x is greater than y and both x and y range between 0 and 1, inclusive. 
     
     
         9 . The light emitting device of  claim 8 , wherein x is about  0 . 1  and y is about  0 . 
     
     
         10 . The light emitting device of  claim 3 , wherein said repeatable stack unit is repeated 15 or more times within said 3DPG structure. 
     
     
         11 . The light emitting device of  claim 3 , wherein said graded layer is approximately 25 nm thick. 
     
     
         12 . The light emitting device of  claim 3 , wherein said delta-doped layer is approximately 1 nm thick. 
     
     
         13 . The light emitting device of  claim 3 , wherein said delta-doped layer is delta-doped with silicon (Si). 
     
     
         14 . The light emitting device of  claim 1 , wherein said first of said semiconductor layers is an n-type layer. 
     
     
         15 . The light emitting device of  claim 1 , wherein said 3DPG structure has a wurtzite crystal structure and is grown along the [000-1] crystal direction. 
     
     
         16 . A current spreading device, comprising:
 a graded layer compositionally graded from a first material to a second material over a grading distance; and   a delta-doped layer disposed adjacent to said graded layer, such that said first material is proximate to said delta-doped layer.   
     
     
         17 . The current spreading device of  claim 16 , wherein said graded layer has a bulk three-dimensional polarization-induced background charge. 
     
     
         18 . The current spreading device of  claim 16 , wherein a repeatable stack unit comprises said graded layer and said delta-doped layer. 
     
     
         19 . The current spreading device of  claim 16 , further comprising an n-type spacer layer disposed adjacent to said graded layer opposite said delta-doped layer such that said graded layer is interposed between said delta-doped layer and said spacer layer. 
     
     
         20 . The current spreading device of  claim 19 , wherein said graded layer and said spacer layer both comprise n-type semiconductor materials. 
     
     
         21 . The current spreading device of  claim 19 , wherein a repeatable stack unit comprises said graded layer, said delta-doped layer, and said spacer layer. 
     
     
         22 . The current spreading device of  claim 21 , wherein said repeatable stack unit is repeated  15 - 20  times within said current spreading device. 
     
     
         23 . The current spreading device of  claim 16 , wherein said first material comprises indium gallium nitride of a first composition (In x Ga 1-x N) and said second material comprises indium gallium nitride of a second composition (In y Ga 1-y N), where x is greater than y and both x and y range between 0 and 1, inclusive. 
     
     
         24 . The current spreading device of  claim 23 , wherein x is about 0.1 and y is about 0. 
     
     
         25 . The current spreading device of  claim 16 , wherein said graded layer is approximately 25 nm thick. 
     
     
         26 . The current spreading device of  claim 16 , wherein said delta-doped layer is approximately 1 nm thick. 
     
     
         27 . The current spreading device of  claim 16 , wherein said delta-doped layer is delta-doped with silicon (Si).