Thick pseudomorphic nitride epitaxial layers

Abstract

In various embodiments, a semiconductor device includes an aluminum nitride single-crystal substrate, a pseudomorphic strained layer disposed thereover that comprises at least one of AN, GaN, InN, or an alloy thereof, and, disposed over the strained layer, a semiconductor layer that is lattice-mismatched to the substrate and substantially relaxed.

What is claimed is: 
     
         1 .- 20 . (canceled) 
     
     
         21 . A light-emitting device comprising:
 a single-crystal aluminum nitride substrate having opposed top and bottom surfaces;   disposed over the top surface of the substrate, a bottom contact layer comprising doped Al n Ga 1-n N n being less than 1;   disposed over the bottom contact layer, a multiple-quantum well layer comprising a plurality of periods each comprising a strained Al x Ga 1-x N barrier and a strained Al y Ga 1-y N quantum well, wherein (i) x and y are different by an amount facilitating confinement of electrons and holes in the multiple-quantum well layer and (ii) y is selected such that an emission wavelength of the light-emitting device is in the range of 210 nm to 320 nm;   disposed over the multiple-quantum well layer, an electron-blocking layer comprising Al u Ga 1-u N;   disposed over the electron-blocking layer, a cap layer comprising Al z Ga 1-z N;   disposed directly on the cap layer, a metallic contact layer; and   disposed between the electron-blocking layer and the cap layer, a strained graded layer comprising Al w Ga 1-w N, wherein a composition of the strained graded layer is graded over its thickness from a value of w=u at an interface between the electron-blocking layer and the strained graded layer to a value of w=z at an interface between the cap layer and the strained graded layer, such that the strained graded layer is less than 20% relaxed.   
     
     
         22 . The device of  claim 21 , wherein the bottom contact layer is pseudomorphically strained with respect to the substrate, such that the bottom contact layer is less than 20% relaxed to its innate lattice parameter. 
     
     
         23 . The device of  claim 21 , wherein a thickness of the bottom contact layer is less than 200 nm. 
     
     
         24 . The device of  claim 21 , wherein a thickness of the strained graded layer ranges between approximately 10 nm and approximately 50 nm. 
     
     
         25 . The device of  claim 21 , wherein a thickness of the strained graded layer is approximately 30 nm. 
     
     
         26 . The device of  claim 21 , wherein z=0. 
     
     
         27 . The device of  claim 21 , wherein a thickness of the electron-blocking layer ranges between approximately 10 nm and approximately 50 nm. 
     
     
         28 . The device of  claim 21 , wherein the electron-blocking layer is doped with a conductivity opposite that of the bottom contact layer. 
     
     
         29 . The device of  claim 21 , wherein the cap layer is doped with a conductivity opposite that of the bottom contact layer. 
     
     
         30 . The device of  claim 21 , wherein the strained graded layer is doped. 
     
     
         31 . The device of  claim 21 , further comprising a homoepitaxial AlN layer disposed between the top surface of the substrate and the bottom contact layer. 
     
     
         32 . The device of  claim 21 , further comprising, disposed between the top surface of the substrate and the bottom contact layer, a second strained graded Al m Ga 1-m N layer, the composition of the second strained graded Al m Ga 1-m N layer being graded over its thickness from a value of m being approximately equal to 1 to a value of m being approximately equal to n, wherein the second strained graded Al m Ga 1-m N layer is pseudomorphically strained with respect to the substrate, such that the second strained graded Al m Ga 1-m N layer is less than 20% relaxed. 
     
     
         33 . The device of  claim 21 , wherein x and y differ by an amount ranging from approximately 0.05 to approximately 0.3. 
     
     
         34 . The device of  claim 21 , wherein the cap layer is lattice-mismatched to the substrate and substantially relaxed to its unstrained lattice constant. 
     
     
         35 . The device of  claim 21 , further comprising an array of misfit dislocations disposed at an interface between the cap layer and the strained graded layer. 
     
     
         36 . The device of  claim 21 , wherein the metallic contact layer comprises an ultraviolet reflector for redirecting light emitted by the multiple-quantum well layer toward the substrate. 
     
     
         37 . The device of  claim 21 , wherein the cap layer comprises a plurality of coalesced islands. 
     
     
         38 . The device of  claim 21 , wherein a thickness of the cap layer ranges between approximately 10 nm and approximately 50 nm. 
     
     
         39 . The device of  claim 21 , wherein the cap layer is pseudomorphically strained, such that the cap layer is less than 20% relaxed to its innate lattice parameter.