Method of fabricating Group III-V compound semiconductor devices using selective etching

Abstract

A solution of hydrogen peroxide H 2 O 2 !, citric acid HOC(CH 2 COOH) 2 COOH.H 2 O!, and a salt of citric acid such as potassium citrate HOC(CH 2 COOK) 2 COOK.H 2 O!, and hydrogen peroxide H 2 O 2 !, in a proper pH range, selectively etches GaAs-containing Group III-V compounds in the presence of other Group III-V compounds. As an illustration, Al y Ga 1-y As is selectively etched in the presence of Al x Ga 1-x As (0≦y<0.2 & x>0.2) when the pH range of the etchant solution is between approximately 3 and 6. The etchant solution described herein may be utilized in the fabrication of, for example, high-frequency transistors exhibiting improved saturated current (I dss ) and threshold voltage (V th ) uniformity.

What is claimed is: 
     
       1. A method of making a semiconductor device comprising the steps of: fabricating a structure having first and second Group III-V compound regions of differing composition, said first compound region including GaAs;   subjecting said structure to an etchant comprising a solution of a citrate buffer and H 2  O 2  with a pH in the range of approximately 3 to 6 so that said first region is selectively etched with respect to the second region, said citrate buffer including citric acid and a salt of citric acid, wherein the concentration . .by volume.!. .Iadd.in moles/liter .Iaddend.of hydrogen peroxide in said solution is greater than the concentration . .by volume.!. .Iadd.in moles/liter .Iaddend.of citric acid in said solution and greater than the concentration . .by volume.!. .Iadd.in moles/liter .Iaddend.of said salt of citric acid in said solution.   
     
     
       2. The method of claim 1 further including the step of formulating said etchant such that said concentration of H 2  O 2  is in the range of approximately 1-2.5 moles/liter when said concentration of citric acid is in the range of approximately 0.2-0.7 moles/liter. 
     
     
       3. The method of claim 2 further including the step of formulating said etchant such that said concentration of H 2  O 2  is in the range of approximately 1-2.5 moles/liter when said concentration of said salt of citric acid is approximately in the range 0.2-0.7 moles/liter. 
     
     
       4. The method of claim 1 further including the step of formulating said etchant such that: said concentration of said salt of citric acid in said solution is in the range of 0.2-0.7 moles/liter, and   one-half of the sum of said concentrations of said citric acid and said salt of citric acid is greater than 0.3 moles/liter and less than 1.0 moles/liter.   
     
     
       5. The method of claim 1 wherein said first compound region consists of Al y  Ga 1-y  As and said second compound region consists of Al x  Ga 1-x  As wherein (0≦y<0.2) and (0.2<x≦1.0). 
     
     
       6. A method of making a semiconductor device comprising the steps of: fabricating a structure by (i) growing one or more layers of the type X a  Y 1-a  As, where X is an atom selected from the group of IIIA atoms and Y is a different atom selected from the group of IIIA atoms, and where (0<a≦1) upon a semiconductor substrate, and   (ii) growing first and second Group III-V compound regions of differing composition upon said one or more layers, said first compound region including GaAs; and subjecting said structure to an etchant comprising a solution of a citrate buffer and H 2  O 2  with a pH in the range of approximately 3 to 6 so that said first region is selectively etched, said citrate buffer including citric acid and a salt of citric acid, wherein the concentration in .Iadd.moles/liter .Iaddend.of hydrogen peroxide in said solution is greater than the concentration .Iadd.in moles/liter .Iaddend.of citric acid in said solution and greater than the concentration .Iadd.in moles/liter .Iaddend.of said salt of citric acid in said solution.     
     
     
       7. The method of claim 6 wherein said second region comprises Al x  Ga 1-x  As where (0.2<x≦1), said second region being grown upon said one or more layers to a thickness selected in accordance with the value of x. 
     
     
       8. A method of making a field-effect transistor comprising the steps of: fabricating a structure by (i) growing one or more active channel layers comprised of Group III-V compounds upon a semiconductor substrate,   (ii) growing a thin etch stop region of Al x  Ga 1-x  As upon said active channel layers where (0.2<x≦1), and   (iii) growing a cap region including GaAs upon said thin etch stop region; and     subjecting said structure to an etchant comprising a solution of a citrate buffer and H 2  O 2  with a pH in the range of approximately 3 to 6 so that said cap region is selectively etched and said etch stop region prevents etching of said active channel layers, said citrate buffer including citric acid and a salt of citric acid, wherein the concentration .Iadd.in moles/liter .Iaddend.of hydrogen peroxide in said solution is greater than the concentration .Iadd.in moles/liter .Iaddend.of citric acid in said solution and greater than the concentration .Iadd.in moles/liter .Iaddend.of said salt of citric acid in said solution.   
     
     
       9. The method of claim 1 wherein said salt of citric acid comprises potassium citrate. 
     
     
       10. The method of claim 6 wherein said salt of citric acid comprises potassium citrate. 
     
     
       11. A method of making a semiconductor transistor device comprising the steps of: fabricating a structure having first and second active channel layers respectively comprised of first and second Group III-V compound regions of differing composition, said first compound region including GaAs and said second compound region being of a thickness of less than 100 angstroms;   subjecting said structure to an etchant comprising a solution of a citrate buffer and H 2  O 2  with a pH in the range of approximately 3 to 6 so that said first region is selectively etched with respect to the second region, said citrate buffer including citric acid and a salt of citric acid, wherein the concentration . .by volume.!. .Iadd.in moles/liter .Iaddend.of hydrogen peroxide in said solution is greater than the concentration . .by volume.!. .Iadd.in moles/liter .Iaddend.of citric acid in said solution and greater than the concentration . .by volume.!. .Iadd.in moles/liter .Iaddend.of said salt of citric acid in said solution.