Method of reducing sodium concentration in a transparent conductive oxide layer of a semiconductor device

Abstract

A method of making a semiconductor device includes forming a semiconductor material stack having a sodium at an atomic concentration greater than 1×1019/cm3, depositing a transparent conductive oxide layer over the semiconductor material stack, such that sodium atoms diffuse from the semiconductor material stack into the transparent conductive oxide layer, and contacting a physically exposed surface of the transparent conductive oxide layer with a fluid to remove sodium from the transparent conductive oxide layer.

What is claimed is: 
     
         1 . A photovoltaic cell comprising:
 a substrate;   a first electrode disposed on the substrate;   a p-doped semiconductor layer disposed on the first electrode and comprising copper indium gallium selenide (CIGS) doped with an alkali metal;   an n-doped semiconductor layer disposed on the p-doped semiconductor layer; and   a second electrode disposed on the n-doped layer and comprising a transparent conductive oxide doped with the alkali metal at an atomic concentration of less than 7×10 19 /cm 3 .   
     
     
         2 . The photovoltaic cell of  claim 1 , wherein the n-doped semiconductor layer is doped with the alkali metal at an atomic concentration of greater than 1×10 20 /cm 3 . 
     
     
         3 . The photovoltaic cell of  claim 2 , wherein the atomic concentration of the alkali metal in the n-doped semiconductor is greater than 2×10 20 /cm 3 . 
     
     
         4 . The photovoltaic cell of  claim 3 , wherein the alkali metal comprises sodium. 
     
     
         5 . The photovoltaic cell of  claim 2 , wherein the atomic concentration of the alkali metal in the second electrode is less than ten times lower than an average alkali metal atomic concentration of the p-doped and n-doped semiconductor layers. 
     
     
         6 . The photovoltaic cell of  claim 1 , wherein a first portion of the p-doped semiconductor layer is disposed within about  100  nm of the n-doped semiconductor layer and comprises the alkali metal at an atomic concentration of greater than 1×10 20 /cm 3 . 
     
     
         7 . The photovoltaic cell of  claim 6 , wherein the alkali metal comprises sodium. 
     
     
         8 . The photovoltaic cell of  claim 1 , wherein the first electrode is doped with the alkali metal at an atomic concentration of greater than 3×10 20 /cm 3 . 
     
     
         9 . The photovoltaic cell of  claim 1 , wherein:
 the second electrode comprises has a first surface and an opposing second surface that faces the n-doped semiconductor layer; and   an atomic concentration of the alkali metal within 50 nm of the first surface is less than 6×10 19 /cm 3 .   
     
     
         10 . The photovoltaic cell of  claim 9 , wherein the alkali metal comprises sodium. 
     
     
         11 . A method of making a semiconductor device, comprising:
 forming a first electrode layer on a substrate;   forming a p-doped semiconductor layer on the first electrode, the p-doped layer comprising copper indium gallium selenide (CIGS) doped with an alkali metal;   forming an n-doped semiconductor layer disposed on the p-doped semiconductor layer; and   forming a second electrode layer on the n-doped semiconductor layer, the second electrode layer comprising a transparent conductive oxide doped with the alkali metal at an atomic concentration of less than 7×10 19 /cm 3 .   
     
     
         12 . The method of  claim 11 , further comprising contacting a selected one of the layers with a fluid to reduce an atomic concentration of the alkali metal therein. 
     
     
         13 . The method of  claim 12 , wherein the contacting the selected layer with the fluid comprises applying deionized water to a physically exposed surface of the selected layer, to remove the alkali metal atoms from the physically exposed surface, for a sufficient duration to permit alkali metal atoms to diffuse from inside the selected layer to the physically exposed surface. 
     
     
         14 . The method of  claim 13 , wherein the deionized water is applied by spraying or immersion. 
     
     
         15 . The method of  claim 13 , wherein the alkali metal is sodium. 
     
     
         16 . The method of  claim 12 , wherein the contacting the selected layer with the fluid occurs after the forming of the n-doped layer. 
     
     
         17 . The method of  claim 16 , wherein the selected layer is the second electrode layer. 
     
     
         18 . The method  claim 12 , further comprising annealing the substrate after forming at least one of the layers. 
     
     
         19 . The method of  claim 18 , wherein the annealing the substrate occurs before the contacting the selected layer with the fluid. 
     
     
         20 . The method of  claim 18 , wherein the annealing the substrate reduces a free carrier concentration in at least one of the layers.