Enhanced p-Type Doping in Polycrystalline CdTe Films: Deposition and Activation

An in situ nonequilibrium method to increase hole density in polycrystalline CdTe thin films to 10(16) cm(-3) using group V substitution on Te is presented. Single-phase CdTe films doped with P, As, and Sb were deposited at 550 degrees C at 100-200 nm/s onto moving cadmium sulfide/high resistance transparent buffer layer/transparent conductive oxide /glass superstrates by vapor transport deposition in Cd overpressure from high purity compound sources. Doping levels before and after activation were determined by capacitance-voltage analysis of diagnostic devices. Secondary ion mass spectrometry depth profiling confirmed dopant incorporation levels of 10(17)-10(18) atoms/cm(3) in as-deposited films. Electronic activation was carried out by post-deposition annealing in Cd or CdCl2 vapor with fast cooling, increasing acceptor concentrations to > 10(15) cm(-3) for P and > 10(16) cm(-3) for As and Sb, compared with mid -10(14) cm(-3) acceptor levels for undoped CdTe films. The activation methods are compatible with post-deposition processing presently used for high-efficiency CdTe solar cells. For the dopants As and Sb, the acceptor concentration increased by substitutional As-Te and Sb-Te formation, respectively, which was validated by cathodoluminescence spectroscopy.