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 1016cm-3using group V substitution on Te is presented. Single-phase CdTe films doped with P, As, and Sb were deposited at 550 °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 1017-1018atoms/cm3in as-deposited films. Electronic activation was carried out by post-deposition annealing in Cd or CdCl2vapor with fast cooling, increasing acceptor concentrations to >1015cm-3for P and >1016cm-3for As and Sb, compared with mid -1014cm-3acceptor 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 AsTe and SbTe formation, respectively, which was validated by cathodoluminescence spectroscopy.