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 10¹⁶ cm^-3 using 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 10¹⁷-10¹⁸ atoms/cm³ 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 >10¹⁵ cm^-3 for P and >10¹⁶ cm^-3 for As and Sb, compared with mid-10¹⁴ 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.