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 °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 CdCl ₂ 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.