Systematic steps towards exactly stoichiometric and uncompensated CdTe Bridgman crystals

Abstract

A modified vertical Bridgman arrangement with Cd extra source and variable CdTe melt surface temperature is used to find out conditions for the growth of near stoichiometric CdTe crystals. Growth experiments were carried out with different temperature courses according to predictions for the optimum temperature program obtained from thermodynamic calculations. The transition point from p- to n-type conductivity for inclusion-free crystals was observed at a CdTe melt surface temperature of 1118°C and a Cd source temperature of 850°C. The incorporation of shallow acceptors (Ag, Cu) as a function of the deviation from stoichiometry during the growth was analysed by photoluminescence, mass spectroscopy and atomic absorption spectrophotometry. The incorporation coefficients of atoms substituting Cd were deduced in dependence on their total concentration in the melt and the Te excess. The maintenance of nearly stoichiometric growth conditions drastically reduces the substitutional impurity fraction, acting as shallow acceptors, and therefore the carrier concentration.