Properties of gallium arsenide purified by zone refining and zone levelling

Bulk GaAs purified in a vertical zone melt (VZM) process by zone refining and zone levelling has been evaluated with emphasis on those properties required for high energy gamma-ray spectrometers. Concentrations for the major electrically active centers: EL2 and the shallow acceptors C and Zn have been determined. Shallow acceptor concentrations have been determined by infrared spectroscopy including electronic 1s-2p and local vibrational mode absorption measurements. EL2 concentrations were determined from near-IR absorption. Zinc was found to be a major contaminant in commercial LEC material, often present in concentrations exceeding that of carbon. Zone refining has established a distribution coefficient, kZn, of 0.6 for zinc. By contrast, the apparent distribution coefficient for carbon, kC, was found to be in the range 0.9 < kC < 1.1. This is substantially nearer 1 than the range of values found in other recent experiments. EL2, the dominant electrically active defect in commerical LEC material, is present in crystals grown from stoichiometric melts with concentrations in the high 1015 cm−3 range. Gallium doping yields a further reduction in EL2 concentration but requires use of Ga with greater than 6–9s purity to avoid contaminating previously zone-refined material.