PROPERTIES OF GALLIUM-ARSENIDE PURIFIED BY ZONE-REFINING AND ZONE LEVELING

Bulk GaAs purified in a vertical zone melt (VZM) process by zone refin ing and zone levelling has been evaluated with emphasis on those prope rties required for high energy gamma-ray spectrometers. Concentrations for the major electrically active centers: EL2 and the shallow accept ers C and Zn have been determined. Shallow acceptor concentrations hav e 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 concentratio ns exceeding that of carbon. Zone refining has established a distribut ion coefficient, k(Zn), of 0.6 for zinc. By contrast, the apparent dis tribution coefficient for carbon, k(C), was found to be in the range 0 .9 < k(C) < 1.1. This is substantially nearer 1 than the range of valu es found in other recent experiments. EL2, the dominant electrically a ctive defect in commercial LEC material, is present in crystals grown from stoichiometric melts with concentrations in the high 10(15) cm(-3 ) range. Gallium doping yields a further reduction in EL2 concentratio n but requires use of Ga with greater than 6-9s purity to avoid contam inating previously zone-refined material.