ANNEALING STUDIES OF LOW-TEMPERATURE-GROWN GAAS-BE

The isochronal and isothermal annealing characteristics of acceptor-doped GaAs:Be grown at low substrate temperatures (300-degrees-C) by molecular-beam epitaxy (LTMBE) have been studied. The Be was introduced in a range of concentrations from 10(16) to 10(19) cm-3. Electrical measurements of as-grown material up to the highest Be concentration of 10(19) cm-3 show that no free holes are contributed to the valence band even though Raman spectroscopy of the Be local vibrational mode indicates that the majority of the Be impurities occupy substitutional sites. It is proposed that Be acceptors are rendered inactive by the high concentration of As(Ga)-related native donor defects present in LTMBE material. The concentration of As(Ga)-related defects in the neutral charge state was estimated from infrared absorption measurements to be as high as 3 x 10(19) cm-3. A distinct annealing stage at 500-degrees-C, similar to that found in irradiation-damaged and plastically deformed GaAs, marks a rapid decrease in the concentration of As(Ga)-related defects. A second annealing stage near 800-degrees-C corresponds to the activation of Be acceptors. The presence of gallium vacancies V(Ga) was investigated by slow positron annihilation. Results indicate an excess concentration of V(Ga) in LTMBE layers over bulk-grown crystals. Analysis of isothermal annealing kinetics for the removal of As(Ga)-related defects gives an activation energy of 1.7 +/- 0.3 eV. The defect removal mechanism is modeled with V(Ga)-assisted diffusion of As(Ga) to As precipitates.