Defects in GaAs grown by molecular-beam epitaxy at low temperatures:: stoichiometry, doping, and deactivation of n-type conductivity

We use a low-energy positron beam to study the influence of doping and stoichiometry on the native defects in GaAs grown by molecular-beam epitaxy at 250 degrees C. Ga vacancies are identified in all samples by measuring the momentum distribution of annihilating core electrons. The charge of V-Ga is negative in Si-doped samples but neutral in undoped and Be-doped material. We propose that the Ga vacancies are complexed with As antisites in undoped and Be-doped samples and with Si impurities in n-type material. The concentration of Ga vacancies depends on the doping and stoichiometry of growth conditions. It follows generally the trends in the V-Ga formation energy as a function of the Fermi level position and stoichiometry. The strong loss of free carriers in the As-rich Si-doped samples is attributed to the formation of Ga vacancy complexes, negative ion defects and inactive clusters of Si atoms. (C) 1999 American Institute of Physics. [S0021-8979(99)07116-9].