POSITRON-ANNIHILATION STUDIES OF NEUTRAL AND NEGATIVELY CHARGED AS VACANCIES IN GAAS

The electronic properties of native monovacancy defects were investigated using techniques based on positron annihilation in n-type Si-doped GaAs. The positron lifetime measurements as a function of temperature have shown the thermal ionization of native As vacancies from negative to neutral charge state: V(As)- --> V(As)0. By combining the results of positron lifetime and two-dimensional angular-correlation-of-annihilation-radiation (2D-ACAR) experiments, the 2D-ACAR distributions of neutral and negative arsenic vacancies were separated from the partial bulk contribution. We found that 2D-ACAR distributions of the As vacancies are different from those of the bulk, and we also observed differences between the distributions of V(As)0 and V(As)-. When compared to the negative charge state V(As)-, the neutral charge state V(As)0 is characterized by a narrower momentum distribution from valence electron annihilations and a lower high-momentum tail from core electrons. These findings lead us to conclude that the electronic and ionic structure of the As vacancies depend strongly on their ionization. We also infer that V(As)- has a smaller open volume than V(As)0. This conclusion is in agreement with previous positron lifetime results and molecular-dynamics calculations.