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

The electronic properties of native monovacancy defects were investiga ted 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 negativ e to neutral charge state: V(As)- --> V(As)0. By combining the results of positron lifetime and two-dimensional angular-correlation-of-annih ilation-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 vaca ncies are different from those of the bulk, and we also observed diffe rences between the distributions of V(As)0 and V(As)-. When compared t o the negative charge state V(As)-, the neutral charge state V(As)0 is characterized by a narrower momentum distribution from valence electr on annihilations and a lower high-momentum tail from core electrons. T hese findings lead us to conclude that the electronic and ionic struct ure of the As vacancies depend strongly on their ionization. We also i nfer that V(As)- has a smaller open volume than V(As)0. This conclusio n is in agreement with previous positron lifetime results and molecula r-dynamics calculations.