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: ${\mathit{V}}_{\mathrm{As}}^{\mathrm{-}}$→${\mathit{V}}_{\mathrm{As}}^0$. 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 ${\mathit{V}}_{\mathrm{As}}^0$ and ${\mathit{V}}_{\mathrm{As}}^{\mathrm{-}}$. When compared to the negative charge state ${\mathit{V}}_{\mathrm{As}}^{\mathrm{-}}$, the neutral charge state ${\mathit{V}}_{\mathrm{As}}^0$ 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 ${\mathit{V}}_{\mathrm{As}}^{\mathrm{-}}$ has a smaller open volume than ${\mathit{V}}_{\mathrm{As}}^0$. This conclusion is in agreement with previous positron lifetime results and molecular-dynamics calculations.