Ionization levels of As vacancies in as-grown GaAs studied by positron-lifetime spectroscopy

The properties of the native monovacancy defects are systematically investigated by positron-lifetime measurements in n-type GaAs with carrier concentrations of n=${10}^{15–}$ ${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$. The native defects present two ionization levels at ${\mathit{E}}_{\mathit{C}}$-30 meV and ${\mathit{E}}_{\mathit{C}}$-140 meV. The first corresponds to a charge transition 1-→0 and the second to 0→1+. The transitions are attributed to ionizations of As vacancy, which may be isolated or part of a complex. In a simple identification of the defect with ${\mathit{V}}_{\mathrm{As}}$, the ionization level at ${\mathit{E}}_{\mathit{C}}$-30 meV is attributed to the transition ${\mathit{V}}_{\mathrm{As}}^{\mathrm{-}}$→${\mathit{V}}_{\mathrm{As}}^0$ and the ionization level at ${\mathit{E}}_{\mathit{C}}$-140 meV to the transition ${\mathit{V}}_{\mathrm{As}}^0$→${\mathit{V}}_{\mathrm{As}}^{+}$. The results show further that the configuration of ${\mathit{V}}_{\mathrm{As}}^{\mathrm{-}}$ is strongly relaxed inwards compared to the structure of ${\mathit{V}}_{\mathrm{As}}^0$.