Optical identification of the gallium vacancy in neutron-irradiated gallium arsenide

Abstract

Gallium arsenide grown by the metallorganic-chemical-vapor-deposition method was irradiated at room temperature with fast neutrons in the fluence range ${10}^{12}$ to 3\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ (1.00 MeV equivalent fluence in gallium arsenide). The effects of neutron irradiation were studied by photoluminescence (PL) spectroscopy in the energy range 0.5--1.55 eV. In the samples irradiated to 3\ifmmode\times\else\texttimes\fi{}${10}^{13}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ and higher fluences, we observe a transition at 1.4745\ifmmode\pm\else\textpm\fi{}0.0003 eV in the PL spectrum recorded at 2 K. Its intensity increases with neutron fluence. Upon increasing the temperature at which the PL is recorded, the transition shifts to higher energies, and an additional higher-energy component appears. At 20 K, the energy of the doublet is 1.4750 and 1.4786\ifmmode\pm\else\textpm\fi{}0.0003 eV. The temperature dependence suggests that the low-energy component is a donor-acceptor pair and the high-energy one is a transition from the conduction band to the acceptor. We identify the acceptor as the gallium vacancy (${\mathit{V}}_{\mathrm{Ga}}$). We deduce its energy to be at 42.2\ifmmode\pm\else\textpm\fi{}0.3 meV above the valence band. Replicas involving the 37-meV LO phonon were observed. Isochronal annealing was performed on samples irradiated to 1\ifmmode\times\else\texttimes\fi{}${10}^{13}$ and 3\ifmmode\times\else\texttimes\fi{}${10}^{13}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. Samples which were irradiated and then annealed to 500 and 550 \ifmmode^\circ\else\textdegree\fi{}C exhibited the same doublet and their phonon replicas in the PL spectrum. This suggests that ${\mathit{V}}_{\mathrm{Ga}}$ is dissociated at 500 \ifmmode^\circ\else\textdegree\fi{}C from a complex formed during neutron irradiation. The effect of neutron irradiation and of isochronal annealing on other transitions observed by PL is presented. These transitions include the sharp transition at 0.702 eV, which has been associated with the arsenic antisite defect (${\mathrm{As}}_{\mathrm{Ga}}$), and the broad transition at approximately 1 eV. The modifications in the infrared PL spectrum are discussed in parallel with deep-level transient-spectroscopy data.