Analysis of the red optical emission in cubic GaN grown by molecular-beam epitaxy

Abstract

The red optical emission band in cubic GaN is examined for the first time by cathodoluminescence as well as steady-state and time-resolved photoluminescence. We establish a close analogy between the red emission band in cubic GaN and the yellow emission in wurtzite GaN, and assign it to a donor-acceptor process involving a shallow donor and a deep acceptor with similar ionization energies in both phases. We show a range of properties of the red emission which clearly indicate its donor-acceptor pair origin, namely the blueshift with excitation intensity, the power-law evolution at increasing excitation, and the blueshift with temperature. The line shape of the red band indicates that the deep acceptor is strongly coupled to the lattice. The photoluminescence decay curves show a fast decay at higher energies within the emission band, while at lower energies the decay is much slower, typical of donor-acceptor pair transitions. An analytical expression for the time-dependent emission intensity for donor-acceptor pair transitions in which one of the centers is strongly coupled to the lattice is derived, extending the theory of Thomas et al. [Phys. Rev. B 140, A202 (1995)]. The observed decay curves are found to be well described by the theory using the donor Bohr radius of 1.7 nm, while temperature quenching of the emission gives an activation energy of 15\ifmmode\pm\else\textpm\fi{}5 meV. Evidence of Coulomb attraction between a shallow donor and a deep acceptor is presented.