Acceptor states in the photoluminescence spectra ofn−InN

Abstract

Interband photoluminescence (PL) and absorption spectra of $n\text{−}\mathrm{In}\mathrm{N}$ samples with Hall concentrations from $3.6\times {10}^{17}\text{to}6\times {10}^{18}{\mathrm{cm}}^{-3}$ were studied. Sample thicknesses were in the range from $12\text{to}0.47\mu \mathrm{m}$. A set of lasers for the PL excitation in the energy range from 2.41 down to $0.81\mathrm{eV}$ was used. The well-resolved structure consisting of three peaks was observed in the PL spectra of the high-quality samples in the energy interval from $0.50\text{to}0.67\mathrm{eV}$ at liquid-helium and nitrogen temperatures. We attributed one of two low-energy features of the spectra to the recombination of degenerate electrons with the holes trapped by deep acceptors with a binding energy of $E_{\mathit{da}}=0.050–0.055\mathrm{eV}$, and the other one was attributed to the LO-phonon replica of this band. The higher-energy PL peak is considered as a complex band formed by two mechanisms. The first one is related to the transitions of electrons to the states of shallow acceptors with a binding energy of $E_{\mathit{sh}}=0.005–0.010\mathrm{eV}$ and/or to the states of the Urbach tail populated by photoholes. The second mechanism contributing to this band is the band-to-band recombination of free holes and electrons. The relative intensities of the two higher-energy PL peaks were found to be strongly dependent on temperature and excitation power. A model approach taking into account the Urbach tails of conduction and valence bands and the acceptor states was developed. The calculations of PL and the absorption spectra have shown that the band gap of InN in the limit of zero temperature and zero electron concentration is close to $0.665–0.670\mathrm{eV}$. The model calculations allowed us to explain the structures of all the spectra observed, their dependence on the excitation power, and the temperature variations of PL and the absorption spectra. The effective masses of electrons at the $\Gamma$ point equal to 0.042 and 0.07 of free-electron mass were tested in calculations. The conductivity band was assumed to be nonparabolic.