Temperature-dependent optical band gap of the metastable zinc-blende structure β-GaN

Abstract

The temperature-dependent (10--300 K) optical band gap ${\mathit{E}}_{0}$(T) of the epitaxial metastable zinc-blende-structure \ensuremath{\beta}-GaN(001)4\ifmmode\times\else\texttimes\fi{}1 has been determined by modulated photoreflectance and used to interpret low-temperature photoluminescence spectra. ${\mathit{E}}_{0}$ in \ensuremath{\beta}-GaN was found to vary from 3.302\ifmmode\pm\else\textpm\fi{}0.004 eV at 10 K to 3.231\ifmmode\pm\else\textpm\fi{}0.008 eV at 300 K with a temperature dependence given by ${\mathit{E}}_{0}$(T) =3.302--6.697\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$${\mathit{T}}^{2}$/(T+600) eV. The spin-orbit splitting ${\mathrm{\ensuremath{\Delta}}}_{0}$ in the valence band was determined to be 17\ifmmode\pm\else\textpm\fi{}1 meV. The oscillations in the photoreflectance spectra were very sharp with a broadening parameter \ensuremath{\Gamma} of only 10 meV at 10 K. The dominant transition observed in temperature-dependent photoluminescence was attributed to radiative recombination between a shallow donor, at \ensuremath{\simeq}11 meV below the conduction-band edge and the valence band.