Absorption, Reflectance, and Luminescence of GaN Epitaxial Layers

Low-temperature absorption, reflectance, and emission spectra from a wide range of epitaxially grown GaN are reported. The best samples, which are far superior to any previously available, have spectral details that allow a definitive analysis. The reflectance results prove that GaN is a direct-gap semiconductor with its lowest exciton state at 3.474 \ifmmode\pm\else\textpm\fi{} 0.002 eV at 2 K. The absorption and luminescence spectra corroborate this observation. Crystal field and spin-orbit splittings have been resolved. The order of the valence-band states is ${\ensuremath{\Gamma}}_{9}>{\ensuremath{\Gamma}}_{7}>{\ensuremath{\Gamma}}_{7}$ with parameters ${\ensuremath{\Delta}}_{\mathrm{cr}}=22\ifmmode\pm\else\textpm\fi{}2$ meV and ${\ensuremath{\Delta}}_{\mathrm{so}}={11}_{\ensuremath{-}2}^{+5}$ meV. A correlation of these results with the emission data has led us to an interpretation in which we associate emission features with LO-phonon-assisted free-exciton decay, with bound-exciton decay at both donor and acceptor sites, and with distant donor-acceptor pair recombination. Binding energies derived from these associations are consistent with values obtained from independent experiments reported earlier. The internal consistency of the absorption, reflectance, and luminescence results provides strong support for our interpretation.