Exciton spectra and electrical conductivity of epitaxial silicon-doped GaN layers

Optical spectra and electrical conductivity of silicon-doped epitaxial gallium nitride layers with uncompensated donor concentrations $N_D-N_A$ up to 4.8 $\times$ 10$^{19}$ cm$^{-3}$ at $T\approx$ 5 K have been studied. As follows from the current-voltage characteristics, at a doping level of $\sim$3 $\times$ 10$^{18}$ cm$^{-3}$ an impurity band is formed and an increase of donor concentration by one more order of magnitude leads to the merging of the impurity band with the conduction band. The transformation of exciton reflection spectra suggests that the formation of the impurity band triggers effective exciton screening at low temperatures. In a sample with $N_D-N_A$ = 3.4 $\times$ 10$^{18}$ cm$^{-3}$, luminescence spectra are still produced by radiation of free and bound excitons. In a sample with $N_D-N_A$ = 4.8 $\times$ 10$^{19}$ cm$^{-3}$, Coulomb interaction is already completely suppressed, with the luminescence spectrum consisting of bands deriving from impurity-band-valence band and conduction-band-valence band radiative transitions.