The first observations of near-infrared interband optical transitions in InN were reported for samples with carrier concentrations close to 1*10¹⁹ cm^-3. Recent progress in the growth techniques has made it possible to grow n-InN films with improved characteristics. For example, the best samples are characterized by carrier concentrations of about 4*10¹⁷ cm^-3 and room temperature Hall mobilities higher than 2100 cm²/Vs.

We present results of the recent optical studies of the interband photoluminescence (PL) and absorption spectra of n-InN samples with Hall concentrations from 3.6*10¹⁷ to 6*10¹⁸ cm^-3. 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 to 0.67 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_da=0.050-0.055 eV and the other one is attributable 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_sh=0.005-0.010 eV and/or to the states of Urbach tail populated by photoholes. The second mechanism contributing to this band is the band-to-band recombination of free holes and electrons. Relative intensities of two higher energy PL peaks were found to be strongly dependent on temperature and excitation power.