Electronic and vibrational states in InN and InxGa1−xN solid solutions

The review presents the results of optical studies of the fundamental physical characteristics of InN, the material which remains the least studied among nitrides of Group-III elements. The results of early optical studies of InN are analyzed and compared with recent data. New experimental facts reported in the review refer to hexagonal single-crystal epitaxial InN layers with an electron concentration of (1−2)×10 18 to 6×10 20 cm −3 , which are grown by molecular beam epitaxy (MBE) and metal-organic vapor-phase epitaxy (MOVPE) on Al 2 O 3 substrates. The aim of this review is to make a joint analysis of optical spectra (absorption, photoluminescence (PL), PL excitation, and photomodulated reflection) near the fundamental band gap. Furthermore, basic structural and electrical characteristics that have been obtained by a whole range of techniques are given for epitaxial layers of hexagonal InN. The principal result of recent studies is that the hexagonal InN crystal is a narrow-gap semiconductor with a band gap of 0.65–0.7 eV. Previously, the band gap of this material was considered to be 1.89 eV. It is shown that the Burstein-Moss effect accounts for the strong difference between the band gap and the optical absorption threshold in InN samples with a high concentration of electrons. The small value of the band gap of hexagonal InN is confirmed by optical studies of In x Ga 1− x N solid solutions at high concentrations of In. Theoretical calculations of the band structure of hexagonal InN crystals are briefly reviewed.