Investigation of optical band gap and optical phonons in indium nitride and indium aluminum nitride films

We have studied the optical band gap in InN thin films and the optical phonon modes in the ternary alloy thin films, In_1−x Al_xN (0 ≤ x ≤ 1). The In_1−xAl _xN films with 0 ≤ x ≤ 1 were grown by Plasma Source Molecular Beam Epitaxy (PSMBE) on sapphire (0001) substrates. The optical properties of InN thin films (∼0.5 μm thick) grown at different substrate temperatures have been measured in order to study the effect of electron degeneracy on the band gap measurement. Hall measurements indicate that the films are n-type with a high carrier concentration (>1020 cm −3). The optical absorption data on these samples show n_e dependent band gap edge and a peak corresponding to plasmon due to strong electron degeneracy. By incorporating the influence of electron degeneracy on the optical absorption data, the calculated value of true band gap energy for these films is found to be ∼0.7 eV. This value is much smaller than the earlier reported values (∼1.9 eV) in the literature, but in better agreement with recent band gap measurements on high quality InN films. In addition, the analysis of the optical absorption data of a high quality non-degenerate InN film with n_e ∼7 × 1017 cm−3, obtained from Cornell University, results in an observed optical band gap energy ∼0.6 eV.; The role of plasmon and LO-phonon damping on the optical measurements of InN films is discussed. The phonon and plasmon damping dramatically modify the spectral features of the optical spectra and destabilize coupled-modes of the system. The phonon damping affects the optical properties in a qualitatively different way than plasmon damping. The low-energy optical transmission region formed in between the coupled-modes in weakly damped situations is investigated. The effect of plasmon and LO-phonon coupling on the analyses of the experimental infrared reflection data of InN films (with different electron densities) is discussed.; UV and visible near-resonance enhanced Raman scattering measurements in wurtzite In_1−xAl_xN (0 ≤ x ≤ 1) thin films are reported. The A₁(LO) and E ₂ zone center phonons have been observed for all the samples. The A₁(LO) phonon frequency shows the expected increase with increasing x. The E₂ mode shows a two-mode behavior supporting the recent theoretical predictions. Due to increased resonance enhancement, strong second- and third-order spectra are also seen in some films.