Optical phonon modes and interband transitions in cubic (formula presented) films

We present a comprehensive study of the phonon mode behavior and the optical interband transitions of cubic (formula presented) films (formula presented) using spectroscopic ellipsometry from the midinfrared to the vacuum-ultraviolet spectral range (0.05-8.5 eV). The (formula presented) layers were grown by radio-frequency plasma-assisted molecular-beam epitaxy and possess free-electron concentrations in the range of (formula presented) A two-mode behavior for the transverse-optical phonon of (formula presented) is observed, which is consistent with theoretical predictions. Due to the high free-electron concentration, the observed (formula presented) fundamental band-gap energy (formula presented) is subject to a strong Burstein-Moss shift and band-gap renormalization. We quantify the amount of both band-gap shifting mechanisms, and provide an estimate for the composition dependence of the (formula presented) band-gap energy (formula presented) which depends approximately linearly on the alloy composition with (formula presented) at room temperature and for carrier-depleted material. For cubic GaN, the L-point interband transition (formula presented) shifts to lower energies with increasing free-electron concentration, whereas the interband transition (formula presented) remains unshifted. Increasing Al content induces a blueshift (redshift) of the (formula presented) for (formula presented) Due to strong broadening effects, both transitions cannot be differentiated anymore for (formula presented).