Infoscience

Journal article

Temperature dependence of hexagonal-GaN optical properties below the bandgap

First, spectroscopic ellipsometry (SE) is carried out at 300 K together with reflectivity measurements versus temperature from 4 to 300 K, in order to determine the temperature dependence of the refractive index of h-GaN films in the transparent region (350 to 600 nm). The SE measurements are carried out at two angles of incidence (72 degrees and 75 degrees) and the refractive index is well described with a Sellmeier's dispersion law. Secondly, the reflectivity related to the A, B and C excitons is detected and analysed within a multi-polariton model including the spatial dispersion. The temperature dependence of excitonic transition energies, broadening parameters and oscillator strengths are deduced between 4.5 and 250 K. The energy variation is analysed in terms of a Bose-Einstein expression. Concerning the behaviour of the broadening parameters at low temperature, the participation of either the acoustic phonons or the E-2 phonon mode to the exciton-phonon interaction is considered. The latter contribution is in better agreement with experimental data.

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