Degenerate wurtzite InN films with electron concentrations in the range of 1018 cm-3 were studied by using spectroscopic ellipsometry (SE) in two spectral ranges. The analysis of the frequencies of the coupled phonon-plasmon modes in the mid-infrared region allows the determination of the plasma frequency and thus of the carrier density in the bulk of the samples. SE from the near-infrared to the visible range provides the dielectric function (DF) around the absorption edge.The electron concentration as an important input parameter ..._ enables accurate determination of Burstein–Moss shift and band-gap renormalization for the analysis of the imaginary part of the DF. Taking into account the in-plane strain of the films, which is caused by lattice mismatch between InN and buffer/substrate, we obtain a zero-density strain-free band gap of 0.675 eV at room temperature.
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