ZnO and the related ternary ZnMgO alloys are known to exhibit a very high exciton binding energy and a strong polar character. Due to these properties, the characteristic features in their absorption spectra cannot be fitted alone by the exciton line shape suggested by Elliott. We present a comprehensive study of the so-called exciton-phonon absorption in ZnMgO alloys based on the analysis of the complex dielectric function (DF). Wurtzite Zn_1-xMg_xO films with Mg contents between x=0 and x=0.37 were grown on C-plane sapphire substrates by plasma-assisted molecular beam epitaxy using a MgO/ZnMgO buffer layer. High-resolution X-ray diffraction measurements indicated composition-independent in-plane lattice parameters, i.e. pseudomorphic growth of the films on the buffer. The optical properties of the films were investigated by temperature-dependent spectroscopic ellipsometry, yielding the DF, photoreflectance (PR) as well as photoluminescence (PL) spectroscopy. In order to determine the characteristic transition energies from the imaginary part of the DF, a model is applied which comprises three parts: free excitonic lines, the exciton continuum and enhanced absorption above the band gap due to exciton-phonon interaction. The fitted free excitonic transition energies are in excellent agreement with the PR data while the corresponding emission peak of the alloys is found at slightly lower energy indicating weak exciton localization. The broadening of the exciton-phonon contribution increases slightly with the Mg content but decreases with temperature. The analysis yields data on the compositional dependence of the Huang-Rhys parameter. The full DF analysis yields the band gaps with considerably improved reliability. In the final part, the influence of the in-plane tensile strain on the determined band gaps and the valence band ordering is discussed. Strain-free values are obtained from k·p calculations using XRD-results as input parameters.

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