The reversed valence band ordering of wurtzite AlN with respect to GaN leads to a very strong dependence of the lowest free excitonic transition energy on stress. The magnitude of strain depends on the epitaxial and thermal mismatch and can be influenced by the substrate choice, i.e. sapphire, SiC or silicon. So far, most studies focussed on the emission properties only, i.e. the analysis of photo- and cathodoluminescence (CL) spectra.

Here, we present for the first time a comprehensive study of the strain influence on the absorption properties, in particular on the energy spacing of the three valence bands at the center of the Brillouin zone and the corresponding oscillator strengths of the excitonic transitions. We compare the shape of the dielectric function (DF) of undoped and Si-doped AlN layers grown on different substrates (sapphire, SiC, Si). The DF from infrared (0.9 eV) up to the vacuum UV (9.8 eV) spectral range is obtained from spectroscopic ellipsometry studies. The use of synchrotron radiation at low temperatures provides superior spectral resolution around the fundamental band gap of about 6 eV. As a result of the valence band ordering, the influence of the extraordinary DF on the ellipsometry spectra of the C-plane layers is more pronounced than for other semiconductors (like GaN). In order to analyze the optical response, we employ an anisotropic optical model for the AlN layers. Its validity is confirmed by measurements of an A-plane AlN film grown on R-plane sapphire substrate.

The excitonic transition energies, extracted from the DF; and the relative oscillator strengths are consistent with the results of strain-dependent k^.p calculations for which the XRD-results serve as input parameters. Finally, the values for the lowest-energetic transition are in excellent agreement with the results of CL studies.

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