It was recently demonstrated, that the quality of zincblende group-III nitrides can be considerably improved if bulk 3C-SiC(001) is used as the substrate for the deposition of the films by molecular beam epitaxy. For example, phase-pure cubic GaN and AlN have been achieved, and intersubband absorption of short-period GaN/AlN MQWs in the infra-red was demonstrated. Despite this progress, fundamental optical properties of these materials have not been reported so far. In this contribution, we present a comprehensive characterization of cubic AlN, GaN and related MQWs. The shape of the dielectric functions (DF) from 0.56 eV up to 9.8 eV, as obtained by ellipsometry, as well as photoreflectance (PR), photo- (PL) and cathodoluminescence (CL) spectra will be discussed in detail.

The DF of AlN for example shows pronounced features emphasizing the excellent quality of the material. In comparison to the hexagonal counterpart, the characteristic interband transitions are appreciably shifted to lower energies. In particular, we find a sharp excitonic onset of absorption at 5.88 eV, from which, taking into account the exciton binding energy, a lowest direct gap at 5.93 eV is obtained. From the pronounced absorption tail we estimate the indirect gap with about 5.2 eV. By low temperature CL we observe for the first time a clear signal originating unambiguously from the cubic AlN layer. Broad and unstructured defect related luminescence bands are detected above the energy position of the bandgap of 3C-SiC extending up to ~ 4.2 eV. Temperature-dependent PR and PL measurements yield an unambiguous indication for strain-induced heavy-light hole splitting in GaN.

Finally, interband transition energies of AlN/GaN MQWs for various well widths, as obtained by ellipsometry, PR, and PL are discussed. A comparison with theoretical calculations will be given.

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