We used photoluminescence, photoluminescence excitation spectroscopy (PLE) and photoreflectance (PR) to study the optical properties of thin films of cubic GaN, deposited by plasma-assisted molecular beam epitaxy on a GaAs (001) substrate. Our results show a clear step-like absorption edge, resulting from the merging of the free exciton with the continuum. Quantitative values for the absorption-edge energy and lifetime broadening are obtained. The dependence of the latter on temperature, as well as some features of the PR spectrum, reveal that the cubic material still presents residual strain and distortions. A secondary absorption-edge due to hexagonal inclusions is also observed in the PLE spectra. (C) 2003 Elsevier Science Ltd. All rights reserved. References: 16
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