Near band-edge optical properties of cubic GaN with and without carbon doping

Recently cubic (c-) GaN/InxGa1-xN/GaN double heterostructures and quantum wells have been grown by MBE and MOCVD [1-3]. Contrary to their hexagonal counterparts, the cubic structures can be grown free from modulation due to spontaneous polarization and strain-induced piezoelectric fields. The spatial separation of the carriers wave function, induced by the quantum-confined Stark effect in the hexagonal phase, is avoided in the cubic structure. Thus, the cubic polytype is expected to increase the optical recombination efficiency in nitride-based quantum wells. Hence, the growth and characterization of high-quality c-GaN epitaxial layers is an essential first step on the road to high performance devices fabricated with this material. In the present work we report the results of a study of the optical properties of cubic GaN thin films, deposited by plasma-assisted molecular beam epitaxy on GaAs (001) substrates. We used photoluminescence (PL), photoluminescence excitation spectroscopy (PLE) and photoreflectance (PR) to study the spectral region near the fundamental absorption edge of these samples. The PLE experiments, carried out for the first time on these type of samples, allow the identification of the optical transitions taking place in the layers. We observe 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. A secondary absorption edge due to hexagonal inclusions is also observed in the PLE spectra. This secondary edge is strongly enhanced due to peculiarities of the PLE technique. This characteristic singles out PLE as a very sensitive tool to detect this type of sample imperfection. The temperature dependence of the absorption edge of the cubic material is measured with PR and analyzed with standard theoretical models. The results obtained by the different techniques are compared to one another and show a high degree of consistency.