We discuss the conditions to be satisfied for the growth of cubic GaN films on GaAs(001) and Si(001) by plasma-assisted MBE. A stoichiometric group V/III flux ratio is required for single-phase growth, which is controlled via the surface reconstruction in real time by means of RHEED. While phase purity of cubic GaN-on-GaAs is induced by a well-defined orientation relationship, the growth on Si(001) often results in a phase transition due to the formation of amorphous SixNy, inclusions at the GaN-on-Si interface, which act as nucleation centers for the growth of the hexagonal GaN phase. A suitable template, such as a thin GaAs or SiC insertion layer, prevents the formation of the SixNy inclusions. Cubic GaN grown under optimized conditions exhibit a narrow excitonic luminescence line at 3.272 eV at 5 K and high luminescence efficiency at room temperature. In addition, we discuss the origin of the n-type background doping in the as-grown layers and we present the new concept of codoping to achieve high p-type conductivity in GaN at room temperature.

We summarize our results on plasma-assisted molecular beam epitaxy of cubic GaN on GaAs(001) and on Si(001) and of hexagonal GaN on 6H-SiC(0001) with emphasis on the nucleation process. A two-step growth sequence must be used to optimize and control the nucleation and the subsequent growth independently. While a perfect epitaxial orientation exists for GaN-on-GaAs due to the coincidence lattice relationship of the two constituents. The same effect is impeded for GaN-on-Si by the growth of SixNy inclusions at the interface which act as nucleation cores for the formation of the hexagonal GaN phase. A suitable template, such as a thin GaAs or SiC insertion layer, avoids formation of the SixNy inclusions. Finally, growth of hexagonal GaN-on-6H-SiC without any buffer layer requires very careful adjustment of the N-to-Ga flux ratio and the substrate temperature, independently for the nucleation stage and for the subsequent layer-by-layer growth. The structural perfection and the optical properties of the resulting 1 μm thick GaN films then reach state-of-the-art quality even without a buffer template.