We identify and discuss the essential strategies for the growth of (Al,Ga)N/GaN and GaN/(In,Ga)N heterostructures on SiC(0001) by both plasma-assisted and reactive molecular-beam epitaxy. Substrate preparation, nucleation, and growth conditions are optimized for simultaneously satisfying the requirements of high structural, morphological, optical, and electrical quality. The results demonstrate that molecular-beam epitaxy is a competitive technique for the growth of group-III nitrides.

GaAsSb bandgap, surface fermi level, and surface state density studied by photoreflectance modulation spectroscopy Appl.The photoreflectance signal from GaN films is sensitive to the ambient medium. A large decrease in the photoreflectance amplitude is observed, when the ambient medium is changed from air to vacuum. This effect is attributed to ultraviolet-light-induced desorption of oxygen from the sample surface leading to a reduction of the surface barrier height. The effect is absent, when a thin Ti layer is deposited on top of the GaN film. A simple model is used to demonstrate that the surface photovoltage can be strongly reduced with a decrease of the surface barrier height.

We investigate the quality of GaN layers directly grown on 6H–SiC(0001) substrates by reactive molecular-beam epitaxy. Despite a pure three-dimensional nucleation, step-flow growth is achieved by in situ adjusting conditions such that the (2×2) reconstruction observed during growth is maximized in intensity. The resulting surface morphology exhibits large terraces separated by mono- and multiatomic steps, and is clearly superior to that obtained by plasma-assisted growth. Furthermore, the structural and optical properties of these layers are comparable to those of layers grown by plasma-assisted molecular-beam epitaxy.

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We study, both experimentally and theoretically, the influence of polarization-induced electric fields on the optical properties of heavily doped (7ϫ10 18 cm Ϫ3 ) GaN/͑Al,Ga͒N multiple-quantum-well structures. To investigate the impact of the strain state on the transition energy, these heterostructures are deposited on either a GaN or an ͑Al,Ga͒N relaxed buffer layer. Furthermore, we show that the recombination dynamics in these heavily doped multiple quantum wells is still controlled by residual electric fields, contrary to the common assumption that flatband conditions are established at this doping level.