Heteroepitaxial growth of GaN on sapphire substrates by high temperature vapor phase epitaxy

This paper presents recent progress in the development of the high temperature vapor phase epitaxy (HTVPE) for a direct deposition of GaN layers on sapphire substrates and the impact of the improved deposition technique on the quality of the HTVPE GaN layers. This technique uses an improved HTVPE reactor for a better control of the growth process. The reactor contains a newly designed Ga evaporation cell made of refractory metals, which significantly reduces the concentration of process-induced impurities. As a result, the impurity concentrations in the HTVPE layers were reduced to or below 1016 cm−3. For a direct deposition of GaN on (0001)-oriented sapphire, a multi-step process was developed that includes the growth of a nucleation layer and a seed layer, which can be used as a template for further overgrowth with an increased growth rate to produce several ten µm thick GaN layers. The properties of the HTVPE layers are characterized by optical and scanning electron microscopy, glow discharge mass spectrometry, Raman spectroscopy, X-ray diffraction, and photoluminescence. The residual stresses and dislocation densities are presented for seed and overgrown layers, and compared with the properties of layers grown by established methods of chemical vapor phase epitaxy. In this context, challenges and prospects of the HTVPE method are discussed.