Thick GaN layers grown by hydride vapor-phase epitaxy: Hetero- versus homo-epitaxy

In this paper, an overview will be given of the growth of thick GaN layers by hydride vapor-phase epitaxy. Two different kinds of substrates were used, that is MOCVD-grown GaN templates on sapphire and GaN single crystals. The layers grown on sapphire-based substrates suffer from the problem of cracking and pit formation. Although the morphology is not mirror-like, the optical and electrical quality of the material is excellent as demonstrated by photoluminescence and Hall-Van der Pauw measurements. The layers grown an Ga-polar GaN single crystals have almost perfect morphologies with only a very low density of pits. For the N-polar substrates the morphology is very rough, exhibiting the same features as are observed for the N-face MOCVD-grown GaN layers, both an sapphire and an N-face GaN single crystals.