The origins of the unintentional formation of polycrystalline AlN during homoepitaxial growth of single-crystal AlN layers by hydride vapor phase epitaxy (HVPE) on single-crystal AlN substrates prepared by physical vapor transport (PVT) were investigated. The formation of polycrystalline AlN in the HVPE layer should be suppressed since it may cause malfunctions in devices such as AlGaN-based deep-UV light emitting devices. Using electron probe microanalysis (EPMA) or transmission electron microscopy (TEM), the polycrystals were found to originate from contamination due to byproducts of the reaction between the source gases, and also from crystallographic defects originating from the AlN substrate.
