Interface formation mechanism of GaN on Al-pretreated ScAlMgO4 (0001) substrates

GaN layers are grown on Al-pretreated ScAlMgO4 (0001) substrates by metal-organic vapor-phase epitaxy (MOVPE) without using low-temperature (LT) buffer layers. The Al pretreatment is performed under a H2 and N2 gas mixture and results in the nucleation of AlN on the ScAlMgO4. High-resolution transmission electron microscopy reveals that the ScAlMgO4 (0001) surface is terminated with the rocksalt phase ScO (111). Because of the chemical nature of the ScO layer, the AlN nuclei are N-polar. However, subsequent GaN MOVPE inverts the polarity to metal-polar in the vicinity of the boundary between the interfacial AlN nucleation layer and GaN layer, which is attributed to the GaN surface during MOVPE being more stable when it is metal-polar. The grown GaN layers exhibit better crystalline quality compared with GaN grown on ScAlMgO4 or sapphire (0001) substrates using conventional LT-buffer technology, thereby demonstrating the promise of the method to produce high-quality GaN epilayers on ScAlMgO4.