Molecular Beam Epitaxy of Al-Polar Wurtzite AlN(0001) on β-Ga₂O₃(-201) Substrates

This study explores the challenging heteroepitaxial growth of wurtzite AlN on monoclinic β-Ga2O3(-201) using plasma-assisted molecular beam epitaxy. By optimizing various nucleation and growth conditions, particularly the Al/N flux ratio, we achieve optimal surface morphology and structural quality. Substrate nitridation growth under N-rich conditions is found to favor the formation of smooth AlN with a sharp nitride/oxide heterointerface, whereas Al-rich conditions lead to the formation of rougher AlN textured along the <0001> direction but with highly twisted grains. Comprehensive structural analyses show the growth of a high-quality AlN(0001) layer with a homogeneous Al polarity on β-Ga2O3(-201), exhibiting an epitaxial relationship of AlN[2-1-10] // β-Ga2O3[020]. The present findings, supported by theoretical calculations reporting the formation of a two-dimensional electron gas with a charge interface density higher than 1013 cm–2, open important perspectives for the development of next generation power electronic devices made of ultrawide band gap semiconductors.