Control of Crystal Morphologies and Interface Structures of AlN Grown on Sapphire by Elementary Source Vapor Phase Epitaxy

Abstract

Elementary source vapor phase epitaxy (EVPE) is an environmentally friendly method for producing aluminum nitride (AlN) crystals from Al metal and N2 gas. Here, we demonstrate that the morphology of AlN grown by EVPE is predominantly affected by the molar flow ratio of the source materials (V/III ratio): high V/III ratios result in AlN film growth, whereas low ratios produce whiskers. These growth characteristics lead to self-separation of AlN films from sapphire, because the local V/III ratio at the AlN/sapphire interface is generally lower than the input V/III ratio due to sapphire decomposition, which generates mechanically weak whiskers at the interface. To avoid excessive separation, which causes rolling of the separated AlN film, we demonstrate that the pretreatment of sapphire with Al and N2, as well as addition of Al2O3 to the Al source, has beneficial effects on the film growth. This controlled interface formation improves the crystalline quality through spontaneous, nanometer-scale lateral overg...