Effects of growth direction and polarity on bulk aluminum nitride crystal properties

Abstract

Aluminum nitride (AlN) single crystals, approximately 25mm in diameter and up to 15mm thickness, were grown by the physical vapor transport (PVT) method in tungsten crucibles. To study the effect of growth direction and polarity, growth was performed using AlN seeds of very different orientations, i.e., Al-polar and N-polar basal and rhombohedral planes. For some growth runs, dual seeds consisting of two halves of switched polarity were used to compare growth under nearly identical settings. Specially roughened seed crystals were prepared to promote large voids in AlN crystals. Stable rhombohedral and (0001) c-plane facets form the Al-polar side of the void, whereas a wavy and less faceted surface occurs at the N-polar side. The growth rate on Al-polar rhombohedral facets is significantly lower compared to the one on the Al-polar basal facet. Regardless of initial seed orientation, the as-grown crystal surface always grows in Al-polar direction and appears to be composed of pyramids bounded mainly by rhombohedral (01−12) and (01−13) surfaces. Finally, the coloration of AlN crystal was found to depend on growth orientation: the material grown on (0001) facet is almost colorless, but when it is grown on rhombohedral facets it demonstrates characteristic amber color. The material grown on N-polar facets appears dark brown and opaque.