On the initial stages of AlN thin-film growth onto (0001) oriented Al2O3 substrates by molecular beam epitaxy

Abstract

Conventional and high-resolution transmission electron microscopy are used to characterize the initial stages of AlN thin-film growth. AlN films are deposited by molecular beam epitaxy onto annealed (0001) oriented α-Al2O3 (sapphire) substrates. During the initial stages of film growth (film thickness ∼25 nm) AlN forms islands of varying alignment with the Al2O3 substrate. Some of the AlN islands are well aligned with the [112̄0]AlN∥[101̄0] Al2O3 and (0001)AlN∥(0001)Al2O3, which matches closed-packed planes and directions. Other islands exhibit either an alignment of one set of planes, i.e., grains are aligned with the (11̄01)AlN∥(112̄0) Al2O3, or are misaligned with respect to the Al2O3 substrate. As the AlN film grows in thickness (film thickness ∼100 nm), the film becomes continuous, and the closed-packed planes and directions of the film and substrate are aligned for the majority of the film. Islands of AlN with an alignment other than this predominant orientation disturb the growth near the AlN/Al2O3 interface and create displacements along the [0001] AlN direction in overlying AlN grains. These misaligned AlN grains provide one source for the formation of planar defects in the epitaxial AlN films. The evolution of the AlN film microstructure and the reasons for the observed orientation relationships are discussed.