Atomic arrangement at the AlN/ZrB₂ interface

Low-dislocation-density GaN films (∼10⁸cm^-2) have been grown on closely lattice-matched ZrB₂ substrates by metalorganic vapor phase epitaxy using low-temperature AlN as a buffer layer. High-resolution electron microscopy images of the AlN/ZrB₂ interface region reveal that the AlN buffer layer does not grow directly on the ZrB ₂ substrate. Instead, the existence of an unintentional intermediate cubic-phase layer (approximately 2 nm thick) has been observed. Misfit dislocations are evident at both interfaces of the intermediate layer. Our analysis indicates that the intermediate layer has a lattice constant a=4.6Å, and that it is a ternary alloy of Zr_xB _yN_z, which should result from a transformation from the hexagonal phase of ZrB₂ due to interdiffusion of nitrogen and boron at the elevated temperature required for growth of GaN. This intermediate cubic-phase layer of Zr_xB_yN_z appears to have been so far unavoidable in the growth of high-quality GaN epilayers on ZrB ₂ substrates by our technique.