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 ZrxB yNz, 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 ZrxByNz appears to have been so far unavoidable in the growth of high-quality GaN epilayers on ZrB 2 substrates by our technique. © 2002 American Institute of Physics.