GaN thin films were grown via metalorganic vapor phase epitaxy on a-plane 4H-SiC substrates on which had been deposited an AlN buffer layer. Atomic force microscopy images revealed that the microstructure of the AlN buffer layer and the subsequently deposited GaN had a highly oriented growth structure where parallel growth features propagated in the [1–1.0] direction. Scanning electron microscopy showed that the interfaces between the substrate, buffer layer, and epi-layer were continuous. Cracking was observed in GaN films having a thickness greater than 800 nm. Plan-view transmission electron microscopy analysis revealed stacking faults and threading dislocations with densities of ∼1.6×106cm-1 and ∼ 3.3×1010cm-1, respectively. X-ray diffraction confirmed that the GaN was deposited epitaxially in the same orientation as the substrate. The average on- and off-axis x-ray full-width half-maxima of the (11.0) and the (10.0) reflections were 948 arcsec and 5448 arcsec, respectively.