Postgrowth thermal processing in the range of 1200-1400°C is shown to improve markedly the quality of thin (∼200 nm) AlN films grown by molecular beam epitaxy on SiC substrates. Comparison of both on-axis (0002) and off-axis (101̄2) x-ray diffraction peaks documents this improvement. Cross-sectional transmission electron micrographs confirm the reduction in dislocations and grain boundaries, while plan-view micrographs demonstrate that threading defect densities can be reduced to ∼3×108/cm2 after annealing. The thermal treatment is particularly effective because of the unusually large temperature window between the onset of a near-zero reactant sticking coefficient at ∼1200°C and AlN thermal decomposition at ∼1400°C. The Al sticking coefficient and the AlN decomposition rate are also reported.