Al2O3 films with different thicknesses have been deposited on n-type (nitrogen-doped) 4H-SiC(0001) epitaxial samples by atomic layer chemical vapor deposition at 300 °C and subsequently annealed in Ar atmosphere at temperatures up to 1000 °C. The Al2O3/4H-SiC structures were analyzed by x-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and transmission electron microscopy (TEM). The XPS and SIMS results indicate that the average composition in the wider interface area does not significantly change due to the annealing. However, as revealed by the TEM investigations in combination with XPS, the as-grown samples exhibit a double interface created by an intermediate suboxide SiOx layer (x<2). After annealing, this intermediate suboxide layer breaks up and transforms into SiO2 islands, resulting in a rather rough interface region and a high concentration of pure Si in the Al2O3 film. Furthermore, a pronounced accumulation of H is found in the rough interface region and this may play a key role for the low density of shallow electron states reported for annealed Al2O3/4H-SiC structures.