Al2O3 grown by Atomic Layer Chemical Vapour Deposition (ALCVD) on n-type 4H-SiC with a nominal thickness of 100nm has been characterized by Grazing Incidence X-Ray Diffraction (GIXD) and Specular X-Ray Reflectivity (SXR) measurements. After post-deposition, the samples were annealed at different temperatures and durations in argon atmosphere. The GIXD results reveal crystallization at temperatures above 900°C, most likely in the form of θ-Al2O3 or γ-Al2O3. However, the formation of a new, non-stoichiometric Al2O3 phase cannot be excluded. The crystalline domain size, evaluated from the peak FWHMs after subtraction of the instrumental broadening, is found to be almost equal (18±1nm), independent of T in the range 900°C≤T≤1100°C and time in the range 1h≤t≤3h. From SXR, mass density profiles are derived. Whereas the as grown film exhibits the lowest mass density, at 800°C a low-density interface layer forms. At the same time, it appears that the initial crystallization starts at the surface. At 900°C, the density increases sharply (this process involves film crystallization) and the film thickness correspondingly reduces. Whereas the density increase and thickness reduction still continue for T>900°C (tendency to the density α-Al2O3), the density of the interfacial layer has a minimum at 900°C and gradually increases for higher temperatures. From Atomic Force Microscopy (AFM) investigations it could be revealed that the starting of the crystallization at 900°C is accompanied with a substantial surface roughening. For annealing at higher temperatures, the surface roughness is in the range of the one of the as-grown sample (about 6Å).