Stoichiometric and pure Al2O3 gate dielectric films were grown on n-type 4H-SiC by a thermal atomic layer deposition process. The electrical properties of both amorphous and epitaxial Al2O3 films were studied by capacitance-voltage and current-voltage measurements of metal-oxide-semiconductor capacitors. A dielectric constant of 9 and a flatband voltage shift of +1.3 V were determined. A leakage current density of 10-3 A/cm2 at 8 MV/cm was obtained for the amorphous Al2O3 films, lower than that of any high-κ gate oxide on 4H-SiC reported to date. A Fowler-Nordheim tunneling mechanism was used to determine an Al2O3/4H-SiC barrier height of 1.58 eV. Higher leakage current was obtained for the epitaxial γ-Al2O3 films, likely due to grain boundary conduction.