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Fermi level (FL) pinning at the Ge valence band results in a high Schottky barrier height for all metal/n-Ge contacts. The origin of this pinning effect has been ascribed to either metal induced gap states or surface states arise from the native defects at the Ge surface, such as dangling bonds. The discrepancy in the reported results/explanations is mainly due to the lack of an explicit characterization of a high quality metal/Ge or metal/ultrathin oxide/Ge junction, which should be ideally single crystalline, atomically smooth and free of process-induced defects or intermixing. We report the Schottky characteristics of high quality metal/MgO/n-Ge junctions with the ultrathin MgO epitaxially grown on Ge. We find the depinning effect displays a weak dependence on the MgO thickness, indicating the interface states due to the native defects on Ge surface are likely to play the dominant role in FL pinning.