We performed temperature dependent current-voltage (I-V) measurements to characterize the electrical properties of Au/Al2O3/n-Ge metal-insulator-semiconductor (MIS) diodes prepared with and without H2O prepulse treatment by atomic layer deposition (ALD). By considering the thickness of the Al2O3 interlayer, the barrier height for the treated sample was found to be 0.61 eV, similar to those of Au/n-Ge Schottky diodes. The thermionic emission (TE) model with barrier inhomogeneity explained the final state of the treated sample well. Compared to the untreated sample, the treated sample was found to have improved diode characteristics for both forward and reverse bias conditions. These results were associated with the reduction of charge trapping and interface states near the Ge/Al2O3 interface.