The native oxide and carbon-contaminant-minimized Ge surface with ammonium hydroxide (NH4OH)-based cleaning for high-k/Ge metal-oxide-semiconductor (MOS) gate stack application is reported. Furthermore, the carbon-free interfacial Ge-oxide layer preparation on the cleaned Ge surfaces was also studied. The thickness of GeO2 on Ge surface before, during, and after different cleaning processes was evaluated by the spectroscopic ellipsometry measurements. The HF rinsing step in the cyclic HF/deionized water cleaning was not effective for a removal of native GeO2 because it cannot form the soluble species by a chemical reaction in the low pH HF solution. A cyclic NH4OH-based cleaning results in a minimum residual GeO2 at 3 and 8 angstrom on Ge(111) and (100), respectively. The fast regrowth of GeO2 on cleaned Ge surfaces under air exposure was observed, regardless of the cleaning methods. However, Auger electron spectroscopy spectra showed less amount of carbon on Ge surfaces with NH4OH-based cleaning than HF-based cleaning. The small amount of residual carbon after NH4OH-based wet cleaning was completely removed by O-2 plasma for a very thin (similar to 10 angstrom) surface oxidation in the remote plasma-enhanced chemical vapor deposition chamber. (C) 2009 The Electrochemical Society. [DOI:10.1149/1.3212848] All rights reserved.