The Impact of HCl Precleaning and Sulfur Passivation on the Al₂O₃/Ge Interface in Ge Metal-Oxide-Semiconductor Capacitors

Surface treatment for Ge substrates using hydrogen chlorine cleaning and chemical passivation are investigated on AuTi/Al₂O₃/Ge metal−oxide-semiconductor capacitors. After hydrogen chlorine cleaning, a smooth Ge surface almost free from native oxide is demonstrated by atomic force microscopy and x-ray photoelectron spectroscopy observations. Passivation using a hydrogen chlorine solution is found to form a chlorine-terminated surface, while aqueous ammonium sulfide pretreatment results in a surface terminated by Ge-S bonding. Compared with chlorine-passivated samples, the sulfur-passivated ones show less frequency dispersion and better thermal stability based on capacitance-voltage characterizations. The samples with HCl pre-cleaning and (NH₄)₂S passivation show less frequency dispersion than the HF pre−cleaning and (NH₄)₂S passivated ones. The surface treatment process using hydrogen chlorine cleaning followed by aqueous ammonium sulfide passivation demonstrates a promising way to improve gate dielectric/Ge interface quality.