The effect of room temperature sulfur passivation of the surface of Ge0.83Sn0.17 prior to high-kdielectric (HfO2) deposition is investigated. X-ray photoelectron spectroscopy(XPS) was used to examine the chemical bonding at the interface of HfO2 and Ge0.83Sn0.17. Sulfur passivation is found to be effective in suppressing the formation of both Ge oxides and Sn oxides. A comparison of XPS results for sulfur-passivated and non-passivated Ge0.83Sn0.17 samples shows that sulfur passivation of the GeSn surface could also suppress the surface segregation of Sn atoms. In addition, sulfur passivation reduces the interface trap density Dit at the high-k dielectric/Ge0.83Sn0.17 interface from the valence band edge to the midgap of Ge0.83Sn0.17, as compared with a non-passivated control. The impact of the improved Dit is demonstrated in Ge0.83Sn0.17 p-channel metal-oxide-semiconductor field-effect transistors (p-MOSFETs). Ge0.83Sn0.17 p-MOSFETs with sulfur passivation show improved subthreshold swing S, intrinsic transconductance Gm,int, and effective hole mobilityμeff as compared with the non-passivated control. At a high inversion carrier density Ninv of 1 × 1013 cm−2, sulfur passivation increases μeff by 25% in Ge0.83Sn0.17 p-MOSFETs.