Termination of Ge surfaces with ultrathin GeS and GeS₂ layers via solid-state sulfurization

Reactions of Ge with S vapor, of interest as a potential approach for forming thin passivation layers on Ge surfaces, have been studied by photoelectron spectroscopy and Raman spectroscopy. Exposure of Ge(100) and Ge(111) to S drives the formation of Ge sulfide near-surface layers. At low temperatures, the reaction products comprise a thin GeS interlayer terminated by near-surface GeS₂. Above 400 °C, exposure to sulfur gives rise to single-phase GeS₂ layers whose thickness increases with temperature. Arrhenius analysis of the GeS₂ thickness yields an activation energy (0.63 ± 0.08) eV, close to the barrier that controls Ge oxidation by O radicals. XPS measurements after extended ambient exposure show a stable, ultrathin near-surface GeS₂ without significant oxidation, indicating that Ge-sulfides may provide an effective surface passivation for Ge surfaces.