Ultrafast Electron Dynamics at the Water-Modified InP(100) Surface

The interaction of water molecules with semiconductor surfaces is relevant to various optoelectronic phenomena and physicochemical processes. Despite advances in fundamental understanding of water-exposed surfaces, the detailed time- and energy-resolved behavior of excited electrons remains largely unexplored. Here, the effects of water exposure on the near-surface electron dynamics of phosphorus-terminated p(2×2)/c(4×2)-reconstructed indium phosphide (100) (P-rich InP) are studied experimentally and matched to theoretical calculations. The P-rich InP surface, consisting of H-passivated P-dimers, serves as a model for other P-containing III-V semiconductors such as gallium phosphide (GaP) or aluminum indium phosphide (AlInP). Electron dynamics near the surface are probed with femtosecond resolution using time-resolved two-photon photoemission (tr-2PPE), a pump-probe spectroscopic technique. Pulsed water exposure preserves electronic states and significantly increases lifetimes at the conduction band minimum (CBM). Density-functional theory (DFT) calculations attribute these findings to suppression of surface vibrational modes in the top P-layer by water exposure, reducing electronic transition probabilities of near-band-gap surface states. The results suggest that many near-surface state lifetimes reported in ultra-high vacuum may change significantly upon electrolyte exposure. These states may thus contribute more strongly to surface reactions than traditionally assumed. Demonstrating this effect for the technologically relevant P-rich InP surface opens new opportunities in this underexplored area of surface electrochemistry.