Oxidation-Induced Polymerization of InP Surface and Implications for Optoelectronic Applications

InP is among the most studied materials for energy-conversion applications including optoelectronics and photoelectrochemical devices. One of the long-standing challenges with this material, and III-V semiconductors more generally, is to understand and control surface oxide formation, which critically impacts device functionality, performance, and durability. We integrate advanced in situ ambient pressure X-ray photoelectron spectroscopy (APXPS) and ab initio simulations to reveal the mechanism of the oxidation process on the InP(001) surface. By interpreting the APXPS results through direct ab initio spectroscopic calculations of surface models, and by comparing calculated and measured work functions, we provide an unbiased picture of the chemical evolution of the thermal oxide. At low temperatures (<573 K), O₂ exposure leads to predominant formation of cross-linked PO_x units dispersed with submonolayer thickness, which grow into an amorphous two-dimensional (2D) film that is kinetically limited to the surface layer. Increased temperature (>573 K) leads to the polymerization of PO_x units and the formation of a complex, inhomogeneous 3D network of surface oxide that is progressively indium rich and phosphorus poor toward the surface. Finally, accelerated phosphorus loss via a hitherto unreported coupled charge-transfer isomer transformation mechanism leads to the formation of a thick, amorphous In₂O₃-like oxide at 773 K with very different optoelectronic and hot carrier transportation properties. In addition to unraveling complex mechanisms of surface oxidation, our results suggest the possibility of deliberately tuning oxide composition by leveraging competition between thermodynamic and kinetic factors.