Ultrafast Electron Dynamics at the P-rich Indium Phosphide/TiO2 Interface

The current efficiency records for generating green hydrogen via solar water splitting are held by indium phosphide (InP)-based photo-absorbers, protected by TiO₂ layers grown through atomic layer deposition (ALD). InP is also a leading material for photonic integrated circuits and computing, where ultrafast near-surface behavior is key. A previous study described electronic pathways at the phosphorus-rich (P-rich) surface of p-doped InP(100) using time-resolved two-photon photoemission (tr-2PPE) spectroscopy. Here, the intricate electron pathways of the P-rich InP surface modified with ALD-deposited TiO₂ are explored. Photoexcited bulk InP electrons migrate through a bulk-to-surface transition cluster of states and surface states and inject into the TiO₂ conduction band (CB). Energy levels and occupation dynamics of CB states in P-rich InP and TiO₂ adlayers are observed, with discrete states preserved up to 10 nm TiO₂ deposition. Thermalization lifetimes of excited electrons > 0.8 eV above the InP conduction band minimum (CBM) are preserved for layer thicknesses up to 2.5 nm. Annealing at 300 °C to achieve crystalline TiO₂ reconstructions destroys interfacial states, affecting charge transfer. These observations enable innovative engineering of the P-rich InP/TiO₂ heterointerface, opening new possibilities for studying hot-carrier extraction, adsorbate effects, surface plasmons, and improving photovoltaic and PEC water-splitting devices.