Optimizing Hot Electron Harvesting at Planar Metal–Semiconductor Interfaces with Titanium Oxynitride Thin Films

Understanding metal–semiconductor interfaces is critical to the advancement of photocatalysis and sub-bandgap solar energy harvesting where electrons in the metal can be excited by sub-bandgap photons and extracted into the semiconductor. In this work, we compare the electron extraction efficiency across Au/TiO₂ and titanium oxynitride (TiON)/TiO_2–x interfaces, where in the latter case the spontaneously forming oxide layer (TiO_2–x) creates a metal–semiconductor contact. Time-resolved pump–probe spectroscopy is used to study the electron recombination rates in both cases. Unlike the nanosecond recombination lifetimes in Au/TiO₂, we find a bottleneck in the electron relaxation in the TiON system, which we explain using a trap-mediated recombination model. Using this model, we investigate the tunability of the relaxation dynamics with oxygen content in the parent film. The optimized film (TiO_0.5N_0.5) exhibits the highest carrier extraction efficiency (N_FC ≈ 2.8 × 10¹⁹ m^–3), slowest trapping, and an appreciable hot electron population reaching the surface oxide (N_HE ≈ 1.6 × 10¹⁸ m^–3). Our results demonstrate the productive role oxygen can play in enhancing electron harvesting and prolonging electron lifetimes, providing an optimized metal–semiconductor interface using only the native oxide of titanium oxynitride.