Engineering Defects and Interfaces of Atomic Layer-Deposited TiO_x-Protective Coatings for Efficient III-V Semiconductor Photocathodes

III-V compound semiconductors offer optoelectronic properties that are well suited for the conversion of solar energy to chemical fuels. While such materials suffer from poor stability under photoelectrochemical (PEC) conditions, atomic layer deposition (ALD) of titanium oxide (TiO_x) has emerged as a powerful approach for creating corrosion protection layers, thereby enabling efficient and robust interfaces. However, the role of defects within TiO_x layers and at the semiconductor/TiO_x interface on the PEC performance remains poorly understood and controlled. Here, we use p-type InP as a model III-V semiconductor to investigate the impact of defects in ALD TiO_x on junction formation, interfacial charge transport, and photocarrier recombination, which underpin characteristics of PEC devices. We show that defect concentrations in TiO_x can be tuned over a broad range, resulting in significant modulation of the optical constants, electrical conductivity, and interface chemistry. While plasma-enhanced ALD yields films with low midgap-state concentrations, it introduces series resistance losses due to oxidation of the substrate. In contrast, thermal ALD suppresses interface oxidation but leads to electronically active defect states within the band gap of TiO_x. By controlling these defect states, the nature of junction formation can be tuned, and high photovoltage photocathodes can be achieved. In particular, ALD TiO_x layers possessing high carrier concentrations form buried InP/TiO_x pn heterojunctions, whereas less defective layers preserve semiconductor/electrolyte junction energetics to achieve large photovoltages and applied bias photon-to-current efficiencies. These results highlight the power of ALD for engineering photoelectrode interfaces and provide a new route for tailoring the junction formation between buried and PEC junctions.