Plasma-Enhanced Atomic Layer Deposition of Hematite for Photoelectrochemical Water Splitting Applications

Hematite (α-Fe ₂O ₃) is one of the most promising and widely used semiconductors for application in photoelectrochemical (PEC) water splitting, owing to its moderate bandgap in the visible spectrum and earth abundance. However, α-Fe ₂O ₃ is limited by short hole-diffusion lengths. Ultrathin α-Fe ₂O ₃ films are often used to limit the distance required for hole transport, therefore mitigating the impact of this property. The development of highly controllable and scalable ultrathin film deposition techniques is therefore crucial to the application of α-Fe ₂O ₃. Here, a plasma-enhanced atomic layer deposition (PEALD) process for the deposition of homogenous, conformal, and thickness-controlled α-Fe ₂O ₃ thin films (<100 nm) is developed. A readily available iron precursor, dimethyl(aminomethyl)ferrocene, was used in tandem with an O ₂ plasma co-reactant at relatively low reactor temperatures, ranging from 200 to 300 °C. Optimisation of deposition protocols was performed using the thin film growth per cycle and the duration of each cycle as optimisation metrics. Linear growth rates (constant growth per cycle) were measured for the optimised protocol, even at high cycle counts (up to 1200), confirming that all deposition is ‘true’ atomic layer deposition (ALD). Photoelectrochemical water splitting performance was measured under solar simulated irradiation for pristine α-Fe ₂O ₃ deposited onto FTO, and with a α-Fe ₂O ₃-coated TiO ₂ nanorod photoanode.