Failure modes of protection layers produced by atomic layer deposition of amorphous TiO2 on GaAs anodes†
Amorphous titanium dioxide (a-TiO2) films formed by atomic layer deposition can serve as protective coatings for semiconducting photoanodes in water-splitting cells using strongly alkaline aqueous electrolytes. Herein, we experimentally examine the mechanisms of failure for p+-GaAs anodes coated with a-TiO2 films (GaAs/a-TiO2). Galvanic displacement of exposed GaAs by Au allowed imaging of pinholes in the a-TiO2 coatings, and enabled collection of quantitative and statistical data associated with pinhole defects. A combination of imaging, electrochemical measurements, and quantitative analyses of corrosion products indicated that extrinsic pinholes were present in the a-TiO2 films before electrochemical operation. During electrochemical operation these pinholes led to pitting corrosion of the underlying GaAs substrate. The dominant source of pinholes was the presence of atmospheric particulate matter on the GaAs surface during deposition of the a-TiO2 layer. The pinhole density decreased substantially when the thickness of the a-TiO2 coating increased beyond 45 nm, and approached zero when the thickness of the film exceeded 112 nm. The density of pinholes in films thinner than 45 nm decreased when the a-TiO2 coating was deposited in an environmentally controlled cleanroom. Pinhole-free GaAs/a-TiO2 devices were also tested via chronoamperometry to quantify the rate of pinhole formation during electrochemistry. The time-to-failure increased with thickness, suggesting that the failure mechanism may involve diffusion or migration through the film. However, other mechanisms may also contribute to the degradation of thicker films (>112 nm). Nevertheless, as previously hypothesized, extrinsic pinhole defects formed during deposition and testing control the short-term protective performance of the a-TiO2 film for GaAs anodes evolving O2 from water.