Silicon protected with atomic layer deposited TiO2: conducting versus tunnelling through TiO2

Abstract

The present work demonstrates that tuning the donor density of protective TiO₂ layers on a photocathode has dramatic consequences for electronic conduction through TiO₂ with implications for the stabilization of oxidation-sensitive catalysts on the surface. Vacuum annealing at 400 °C for 1 hour of atomic layer deposited TiO₂ increased the donor density from an as-deposited value of 1.3 × 10¹⁹ cm⁻³ to 2.2 × 10²⁰ cm⁻³ following the annealing step. Using an Fe(II)/Fe(III) redox couple it was shown that the lower dopant density only allows electron transfer through TiO₂ under conditions of weak band bending. However it was shown that increasing the dopant density to 2.2 × 10²⁰ cm⁻³ allows tunneling through the surface region of TiO₂ to occur at significant band bending. An important implication of this result is that the less doped material is unsuitable for electron transfer across the TiO₂/electrolyte interface if the potential is significantly more anodic than the TiO₂ conduction band due to moderate to large band bending. This means that the lesser doped TiO₂ can be used to prevent the inadvertent oxidation of sensitive species on the surface (e.g. H₂ evolution catalysts) as long as the redox potential of the material is significantly more anodic than the TiO₂ conduction band. Conversely, for situations where an oxidative process on the surface is desired, highly doped TiO₂ may be used to enable current flow via tunneling.