Direct probe of heterojunction effects upon photoconductive properties of TiO2 nanotubes fabricated by atomic layer deposition

This study investigated Schottky- and ohmic-contact effects upon the photoresponses of ITO/TiO2/Si and Ti/TiO2/Si nanotube-based photodiodes. The TiO2 tube arrays were fabricated by atomic layer deposition (ALD) and shaped by an anodic aluminum oxide (AAO) template on a p-type Si substrate. The contact area between the electrode (Ti or ITO) and the TiO2's tip was varied by tuning the tube's inner wall thickness with ALD, providing a direct and systematic probe of the heterojunction effects upon the photodiodes' responses. Results show that the Ti/TiO2/Si diode exhibits a highly thickness-dependent photoresponse. This is because the photocurrent is driven by the p-n junction at TiO2/Si alone and it faces no retarding at the ohmic contact of Ti/TiO2. For the ITO/TiO2/Si diode, the Schottky contact at ITO/TiO2 regulates photocurrent overriding TiO2/Si as a result of higher efficiency in photogeneration, leading to the opposite response compared with the Ti/TiO2/Si diode. Respective energy band diagrams are provided to support the statements above, and a consistent picture is obtained for both time response and quantum efficiency measurements.