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/TiO(2)/Si and Ti/TiO(2)/Si nanotube-based photodiodes. The TiO(2) 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 TiO(2)'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/TiO(2)/Si diode exhibits a highly thickness-dependent photoresponse. This is because the photocurrent is driven by the p-n junction at TiO(2)/Si alone and it faces no retarding at the ohmic contact of Ti/TiO(2). For the ITO/TiO(2)/Si diode, the Schottky contact at ITO/TiO(2) regulates photocurrent overriding TiO(2)/Si as a result of higher efficiency in photogeneration, leading to the opposite response compared with the Ti/TiO(2)/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.

Effect of geometric nanostructures on the absorption edges of 1-D and 2-D TiO2 fabricated by atomic layer deposition.

2-Dimensional (2-D) TiO2 thin films and 1-dimensional (1-D) TiO2 nanotube arrays were fabricated on Si and quartz substrates using atomic layer deposition (ALD) with an anodic aluminum oxide (AAO) template at 400 °C. The film thickness and the tube wall thickness can be precisely controlled using the ALD approach. The intensities of the absorption spectra were enhanced by an increase in the thickness of the TiO2 thin film and tube walls. A blue-shift was observed for a decrease in the 1-D and 2-D TiO2 nanostructure thicknesses, indicating a change in the energy band gap with the change in the size of the TiO2 nanostructures. Indirect and direct interband transitions were used to investigate the change in the energy band gap. The results indicate that both quantum confinement and interband transitions should be considered when the sizes of 1-D and 2-D TiO2 nanostructures are less than 10 nm.

The heterojunction effects of TiO2 nanotubes fabricated by atomic layer deposition on photocarrier transportation direction.

The heterojunction effects of TiO2 nanotubes on photoconductive characteristics were investigated. For ITO/TiO2/Si diodes, the photocurrent is controlled either by the TiO2/Si heterojunction (p-n junction) or the ITO-TiO2 heterojunction (Schottky contact). In the short circuit (approximately 0 V) condition, the TiO2-Si heterojunction dominates the photocarrier transportation direction due to its larger space-charge region and potential gradient. The detailed transition process of the photocarrier direction was investigated with a time-dependent photoresponse study. The results showed that the diode transitioned from TiO2-Si heterojunction-controlled to ITO-TiO2 heterojunction-controlled as we applied biases from approximately 0 to -1 V on the ITO electrode.