title: Direct probe of heterojunction effects upon photoconductive properties of TiO2 nanotubes fabricated by atomic layer deposition.
abstract: 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.
authors.affiliation: Department of Materials Science and Engineering, National Chiao Tung University, Hsin-chu, Taiwan, Republic of China.
authors.forename: Yung-Huang
authors.initials: YH
authors.lastname: Chang
authors.forename: Chien-Min
authors.initials: CM
authors.lastname: Liu
authors.forename: Yuan-Chieh
authors.initials: YC
authors.lastname: Tseng
authors.forename: Chih
authors.initials: C
authors.lastname: Chen
authors.forename: Chia-Chuan
authors.initials: CC
authors.lastname: Chen
authors.forename: Hsyi-En
authors.initials: HE
authors.lastname: Cheng