Abstract
This study investigated Schottky- and ohmic-contact effects upon the photoresponses ofITO/TiO2/Si andTi/TiO2/Si nanotube-basedphotodiodes. The TiO2 tube arrays were fabricated by atomic layer deposition (ALD) andshaped by an anodic aluminum oxide (AAO) template on a p-type Sisubstrate. The contact area between the electrode (Ti or ITO) and theTiO2’s tipwas varied by tuning the tube’s inner wall thickness with ALD, providing a direct and systematicprobe of the heterojunction effects upon the photodiodes’ responses. Results show that theTi/TiO2/Si diode exhibits a highly thickness-dependent photoresponse. Thisis because the photocurrent is driven by the p–n junction atTiO2/Si alone and it faces no retarding at the ohmic contact ofTi/TiO2. For theITO/TiO2/Si diode, the Schottkycontact at ITO/TiO2 regulatesphotocurrent overriding TiO2/Si as a result of higher efficiency in photogeneration, leading to the opposite response compared with theTi/TiO2/Si diode. Respective energy band diagrams are provided to support the statements above, anda consistent picture is obtained for both time response and quantum efficiencymeasurements.
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