| Summary: | 博士 === 國立交通大學 === 材料科學與工程學系 === 99 === Vertically aligned TiO2 nanotubes and TiO2 thin film are fabricated on p-type Si substrates by using atomic layer deposition system with anodic aluminum oxide template at 400 ?aC. The wall-thickness of TiO2 nanotubes and the thickness of TiO2 thin films can be controlled precisely by controlling the deposition cycle number. A polycrystalline anatase structure for TiO2 nanotubes and TiO2 thin film was confirmed by XRD and selected area diffraction pattern. The absorbability for TiO2 nanotubes and TiO2 thin film was improved as an increase of the film thickness. Due to the larger surface area, the fabricated TiO2 nanotubes exhibit an excellent performance on Photoluminescence characteristics, compared with TiO2 thin film.
Metal-semiconductor-Metal (MSM) structured Ti/TiO2/Ti detectors exhibited a highly thickness-dependent photoresponse due to the absorbability was enhanced as an increase of thickness. For the ITO/TiO2/Si diode, the photocurrent is controlled by either the TiO2/Si hetero-junction (p-n junction) or the ITO/TiO2 hetero-junction (Schottky contact), which is in the opposite direction. In short circuit, no matter positive or negative electrode applied on ITO, the TiO2/Si hetero-junction dominates the photocarrier transportation direction due to the larger space charge region and potential gradient. For photocurrent sources, the detail transfer process from TiO2/Si hetero-junction to ITO/TiO2 hetero-junction was examined in the time-depended photoresponse at the biases of 0 V to -1 V.
Schottky-contact and Ohmic-contact effects upon the photoresponses of ITO/TiO2/Si and Ti/TiO2/Si nanotube-based photodiodes were investigated. 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.
The hetero-junction effects on the photoresponses of TiO2 and TiO2/AAO nanotube-based photodiodes were investigated. Due to the space confinement effect, TiO2/AAO nanotubes revealed excellent PL characteristics, compared with TiO2 nanotubes. In short circuit, due to the recombination of photogenerated electron-hole pairs upon PL spectra, the performance of quantum efficiency in TiO2/AAO nanotubes was not splendid. It was improved for TiO2/AAO nanotubes on quantum efficiency by restraining the recombination in high field and the injecting the photogenerated electron from AAO depletion region.
For the nano-scale framework, the classical knowledge of energy band diagram was successfully used to explain the mechanism of the carrier transportation direction under UV on/off illumination. These works emphasize the equal importance of the hetero-junctions as compared with materials’ photoconductive properties.
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