Photoconductivity in ZnO nanowires
Date Issued
2010
Date
2010
Author(s)
Chen, Ming-Wei
Abstract
In this thesis, the mechanisms of photoconductivity in ZnO nanowires (NWs) are discussed in detail, and the improvement methods for nanowire photodetectors (PDs) are proposed and discussed.
First of all, since surface effects are widely recognized to greatly influence the properties of nanostructures, we report the evaluation of the surface effect on the photocarrier relaxation behavior by using a single ZnO NW ultraviolet (UV) photodetector. The pronounced surface effect leads to the enhanced-mode device behavior and the space-charge-limited transport of a single ZnO NW PD in dark. In the recovery phase, the decay of photocurrent is found to be strongly related to the power of UV light and the diameter of NWs, indicating that the photocarrier relaxation behavior is dominated by the surface band bending. A model for the relaxation behavior based on the surface band bending of NWs is proposed to interpret the experimental results.
Second, we demonstrate the Au nanoparticle (NP) decoration as an effective way to enhance both photocurrent and photoconductive gain of single ZnO NW PDs through localized Schottky effects. The enhancement is caused by the enhanced space charge effect due to the existence of the localized Schottky junctions under open-circuit conditions at the NW surfaces, leading to a more pronounced electron-hole separation effect. Since the band-bending under illumination varies relatively small for an Au NP-decorated ZnO NW, the decay of gain is less prominent with increased excitation power, demonstrating the feasibility for a PD to maintain a high gain under high-power illumination.
Finally, an efficient way to fabricate ZnO nanobelt (NB) networks as UV PDs is proposed. The network PDs are demonstrated to show high sensitivity to UV light and exhibit fast photoresponse and recovery behaviors. The high performance is resulted from an additional conduction mechanism that is not available for single nanowire devices. The NB-NB junctions in a network device act as energy barriers to hinder the carrier transportation, and the reduction of energy barriers under illumination accounts for the high photosensitivity of NB network PDs. These results suggest that ZnO NB networks could be promising for inexpensive PDs and applicable on other large area nanostructure-based optoelectronics devices.
Subjects
ZnO
nanowire
photodetecto
photocurrent
Au nanoparticle
Type
thesis
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