Fabrication Of ZnO Nanowire Photodetectors And Surface Passivation

Semiconductor nanowires have been studied intensively as potential active materials in optoelectronics and electronics due to the advantages in structure, function, and fabrication. ZnO is an important wide gap semiconductor with a band gap of 3.3eV, and has a strong absorption in ultraviolet light, thereby leading to evident electronic transitions from valence band to conduction band. ZnO nanowires-based ultraviolet photodetectors exhibit promising properties, specifically ultrahigh photocurrent gain.However, with the development of the nanowire optoelectronic researches, a series of problems still need to be solved. The traditional fabrication of the device has limited the features of the nanowires and a new structure is the need to develop the nanowire optoelectronic application. To date, ZnO nanowires are often exposed in air for light transmission in most photodetectors, hence unavoidably resulting in H2O/O2 adsorption on the nanowire surface, as well as degradation in the response time. In view of above discussion, a sequent study has been carried out, including growth of the ZnO nanowires, fabrication of the photodetectors and passivation of the surface states. Our works are as followed:1. We developed a method from growth of the ZnO nanowires by chemical vapor deposition without catalyst to fabricate the lateral electrodes on nanowires and finally realized an ultraviolet photodetector. The feature of the lateral electrode architecture leads to short the distance between excitation of the photogenerated carriers to the electrodes. Carriers directly drift to the electrodes rather than recombination during their diffusion along the axial nanowires. The integrated architecture is beneficial in transporting carriers across the diametrical direction and in electronic injection/extraction. The Schottky phototdetector exhibits an on/off ratio of 3 orders and a favorable rectification characteristic.2. Various surface passivation were performed, such as high-intensive ultraviolet irradiation in oxygen atmosphere for a long duration, which exhibited strong oxidization to repair the oxygen vacancies as well as reduced the H2O/O2 desorption. Comparing to the nanowires without surface treatment, the passivation of the surface states reduced the dark current from 20 pA to 2 pA at-1 V bias and decay time decreased from 20s to 2s. Compared with the device structure that the electrodes locate at both ends of a nanowire, lateral electrode structure can preferably achieve photocurrent with zero bias. A physical model based on surface band theory was developed to understand the origin of the performance improvement of the photodetector.