Photoluminescence And Photoresponse Properties Of ZnO Nanorods

In order to develop nano-scale devices based on ZnO, controllable and reproducible fabrication of ID ZnO nanostructures is essential. Due to the large aspect ratio of ZnO nanostructures, surface states are expected to play an important role in the optical and electrical properties. Deep insights into how the surface states affect the optical and electrical properties of ZnO are the key points for the optoelectronic applications of ZnO nanostructures. In this thesis, ZnO nanorods with different size were fabricated first, and then the photoluminescence and photoresponse properties of ZnO nanorods were analyzed.The primary results are described in the following.(1) ZnO nanorods with different size have been grown by general and PS template assisted CVD on Si (100) wafer using Au as the catalyst.(2) It is found that strong surface exciton recombination is present in ZnO nanorods with average diameter as large as 500nm. Coating the nanorods by Al2O3 significantly reduces the surface state-related emissions, indicating that surface passivation rather than surface band-bending mechanism dominates.(3) We also provide evidence that the long controversial 3.3 leV emission in ZnO is not related to surface states but a free-to-bound transition involving an unknown acceptor level of 125 meV.(4) In the visible spectral region, an orange emission around 2.1 eV together with the normal green emission is observed in the thick nanorods. The little change in intensity after Al2O3 coating allows us to conclude that the visible emissions are unlikely from the surface. Based on a DAP-like transition model, we are able to interpret the blue shift of the orange emission with increasing temperature and attribute the emission to zinc vacancy defects.(5) It is found that persistent photoconductivity (PPC) exists in ZnO nanorods by UV and visible light illumination. By comparing the photoresponse to UV and visible light, we suggest that the PPC originates from the defect states, most likely surface states.