Study Of ZnO Nanostructures And Devices

Zinc oxide, as a semiconductor with a direct wide band gap of3.37eV andexciton energy of60meV, favors the excitonic emission even at high temperature.Therefore, the ZnO has the great potential for the production of optoelectronic devices,such as light-emitting diodes and laser diodes. Because of the lower surface freeenergy of the (002) plane, ZnO usually is grown along the c-axis orientation. As aresult, one-dimensional ZnO nanostructures are more successfully to be synthesized,such as nanowires, nanobelts, nanotubes, nanoring etc. ZnO is a material whichexhibits piezoelectric and optoelectronic multiple properties. Especially for thelow-dimensional ZnO nanostructures, many useful effects can be detected due to thequantum confinements. Therefore, ZnO nanostructures have the promising potentialsin fabricating nano-optoelectronics and nano-electronics devices etc. The main workof this dissertation is based on the applications of ZnO nanostructures in the field ofresistive-switching random access memory and gas sensors devices. The dissentationis formed by the following aspects:1. The ZnO nanostructures and films were successfully synthesized by reactivevapor deposition method and radio frequency magnetron sputtering system,respectively. The low temperature (10K) PL spectra of the ZnO nanorods reveal thatthe nanorods have high quality as indicated by the negligible deep level emission. Thetemperature dependent PL spectra shows that the positions of the near band-edgeemission peaks show a redshift with increasing temperature and the intensity of D⁰Xdecreases with increasing temperature, whereas the free exciton becomes stronger andfinally becomes the dominated. The post annealing in oxygen ambient can improvethe crystalline quality and optical property of the ZnO films and presents the bestquality at the annealing temperature of700℃.2. Resistive-switching random access memory (RRAM) devices based onrandomly oriented nanowire networks (ZnO NWNs) was fabricated and researched.The Au/ZnO NWNs/ITO (MIM) structure was successfully fabricated, in which the ITO was played as both the substrate for growing ZnO and the bottom electrode forthe RRAM structure. The Au top electrode was deposited by thermal evaporationmethod using a metal shadow mask. I-V curves of the MIM structure results revealthe reproducible bipolar resistive property with a high R_on/R_offratio (¹0⁴). Theswitching mechanism is confirmed in terms of the formation and rupture ofconductive filaments, with oxygen vacancies localized on the ZnO NWNs surfaceinvolved in.3. Physical type gas sensors operated by field ionization based on Pdnanoparticle-capped ZnO (Pd/ZnO) nanorods was fabricated and researched. Theresults show that the Pd/ZnO nanorod-based sensors, compared with the bare ZnOnanorod, have lower breakdown voltage for the detected gases with good sensitivityand selectivity. This precise breakdownvoltage is a fingerprinting property forindividual gas.