Investigation On The Optical Properties Of ZnO And Sodium Doping Grown By MOCVD

In the past decade, there has been widespread effort in the development of zinc oxide as a Ⅱ-Ⅵ semiconductor material. Potential advantages are claimed in many research fields including optoelectronic, piezoelectric, pyroelectric, ferroelectric and bio-engineering, for its unique electrical and optical properties. In addition, with a large exciton binding energy of60meV and a wide direct bandgap of3.37eV at room temperature, ZnO is considered as a promising candidate for short-wavelength optoelectronic devices. Continuous efforts have been made in transparent conductivity, p-type doping and controllable emissions in ZnO.ZnO films and nanostructures grown by metal-organic chemical vapor deposition (MOCVD) were studied in this dissertation. With an emphasized attention paid on its optical properties, characterization for the materials and devices were carried out frequently and the influences of defects and impurities were discussed. Moreover the electrical and optical properties of sodium-doped ZnO were also studied tentatively. The work included:1. Films and nanorods were grown on (1120) sapphire substrates in different processings by MOCVD. These nanodos were well single-crystallized and of great optical quality for investigation:(1) Dominant free exciton emissions by low temperature photoluminescence were firstly reported in our nanorods. The giant enhancement of free exciton emissions was attributed to enhancement of crystallization and the depression of hydrogen in ZnO. The optical signal of hydrogen was confirmed by PL and SIMS. Hydrogen can be annealed out by post-annealing to reduce the commonly intensive bound exciton emissions. It paved a way to prepare ZnO films and nonorods with a low free electron concentration even to realize p-type by reducing hydrogen involved donors.(2) A surface defect involved emission at3.338eV in ZnO was revealed by low temperature photoluminescence. The negative thermal quenching behavior of this emission was translated by a multiple-level model. Correlation between the3.338eV emission and the commonly reported surface exciton3.368eV emission was explained as strongly-weakly localized excitons.(3) Aluminum nanoparticles were prepared by the dewetting method. The surface plasmon resonance (SPR) of silver and aluminum was compared by the Uv-Vis spectroscopy. And an enhancement ratio of～3in near band edge emissions was achieved by capping the ZnO films and nanorods with aluminum. Metal-insulator-semiconductor structured diodes were also prepared to realize electroluminescence. It was found that Al2O3as the insulator layer had a better performance and a smaller threshold current instead of diodes with SiO2layers. A structure to enhance output power of electically pumped diodes by Al SPR was proposed, and it was also believed to reduce threshold by rough interface scattering.2. A new MO source for sodium doping in ZnO was proposed. ZnO:Na films and nanorods were successfully grown by MOCVD and a doping concentration of0.025at.%was confirmed by SIMS and ICP-AAS. Homojunction diodes of ZnO:Na/ZnO were fabricated tentatively.3. One MOCVD system with a newly designed reactor was set up successfully. The reactor was carefully designed to isolate the transport of Zn MO species and H2O to avoid pre-reaction and it will be used for epitaxial growth of ZnO films.