Ultraviolet Band-Edge Emission from Zinc Oxide Nanostructures

Abstract

Zinc oxide is a wide band gap (3.37 eV) semiconductor with a high exciton binding energy (60 meV), of interest for optoelectronic applications due to both its ultraviolet (UV) and visible emissions. This dissertation shows four methods of tuning the UV band-edge emission of ZnO nanostructures. First, in a ZnO/MgO core-shell nanowire there exists a UV photoluminescence enhancement that varies with MgO thickness due to the formation of Fabry-Perot and whispering gallery optical cavity modes. Second, exciton-plasmon coupling in ZnO/MgO core-shell nanowires leads to enhanced emission of the UV luminescence. The combination of optical cavity formation and Ag plasmons leads to two enhancement mechanisms that depend on the MgO spacer thickness. Third, the structure of the MgO shell in the core-shell nanowires depends on the ZnO surface conditions. Changing the growth parameters of the ZnO nanowires tunes the ZnO m-plane surface conditions and thus the MgO shell structure. A smooth MgO shell supports enhanced UV photoluminescence through the formation of guided-wave optical modes, while a rough MgO shell supports neither the enhancement nor the guided modes. Fourth, temperature-dependent studies of exciton-phonon coupling elucidate that the vertically oriented nanowires, grown hydrothermally, have fewer defects and a sharper UV band-edge emission than the previously studied randomly oriented nanowires, further indicating that growth protocol has a critical influence on the UV optical properties of ZnO. Additionally, a novel structure, ZnO “nanopopcorn” showed strong exciton-phonon coupling, highlighting its high defect density. In summary, there is a range of mechanisms that can be used to enhance, modify, and control the UV band-edge emission in ZnO nanostructures, with vertically oriented ZnO nanowires having the most promise for enhanced UV emission optoelectronic and all-optical devices, from on-chip waveguides, lasers and LEDs, to scintillators and sensors.