Optical properties of zinc oxide nanotips and their device applications

Zinc oxide (ZnO) nanostructures are emerging as the key building blocks for nanoscale optoelectronic and electronic devices. ZnO has a large exciton binding energy (~ 60 meV), which makes its nanotips ideal for studying excitonic emissions in one-dimensional systems even at room temperature. ZnO nanowires show a strong exciton-polariton interaction, promising for fabricating UV nanolasers. The large and fast photoconductivity in high quality ZnO is suitable for making UV photodetectors. ZnO nanotips can be grown on various substrates, including glass, Si, and GaN, at low growth temperature (~ 400° C) by metal-organic chemical vapor deposition (MOCVD) that provides the potential of the integration of ZnO nanotips with Si based microelectronics and GaN based optoelectronics devices. To date, most of the research has been focused either on ZnO films, or on "pick-and-place" manipulation of randomly dispersed ZnO nanowires to study their physical properties. In this dissertation work in-situ n-type doping of ZnO nanotips during MOCVD is studied, including the doping effects on optical properties and electrical conductivity. Nanoscale tunneling current-voltage characteristics of the ZnO nanotips show the conductivity enhancement due to Ga doping at the proper range of doping concentration. At low or moderate doping levels, the increase in photoluminescence (PL) intensity from Ga doping is attributed to the increase of Ga donor related impurity emission. The excitonic emissions of ZnO nanotips are investigated using temperature-dependent PL spectroscopy. The sharp free exciton and donor-bound exciton peaks are observed at 4.4K, confirming high optical quality of the ZnO nanotips. Free exciton emission dominates at temperatures above 50K. The thermal dissociation of these bound excitons forms free excitons and neutral donors. Temperature-dependent free A exciton peak emission is fitted to the Varshni's equation to study the variation of energy bandgap versus temperature. A prototype of ZnO nanotips/GaN light emitting devices has been demonstrated using an n-ZnO nanotips/p-GaN heterostructure. The electroluminescence with a peak wavelength of 406nm is primarily due to radiative recombination from electron injection from n-type ZnO nanotips into p-type GaN. A novel integrated ZnO nanotips/GaN LED has been fabricated for enhanced light emission efficiency. A Ga-doped ZnO transparent conductive oxide (GZO) film and ZnO nanotips are sequentially grown on top of a GaN LED, serving as the transparent electrode and the light extraction layer, respectively. Compared with the conventional Ni/Au p-metal LED, light output power from the ZnO nanotips/GZO/GaN LED is improved by 1.7 times. The enhanced light extraction is attributed to the increased light scattering and transmission in the ZnO/GaN multilayer.Ph.D.Includes bibliographical references (p. 124-132)