Research On The Properties Of Na Doped ZnO

ZnO is an attractive wide-band-gap semiconductor with a direct band gap of3.37eV at room temperature. The large exciton binding energy (60meV) and well-developed bulk, film growth processes promise its optoelectronic applications, such as light emitting diodes (LEDs), transparent conductive layer, photodetector and gas sensor. Due to the asymmetric doping limitations, n-type ZnO can be easily obtained via intrinsic or extrinisic dopants, whereas it has thus-far proven very difficult to obtain high quality and stable p-type ZnO. Recently, Na has been considered as an effective p type dopant in ZnO. There has been many reports on p-type Na-doped ZnO using various methods. Theoretical studies indicate that Na doping produces shallow acceptor state and small relaxation for Nazn.Moreover, p-type ZnO thin film and ZnO-based p-n homoj unction fabricated by various methods prove Na as an effective p-type dopant in ZnO. In this thesis we focus on growth and characterization of Na doped p type ZnO obtained by pulsed laser deposition (PLD), diffusion and ion implantation methods. The thesis includes:1. Na δ-doped p-type ZnO thin films were fabricated on quartz substrates with the structure of ZnO/Na (δ-layer) multi-layers by PLD. NaF ceramic target was used as Na source. The effects of oxygen pressure and substrate temperature on the electrical properties of δ-doped ZnO thin films were discussed. An optimized result with a resistivity of29.8Ωcm, Hall mobility of0.263cm2/Vs, and hole concentration of7.9×1017cm-3was achieved, and electrically stable over several months. The work is of interest for developing a novel method to realize p-type ZnO thin films doping with Na.2. Na-diffused p-type ZnO thin films have been realized via pulsed laser deposition using NaF ceramic target followed by rapid thermal process in nitrogen. An optimized result with a resistivity of426.7Ω·cm, Hall mobility of7.54cm2/Vs, and hole concentration of1.94×1015cm-3was achieved, and electrically stable over several months. Hall-effect measurements supported by X-ray photoelectron spectroscopy indicated that diffusion temperature and diffusion time played a key role in optimizing the p-type conduction of Na-diffused ZnO thin films. Furthermore, ZnO-based p-n homoj unction was obtained by fabrication of a Na-diffused p-type ZnO layer on an undoped n-type ZnO layer.3. Three-folder Na+ion implantation is applied to form a plant region of Na doped ZnO. Low temperature PL shows that a well-resolved bound exciton line at3.352eV with a linewidth of～8meV emerges from the850℃,6min annealed sample and is likely related to the formation of NaZn acceptor. ZnO-based p-n homojunction was obtained by three-folder Na+implanted layer. Also one time Na+implanted single crystal ZnO was studied.4. The effects of annealing temperature on excitonic emissions from Na+ion implanted ZnO nanorods are studied and annealing at between600and800℃can effectively repair the implantation as evidenced by the enhanced excitonic emissions. A well-resolved bound exciton line at3.352eV with a linewidth of～2meV emerges from the800℃annealed sample and is likely related to the formation of NaZn acceptor. When the annealing temperature is increased, the intensity of the I6-8line decreases while that of I3increases, suggesting enhanced ionization of neutral donors at elevated temperature.5. Unusual vibronic fine structures of the～3.0eV violet emission from ion-implanted ZnO nanorods has been investigated by photoluminescence (PL). A set of equally separated peaks with energy spacing of72meV, which corresponds to the longitudinal optical (LO) phonon energy of ZnO, were well resolved in the low temperature PL spectra. The overall emission band can be perfectly described by LO phonon-assisted transition with the Huang-Rhys factor of1.98, indicating intermediate electron-phonon coupling. The thermal quenching of PL gives rise to activation energy of～160meV, which is attributed to the energy level of acceptor-like defects introduced by implantation.