Research On Fabrication And Doping Modification Of ZnO Thin Films And Nanorods

ZnO nanomaterials have attracted significant attention in the last decades due to their promising properties. High quality pure and doped ZnO nanomaterials are desired in a wide range of applications including UV lasers and detectors and high transparent solar cell electrodes. In particular, ZnO nanorods are receiving increasing interests due to their simple processing techniques and a high surface to volume ratio. However, there are several unanswered questions, especially those are related to the doping effects and the origin of some luminescence bands. In this work, the structure, morphology, optical and electrical properties of pure and doped ZnO thin films and nanorods were investigated as following.(1) Effects of lithium doping on the properties of ZnO thin films were investigated. Among the commonly investigated elements, lithium is known as a shallow acceptor in ZnO. Therefore, it is crucial to investigate the effect of lithium doping on the properties of lithium-doped ZnO. In this work, the lithium-doped ZnO thin films were deposited with radio frequency (RF) magnetron sputtering onto a quartz substrate. Effects of sputtering parameters (substrate temperature and argon to oxygen flow ratio) and annealing on the properties of lithium-doped ZnO thin films were investigated. XRD studies revealed that the films had a hexagonal-wurtzite crystal structure with preferred growth orientation along the c-axis. The results also have shown that under argon: oxygen flow ratio of10:1sccm, the crystallinity was improved and caused a significant increase in the crystallite size (818nm). The sputtering parameters showed minor effect on the optical band gap values. All lithium-doped ZnO films exhibited a broad UV-violet emission band centered on407nm and attributed to the radiative recombination processes near the band edge. A Hall mobility of~33.3cm2/V-s, concentration (n) of~7.6×1018cm-3and resistivity of~39·7Ω·cm were obtained for the film deposited at500°C under argon: oxygen flow ratio of10:1sccm. The results indicated that the sputtering parameters play a crucial role in the structure, morphological, optical and electrical properties.(2) Effects of zinc (Zn) and hydroxyl (OH-) sources on the structure, morphology and optical properties of ZnO nanorods were investigated. XRD patterns showed that,(002) peak of ZnO grown in a solution containing hexamethylenetetramine (HMT) as the hydroxyl precursor is more significant. SEM images showed that, using hexamethylenetetramine as the hydroxyl source and zinc acetate as the zinc source, highly aligned ZnO nanorods can be obtained. From PL spectra, a strong near-band-edge emission was observed for the nanorods that were prepared from hexamethylenetetramine and zinc acetate as hydroxyl and zinc sources, respectively. Additionally, the relative intensity ratio of the near-band-edge emission (NBE) to the green emission (INBE/IGE) increased from~2.6for the as-grown ZnO nanorods to~68.7when the nanorods were annealed under oxygen.(3) The concentration effects of single doping elements (copper, sulfur and nitrogen) and codoping elements (copper: sulfur and lithium: nitrogen) on the properties of the grown nanorods were investigated. The nanorods were grown by a hydrothermal method onto a quartz substrate. For copper doped ZnO nanorods, SEM studies revealed that adding copper into the ZnO host lattice is an effective factor for the formation of copper-doped ZnO nanorods with large diameter. It was also found that, the near-band-edge emission deceases with the increase in the copper content. For sulfur doped ZnO nanorods, the nanorods diameter and length increases with sulfur concentration. The sulfur doped ZnO nanorods found to be transparent in the visible region. For nitrogen doped ZnO nanorods, XRD analysis indicated that nitrogen doping resulted in a slight increase in the the crystallite size while the optical studies revealed the reduction in the optical band gap compared to the standard ZnO. For the codoped nanorods, it was found that increasing the doping concentration on oxygen site compared to the doping concentration on zinc site has led to the formation of thicker nanorods. Also, it was observed that, the UV emission band is quenched while the visible emission enhanced remarkably due to the incorporation of the dopants in the host lattice.