Optical Study Of ZnO-based Quantum Wells Prepared By Pulsed Laser Deposition

ZnO, a wide band gap II-VI oxide semiconductor, is considered a promising candidate of optoelectronic industry and offers potential to be used in blue/ultraviolet (UV) region. Because of its large exciton binding energy (60 meV), ZnO offers attractiveness in exhibiting exciton stability at room temperature, thus is interesting for optoelectronic device applications, such as light emitting diodes (LEDs), laser diodes, and UV detectors. For example, ZnO is expected leading to lasing action based on exciton recombination even above room temperature. Further progress to design device needs fabrication of high-quality ZnO-based heterostructures including quantum well structures, which show increased radiative efficiency due to quantum confinement (QC) effect. This dissertation focuses on the fabrication of ZnO-based multiple quantum wells (MQWs) and investigations in the terms of crystallinity, photoluminescence (PL), QC effects, as well as thermal stability. Main achievements in this dissertation were summarized below.(1) Ten-periodic ZnO/ZnMgO MQWs with well-defined multilayer structure were fabricated using pulsed laser deposition (PLD) method by optimizing the growth conditions of ZnO and ZnMgO single-layer films on c-plane sapphire substrates. The well-layer thickness in the MQW samples varied from 1.4 to 3.0 nm. Excited by a 325-nm He-Cd laser, all the MQW samples emitted intensive ultraviolet light, which was tuned from 3.38 to 3.52 eV at room temperature by the QC effect. In addition, the MQW samples exhibited extremely enhanced multiple-phonon resonant Raman scattering (RRS), which is the first observation in the PL spectra of ZnO-based MQW samples. Spectral analysis shows that the enhanced RRS can be ascribed to the ingoing and outgoing resonance with the barrier layers in one hand. On the other hand, the enhanced multiple-phonon RRS in the MQW samples was found highly in accordance with that in ZnMgO single-layer film, indicating that MgO contents are same in the barrier layers of the MQWs and in the ZnMgO epilayer deposited under the same condition, thus could be regarded as a solid evidence supporting that the MQW samples have well-defined multilayer structures. Using the PL spectra of ZnMgO single-layer film and the energy of longitudinal optical (LO) phonon determined by RRS, MgO content was estimated to be?15% in the barrier layer, and then the offsets between ZnO and ZnMgO were further calculated to be 338 meV in conduction band and 38 meV in valence band. Using the offset values, the PL shifts induced by QC effect as a function of well-layer thickness under Kronig-Penney model were calculated, in good agreement with the experimental results determined from the PL spectra taken at?12K. Furthermore, the ZnO/ZnMgO MQWs were found thermally stable at temperatures below 600?, and the multilayered structure will be destroyed by further elevating the annealing temperature above 700?.(2) As a fluorescence killer, Co2+ ions doped in ZnO will result in PL quenching of exciton recombination, but the PL quenching mechanism still remains an open question. In this work, the PL quenching was systematically studied in the first time using ZnCoO/ZnMgO MQW samples, which were grown on c-plane sapphire substrate with a?20 nm thick ZnO buffer layer using PLD method. The ZnCoO/ZnMgO MQW samples exhibited the enhanced multiple-phonon RRS similar to that in the ZnO/ZnMgO MQWs, evidencing that the multilayer structures are good enough for the formation of periodic quantum wells. In compared with the ZnCoO single-layer film grown under the same condition, the QC effect in the ZnCoO/ZnMgO MQWs made an increased optical emission of Co2+ ions at ?1.80 eV, which is at an energy level of highly localized Co2+ 3d electronic states, but maintains PL quenching of exciton recombination. Therefore, the PL quenching of exciton recombination was attributed to the energy transfer between ZnO excitons and the localized Co2+ 3d states, and then fluorescence resonance energy transfer (FRET) was proposed to be the mechanism responsible for the PL quenching of exciton recombination and the enhancement in the optical emission of Co2+ ions at?1.80 eV.(3) An extraordinary optical emission with a broad spectral feature was observed in the near-band-edge (NBE) region centered at?3.0 eV in the highly epitaxial ZnO films deposited on sapphire by PLD. The ZnO samples with extraordinary NBE emission were studied in comparison with the annealed sample and the bulk ZnO single crystal using temperature-dependent PL, PL excitation (PLE), and time-resolved PL. Experimental results revealed that the NBE optical emission cannot be assigned to any specific defect, thus a band-tail model was suggested. In the band-tail model, various defects, chemical disorders, and the lattice strains result in the formation of localized electron states below the energy level of conduction band minimum, and then the extraordinary NBE emission originates from the radiative recombination of excitons in the process of thermal relaxation to the lowest states in the band tail. The band-tail model was proved by the quantitative fitting to the temperature-dependent and time-resolved PL spectra. In addition, a nitrogen-plasma-assisted PLD system was developed for deposition of N-doped Zn? films. The electric transport properties and PL spectra were studied as a function of oxygen partial pressure.