| ZnO-based semiconductors have been recognized as one of the most promising candidates for light-emitting diodes (LEDs) and laser diodes (LDs) operating in the visible and ultraviolet region, owing to their direct wide band gap (3.37eV) and large binding energy of excitons (60meV). An important step to design ZnO-based devices is the realization of band-gap engineering to create barrier or well layers and quantum wells in heterostructures. Fabrication and characterization of alloys such as ZnMgO, ZnCdO or ZnSO are very important from the viewpoint of band gap engineering as well as p-n junctions. The band gap of ZnO can be tuned by alloying with CdO (form ZnCdO alloy) to achieve luminescence from ultraviolet to blue and even green light, which is essential to construct appropriate ZnCdO-related heterostructures or quantum well structures in order to apply in the ZnO-based optoelectronic devices.In this paper, high quality ZnO thin films, ZnCdO alloy thin films, ZnSO alloy thin films and ZnNiO alloy thin films were prepared on quartz by pulsed laser deposition (PLD). By optimizing the gowth parameters, the smooth ZnO films were achieved, the bandgap could be modulated in ZnCdO and ZnSO films, and room temperature ferromagnetism was achieved in ZnNiO films. After that, ZnCdO/ZnO quantum wells were fabricated on sapphire by PLD. The optical properties of these quantum wells were analyzed by photoluminescence (PL) spectra.1. High quality ZnO thin films, ZnCdO alloy thin films, ZnSO alloy thin films, and ZnNiO alloy thin films were prepared by pulsed laser deposition (PLD). The dependence of these films on substrate temperatures, oxygen pressures, targets, etc was investegested. By changing Cd contents in ZnCdO targets, the maximum Cd content of9.6at.%in ZnCdO film could be achieved without second phases and the bandgap ZnCdO film could be modulated from2.88eV to3.26eV. By changing substrate temperatures, the maximum S content of13.8at.%in ZnSO film could be achieved without second phases and the bandgap of ZnSO film could be modulated from3.11to3.26eV. The relationship of electrical and magnetic property was investigated by comparing ZnNiO thin films and Na co-doped ZnNiO thin film. Room-temperature ferromagnetism and better electrical property of ZnNiO film with the Ni content of3at.%were obtained. By Na co-doping, the electrical property was changed from n-type to weak p-type and the room-temperature ferromagnetism was enhanced remarkably with a coercive force of-100Oe and a saturation magnetization of0.08μB/atom. We believe that p-type Na’can enhance the ferromagnetic interaction in ZnNiO film.2. ZnCdO/ZnO heterojunctions with different Cd contents were fablicated by PLD and investigated by X-ray photoelectron spectroscopy (XPS) measurements. Valence band offset of0.06eV and0.2eV in ZnCd0.05O/ZnO and ZnCd0.1O/ZnO were obtained by calculation, respectively. We drew the structure diagrams of ZnCdO/ZnO heterojunctions. A series of ZnCdO/ZnO quantum wells with various well widths were fablicated on c-sapphire by PLD. The optical property of ZnCdO/ZnO multiple quantum wells (MQWs) is better then single quantum well. XRD, STEM, EDS analysis reveals that MQWs exhibit periodic structure with sharp interface. The well layer emission shows S type shift in the temperature dependent PL spectum as temperature increasing, which is caused by localization effect of excitons. We gave a behaviour model of localized excitons with various temperatures and deduced a potential trough of-11eV in MQWs. Both quantum confinement effects and quantum-confinement Stark effects in MQWs can be observed up to room temperature. By using a model of One-dimensional finite potential well, we revealed the competitive relationship of quantum confinement effects and quantum-confinement Stark effects in MQWs and analyzed the emission mechamism of exiton in build-in electric field with various well widths. We can modulate the well emission energy of MQWs from2.90to3.085eV by varying the well widths. |
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