Pld Prepared Zno Thin Films And Their Structure And Luminescence Properties

ZnO is a wide band gap semiconductor of wurtzite structure with a band gap of 3.37eV at room temperature , it has many applications, such as transparent conducting films, surface acoustic wave devices, gas sensors and photoelectronic devices. The report of ultraviolet laser emission of high quality ZnO thin films at room temperature promoted people's interesting in ZnO researching.In this thesis, we report that ZnO thin films on different substrates of Si, SiC and Al₂O₃ are prepared by pulsed laser deposition and the growth conditions are optimized. Furthermore we employed some normal analysis methods and synchrotron radiation experiment techniques to investigate the influence of different growth conditions on structure, optical and electric properties of ZnO thin films. The major results are listed as the following:1. Growth of ZnO thin films on Si substrate and investigation of their structure and optical propertiesWe have prepared highly c-axis oriented ZnO on Si substrate at different growth conditions, such as substrate temperature, oxygen partial pressure, laser energy density and laser frequency, and investigated the effects of the growth condition on the crystallinity of ZnO thin films, and analyzed the reason of the influence. Then we prepared ZnO thin film with good crystallinity at the optimized grow condition. X-ray ω rocking curve result indicates that the FWHM of its (002) peak is 1.3° .Moreover we used XAFS and XPS to study the local structure, chemical state and element relative content of ZnO grown at different substrate temperature (300°C and 500°C).The results of XAFS and XPS show that the crystallinity of the ZnO thin films grown at 500°C is better than that of the ZnO thin films grown at 300℃,but the O/Zn proportion of the ZnO thin films grown at 500°C is smaller than that of ZnO thin films grown at 300℃.GID was used to investigate the lattice relaxation inside thin films, its result indicated that both a-axis lattice constants of the two sample decreased with the increasing of the detected depth of X-ray from surface of thin films to the interface between films and substrate, which shows the stress inside the thin films is not equal . Furthermore, PL results suggest that UV emission of ZnO is relative to its crystallinity. We also observed the violet emission with about 430nm of ZnO films excited by synchrotron radiation at 18K. The violet luminescence is ascribed to the defects related to the interface traps existing at the grain boundaries, which probably originate from interstitialzinc(Zn_i).2. Influence of Mn-doped on the structure and optical properties of ZnO thin filmsHighly c-axis oriented Zn_0.9Mn_0.1O thin films have been fabricated on Si substrates by pulsed laser deposition. Photoluminescence results show that Mn atoms doping induce the blue-shift of UV emission. At the same time the intensity of UV emission decreases, while green emission increases. X-ray diffraction, X-ray absorption fine structure, X-ray photoelectron Spectroscopy and Raman have been employed to characterize the influence of Mn-doping on properties of ZnO thin films. The results indicate that Zn_0.9Mn_0.1O alloy formed after Mn doped in ZnO. Mn atoms have entered into the ZnO crystal lattice and substituted Zn atoms with Mn²⁺ state. As a result, the band gap of Zn_0.9Mn_0.1O increases, which is associated with the UV emission blue-shift. Raman results show that Mn doping influences the crystal lattice vibration of ZnO, and prompts the increasing of stress between film and substrate. Furthermore, the reaction between the doped Mn⁴⁺ and interstitial Zinc atoms (Zn_i) induces the crystallinity of sample worse, and makes Mn⁴⁺ turn to Mn²⁺. Thus, interstitial Zinc atoms (Zn_i) decreases and the oxygen vacancy (Vo) increases, which accounts for the intensity decreasing of UV and violet emission, and the increasing of green emission of the films samples.3. Preparing ZnO thin films on SiC single crystal or SiC buffer and studying the interface of ZnO/SiCWe used PLD technique to prepare highly c-axis oriented ZnO on 3C SiC/Si substrate grown by MBE at different growth conditions, such as substrate temperature, oxygen partial pressure, and investigated the effects of the growth condition on the crystallinity of ZnO thin films. At the same time we also analyzed the cause of the influence. Besides, we have prepared ZnO thin films with good crystallinity on 6H-SiC single crystal substrate at the optimized growth condition by PLD. X-ray ω rocking curve result shows that its FWHM of (002) peak is only 0.47°. The results of synchrotron radiation X-ray grazing incidence diffraction shows the actual lattice mismatch between film and substrate is only 5.84%, and X-ray Φ scan shows that the six-fold symmetry of ZnO(110) planes family, which indicates that we have prepared single crystal ZnO thin film on 6H-SiC single crystal substrate by PLD. We also investigated the effect of SiC and graphite buffer prepared by PLD in situ on the electric properties of ZnO thin films, the result indicates that the buffer can improve the I-V properties of ZnO/Si p-n junction greatly.The adsorption and the thermal oxidation of Zn on 6H-SiC surface and the interface formation of ZnO/SiC have been investigated by using synchrotron radiation photoelectron Spectroscopy (SRPES) and X-ray photoemission (XPS). The results show that at the initial stage of Zn adsorption, Zn can capture and bond with 0, which was remained on the SiC surface. With increasing of Zn coverage, the surface exhibits metallic characterization. When the deposited Zn film is annealed at 180°C in oxygen flux with the pressure of 2×10^-4Pa, it could be partly oxidized to form ZnO. Part of Zn atoms could be escaped from the surface due to its low evaporation temperature in UHV condition. While annealed at 600°C in same oxygen flux, the total deposited Zn atoms could be oxidized to form ZnO. During annealing process, the substrate is also oxidized, which induces a thin layer of silicon oxide existing at the interface of ZnO/SiC. By using the results of SRPES and XPS, the valence band offset of ZnO/SiC is calculated to be 1.1eV. 4. Preparing ZnO thin films on A1₂O₃ and studying their structure propertiesWe have grown highly c-axis ZnO thin films on Al₂O₃ at different substrate temperatures and oxygen pressures by PLD.XRD and RHEED have been employed to investigate the influence of substrate temperature and oxygen pressure on the crystallinity and growth mode of ZnO thin films. X-ray rocking curve results exhibit that the FWHM of (002) peak of ZnO thin films prepared on Al₂O₃ at the optimized condition is only 0.46°, indicating that we have grown single crystal ZnO thin film. The results of Synchrotron radiation X-ray grazing incidence diffraction show that with the increasing of the detected depth of X-ray from the surface of film to the interface between the film and substrate, the a-axis lattice constants of the ZnO thin films grown at 450°C and 650°C increase obviously, while the a-axis lattice constant of the ZnO thin film grown at 750°C decreases slightly, which indicates that the substrate temperature play important role to affect the lattice relaxation in ZnO thin films. The results of synchrotron radiation X-ray grazing incidence reflection shows that thickness and surface roughness of ZnO thin films grown at 650°C and 0.13Pa are 46.2nm and 0.63nm respectively, and the roughness at the interface is 1.41nm.