Studies On The Preparation And Characteristic Of ZnO Nanomaterials

ZnO is a promising wide band-gap semiconductor (Eg =3.37eV) and attracts asmuch attention as GaN for its potential utilization in short-wavelength light-emitting/detecting devices. In comparison to GaN, its optoelectronic characteristics are littleaffected by the defects, and it has higher exciton binding energy (60 meV at roomtemperature), lower growth temperature and cost. Due to their excellent physical andchemical properties, ZnO films have many realized and potential applications such asdisplay devices, sensors, piezoelectric transducers, transparent conductor, surfaceacoustic wave devices and lasers operating in the near ultraviolet and blue spectralrange.ZnO thin films can be deposited by various methods, such as sputtering, metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), atomiclayer epitaxy (ALE), chemical vapor deposition (CVD), e-beam evaporation, spraypyrolysis, sol-gel, pulsed laser deposition (PLD), etc. PLD is a newly developed filmgrowth technique. In this technique, high density laser ablates the target and producesZnO plume depositing on heated substrate in high vacuum background. PLD hasadvantages in comparison with other methods, such as deposition in relatively highoxygen pressure, lower temperature of film crystallization because of high particleenergy in laser produced plasma plume. In this paper, a pulsed laser deposition system was used for the epitaxy of ZnO thinfilms on Si (111) substrate. The influences of different growth conditions on features ofZnO thin films in several aspects are studied, especially in the impacts of differentsubstrates temperatures, laser energy density and oxygen pressure, respectively. Thecharacteristics of ZnO film grown on different growth conditions were studied bymeasurements of X-ray diffraction (XRD), scanning electron microscopy, atomic forcemicroscopy (AFM), photoluminescence and Raman scattering.We investigate the effects of the substrate temperature on the crystal andluminescent quality of the ZnO thin films by depositing films at variable substratetemperature keeping all the rest of the parameters unchanged. It is found that arelatively high substrate temperature induces high quality films. The substratetemperature of 400 ℃ is the optimum condition of crystalline for the Si (111) substrate.Photoluminescence (PL) results indicate that the thin films show the UV and blueemissions. The blue emission is related to the oxygen vacancies in the ZnO film. Theintensity of the blue emission increases with the increases of substrate temperature. Thereason is that high substrate temperature increases the opportunity for the oxygenoutgassing of the film which induced increase of oxygen vacancies.We investigate the effects of the laser energy density on the crystal andluminescent quality of the ZnO thin films by depositing films at variable laser energydensity keeping all the rest of the parameters unchanged. It is found that the laser energydensity of 62.5 J/cm2 is the optimum condition of crystalline for the Si (111) substrate.Photoluminescence (PL) results indicate that the intensity of the blue emission increaseswith the increases of laser energy density. The reason is that high laser energy densityincreases the opportunity for the interdiffusion between ZnO film and Si substrate andthe formation of SiO2, which results in the increase of oxygen vacancy.ZnO thin films were deposited at different oxygen pressures. After deposition, thesamples were investigated by XRD, AFM and Raman spectroscopy. It is found that theintensity of ZnO (002) peak increases while the FWHM of ZnO (002) peak decreaseswith the increase of the oxygen pressure. This means that the crystallinity of thespecimens is improved when increase the oxygen pressure. The Raman results indicatethat oxygen ambient plays an important role in removing defects due to oxygen vacancy.It is found that the oxygen pressure of 1.3 Pa is the optimum condition of crystalline forthe Si (111) substrate.