Research Of Characteristics Of ZnO And ZnO/MgO Multilayer Films Fabricated By PLD

ZnO is a compound semiconductor with wide direct band gap of 3.37eV and a large exciton binding energy of 60meV at room temperature. Theoretically, it can realize stimulated ultraviolet (UV) emission at room temperature. ZnO is also an ascendant piezoelectric, gas sensitive and optoelectronic material. There are many growth methods for ZnO films such as magnetron sputtering, chemical vapor deposition, sol-gel, spay pyrolysis, thermal oxidation and molecular beam epitaxy (MBE). Recently, some new growth methods such as pulsed laser deposition (PLD), metal organic chemical vapor deposition (MOCVD), atomic layer epitaxy and laser molecular beam epitaxy have been widely used. Because ZnO thin film has so many advantages in structural and electrical properties and can be grown through many methods, it can suit many different applied demands. ZnO thin films can be used to fabricate transparency electrode, piezoresistor, cell battery window, surface acoustic wave device, gas sensor and light-emitting diode etc.. In short wave region, ZnO can be used to fabricate UV light emitting and UV laser devices, which is very important for improvement of memory density and optical information access speed.PLD is a newly developed film growth technique which can fabricate high-quality films. In this technique, high density laser ablates the target and produces ZnO plume depositing on heated substrate in high vacuum background. Many kinds of substrates can be used to grow ZnO thin films. At present, the most abroad used substrate is Al₂O₃ and high-quality ZnO films have been obtained on it. Si is the most important and abroad used semiconductor material. It is cheap in price and has mature processing technique. In this paper, ZnO films were prepared on Si (111) substrates at various substrate temperatures, pulsed laser repetition frequencies and oxygen pressures. The films were examined by X-ray diffraction (XRD), photoluminescence spectra (PL), scan electron microscopy (SEM), atomic force microscopy (AFM), surface profiler and electronic probe. Through the research of the structural and optical properties of ZnO films, optimized conditions for growing ZnO films were obtained. The results suggested that high quality ZnO films with highly c-axis oriented can be prepared by PLD. The narrower and stronger UV peak of ZnO films with excellent crystallinity can be found from the samples grown at about 650°C, 50Pa and 5Hz.Light emission mechanics of ZnO films were also studied. It is found that the UV emission is origin from free exciton recombination radiation. The green peak is closely relatedwith oxygen atom at the zinc position in the crystal lattice, OZn and blue peak is from oxygen vacancy, Vo.One of the merits of PLD technique is that the growth of films can be monitored in situ. In our experiments, we observed the growth of ZnO thin films maily using reflection high-ernergy electron diffraction instrument. The growth procedure of ZnO thin films was examined by RHEED and the growth mode of ZnO films was analysed. The RHEED images of Si (111) and A12O3 substrates were analysed in structure. Two kinds of RHEED electron diffraction dot matrixes of ZnO films were found and indexed.MgO has a band gap of about 6.7eV and different optical index of refraction with ZnO. For the purpose of modulating the ernergy band gap structure and light emission properties of ZnO and making Bragg reflector to improve the UV emission of ZnO films using their different optical index of refraction, ZnO/MgO multilayer films with different growth periods were fabricated using ZnO and MgO targets. The structure and the PL properties of ZnO/MgO multilayer films were explored. Compared with highly c-axis orientation ZnO films, in the multilayer films the position of (002) peak of ZnO have a excursion toward larger diffraction angle and UV peak of ZnO have a blue shift because of the doping of Mg. These made the energy band gap of ZnO in multilayer films increas about 0.05 eV than that of ZnO films. From TEM images, the film is found to be polycrystal grains and have more crystal direction than ZnO films. In TEM images, the crystal characters and area distribution of the multilayer films were obtained. The TEM images suggest that the doping of Mg atoms into ZnO lattice resuted in that distorted lattice in the (001) plane and the increase of lattice constant a and the decrease of lattice constant c of hexangular ZnO. The other area formed MgO and ZnxMgi.xO polycrystal grains determined by the different diffusion extent. All these work made a primary foundation for later research.