Pulsed Laser Deposited GaN Film And The Effect Of ZnO Buffer Layer

In recent years, Wurtzite structured Gallium nitride (GaN) has attracted much attention for fabrication of ultraviolet (UV), blue and green light emitting diodes (LEDs), laser diodes (LDs), UV-detectors and devices operating in high-temperature, high-frequency and high-power conditions. It is difficult to grow large-sized single crystals of GaN, so hetero epitaxy has been used for the growth of high quality GaN films on various substrates. Si substrates for growing GaN films have many advantages, it can be considered as an attractive substrate for the growth of GaN films. However, Single crystal GaN films on Si substrates have not been obtained because of the large lattice mismatch (17%) and the large difference in thermal expansion coefficients (56%) between GaN and Si. Buffer layers, such as AlN, SiC, ZnO, may improve the quality of GaN films.GaN thin films can be deposited by various methods, such as sputtering, metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) and pulsed laser deposition (PLD), etc. PLD is a newly developed film growth technique. In this technique, high density laser ablates the target and produces plasma depositing on heated substrate in vacuum background. PLD has advantages in comparison with others methods such as lower temperature of film crystallization.In this research, GaN thin films were deposited on Si (111) substrates using ZnO buffer layers by pulsed laser deposition (PLD). High-quality GaN thin films were obtained after annealing at 950℃for 15 min in a NH3 atmosphere. We studied the advantage and disadvantage of the GaN films deposited by PLD, analyzed the effect of the annealing on ZnO/Si films, we mainly studied the effect of ZnO buffer layers on the annealing of GaN/ZnO/Si film, and the results of the experiment have also been analyzed. The main content is as follow:1. We analyzed the principle of the PLD technique at first and deposited the GaN film and ZnO film 2. Wurtzite structured ZnO film was obtained by PLD on S(i111)substrate directly. We analyzed the effect of annealing on ZnO film. Because GaN has strong rigidity and high melt point. The GaN thin films were deposited by PLD at first and then annealed in NH3 ambient for 15 min. A Quanta-Ray DCR-3 Nd:YAG laser was focused on a rotating target which is the same component as the preparing films. The anneal process was carried in an open tube furnace. The main test methods in experimental are X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and Raman spectrum.3. We studied the effect of the ZnO buffer layer on the crystalline quality, composition and surface morphology of the GaN/ZnO/Si film. The results of the experiment have also been analyzed, crystalline ZnO buffer layer improved nucleation in the deposited progress and the growth in the annealing progress of GaN films4. We investigated the effect of the annealing temperature or annealing time on GaN film keeping all the rest of the parameters unchanged. Through analysis of the measured results, a conclusion was drawn that annealing temperature and annealing time affected the structural properties. Zn-O bonds are destroyed when the GaN films are annealed in ammonia (NH3) ambience, O and Zn atoms depart from their positions, N and Ga atoms fill in the empty positions and form a hexagonal structure of a special component, the structure is propitious to the epitaxial growth of GaN, with the annealing time increase, in the end, formed Crystalline GaN film. The optimum annealing temperature is 950℃and the least annealing time is 15 min under our experimental conditions.