The Study Of Pulsed Laser Deposition Of GaN Thin Films

In recent years, the wide band gap semiconductor has attracted much attention and advanced rapidly, mainly due to its promising applications in short-wave light emitting devices,Photodetectors,as well as anti-adiation,high frequency and high power electronic devices. Wurtzite structured Gallium nitride (GaN) is an excellent III-V nitrides semiconductor material which has large direct energy band gap of 3.4eV at room temperature. It is the ideal material for fabrication of ultraviolet (UV), blue and green light emitting diodes (LEDs), laser diodes (LDs). These light sources have promising applications and potential market demands for the high-density storage of opto-information, high-speed laser print, high-brightness and dynamic display in all colors, solid light sources, signal detectors and communication. In addition, GaN has been attracted much attention as a candidate for fabrication of high temperature, high frequency and high power devices. With the realization of some pivotal technical methods and the development of materials growth and devices techniques, the research on GaN has attracted more and more attention and advanced rapidly.It is still very difficult to grow large bulk single crystal of GaN at present, heteroepitaxy on various substrates is necessary for the growth of GaN films and the development of GaN-based devices. Now, MOCVD, MBE and HVPE have become dominating techniques to grow GaN materials. However, these technical are very complicated and the equipments are very expensive which confined the fabrication of GaN material. So many research institutes and universities in the world are trying to grow GaN materials with simple and lower cost methods. Pulsed laser deposition is a newly advanced films growth technique developed in recent years. The advantages for fabricating films by PLD method are that the growth is easily controllable by changing the experimental parameters, and high quality films can be produced at lower growth temperature.At present, almost all high quality GaN films are grown on sapphire,but sapphire itself is insulate,hard to incise, low thermal conductivity as well as difficult technics and high cost for devices. Accordingly,Si is regarded as one of the most promising substrate for growing GaN films due to its large size,high quality and relatively low cost. Furthermore , the GaN epitaxy on Si will facilitate the integration of microelectronics and optoelectronics. Therefore, the investigation of GaN epitaxy on silicon has practical significance. However,there are several difficulties to grow single crystalline GaN films directly on the Si substrate: 1) the high lattices mismatch and the differences in the thermal expansion coefficient between GaN and Si substrate; 2) the poor wetting of GaN on Si, 3) the nitridation of the Si surface during growth of nitrides.To overcome these impediment,lots of methods were used. Annealing is one of the most useful methods to improve the quality of GaN thin films. In this paper, we first fabricate the GaN films on Si substrate by PLD. Then we anneal the samples in NH3 at different temperatures. Measured results reveal that the grown samples are hexagonal wurtzite structure, the annealing temperature plays an important role in the fabrication of high quality GaN. With the annealing temperature increase, the structure, surface modality are effectively improved, and the optimum annealing temperature for GaN films deposited on Si (111) substrate at the temperature of 800 oC with laser energy of 360 mJ is 1100 0C in ammonia ambience for 15 min. If the annealing temperature above 1150 0C, the crystalline quality of the films will decline because of the volatilization of GaN is violent. That is to say that there is a transforming point of annealing temperature for GaN thin films from improving its quality to degrading that. In addition, two step methods are also used extensively to improve the quality of GaN films. That is to say buffer layers are introduced to decrease the effects of the lattice mismatch and thermal expansion coefficient difference between GaN and Si. At present, SiC and ZnO are widely used as the buffer layers of GaN thin films. In this paper, we investigate the effect of AlN and GaN to the growth of high quality of GaN thin films. A buffer layer is firstly grown on the Si substrate at low temperature, and then the GaN film is grown on the buffer layers. Results indicate that the quality of the film improved remarkably by using buffer layer, and the GaN buffer layer is better than AlN buffer layer to fabricate high quality GaN films using PLD.