Epitaxial Growth And Properties Of Hexagonal GaN Films On Silicon By MOCVD

Wide bandgap semiconductor Gallium Nitride and its related alloy materials, have great potentials in the applications of short wavelength LEDs and LDs, high temperature, high power and high frequency electronic devices. However, it is very hard to grow GaN bulk crystals, so the heteroepitaxial growth of high quality GaN thin films is the premise for the development of GaN-based devices. Sapphire is the most commonly used substrate for the GaN epitaxy growth, GaN-based devices are usually fabricated on this substrate. But sapphire shows many disadvantages, such as insulating nature, low thermal conductivity, high defect density, hard to cleave and expensive, which not only lead to the complex device fabrication process, but also limit the development of high power devices. Silicon as a good alternative substrate can make up these shortages. Therefore, the investigation of GaN epitaxy on silicon is of extreme practical importance.In this thesis, based on a comprehensive review of the research history and current status of GaN material preparation and GaN-based devices processing, we conducted a detailed study of GaN epitaxy on silicon substrate using our newly-bulit MOCVDsystem.The main content of this thesis is as follows:1. We present the epitaxial relationship between GaN films and Si(111) substrate and the growth model of GaN on silicon. GaN films with c-axial orientation was successfully grown on Si(111). There are two models of GaN on silicon. Without the buffer layer, the film shows polycrystal due to the three dimensional growth model. With the buffer layer, the growth model of the film becomes two dimensional.2. The high crystalline quality hexagonal GaN films on Si(111) have been grownusing high temperature A1N (HT-A1N) buffer layer, with the FWHM of (002) peak by X-ray rocking curve is 560arcsec. Our research results indicated that extending the growth time of A1N, some triangle islands appeared on the top surface of A1N and the crystalline quality of GaN decreased, yet these islands can release the tensile stress in GaN films, which are good for no cracking films growth.3. The optimization of GaN films growth parameters can improve the crystalline quality of GaN. The mixture of N₂ and H₂ were used as the top flow in our two-flow MOCVD system. The flux ratio of N₂/H₂ (1:1), the proper total top flow flux and the low flow rate of TMG are good for the crystalline quality of GaN.4. HRXRD and SEM were performed to characterize the defects in GaN films. The results indicated the threading dislocation density is 10⁹ cm^-2, which is the best without using ELOG technology. We contribute the hexagonal-shaped etch pits on the surface of GaN films to the following two reasons, one is the incomplete coalescence of GaN pyramids, the other is nanotube on the surface.5. PL, Raman and Hall were used to characterize the optical, stress and electronic properties. Si diffusion under high temperature may induce the yellow luminescence in PL spectra. Raman spectra results showed that GaN films were in tensile stress, which served as the direct cause of the cracks found in some of our samples, and A₁(LO) mode did not appear in the spectra when the GaN crystalline quality is poor. Hall tests showed the background electron carrier density can reduce to 10¹⁶cm^-3 , but mobility was still low because of the strong impurity scattering. Furthermore, Al_xGa_1-xN films with x=0.06 have been successfully grown on GaN/Si substate.