Epitaxial Growth And Characterization Of GaN And Al_xGa_1-xN Films

In recent years, Gallium Nitride as a wide band gap semiconductor has attracted more and more attention for its applications in blue, green light emitting diodes (LED). laser diodes (LD). and high-temperature, high-frequency, high-power electronic devices, such as HEMTs. However, the majority of GaN-based devices are fabricated on sapphire substrate, which has many disadvantages to the device fabrication processes, such as insulating nature, hard to cleave. Silicon substrate is an excellent alternative to sapphire, which is cheaper and has much higher crystalline quality. Most importantly, it is conductive and can serve as electrodes itself. In this thesis, focused on the fabrication of GaN-based devices on Si substrates, we successfully grew GaN epilayers on Si (111) using MOCVD technique, laying foundation to subsequent fabrication of GaN-based devices.The main content of this thesis is as follows:1, The optimization of film growth parameters for our newly-built MOCVD system and system troubleshooting. The optimized parameters include substrate temperature for epilayer growth, film growth rate (mole flow rate of TMGa), and rotation speed of substrate. According to the characterization results, GaN has a hexagonal structure, with a band gap of 3.4eV. The FWHM for (0002) peak of GaN over sapphire substrate is 21.5 arcmin. Meanwhile, some problems occurred during our experiments have been solved, with some parts of MOCVD system being improved.2. The epitaxial growth and characterization of GaN epilayers on Si(lll). Low-temperature GaN buffer layer and A1N buffer layer have been used for the growth of GaN on Si. High-quality GaN films have been grown using a high-temperature A1N buffer, with low mole flow rate of TMA1. Moreover, the crystalline quality of GaN can be greatly improved through in-situ annealing in NI atmosphere, with the FWHM of GaN (0002) being decreased from l050arcsec-2-to 560arcsec. This FWHM value is one of the best results which have ever been reported in literature.5. Micro-Raman spectra and SIMS characterizations have also been carried out on our GaN samples. Raman spectra results showed that our GaN films are in tensile stress, which serves as the direct cause of the cracks found in some of our samples. SIMS profile showed that interdiffusion at the GaN-Si interface is significant. Silicon's diffusion into GaN can serve as self-dopant in epilayers. In order to eliminate Si's self-doping effect on GaN. the thickness of GaN must be more than 0.5um.4. The growth of AlxGai.NN layers using MOCVD system. X-ray diffraction results showed that AlGaN films have been successfully grown on GaN/Si substrate. The FWHM value for AlGaN(0002), AlGaN(0004) are 710arcsec. 506arcsec, respectively. The mole fraction of Al, as indicated in the results of co-26 scan, is 22.8%, which can meet the demands of LED devices.