Study Of Low-dimensional SiC Grown By VLS Process

Silicon carbide (SiC), owing to its high breakdown field, high thermal conductivity, high saturated electron drift velocity, large energy band gap and excellent chemical stability, has been considered as one of the most promising semiconductor for applications in high power, high frequency and high temperature devices.Vapor-liquid-solid (VLS) process is a common technique used in nanorods and whiskers growth. It involves the formation of a liquid droplet on top of the growing crystal, which acts as"catalyst". The catalyst leads to the dissolution of the carbon and silicon atoms from the decomposition of the gaseous reactant. Due to the assistance of the liquid solution during VLS process, the growth of SiC occurs at the liquid-solid interface like in the liquid phase epitaxy (LPE) process. So the VLS process has the advantages of both CVD technique and LPE technique. Because of the liquid droplet, the quality of the grown SiC nanorods is excellent. In this dissertation, VLS process is used to grow SiC. The samples are characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, atomic force microscopy, Electron backscatter diffraction, transmission electron microscopy and micro-Raman spectroscopy.First, SiC whiskers are fabricated on Si substrate by VLS mechanism as shown in chapter 3. Ni is used as the catalyst. The processes of VLS mechanism are studied. The factors such as growth temperature, growth pressure, the thickness of the catalyst and so on, which will affect the SiC whiskers growth, are also investigated. Nano-scale and micro-scale SiC whiskers are fabricated successfully.Then, SiC micro-pillars are grown on patterned Si substrates by VLS mechanism as shown in chapter 4. There are lots of circular features on the patterned Si substrates before our experiments. Ni is also the catalyst. The diameter of the grown SiC micro-pillars reaches to 20μm. The full width at half maximum (FWHM) value of the 3C-SiC TO mode is about 8 cm-1. The residual stress in 3C-SiC micro-pillars was only 0.3 GPa. The studies showed that the crystalline quality of the VLS grown 3C-SiC micro-pillars was excellent. The"over-growth"was observed, during SiC micro-pillars fabrication. So the VLS process is utilized to homoepitaxially grow 6H-SiC thin films as shown in chaper 5. Compared to the conventional CVD growth, micro-pipes are almost closed by the VLS mechanism. However, on the surface, the step height is about 15 nm and the root-mean-square roughness (RMS) is about 4.5 nm. In order to improve the RMS of the grown thin film, a"two-step"method is adopted. In the first step, VLS growth is used to eliminate the formation of micro-pipes, and the subsequent step based on the conventional CVD process is employed to improve the surface roughness. The RMS of the grown thin film is decreased to 2.5 nm. From the SEM, AFM and HRXRD characterizations, it is shown that thin films grown by the two-step method not only exhibit closed MPs but also improve surface morphology of the epilayers. The carrier concentration and the mobility is 7.9×1018 /cm3and 200μ(cm2/Vs).At last, the porous SiC thin films are also grown on Si substrates successfully as shown in chaper 6. Methyltrichlorosilane (MTS) is used as the reactant gas. The porous SiC thin films are used as the buffer layer to epitaxy 3C-SiC thin film. Because of the roughening interface, the stress between the Si substrate and SiC thin film should be released. 3C-SiC thin film grown on porous SiC buffer layers has a high crystalline quality. Inωscanning, FWHM value of the SiC(100) peak is only 0.8°.