Solution Growth Of SiC Crystals With Different Morphologies

As one of the third generation wide bandgap semiconductors, silicon carbide (SiC) has been developed accompanying the progress of elemental semiconductors, e.g. Si, of the first generation and compound semiconductors, e.g. GaAs, GaP and InP, of the second generation. Due to the excellent physical and chemical properties, SiC bulk crystals and SiC nanomaterials have tremendous potential applications in the fabrication of optical and electronical devices that can operate at high temperature, high frequency, high power, and in harsh conditions. Based on the previous research, the syntheses and growth mechanisms of SiC bulk crystals and SiC nanomaterials using metal silicide melts as fluxes are investagated in the thesis.Solution growth of SiC crystals was practiced by heating FeSi flux in a graphite crucible. The experimental result revealed that carbon from the crucible dissolved and SiC crystals grew out of the melts. The XRD pattern of the SiC crystals demonstrates that the crystals mostly belong to 3C-SiC, with a few of 6H-SiC which are usually ascribed to the stacking defaults or other faults in 3C-SiC. The Raman spectrum confirms again that the SiC crystals are basically 3C-SiC but with some stacking faults and high level metal doping. According to the typical microstructure of the SiC crystals, the planar-nuclei-layer growth mechanism is proposed for the SiC crystal grown from the melts.In the study of the solution growth of SiC crystal, SiC nanowires were found on the surface of the melts when the relatively high oxygen impurity was present in the growth atmosphere. Based on the important phenomenon, the syntheses of SiC nanomaterials from FeSi and NiSi flux were performed, and a great deal of high-quality SiC nanowhiskers were obtained. The characterization of the products shows that the nanowhisker is 3C-SiC (a=4.364A), the fringe separation corresponding to (111) plane is 0.25nm and the growth direction is [111]. The Raman spectrum suggests that the SiC nanowhisker is highly crystalline. Compared to the standard spectrum, it has a little blue shift maybe due to size effect.The SiC nanowhisker formation has two stages, nucleation and growth stages. The SiC nanowhiskers root on the surface of the melt, and on the other hand, with solidifiedliquid droplets attaching to the tips. Base upon these, a growth mechanism combining solid-liquid-solid (SLS) mode and vapor-liquid-solid (VLS) mode is proposed. During the nucleation, carbon from the graphite plate (S) dissolved in the melt (L), in which Fe /Ni enhanced the carbon solubility in the melt, and the supersaturated carbon then reacted with the Si by the SLS mode to form SiC embryos (S). In the growth stage however, the vapor-liquid-solid (VLS) reaction of CO and SiO catalyzed by iron within the liquid droplets at the tips of growing SiC nanowhiskers leads to the SiC growth along [111] direction.Due to the poor wetting between NiSi melt and graphite at relatively low temperature, the NiSi melt did not spread but forming liquid balls on the graphite plate, and SiC nanowhiskers grew at the surface of liquid ball. Due to the reaction of residual intermediate products, amorphous SiO₂ wrapped SiC whiskers were formedA great abundance of SiC whiskers also formed on the wall of graphite crucible in the NiSi experiment. XRD patterns demonstrate that the whiskers mostly belong to 3C-SiC, with a few of 6H-SiC. Different from the nanowhiskers grown on the surface of the melt on the graphite plate, the whiskers did not have liquid droplets attached at the tips and direactly grew on bare graphite surface, which indicate that they grew along [111] direction following vapor-solid (VS) mechanism.