摘要
SiC作为第三代宽带隙半导体材料,由于它具有宽带隙、高临界击穿电压、高热导率、高载流子饱和漂移速度等特点。在高频,大功率、耐高温、抗辐照的半导体器件及紫外探测器和短波发光二极管等方面具有广阔的应用前景。由于SiC纳米结构处在纳米尺度,因此其除了保持上述优良特性外,还具有许多大尺寸晶体所不具备的性质,其必将在未来纳米器件的制造中有着极为重要的应用。所有这方面的研究是以SiC纳米结构的制备研究为先导。本文在前人研究的基础上通过热蒸发法研究了C-Si-O、C-Si-O-Fe、C-Si-O-Ni、C-Si-O-Al、C-Si-O-Zn-S等体系下SiC以及其他纳米结构的制备;得到了一系列重要结论和一些创新性成果。
在C-Si-O体系下纳米结构的生长中我们发现在坩埚的不同部位生成了不同的产物。在放置碳纳米管的氧化铝基片上同时得到了SiC纳米线和SiC纳米管,而在装有纯硅的高纯石墨坩埚内壁上则生成了SiC/SiO_2纳米电缆。同时生成SiC纳米线和SiC纳米管是因为作为碳源的部分碳纳米管中含有残留催化剂颗粒而另一部分不含这些催化剂颗粒。含有催化剂颗粒的位置将以碳纳米管作为碳源通过VLS机制长成SiC纳米线;不含金属催化剂颗粒的碳管将通过VS机制由外到内逐层转变为SiC纳米管。高纯石墨坩埚内壁上由于炉壁循环水的作用其在降温阶段总是具有比氧化铝基片上具有更低的温度,也就是说该位置具有更有利于SiO_2生成的热力学条件;因此最终该位置在升温和保温阶段生成的SiC纳米线上将沉积一层SiO_2从而得到SiC/SiO_2纳米电缆。
SiC纳米线的制备中温度对结果的形貌有着显著的影响,在低温下通常得到表面光滑的圆柱状SiC纳米线而温度较高时由于具备了足够的动力学条件表面原子将发生重排得到具有{111}系列小面侧面的六棱柱状SiC纳米线。
在SiC/SiO_2复合结构的制备中作为核心的SiC纳米线的形貌差异也会对制得的SiC/SiO_2复合结构形貌带来明显的影响;在温度较低核心为表面光滑的圆柱状SiC纳米线时降温阶段生成的SiO_2将均匀的平铺在SiC纳米线核心上面从而形成SiC/SiO_2纳米电缆而当温度较高核心为具有{111}系列小面侧面的六棱柱状SiC纳米线时/SiO_2会在突出的位置聚集并最终聚集为一个一个分立的球状个体;从而生成糖葫芦状SiC/SiO_2复合结构。
C-Si-O体系中不同元素的加入会对体系中得到纳米结构的尺寸,形貌,产量带来不同的改变。C-Si-O-Fe体系中得到了直径尺寸逐渐增加的六棱柱状SiC纳米线,不同Fe含量会对其直径的渐变规律以及SiC纳米线生长速度带来有规律的影响。随着Fe含量的增多,SiC纳米线的纵向生长速度降低而且产物中SiOx的产量增多,也就是说Fe催化剂的存在相对更有利于SiOx的生成。同样温度下C-Si-O-Ni体系中得到的主要产物为圆柱状SiC纳米线,这表明影响除了温度外不同的催化剂也会对SiC纳米线到底是圆柱状还是六棱柱状带来影响;同时C-Si-O-Ni体系中Ni的含量的改变对所得产物形貌的影响相对并不明显,至于该体系下其他温度条件下是否会有不同的结果将有待他人继续研究。C-Si-O-Al体系中的产物为超薄的纳米带。
C-Si-O-Fe-Zn-S体系中ZnS的存在非常明显的增加了所得产物的尺寸,该体系下的到的SiC晶须直径较粗,达到500nm左右,此时由于SiC晶须尺寸较大因此其VLS机制生长中生长端的催化剂颗粒很少有脱落的现象,这也从侧面印证了其他体系中晶须末端本来也有同样的催化剂颗粒,只是由于其他体系下SiC纳米线尺寸相对较小因而催化剂颗粒较容易脱落;将催化剂颗粒移除后我们可以看到这些晶须的生长端有两种典型的形貌,一种是其他体系中最为常见的扁平状的末端,也就是说他们的生长为二维形核生长机制;而与其他体系不同的一点是该体系有部分晶须末端为圆锥结构,也就是说有部分晶须的生长是由螺型位错机制所控制的,同时也表明在SiC纳米线的生长中螺型位错机制并非主流机制,至于为何尚有待他人做进一步的深入研究。
As one of the third generation wide band-gap semiconductor, silicon carbide (SiC) has wide energy band gap at room temperature, high breakdown electric field, high thermal conductivity, and high elelctron drift velocity. These excellent properties make SiC become a promising semiconductor material for the fabrication of optic and electronic devices, especially for the devices used at high temperature, high power, high irradiation and high frequency. Besides all the mentioned properties above SiC nanostructures have many other remarkable optical, electrical and mechanical properties which bulk SiC crystals don't have, therefore they have great potential applications as interconnects or functional components in nanoscale electronics, optoelectronics and mechanical devices. All researches in this aspect take the preparartion of SiC nanostructures as their starting points. In this thesis we studied the preparation of SiC nanostructures and other nanostructures in C-Si-O, C-Si-O-Fe, C-Si-O-Ni, C-Si-O-Al, C-Si-O-Zn-S systems by thermal evaporation method. We obtained some important conclusions and some innovative achievements.
In C-Si-O system, we obtained SiC nanowires and SiC nanotubeson the alumina grid where multi-wall carbon nanotubes (MWCNT's) were situated, while SiC/SiO_2 nanocables on on the inner wall of the graphite crucible where Si melt was contained. It is believed that SiC nanowires were formed from those MWCNT's still with metallic catalyst particles attached by Vapor-Liquid-Solid (VLS) mechanism, and SiC nanotubes were obtained by Vapor-Solid (VS) mechanism due to the tranforamtion of other MWCNTs without catalyst particles attached (falling down). In both situations, CNTs supplied C and Si vapor due to the evaporation of Si was Si source In the cooling stage temperature in the inner wall of the graphite crucible will be always lower than the alumina grid, so the SiC nanowires on the inner wall could firstly reach the reaction temperature for the deposition of SiO_2 and then always be prior to form SiC/SiO_2 nanocables.
Temperature plays an important role for the synthesis of SiC nanowires. At low temperatures the SiC nanowires produced are cylindrical shaped with smooth surface. When the temperature is high enough, Si atoms and C atoms on the surface of SiC nanowires are rearranged as the kinetic qualification could be conquered to grow hexagonal prism shaped SiC nanowires with {111} plane side facets.
For the syntheses of SiC/SiO_2 composites the two different shapes of SiC nanowires will determine the geometrical of the products. For low temperature the core is cylindrical SiC nanowires with very smooth surface, and SiO_2 thus formed on the cooling process would wrap on the SiC core uniformly to produce SiC/SiO_2 nanocables. At high temperature however, because the core is hexagonal prism shaped SiC nanowires with continual side facets, SiO_2 formed on cooling do not wrap on the core but rather assemble to some spherical particles to become Tang Hum-like SiC/SiO_2 composites at last.
The joining of other elements in C-Si-O system may bring about many changes in size, shape and yield etc. to the nanostructures. In C-Si-O-Fe system, we find that Fe addition favored the formation of hexagonal prism ans at the same time affected the diameter and growth rate regularly. With increasing Fe content, the diameter of the produced SiC nanoprisms increased from about 50 nm to hundreds of nanometer, and the yield of SiOx was enhanced. In C-Si-O-Ni system, cylindrical SiC nanowires were mostly obtained at the temperature where nanoprism should be formed in other systems. It demonstrates that not only temperature but also new elements can affect the shape of the synthesized nanostructures. In C-Si-O-Al system we obtained ultra-thin nanobelts.
The addition of ZnS in C-Si-O-Fe system further increased the diameter of the products to about 500 nm. Furthermore, big size metal catalyst particles were often found on the top of these SiC nanowires. After these metal catalyst particles removed by erosion we can see two types of tips, one was even tip, we believe these nanowires growth under two dimensional nucleation mechanism; other was thiner needle tip, which is very close to those nanowires growth under screw dislocation growth mechanism. That is to say some of the nanowires growth controlled under screw dislocation growth mechanism, but this is not the major mechanism, the reason of such phenomena still needs further investigation.
引文
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