【作者】 曹连振；

【导师】 宋航； 蒋红；

【作者基本信息】 中国科学院研究生院（长春光学精密机械与物理研究所） ， 凝聚态物理， 2010， 博士

【摘要】 SiC作为第三代核心半导体材料之一,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,在紫外探测器和短波发光二极管及高频、大功率、高温、抗辐照的电子器件等领域具有广阔的应用前景。SiC零维和一维纳米材料,由于处在纳米尺度,除具有体材料所具有的优异性质外,还具有其大尺寸材料所不具有的性质,在光催化、光电子器件和场发射显示等方面已显示了巨大的应用价值,因此研究SiC零维和一维纳米材料具有重要的意义。目前,SiC零维和一维纳米材料的研究主要集中在两个方面:用不同的方法制备SiC零维和一维纳米材料;对制备的SiC纳米材料进行光学和发射特性表征研究。所采用的制备SiC纳米材料方法都有各自的优点,但是这些方法也存在一些缺点:产物较少,易引入杂质,制备温度高,成本较高等。特别是由这些方法制备的SiC纳米材料大都存在大量的表面态,导致发光效率降低,并且SiC一维纳米材料在场发射显示的应用中需要较高的开启电场,限制了SiC纳米材料的实际应用。在SiC表面包覆一层SiO2保护层,实现纳米SiC/SiO2核壳结构,可以减少SiC材料本身的表面态,提高发光效率,而且薄的SiO2层,还可以改善SiC一维纳米材料的场发射特性。因此采用一些简单易行的方法来制备SiC/SiO2核壳结构纳米材料,并研究其光学及场发射特性,具有重要的物理意义和应用价值。本论文的研究内容主要包括以下几部分:1).首先设计了一个采用两步生长的办法来合成SiC/SiO2核壳结构纳米颗粒的方案,并采用热化学气相沉积(T-CVD)和热退火技术,首次合成了零维6H-SiC/SiO2核壳结构纳米颗粒。用扫描电镜(SEM)、透射电镜(TEM)、能谱(EDX)、X射线衍射(XRD)等手段对制备的纳米颗粒进行了表征。研究了影响纳米颗粒合成的因素。2).利用固相SiO2与催化剂混合,在CH4气氛下,成功合成了“线状”、“珊瑚状”和“芦荟状”三种不同形貌的SiC/SiO2核壳结构一维纳米材料。分析认为纳米材料的生长方式为典型的“气-液-固(VLS)”生长机制且为顶部生长模式。生长过程中所引入的Fe催化剂对纳米材料的形成起着重要作用,同时催化剂的形貌不同是形成不同形状纳米材料的主要原因。3).用光致发光谱(PL)分别研究了零维和一维SiC纳米核壳结构材料的光致发光性质。研究表明,对于SiC纳米颗粒,未退火前的发光主要是SiC表面的缺陷发光;退火后表面缺陷发光消失,SiC的本征发光强度明显提高。表明有SiO2包覆的SiC纳米材料的发光效率得到改善。以“线状”的β-SiC/SiO2一维核壳结构纳米材料为例,研究了其室温和变温光致发光谱。研究发现“线状”纳米材料的室温宽带峰可以分解为主峰分别为2.3eV和2.1eV的两个发光峰组成。2.3eV的发光是SiC的带边发光,2.1eV的发光来源于表面缺陷态。4).研究了“线状”、“珊瑚状”和“芦荟状”的β-SiC/SiO2一维核壳结构纳米材料的场发射特性。结果表明,三者都具有较低的开启电场和较高的发射电流密度,是良好的场发射阴极材料。β-SiC/SiO2一维核壳结构纳米材料具有良好的场发射特性的原因主要有三部分:一是,核壳结构纳米材料都具有大量有效的发射点;二是,核壳结构纳米材料都具有很大的长径比,使得在材料的顶端容易形成强的局部电场,使电子易于克服表面势垒发射到真空中;三是,包覆的具有小的电子亲和势的10nm左右的非晶SiO2薄层,可以改善核壳结构SiC纳米材料的场发射特性。更多还原

【Abstract】 As an important third-generation semiconductor, silicon carbide, with wide bandgap, high breakdown voltage, high thermal conductivity, and high saturation drift velocity, appears to be a promising material for UV detector, short-wave light-emitting diodes and electronic devices that can be operated at high power, high frequency, high temperature and in harsh environment. Zero-dimensional and one-dimensional SiC nanostructure materials, as in the nano-scale, have some new properties compared to the bulk-SiC material. By now, the zero-dimensional and one-dimensional SiC nanostructure materials display huge potential application in photocatalytic, optoelectronic devices and field emission display etc. The investigation of SiC zero-dimensional and one-dimensional nanostructure materials is an attractive and important work. At present, the research of SiC zero-dimensional and one-dimensional nano-materials has been focused on two aspects: preparation of SiC nanomaterials with different methods, and characterization of optical and field emission properties. However, these preparation methods have some disadvantages, such as, low yield, high impurity, high preparation temperature, high cost and so on. The SiC nanomaterials prepared by these methods usually have a large number of surface states, resulting in lower luminous efficiency. In particular, the application of SiC one-dimensional nanomaterials on field emission display needs high turn-on field. Fabricating SiC/SiO2 core-shell zero-dimensional and one-dimensional nanomaterials by coating a SiO2 protection layer can reduce the surface states and improve the luminous efficiency of SiC material. At the same time, a thin SiO2 layer can enhance the field emission characteristics of SiC one-dimensional nanomaterials. Thus, creating some simple and effect methods to prepare SiC/SiO2 core-shell structure materials and study their optical and field emission characteristics have important physical meaning and practical value. Major contents of study in this thesis were summarized as follows:1). First we designed a two-step approach to synthesize the SiC/SiO2 growth of core-shell nanoparticles. By thermal chemical vapor deposition (T-CVD) and thermal annealing process, we successfully synthesize zero-dimensional 6H-SiC/SiO2 core-shell nanoparticles. The scanning electron microscopies (SEM), transmission electron microscopy (TEM), energy dispersive x-ray detector spectroscopy (EDX), X-ray diffraction (XRD) were used to characterize the nano-particles. The effect factors of fabrication of nano-particles were also analysized.2).‘Normal-shaped’,‘coralloid’and‘aloetic-shaped’β-SiC/SiO2 one-dimensional nanowires were synthesized by using simple synthetic method. The growth mechanism of nanowires is‘vapor-liquid-solid (VLS)’with a‘tip growth model’. The Fe nanoparticles play a key role in the formation of nanowires and the catalyst morphology is the key factor for synthesizing different shapes of SiC nanowires.3). Photoluminescence (PL) spectrum was used to characterize the optical properties of zero-dimensional and one-dimensional SiC core-shell structure nanomaterials. The results showed that, before annealing in oxygen atmosphere, luminescence of SiC nano-particles were mainly from SiC surface defect emission, emission from surface defects disappeared and the intrinsic emission intensity of SiC obviously enhanced after annealing. This indicated that the luminous efficiency of nano-SiC coated with SiO2 materials has been improved. Room temperature and variable temperature photoluminescence of the‘normal shaped’β-SiC/SiO2 one-dimensional core-shell structure nanowires were also investigated. It was found that the PL boradband of nanowires can be decomposed as two emission peaks, with maximum peak at 2.3 eV and 2.1eV, respectively. It was proved that he 2.3 eV luminescence from SiC band edge emission and 2.1eV luminescence from surface defect states.4). Field emission properties of the‘normal-shaped’,‘coralloid’and‘aloetic-shaped’β-SiC/SiO2 one-dimensional nanowires were investigated detailedly. The results show that all nanowires have a low turn-on field and high emission current density. So the three nanowires are good field emission cathode materials. The reasons of the SiC core-shell nanowires with good field emission characteristics can be summarized as follows: first, core-shell nanowires have a large number of effective emitting point; Second, core-shell nanowires have a great aspect ratio, makes it easy to form strong local electric fields can be easily formed on the tips. This electric field will make the electron overcome the surface barrier to the air with ease and then greatly enhance the electron emission; Third, the thin SiO2 shell with a small electron affinity. By coating with 10nm SiO2 shell, the field emission properties of SiC nanowire emitters can be enhanced.更多还原