【作者】 万雨挺；

【导师】 沙健； 王业伍；

【作者基本信息】 浙江大学 ， 凝聚态物理， 2011， 博士

【摘要】 准一维硅纳米结构包括硅纳米线和纳米锥等。由于其奇特的物理和化学性质并且将可能在光致发光、生物传感器、场效应管、太阳能电池、锂离子电池、热电材料和场发射器件等诸多方面有潜在的应用前景,因此准一维纳米结构越来越被人们所关注。为了满足硅纳米结构的科学研究和技术应用等方面的需求,人们相继发展了多种方法来合成高质量的硅纳米线和纳米锥,例如气液固法(VLS),氧化物辅助法(OAG)和金属辅助化学腐蚀法(MaCE)等。尽管目前人们已经在硅纳米结构的生长方向、形貌和位置的控制等方面取得了巨大进展,但是准一维硅纳米结构的生长依然存在许多问题和挑战；虽然金属催化生长的机理已经被广泛的研究并理解,但很多具体的中间过程还是不清楚,例如硅的氧化过程,金属催化剂的迁移过程和纳米锥的径向生长过程等等。明晰这些过程不仅有利于准一维硅纳米材料的形貌控制,更有利于其杂质和缺陷等的控制,因为硅中的杂质和缺陷对其物理性能有着非常重要的影响。由于硅基半导体产业非常成熟,人们试图拓展硅在微电子产业以外领域的应用,比如场发射器件。准一维硅纳米材料长径比大且功函数较低,但是由于其热导率差,在场发射器件应用上受到限制。因此人们通常在硅纳米结构上覆盖其它具有良好场发射性能的材料,以有效地提高其场发射性能。我们知道,碳化硅是一种是重要的宽带隙场发射材料,能在高温、高频、高功率等极端条件下工作,因此我们在硅线阵列上生长碳化硅纳米线,以期降低其开启电场同时保持其场发射电流稳定性。基于此,本论文在硅线生长动力学以及其场发射性能等方面展开研究工作,主要工作内容分为以下几个部分：(1)以金为催化剂,采用化学气相沉积法成功地在Si(111)和(100)衬底外延生长了有序的硅纳米线。实验结果表明,外延生长对腔体中的氧含量非常敏感。当系统中的氧含量较高时,金不仅催化硅纳米线的生长也催化硅的氧化,导致纳米线停止生长,纳米线的低温热氧化实验验证了金催化硅氧化的过程；当系统中的氧含量较少时,金原子在硅线表面迁移,导致其电学性能变差,该现象在纳米线的直径较大或者金在共熔液滴中过饱和时尤为明显。(2)我们提出了一种新方法来合成硅纳米线。首先将草酸铜直接分散在氧化铝模板衬底上,然后在系统温度达到590℃时,向CVD腔体中通入硅烷气体生长硅纳米线。通过研究催化剂形成过程,我们发现,首先草酸铜自组织地热分解为Cu和Cu2O纳米颗粒,然后与硅烷反应形成Cu3Si,进而成为硅纳米线的成核中心。纳米线平均直径大约为20nm。这是一种廉价有效的硅纳米线制备方法。(3)通过VSS机制制备硅纳米锥,并详细地研究了硅纳米锥轴向和径向生长过程。为了避免由成核时间所导致的测量误差,我们首次设计并制备了多段硅纳米锥。然后建立模型解释了总气压和氢气含量等因素对硅纳米锥生长动力学的影响,我们发现Langmuir吸附决定了硅尖锥轴向生长速率对总气压的依赖关系,而氢在硅纳米锥表面的覆盖则会大大降低其径向生长速率。最后,通过对总气压和氢气含量的调节可以实现硅纳米锥锥角的有效控制。这些研究结果可能应用于硅基太阳能电池。(4)使用NiO为催化剂在有序的硅纳米线上制备β-SiC纳米线。普通衬底上的SiC场发射开启电场为3.4 V/μm,而在硅线阵列上的SiC场发射开启电场低达2.2 V/μm,与文献报道的有序SiC纳米线相近。这种结构的场发射性能也很稳定。经过一系列地系统研究,我们对硅线(锥)的VLS和VSS生长过程有了更深入地理解,并对其在场发射等领域的应用做了初步的工作。更多还原

【Abstract】 Quasi-one-dimensional silicon nanostructures, including silicon nanowires (SiNWs) and silicon nanocones (SiNCs), have attracted more and more interest for their novel physical and chemical properties and potential wide applications in photoluminescence, biology sensor, FET, solar cells, Li battery, thermoelectric materials and field emission devices. In order to meet the need of scientific study and technical application of silicon nanostructures, many methods have been developed continuously to synthesize high-quality SiNWs and SiNCs, such as vapor-liquid-solid (VLS), oxygen-assisted growth (OAG), metal-assisted chemical etching (MaCE) and so on. Although much progress has been made in the control of nanostructures’ orientation, morphology and position, there are still many problems and challenges in the synthesis of them. Concretely speaking, although the VLS growth mechanism has been extensively studied and understood, many detailed medium processes remain unclear, i.e. the oxidation of silicon, the migration of catalyst and the radial growth of SiNCs etc. To understand these processes will not only be advantageous in the control of the morphology of quasi-one-dimensional silicon nanostructures but also in the control of contaminants and defects, which have important influence on their physical properties.Semiconductor industry based on silicon has been so well developed that the applications of silicon are being extended to other fields beside microelectronics such as field emission devices. Quasi-one-dimensional silicon nanostructures have high aspect ratio and low work function, but due to the low thermal conductivity the application of silicon in field emission has been restricted. Therefore, other good field emission materials are usually used to cover silicon nanostructures in order to improve the field emission performance effectively. As we know, SiC is a kind of important wide band gap semiconductor that can be operated in harsh conditions such as high temperature, high frequency and high power. So we intend to employ SiC nanowires to cover the surface of silicon arrays for the purpose of lowering the turn-on field and holding the stable current. Based on the above mentioned, the main work of this thesis is focused on the growth kinetics and field emission properties of quasi-one-dimensional silicon nanostructures, divided into four parts as follows:(1) Ordered SiNWs were successfully grown on silicon (111) and (100) wafer epitaxially using gold as catalyst via chemical vapor deposition. The results show that the epitaxial growth was very sensitive to the oxygen content in the chamber. When the oxygen content was high, gold not only catalyzed the VLS growth but also the oxidation of silicon, which stopped the growth. This was demonstrated by the experiment of low-temperature oxidation of SiNWs. When the oxygen content was low, gold atoms migrated on the surface of silicon nanowires, degrading the electric conductivity. The migration of gold was enhanced when the diameters of SiNWs were large or gold was supersaturated in the eutectic drop.(2) A novel approach to prepare SiNWs was proposed. First, copper oxalate was dispersed on the AAO substrate directly. When the system temperature reached 590℃, silane was flowed to the CVD chamber to grow silicon nanowires. The process of the formation of catalyst was studied. It was found that copper oxalate was thermally decomposed to be Cu and Cu2O nanoparticles self-assembly. Then the nanoparticles reacted with silane sequentially to form Cu3Si, which served as the nuclei for the growth of SiNWs. The as-grown nanowires are as thin as 20 nm in diameter in average. It is a cheap and efficient method for the synthesis of SiNWs.(3) SiNCs were synthesized through the VSS mechanism. Detailed study of the axial and radial growth of SiNCs was carried out. In order to avoid the measuring error that rised from the nucleation time, multi-segment SiNCs were first designed and fabricated. A model was developed to investigate the growth kinetic of SiNCs dependent on pressures of silane and hydrogen. It was found that Langmuir adsorption contributed to the dependence of the axial growth rate on total pressure, while hydrogen coverage on the surface of SiNCs inhibited the radical growth rate greatly. In the end, we also demonstrated the controllable cone angles of SiNCs by modulating the total pressure and hydrogen content, which might be applied in silicon-based solar cells. (4) (3-SiC nanowires were grown on aligned SiNWs using NiO as catalyst. The turn-on field of SiC on silicon substrate was 3.4 V/μm, while it was 2.2 V/μm for SiC on silicon arrays substrate, which approached the ordered SiC nanowires in the literature. Besides, the SiC coated silicon arrays posed to be a kind of stable field emission structure.After a series of systematic study, better understanding of the VLS and VSS process of SiNWs (SiNCs) has been achieved and primary work of their application in field emission has been accomplished.更多还原