Synthesis Of Quasi-one-dimensional Si Nanomaterials And The Field Emission Property

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.