Growth of Zero- and One-dimensional Si-based Nanostructures on Self-assembled Hexagonal Au Particle Networks

• Main Authors: Tian-Fu Chiang, 蔣天福
• Other Authors: Lih-Juann Chen
• Format: Others
• Language: en_US
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/32444330609839954814

博士 === 國立清華大學 === 材料科學工程學系 === 93 === The growth mechanism of SiOx nanowire on the self-assembled hexagonal Au particle networks has been investigated. Self-assembled hexagonal networks with discrete Au particles on different substrates were generated in samples annealed under N2 flow. A vapor transport process was used for the growth of SiOx nanowires. Two kinds of solid sources were used in this work: (1) high purity Si powder (purity 99.96 %) mixed with approximately 50 wt. % SiO2 powder (Si + SiO2); and (2) high purity Si powder only. These powders were placed in an alumina boat as the source material and positioned at the high-temperature (1200 0C) zone of a tube furnace. Ar gas was used as the carrier gas. The nanowires were grown onto Al2O3 (00•1) and Si (111) substrates located at the low-temperature (900 0C) zone of the furnace. After 3-5 hr of reaction, SiOx nanowires were found to grow on the Au particles. The resulting samples were characterized by scanning electron microscopy, transmission electron microscopy. The simulations were performed with the Vienna ab initio simulation package, using the non-normconserving ab initio Vanderbilt pseudopotentials method. On Al2O3 (00•1) substrate, preferred growth of SiOx nanowires on patterned facetted Au particles has been achieved with Si + SiO2 as the solid source after annealing at 900 0C for 5 hr. The {111} facets of Au particles were found to be parallel to the Al2O3 substrate and free from nanowire growth. On the other hand, SiOx nanowires were found to grow on {001} facets with a high density of ledges on the surface. Furthermore, nanowires were found to grow on all over the Au particles when pure Si was used as the solid source. The results are consistent with those obtained by the first-principle total energy and molecular dynamics computation and simulation. The preferred growth may be exploited to grow silica nanowires controllably in designated directions. On the Si (111) substrate, the nanowires grew selectively on the Au particles. The facets of Au particle tended to disappear and the Au particle became more spherical after nanowire growth. Nanowires were grown selectively on spherical Au particles on Si (111). The Si atoms in the vapor and from the substrate could be incorporated into Au particle to form Au-Si liquid droplet on Si (111) substrate at 900 0C. The liquid droplet continued to dissolve the Si atoms from the vapor and became saturated to induce the growth of Si nanowires selectively on Au particles. Subsequently, the Si nanowires were oxidized to grow the amorphous Si oxide nanowires. Therefore, the nanowires grown at Au particle network on Si substrate were designated as SiOx nanowires. The phase formation sequence of Ti on a single SiNW during heat treatment in ultrahigh vacuum has been studied by in-situ TEM observation and HRTEM examination. A SiNW with a diameter of about 30 nm and a length of more than 10 μm was obtained by using SiO + C powders as solid source. Amorphous layer was observed in the SiNW after Ti deposition in UHV-TEM at room temperature. The Ti5Si3, Ti5Si4 and C49-TiSi2 were found to appear near the surface of the SiNW annealed at 400, 600 and 700 0C, respectively. After annealing at 800 0C, the whole SiNW transformed to C49-TiSi2 nanowire. For C49-TiSi2 nanowires annealed up to 850 0C, no C54-TiSi2 phase was found. The reduced density of nucleation sites on the SiNW was thought to be the main reason for the retarded transformation of C49- and C54-TiSi2 phase. The existence of medium-range ordering structures or nanocrystallites in as-deposited amorphous SiGe thin films has been demonstrated by high resolution transmission electron microscopy in conjunction with auto-correlation function analysis. The density of nanocrystallites decreases in amorphous SiGe samples annealed at 300-350 0C then increases in samples annealed at 400-450 0C with annealing temperature. The observations can be interpreted in terms of free energy change with annealing temperature.