Investigation of metal-induced crystallization of silicon thin films and silicon based nanostructures

Abstract

The main focus of this thesis is on low temperature processing of polycrystalline Silicon (Si) thin films and Si nanorods. It has also been extended to induce the growth of more complicated nanostructures, such as superlattice and nanodots. The fabrication techniques for all of them, however, are based on catalytic metal assisted growth. As you read through this thesis, you will find that we start off with preparation of two dimensional (2-D) structure, then 1-D structure like nanorods, and finally nanodots, which are of 0-D geometry. Owing to the world energy crisis and the hazard of global warming due to carbon emission, it is meaningful and highly desirable to develop renewable energy technologies. Si based photovoltaic, by far, has been considered as one of the most promising candidates for replacing the conventional energy sources, like fuel and oil. Nevertheless, solar photovoltaic devices based on bulk crystalline Si (c-Si) is too expensive for large scale solar energy harvesting. An alternative is to use polycrystalline Si (poly-Si) thin films. The present most mature chemical vapor deposition (CVD) technique of fabricating Si thin film solar cells involves annealing the films at high temperature of above 900 ℃. For this reason, there is less degree of freedom on choosing cheap substrate, such as soda-lime glass. With currently fast growth of thin film technology, we aim to fabricate poly-Si thin film with large grain on inexpensive and transparent soda-lime glass substrate at low temperature, say 450 oC. Herein, we have adopted metal induced or metal mediate Si crystallization method to fabricate poly-Si on glass substrate. It is compatible to use Aluminum (Al) because of its low cost and being a standard electronic material, which can act as p-type dopant. The Al catalytic layer is deposited at room temperature under high vacuum by electron beam (ebeam) evaporation. Without breaking the vacuum, the Si was subsequently deposited at 450 ℃. During this process, it is very likely and highly reproducible to obtain poly-Si with grain size of more than 5 μm. Post annealing at the same temperature was used to improve the crystallinity of Si. The physical mechanism has been investigated and studied based on various characterization techniques, such as X-ray diffraction, micro-Raman spectroscopy, Field Emission Scanning Electron Microscopy and Transmission Electron Microscopy. The as-prepared poly-Si thin film can be used as a seed layer for the homo-epitaxial growth of a thick poly-Si absorbing layer. As a consequence, the IV characteristics of a solar cell with a lateral structure of glass/ITO/poly-Si (p ⁺ type)/poly-Si (p type)/a-Si (n-type)/Al are also presented. It has been well known that Si, as an indirect bandgap semiconductor material, exhibiting relatively low absorption capability in the solar spectrum. In order to enhance the light absorption it is vitally important to introduce micro- or nano- scale structures on Si thin films. In this respect, Si nanorods fabricated by vapor-liquid-solid (VLS) method on (111) oriented Si wafer has been studied in this project. The initial growth involves deposition of a very thin Au layer coating on Si wafer. Upon thermally treated at 500 ℃, this thin Au layer turns into numerous nano-sized droplets uniformly distributed on the Si wafer surfaces. During the subsequent deposition of Si, the substrate temperature is raised to 600 ℃. The continue supply of Si atoms leads to the nucleation and crystallization of Si at the site of the Au droplet. The cap which is made of Si-Au eutectic guides the growth direction of Si nanorods (SiNRs). From the experiment, we found all the SiNRs are well aligned vertically. Apart from the poly-Si thin films fabrication and Si nanorods growth, long range order Si twinning superlattice based on the interdiffusion of Al-Si has been demonstrated. This promising discovery differs from the conventional epitaxial growth of superlattice structure with two well lattice-matched materials or the molecular beam epitaxial growth of twinning superlattice by inserting boron (B) layer in between. It opens up a new method in dealing with single crystalline phase superlattice of a single material on amorphous substrate, such as glass. The final part of this thesis is devoted to studies of the most popular 2-D graphene. A complex nonlithographic patterning technique will be demonstrated. Large area and uniform crystalline Si nanodots (c-SiNDs) fabricated on CVD-made single layer graphene is expected to be useful for future nanoelectronic applications.