|M.Sc Student||Aharonovich Igor|
|Subject||Growth and Structure of Silicon-based Nanowires|
|Department||Department of Materials Science and Engineering||Supervisor||Professor Emeritus Yeshayahu Lifshitz|
The miniaturization of microelectronic devices by lithography is reaching its technological limit and alternatives are requested. Si nanowires (SiNWs) are most suitable candidates to serve as building blocks for future electronics. Implementing SiNWs in industrial processes requires high quality SiNWs with well defined properties. The present work aims at the controlled growth of Si based NWs and the understanding of the correlation between their growth parameters, structure and properties.
Currently, SiNWs are synthesized by either a catalytic Vapor-Liquid-Solid (VLS) process or a non catalytic Oxide-Assisted-Growth (OAG). A typical VLS process begins with a liquid droplet of a metal catalyst which dissolves a gaseous silicon reactant followed by nucleation and subsequent growth of the SiNW. The OAG mechanism is based on a SiOx droplet which collects the incoming SiO vapor clusters that condense on the SiOx droplets. Afterwards, a Si-Si bond is formed, and a silicon crystal is nucleated. A SiO2 shell wraps the Si core and suppresses the lateral growth of the NW.
Laser ablation and thermal evaporation techniques were employed to construct silicon based NWs through these mechanisms. In both techniques silicon containing vapor is transferred by a carrier gas to heated substrates where NWs are expected to grow. The growth parameters investigated in both techniques included: (1) the target material; (2) the carrier gas composition, flow and pressure; (3) the substrate type; (4) the substrate temperature; (5) the effect of a metal catalyst.
The obtained NWs were investigated using High Resolution Scanning and Transmission Electron Microscopes with Energy Dispersive Spectroscopy, Electron Energy Loss Spectroscopy, X-ray Diffraction, Fourier Transform Infra Red Spectroscopy, Photo Luminescence, X-ray Photoelectron Spectroscopy and Auger.
The main results are: (1) Laser ablation yielded amorphous SiOx NWs and a mechanism for this growth was suggested; (2) Thermal evaporation yielded single crystalline SiNWs embedded in a SiOx sheath. The nucleation of SiNWs as well as the density were found to highly depend on the substrate type; (3) A metal catalyst was essential for the NW growth in both growth systems.
Finally, our SiNWs were compared to different SiNWs grown by decomposition of silane on a metal catalyst and by thermal evaporation without a metal catalyst. The SiNWs produced by silane exhibit a straight morphology and single crystalline Si core with a very thin oxide shell. In contrast, the SiNWs produced by thermal evaporation without metal catalyst revealed different morphologies such as nanochains.