Modeling of physical and electrical properties of boron doped silicon microneedle grown by vapor liquid solid (VLS) mechanism

Abstract

Silicon microneedles are currently of great interest as one of the most useful and powerful forms of Si microstructures that could provide a promising option for emerging electronic systems. Microneedle have novel properties for various applications such as sensor, fabrication of active devices like transistors, solar cell etc. Si microneedles can be fabricated by various techniques and the Vapor-liquid-solid process (VLS) is a mechanism of forming such needle like crystal directly. Highly conductive doped Si microneedles are required for sensing small signals and for device fabrication. By in-situ doping into the VLS growth method, more conductive needles can be grown. Boron-doped p-type silicon micro needles have been grown via in-situ doping VLS mechanism using gold (Au) as a catalyst with di-silane (Si2H6) as Si source and diborane (B2H6) as dopant source. Experimental data shows that doping changes the growth kinetics and electrical properties of the needles. Thus boron doping brings about new challenges such as control of the sizes, structures, and properties of microneedles during the synthesis step. Hence the purpose of this thesis work was to study and analyze the physical and electrical characteristics of boron doped Si-micro needles in detail with proper mathematical modeling. At first, the effect of doping level on the growth rate of the microneedle was analyzed. A mathematical model relating growth rate with doping level was derived which is compatible with the experimental result. Similarly the effect of microneedle diameter on the growth rate has been explained with mathematical relation based on the experimental data and related research works. Again the dependency of the diameter of the needle on doping level and other initial conditions of VLS growth was analyzed and explained in this work. At last the electrical characteristics of the boron doped Si microneedle was investigated and explained using the physics of metal-semiconductor junction, Schottky barrier and band theory. All the results of this work have been compared with experimental data to justify the compatibility of the models. The mathematical model and analysis in this thesis will be very helpful to anticipate the size of such Boron doped microneedles and hence to fabricate microneedles of desired length and width for certain applications. The analysis of the electrical properties work will also be helpful to improve the J-V characteristics in future while fabricating vertical devices like diodes, transistors with Si microneedles.