Abstract:
Semiconducting microstructures and nanostructures are promising candidates in MEMs, optoelectronics, sensor circuits and photovoltaics. Needle like Si microstructures have found several novel applications. Si microneedles can be fabricated in many ways, however, the Vapor Liquid Solid (VLS) process is one of the most prominent method for fabricating needle like crystals directly.
Highly conductive doped Si microneedles are required for sensing small signals and for device fabrication. By in situ doping into VLS growth mechanism, more conductive needles can be grown. Phosphorus doped n-type silicon microneedles have been grown via in situ doping VLS method using gold (Au) as catalyst particle with disilane (Si2H6) as Si precursor source and phosphine (PH3) as the dopant source. Experimental data shows that the doping changes the growth kinetics and electrical properties of the needles. Thus phosphorus doping brings about new challenges such as control of 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 phosphorus doped Si microneedles in detail with proper mathematical modeling.
At the very first, the effect of changing doping level on the growth rate has been 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 phosphorus 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 phosphorus doped microneedles and hence to fabricate microneedles of desired length and diameter for certain applications. The analysis of the electrical properties will also be helpful to improve the I-V characteristics in future while fabricating vertical devices like diodes, transistors with Si microneedles.
