Growth and characterization of ZnO and SiC nanowires

Abstract

The synthesis of semiconductor nanowires has been studied intensively worldwide for a wide spectrum of materials. Such low-dimensional nanostructures are not only interesting for fundamental research due to their unique structural and physical properties relative to their bulk counterparts, but also offer fascinating potential for future technological applications. Deeper understanding and sufficient control of the growth of nanowires are central to the current research interest. The objective of the thesis work is synthesizing semiconductor nanowires using various growth processes, with a focus on the spontaneous growth process, which offers an opportunity for the control of spatial positioning of nanowires. Zinc oxide (ZnO) based and Silicon carbide (SiC) based nanowires have been concentrated to synthesize using vapor-solid (VS) and vapor–liquid–solid (VLS) techniques respectively. ZnO is one of very interesting semiconductor material because of its physical and chemical properties. Also, it is well known that high n-type conductivity can be achieved by alloying zinc oxide with group III elements (such as Al, In or Ga) in ternary or even quaternary oxide compounds, in order to obtain transparent conducting oxides (TCOs). In this part of work, there were two major materials have been synthesized such as vertically aligned ZnO nanorods and ternary Zn(In,Ga,Sn)O nanorods using vapor phase technique. First, solution-free and catalyst-free vertically aligned ZnO nanorods have been synthesized by thermal CVD reactor at relatively low temperature (< 500 °C) to produce high-surface 3D photoanode on glass substrate. Different TCOs films such as Al doped ZnO films deposited by PED, RF-sputtering techniques and ITO were considered for the growth as starting seeding layer for the nanorods. The aim of this work is mainly focused to control the thickness and length of these nanostructures by varying not only the growth parameters, such as amount of Zn evaporation, but also substrate characteristics, such as grain size of Al doped ZnO and ITO seeding films. Second, Indium Zinc oxide nanorods (IZO-NRs) have been obtained at temperatures lower than 500°C using same CVD system, with a resulting indium concentration larger than 1%. The growth of these ternary oxide nanostructures has been obtained at relatively low temperature, starting from the corresponding metals, thanks to the direct deposition on the growth substrate of an In layer, which in its molten state and upon mixture with Zn acts as growth seed. The obtained indium concentration corresponds to the value required to get metallic behavior and make this ternary oxide a TCO (transparent conducting oxide), while the used temperature range makes it compatible also with commercial glass substrates. Same technique have been used to obtain GaZnO and SnZnO nanostructures. Among many kind of semiconductor, SiC is an important wide band gap IV-IV semiconducting material and it exhibit excellent, unique physical and mechanical properties at nano-scale, which lead to their potential applications for being used as the building blocks in nanoelectronics and nanooptoelectronics. Also, it has biocompatibility and inertness can be exploited for biomedical applications. In this part of work, there were two types of SiC nanowires have been synthesized using VLS growth technique. First, Cubic SiC nanowires were successfully grown using home-made induction heated Vapor Phase Epitaxy (VPE) reactor on Si (100) and Si (111) substrate using nickel (Ni) and Iron (Fe) as a catalysts. The main aim of this work is to optimize the condition to grow SiC nanowires with Ni and Fe catalyst. The size and shape of the nanowires has been controlled using temperature and gas flow rate. Second, self-assembled SiC core with SiO2 shell coaxial nanowires using Ni and Fe catalyst have been synthesized by thermal CVD reactor. The growth conditions were optimized for both catalyst using temperature, gas flow rate. This SiC /SiO2 coaxial core/shell nanowires (NWs) are intriguing as novel nanostructured to be functionalized because of the 3C-SiC biocompatibility and of the presence of a SiO2 native shell that favours surface functionalization. Those findings are encouraging in the prospective to employ this functionalized system for different nano-medical applications such as targeted therapy against deep tumor cells.