Silicon nanowire growth using low pressure chemical vapor deposition, and optical and optoelectrical properties tailoring.

In this thesis, the author describes the research work done during the three-year master‟s studies, focusing on chemical vapor deposition (CVD) synthesis of silicon (Si) nanostructures, and the optical and optoelectrical properties tailoring of Si nanowires (SiNWs). Firstly, as the major project, various Si nanostructures were grown using the First Nano Easy Tube 3000 low pressure chemical vapor deposition system (LPCVD), located at Nanyang Nano-Fabrication Centre clean room, in Nanyang Technological University, Singapore. At low processing temperature (400-700 °C) with Silane (SiH4) as precursor, SiNWs with high crystalline quality can be achieved via Au particles or films – catalyzed growth. SiNWs are also grown on other metal substrates: nickel silicide forms with various morphologies on nickel foam substrate under different deposition temperatures; on copper mesh, only polycrystalline Si film coating is achieved. On the other hand, SiCl4 is employed as Si precursor at elevated temperatures (> 800°C). Gold and copper catalyzed growths show different morphology evolution trends versus processing pressures, but the similar periodic diameter oscillations of nanowires at low deposition pressure, which is induced by the fluctuation of catalyst configuration periodically disturbed by supersaturation processes. Interestingly, with only a germanium (Ge) film, SiNWs with two-segment structures are realized through silicon tetrachloride (SiCl4) deposition processes. Raman spectroscopy and high resolution transmission electron-microscopy (HRTEM) clearly identify the composition and illustrate the growth mechanism, in which Si-Ge alloy were formed at the initial stage followed by pure Si crystal growth. Though metal impurities, which are absorbed from the deposition chamber at elevated temperatures, are found at the tip of nanowires, this type of unconventional growth has potential applications to sensitively detect the chamber impurities and to grow SiNWs utilizing metal wastes from the reaction chamber. Moreover, the optical and opto-electrical properties of as-grown SiNWs are tailored by noble metal nanocrystals decoration, due to the coupling of surface plasmon to nanowires. A galvanic displacement based strategy is employed to achieve in situ nucleation of Ag/Au nanoparticles on SiNWs surface, and a post-growth finally leads to higher-order Au nanostructures such as bow-tie dimers, nanorods and nano-prisms along a single Si nanowire. The presence of Au nanostructures significantly enhances the optical properties of nanowires, and various morphologies show different enhancement ratios on the Si Raman scattering intensity. In addition, a surface plasmon enhanced photocurrent by almost 100% is demonstrated on single-SiNW field effect transistor (FET) devices, which is higher than those from previous reports. This study shows that SiNWs decorated with metallic nanostructures by in situ galvanic displacement exhibit significant promise towards high efficiency photodetection and light harvesting applications. Last but not the least, through CVD processing modification, vertically aligned Si wire arrays are grown with our LPCVD system, which holds strong promises for photovoltaic and photoelectrochemical applications. Photolithography is employed to pattern the substrate surface, followed by copper catalysts evaporation. SiO2 barrier layer is found necessary to avoid merging of patterned catalysts, and to restrict the wire growth from each pocket. By tuning the thickness of catalyst film and adjusting the pre-heating process, well aligned silicon micron wires (SiMWs) array are ready to be grown in large scales. Furthermore, based on the wire arrays, the both-end exposed wire-in-polymer (BEEWIP) structure is achieved by infiltration of PDMS (supporting layer) and gelatin (sacrificial layer) dual layer polymers. Consequently, the concept for arrayed SiMW-TiO2 shell tandem water splitting cell is proposed. Compared with conventional homo-junction, this ingenious design has several advantages: the adopted dual-band gap heterostructure allows utilization of a larger part of the solar spectrum, as unabsorbed longer wavelength light that passes through TiO2 can be further absorbed by Si; efficient electron-hole separation within the Si-TiO2 interface can be achieved due to band bending; flexible polymer embedded SiMW-TiO2 array with exposed both ends allows for simultaneous O2 and H2 generation while separated collection. All the three parts of work as mentioned above are presented in the following chapters. Moreover, successive work and research plan will be clearly illustrated in the last section of this thesis.​Master of Scienc