Synthesis And Properties Of Lightly Doped Silicon Nanowires

Porous Si has been extensively investigated for its lighting emitting properties and potential applications in optoelectronics. Usually, porous Si is synthesized by applying a voltage bias to a Si substrate immersed in hydrofluoric (HF)-containing aqueous or organic solutions or by chemical etching. With advancement of the semiconductor nanotechnology, based on the above synthetic method for producing porous Si, a metal-assisted deposition and chemical sacrificial etching process has been exploited to fabricate single-crystalline porous Si arrays, which show excellent optical properties and may be useful as photocatalytic substrates or active nanoscale optoelectronic devices for energy harvesting, conversion, and biosensing. It is found that the doping level of the Si wafers is a key factor determining the surface roughness or porous structures of the Si nanowires, and only the Si wafers with highly or heavily doped p-typed wafers (resistivity:< 0.005Ω·cm) and n- typed wafers (resistivity:0.008-0.02Ω·cm) can be etched to produce porosities inside them. So, the fabrication of the porous Si nanowires from lightly doped Si wafers by the above described chemical etching method has not yet been accomplished and remains a challenge. Porous silicon, silicon nanowires and lightly doped porous silicon nanowires with various morphologies were successfully fabricated using a chemical etching method. The morphologies and crystal structure of the as-synthesized products were characterized in detail. Photoluminescence properties, electrical measurements and superhydrophobic properties were also carefully carried out. The main results are summarized as follows:(1) Lightly doped porous silicon nanowires with different microstructure morphologies have been fabricated on silicon wafer via the chemical etching method, through controlling the composition of etching solution, etching temperature and etching time. The morphology and crystal structure of the as-synthesized samples were characterized by SEM, TEM, HRTEM, SAED in detail. Si lattice fringes are continuous between the surface and the interior of the as-fabricated lightly doped porous Si nanowire, indicating that the surface of this Si nanowire is not native oxide and Si lattice integrity is not destroyed at the surface of the nanowire during the etching process. In addition, the formation mechanism of the product was seriously studied. An addition of H2O2 into the etchant played a significant role in the density and homogeneity of the lightly porous Si nanowires. An increase of the concentration of H2O2 will enhance the etching of the Si nanowire. The etching to this products partly penetrated the nanowires in the lateral direction.(2) The electrical measurements of lightly doped silicon nanowires were carried out using a new STM-TEM holder. The relative X, Y, and Z positions of the gold cantilever and a platinum cantilever with a mounted si nanowire were adjusted through the nanoscale precision piezodriven manipulation of the silicon nanowires inside the TEM, so as to build a silicon nanowire bridge between the gold and platinum cantilever. Silicon nanowire with a lower work function than those of platinum and gold makes a schottky contact between them. Both non-porous silicon nanowires and porous nanowires with the voltage range from -10 to 10V for 4000 milliseconds and I-V data were obtained. The current value of porous silicon nanowires varies from-4.5 to 4.5nA, while the current value of nonporous silicon nanowires varies from-1.0 to 1.5nA. The range of the current variation of porous silicon nanowires is-3 times as large as those of non-porous silicon nanowires at the same applied voltage. It is stated that the porous silicon nanowires possess improved electrical property and significantly increase conductance, compared with non-porous silicon nanowires. The porous silicon nanowires may expand opportunities for nanoscale optoelectronic devices, energy harvesting, conversion and sensors.(3) Porous silicon and silicon nanowires arrays with different morphologies have been prepared on the surface of the Si wafer via a chemical etching, through controlling the etching solution and etching temperature. Porous silicon was obtained at room temperature; straight and vertical Silicon nanowire arrays were obtained when heating. The morphology of nano-scale silver and silicon nanowire arrays were revealed in detail by SEM imaging. The effects of silver nitrate concentration and etching time on the morphologies of silicon nanowires were also studied. The as-synthesized silicon nanowires can be used as original silicon in order to prepare silicon nano-particles owing to its high activity.(4) The photoluminescence properties of the resulting silicon nanowires were seriously investigated. Using the excitation of 470nm laser beam from a diode laser, the silicon nanowires dispersed in ethanol solution apparently emitted a strong green emission band centered at 560nm. The PL emission can be attributed to the quantum confinement. Detailed experiments have been carried out to investigate the wetting behavior of silicon surfaces. Both the starting silicon wafer and the silicon nanowires substrates were coated with a polydimethysiloxane (PDMS) film via a vapor deposition technique, heating at 240℃. The modified silicon wafer was hydrophobic with a contact angle of 100°. However, the modified silicon nanowires combined with a contact angle of 155°became superhydrophobic. The contrast results suggest that different microstructure morphologies made a great impact on the wetting properties of solid surfaces.