Studies of electrical and optical properties of surface-modified silicon nanowires

Abstract

Semiconductor nanowires represent an ideal system for investigating low dimensional physics and are expected to play a vital role as both interconnects and functional components in nanoscale electronic and optoelectronic devices. Since silicon is of great technological importance in microelectronics, silicon nanowires (SiNWs) have attracted extensive study interests in recent years. It is well accepted that the surface conditions of SiNWs are crucial to their properties and applications. Consequently, it is significant to investigate the properties of surface-modified SiNWs. Numerous methods have been developed for synthesizing SiNWs. This thesis focuses on three approaches for preparing SiNWs: i) thermal evaporation in high temperature tube furnace, ii) hot filament chemical vapor deposition (HFCVD), iii) metal-assisted chemical etching. Millimeter-long and uniform silicon nanocables were synthesized by simple thermal evaporation of SiO powder mixed with a small amount of Sn. The nanocables possess single-crystal Si cores and compact amorphous SiOx shells. Each nanocable has a droplet head of Si-Sn eutectic and an uncovered Si core at the tail. These structural features are feasible to fabricate metal-oxide-semiconductor field-effect transistors (MOSFETs) device. Characteristic I-V curves from demonstrated devices reveal that the Si nanocables are p-type semiconductor. Large-area upstanding SiNWs were synthesized by HFCVD using powder silicon monoxide (SiO) as Si source under high vacuum. Gold nanoparticles are catalyst, which are formed on Si substrate by in-situ reduction of gold chloride. The diameter and density of SiNWs could be modified in some range by controlling the size and distribution of the Au dots on substrate. The SiNWs are single crystalline with a very thin amorphous sheath. Organic inorganic heterojunctions based on copper phthalocyanine (CuPc) and this kind of SiNWs has been fabricated and photovoltaic effect has been obtained. Large-scale uniform SiNWs arrays have been prepared by Ag nanoparticles (AgNPs) catalyzed room temperature mild chemical etching in aqueous solution of hydrofluoric acid (HF) and hydrogen peroxide (H2O2). All SiNWs obtained are epitaxial single crystals with same types and doping levels as the substrate. Six kinds of SiNWs arrays have been used with different types, surface orientations and doping levels. The etching conditions, such as the concentration of H2O2 in etchant, etching temperature and etching time affect and modify the morphology of SiNWs arrays. A single nanowire from p-Si (111) wafer with 8~13 Ω⋅cm resistivity has been fabricated to FET device to study the electrical transport property. Organic inorganic hybrid solar cells based on these SiNWs arrays have been demonstrated. The used SiNWs arrays were prepared from HFCVD growth and AgNPs-assisted chemical etching, which worked as n-type part. A layer of p-type organic semiconductor copper phthalocyanine (CuPc) was spin coated on SiNWs array surface. Then a gold film was covered on CuPc layer to act as transparent anode. The obvious photovoltaic effect has been obtained from our devices. For SiNWs array fabricated by chemical etching method, the AgNPs were secondly grown on every SiNWs’ surfaces through galvanic replacement reaction. The hierarchical nanostructures (AgNPs@SiNWs array) exhibit strong surface enhanced effect in Raman spectroscopy. Highly care pollutants Sudan dyes (Sudan I, II, III, IV and G) and triphenylmethane dyes (malachite green, leucomalachite green, crystal violet and leucocrystal violet) have been detected by surface-enhanced Raman spectroscopy (SERS) at trace level. The immunocomplex formed by mouse IgG and goat-anti-mouse IgG showed different Raman bands on the SERS substrate, which was used as label-free immunoassay readout. Different DNA components emitted their characteristic Raman peaks, which could be utilized to DNA sequencing. The SERS signals also worked as indicator for DNA hybridization. The surface conditions changed when normal cells became cancer cells. Therefore, the distinguished SERS signals of normal cells and cancer cells on this substrate may provide a possibility of early stage diagnosis of cancer.

Date of Award	2 Oct 2007
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Ning Bew WONG (Supervisor)