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Title: Electronic properties of silicon nanowires
Other Titles: Na mi gui guan de dian zi te xing
Authors: Au, Frederick Chi Kan (區梓芹)
Department: Dept. of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 2005
Publisher: City University of Hong Kong
Subjects: Nanowires -- Electric properties
Notes: CityU Call Number: TK7874.85.A9 2005
Includes bibliographical references.
Thesis (Ph.D.)--City University of Hong Kong, 2005
xi, 94 leaves : ill. ; 30 cm.
Type: Thesis
Abstract: One-dimensional (1D) semi-conducting nanowires, especially silicon nanowires (SiNWs), are of both scientific and technological interest. They not only exhibit interesting electronic and optical properties intrinsically associated with their low dimensionality and the quantum confinement effect, but also expect to play an important role as both interconnects and functional components in the fabrication of future miniature nano-scale electronic and optoelectronic devices. As a result, the study of the size and dimensionality effect on the electronic properties of semi-conducting nanowires is crucial. In the beginning of the study, the electron field emission properties of SiNWs are first studied based on current–voltage (I-V) measurements and the Fowler–Nordheim equation. The electron field emission increased with decreasing diameter of SiNWs. The turn-on field of SiNWs emitters with a nominal diameter ~ 10 nm is as low as 4.5 V/μm. A hydrogen plasma treatment of SiNWs for reducing the oxide layer improves the electron emission uniformity of the emitters. The electrical transport properties of SiNWs are investigated by fabricating SiNWs nano-devices by means of e-beam lithography and focus ion beam (FIB) method. Rapid thermal annealing (RTA) of the fabricated devices results in more stable and higher conductance through SiNWs. RTA could induce metal diffusion through the oxide layer to form good-quality metal–SiNW ohmic contact as well as diffusion of metal dopants into SiNWs. The resistivity of SiNWs is in the range of 10 – 10000 ohm-cm. Temperature-dependent resistance measurements of SiNWs are fitted into thermal activation form at high-temperature region, while they show no evidence of Coulomb blockade effects at temperatures down to 4.2 K. The surface structures and the electronic band gaps of SiNWs have been studied by means of scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) respectively. The surfaces of SiNWs are oxide removed and terminated by hydrogen through hydrofluoric acid treatment. STM studies of SiNWs reveal atomically resolved images that can be interpreted as hydrogen-terminated Si (111)-(1 × 1) and Si (001)-(1 × 1) surfaces corresponding to SiH3 on Si (111) and SiH2 on Si (001), respectively. STS measurements of the electronic density of states (DOS) of SiNWs are used to evaluate the corresponding electronic energy gaps. The energy gaps are found to increase with decreasing SiNW diameter from 1.1 eV for 7 nm to 3.5 eV for 1.3 nm, in agreement with previous theoretical predictions and demonstrating the quantum size effect in SiNWs. Furthermore, the electronically active defects in SiNWs are investigated with the help of electron spin resonance (ESR) and photothermal deflection spectroscopy (PDS) measurements. ESR measurements on the as-grown SiNWs reveal several paramagnetic defects: Dangling bonds or Pb-centers at the interface of the crystalline core and the surrounding oxide, E’-centers and EX-centers located in the oxide. A total spin density of 1.5 x 1018 cm-3 has been found for the as-grown SiNWs. H-termination of SiNWs via hydrofluoric acid decreases the spin density drastically by a factor of 50. PDS measurements of the as-grown and H-terminated SiNWs show that the optical absorption properties of the SiNWs above photon energies of 1.5 eV are essentially those of microcrystalline silicon. The PDS experiments also confirm the decrease in defect density after the hydrofluoric acid etching.
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