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Title: Synthesis of semiconductor micro/nanowires and their applications in devices
Other Titles: Ban dao ti wei na mi xian de zhi bei ji qi qi jian yan jiu
Authors: Luo, Linbao (羅林保)
Department: Department of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 2009
Publisher: City University of Hong Kong
Subjects: Nanowires.
Notes: CityU Call Number: TK7874.85 .L86 2009
xviii, 129 leaves : col. ill. 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2009.
Includes bibliographical references.
Type: thesis
Abstract: In the work described in this thesis, the synthesis of silicon microwires and nanowires via metal-assisted HF etching solution was performed, and the as-prepared silicon microwires or nanowires were constructed into field effect transistors (FETs), heavy metal ions sensors, and p-n junctions. Furthermore, silver and gold nanoparticles grown within periodic silicon microholes via electrochemical plating showed a very strong surface enhanced Raman scattering effect. Finally, selenium nanowires were prepared via a simple solution method, and photodetectors from selenium nanowires were studied systematically. The main results of these five focuses are as summarized next. Firstly, large-area uniform silicon microwires (SiMWs) arrays were fabricated by gold-assisted chemical etching of silicon wafers. The lengths of the SiMWs were accurately controlled by the etching time. By over-developing the patterned photoresist dots for different times, SiMWs with diameters of 2.5, 2.0, 1.5, 1.0 and 0.6 μm were easily obtained. Additionally, since the etching directions were found to be always perpendicular to the wafer surface, SiMWs with growth directions along [100], [110], [111] and [311] were fabricated through etching wafers with corresponding orientations. The main factors affecting the morphology of the final product, such as etching time, thickness of metal film and resistivity of wafers were systematically studied. Finally, the transport characteristics of field effect transistor from individual SiMW revealed a typical p-type semiconducting behavior, with an on/off ratio as high as 103, a hole mobility of 43.9 cm2V-1S-1 at Vds= - 0.06 V and a hole concentration of 8.90 ×1016 cm-3. Secondly, based on the silicon nanowires (SiNWs) that were synthesized by chemical etching method, high-sensitivity detection of toxic heavy metal cations such as Hg2+ and Cd2+ ions was demonstrated using single-wire SiNW-FET. The conductance of FET fabricated from thermally oxidized SiNWs functionalized with MPTES showed high sensitivity to Hg2+ and Cd2+ ions at concentration down to 10-7 and 10-4 M, respectively. The sensing was due to the chemical gating effect induced by the strong chelation between surface-modified thiol groups of 3-mercaptopropyltriethoxysilane (MPTES) and the positively charged Hg2+ and Cd2+ cations. A linear relationship between the concentration of metal ions and the current change was observed for different concentration range from 10-7 to 10-4 M for Hg2+. Additionally, the SiNW sensors could also be recycled with nearly the same sensitivity as before. Finally, comparative experiments showed the great selectivity of SiNW-FET sensors to Hg2+ and Cd2+ over other metal cations. Thirdly, large-area gold and silver nanoparticles embedded in periodic silicon microhole arrays were prepared via simple electrodeposition. Such metal nanoparticles within Si microholes showed a strong surface-enhanced Raman scattering (SERS) effect, which enabled the detection of rhodamine at 10-8 M and 10-9 M on gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) substrates, respectively. The SERS activity of substrates depended on the metal particle morphology. Microfabrication combined with electrodeposition is shown to be a viable approach for preparing reproducible SERS substrates for sensitive Raman detection. Fourthly, hydrogen termination on silicon nanowires was found to attract electrons strongly from the SiNW core, which would lead to the transformation of the nanowires transport property from the original n-type to p-type. Based on this discovery, p-n silicon junction arrays were fabricated by partially controlled etching. I-V curves of the as-obtained arrays showed a distinctive rectifying behavior with a low turn-on voltage at a forward bias of around 0.8 V. The leakage current was less than 0.1 nA at a reverse bias of up to 3 V. Lastly, selenium nanowires with a diameter of about 70 nm and a growth direction along [001] were fabricated via a facile solution method. Photoconductive properties of Se wires were systematically characterized via photodetectors made of single Se nanowires. The photodetectors exhibited a high light on-off current ratio (Ilight/Idark) of 450, and a fast light response speed of millisecond rise/fall time with excellent stability and reproducibility. The response time strongly depended on the intensity of the illumination light: the rise time and fall time for a typical photodetector is 0.68/1.85, 0.53/1.70, 0.54/1.65, 0.51/1.59, and 0.49/1.58 ms for light intensities of 0.18, 0.26, 0.43, 0.96, and 1.89 mW/cm2, respectively, and the relationship between the light intensity and the photocurrent can be fitted by using a simple power law. The diameters of the nanowire were found to have a significant influence on the response speed with smaller Se nanowires showing higher response speed. Finally, the mechanisms of photoconduction and factors affecting the performance of the photodetectors were elucidated.
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