Semiconductor nanowires from oxides
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Semiconductor nanowires from oxides S.T. Lee,a) Y.F. Zhang, N. Wang, Y.H. Tang, I. Bello, and C.S. Lee Center of Super-Diamond and Advanced Films and Department of Physics and Materials Science, The City University of Hong Kong, Hong Kong
Y.W. Chung Department of Materials Science and Engineering, Northern University, Evanston, Illinois (Received 6 July 1999; accepted 13 September 1999)
Highly pure, ultralong, and uniform-sized semiconductor nanowires in bulk quantity were synthesized by thermal evaporation or laser ablation of semiconductor powders mixed with oxides. Transmission electron microscopy study shows that decomposition of semiconductor suboxides and defect structures play important roles in enhancing the formation and growth of high-quality nanowires. A new growth mechanism is proposed on the basis of microstructure and different morphologies of the nanowires observed.
I. INTRODUCTION
III. RESULTS AND DISCUSSION
Semiconductor nanowires have been synthesized by the vapor–liquid–solid (VLS) reaction,1,2 photolithography combined with etching,3–5 and scanning tunneling microscopy.6,7 In this paper, we show that thermal evaporation (assisted by laser) of semiconductor powders mixed with oxides is an effective method for synthesizing ultralong (in millimeters), highly pure (no contamination), and uniform semiconductor nanowires in bulk quantity. The present approach, nevertheless, can be expanded to synthesize large quantites of onedimensional nanostructured materials for use in fundamental research and technological applications. II. EXPERIMENTAL
The experimental apparatus consists of an evacuated quartz tube (500 torr, Ar flow) placed inside a tube furnace. A solid source is positioned inside the tube. Each target source used in the present work is composed of one of the following pairs of highly pure powders: Si/SiO2, Si/Fe2O3, or Ge/GeO2. The typical experiment was carried out by using an excimer laser (248 nm, 10 Hz, 400 mJ/pulse) to ablate the target in an evacuated quartz tube containing Ar (500 torr). The temperature around the target was 1200 °C. Si nanowire products (spongelike, dark red in color) were formed on the inner wall of the quartz tube near the water-cooled finger after 2-h laser ablation.8,9 The optimal temperatures of the substrates for the nucleation and growth of Si and Ge nanowires are 930–950 and 690–700 °C, respectively. a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 14, No. 12, Dec 1999
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As reported by different groups, laser ablation or thermal evaporation of metal-containing Si powder can produce Si nanowires.8–12 Assuming that metallic or metal–silicide nanoparticles form and act as the critical catalyst during growth, the VLS reaction forms the basis for the postulated growth mechanism of Si nanowires. In the classical VLS reaction, Au particles on Si substrate are generally used as the mediating solvent since Au and Si f
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