Oriented Si nanowires grown via an SLS mechanism
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ABSTRACT Highly oriented silicon nanowires were grown on Si (I 1l) substrate via a solidliquid-solid (SLS) mechanism. Unlike the well known vapor-liquid-solid (VLS) mechanism of whisker growth, no gaseous or liquid Si source was supplied during growth. Ni was used as the liquid forming agent and mixture of H2 and Ar was introduced in the experiment. Oriented silicon nanowires grew at 950*C and the ambient pressure kept at about 200 Torr. The oriented silicon nanowires have a length around I il m and uniform diameter about 25nm. Selected area electron diffraction showed that silicon nanowires are completely amorphous. The approach used here is simple and controllable, and may be useful in large-scale synthesis of various nanowires. INTRODUCTION The pioneering work of the discovery of multi-walled carbon nanotubes by lijima' has stimulated intensive research interests on one dimension nanomaterials. Compared with the indirect gap nature of bulk Si, silicon nanowires (SiNWs) are a novel one-dimension nanostructures whose physical properties are modified through quantum confinement effect 2". It has been reported that silicon nanowires can emit full visible band light5 . Molecular level semiconductor/metal hetero junctions using silicon nanowire/carbon nanotube were synthesized, which show typical rectifying effect of a nano diode6 . These results reveal that silicon nanowires are a promising material in the fundamental research and potential application. Several methods, such as laser ablation2 , physical evaporation3 , or thermal decomposition of silane7 , were employed to synthesize silicon nanowires, and controlled growth of the nanowires is yet to be achieved, which is important in device application. In this paper, we will report highly oriented amorphous silicon nanowires were controllable grown on Si substrate in a new large-scale and low-cost. Though the Si nanowires are of amorphous nature, the approach described here proved to be effective to realize the controlled growth of nanowires. EXPERIMENTALS Heavy-doped (1.5 X 10 2 £/cm) n-type Si (111) wafers were used as substrates. The silicon substrates were cleaned ultrasonically in pure petroleum ether and in ethanol in turns for 5 min, and leached in distilled water, then dried. A thin layer of 40 231 Mat. Res. Soc. Symp. Proc. Vol. 581 © 2000 Materials Research Society
nm nickel was thermal deposited on the substrate. The substrate was placed in a quartz tube which was heated in a tube furnace. A three-step heating procedure was involved in the growth. Firstly, the system was heated to 8001C, and a mixture of H 2 (36sccm)/Ar (%'sccm) were introduced to the tube. The temperature was then raised from 800"C to 950"( and pure Ar (100scem) was used as a carrier gas in this step. The ambient pressure of the tube was kept near 750 Torr by adjusting the exit valve, then evacuated the tube. This procedure was repeated three times. Finally, the temperature was held at 9501C at the pressure of about 200 Torr for one hour. A mixture of H2 (4sccm) and Ar (36sccm) was in
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