Bulk-quantity Si nanowires synthesized by SiO ...

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grow Si nanowires by the vapor}liquid}solid (VLS) growth method [1}4]. However, it was di$cult to grow nanowires in bulk-quantity with diameter below 20 nm ...
Journal of Crystal Growth 212 (2000) 115}118

Bulk-quantity Si nanowires synthesized by SiO sublimation Y.F. Zhang, Y.H. Tang, C. Lam, N. Wang, C.S. Lee, I. Bello, S.T. Lee* Center of Super-Diamond & Advanced Films (COSDAF) and Department of Physics & Materials Science, City University of Hong Kong, Hong Kong, People's Republic of China Received 22 October 1999; accepted 10 January 2000 Communicated by G.B. Stringfellow

Abstract High-purity Si nanowires in bulk-quantity have been grown from SiO in a high-temperature tube furnace. Thermal sublimation of SiO powders produced SiO vapor, which was transported and deposited on the inner wall of the tube at &930C. We proposed that the SiO deposit underwent a disproportionation reaction to form nanowires containing Si and SiO . Each wire consisted of a single crystalline Si core and an oxide sheath, 6}28 nm in diameter and up to 1 mm in  length. This work gave strong support to the recently proposed oxide-assisted growth mechanism. The yield of Si nanowires increased with sublimation temperature and pressure. Under suitable deposition conditions, Si nanowire growth rate of more than 10 mg/h could be routinely obtained.  2000 Elsevier Science B.V. All rights reserved. Keywords: Nanoscale material; Si nanowire; SiO; Thermal sublimation

The synthesis of Si nanowires has attracted much attention in recent years. Some e!orts were made to grow Si nanowires by the vapor}liquid}solid (VLS) growth method [1}4]. However, it was di$cult to grow nanowires in bulk-quantity with diameter below 20 nm using the VLS method partly because the diameter was limited by the minimum size of the liquid metal catalyst that could be produced under equilibrium conditions [5,6]. Moreover, the processes based on this mechanism have the inherent limitation that the nanowires are invariably contaminated by metal catalysts. Recently, we reported an oxide-assisted growth method to synthesize high-purity Si nanowires by laser ablation of

* Corresponding author. Fax: #852-2788-7830. E-mail address: [email protected] (S.T. Lee).

a mixture of Si and SiO [7}9]. Unlike the VLS  method, the new method produced Si nanowires whose growing tips were made of SiO (x+1) V instead of metal. In this letter, we report a simple and more e$cient synthesis method for Si nanowires, which is based on sublimation of SiO. The experimental apparatus employed in this work consisted of an Al O tube placed inside   a tube furnace. Three grams of SiO powder (purity: 99.98%, particle size: &45 lm, density: 2.1 g/cm) were placed at the center of the Al O tube, where   the temperature for SiO sublimation was controlled by the furnace. High-purity argon was used as the carrier gas, which #owed through the preevacuated Al O tube at 50 SCCM and trans  ported the SiO vapor downstream. The SiO vapor was found to deposit and accumulate only at the tube wall where the temperature was lower than

0022-0248/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 2 3 8 - 4

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Fig. 1. (a) TEM image of a Si nanowire sample obtained from the sponge-like product inside the Al O tube at 4;10 Pa and 12003C   sublimation temperature; (b) high-resolution TEM image of nanowires showing a crystalline Si core and an amorphous SiO sheath. 

&9503C. Si nanowires grew in the region where the temperature ranged from 920 to 9503C. In the region where the temperature was lower than 9203C, powder of nanoclusters, which was very #exible and sticky, was deposited. Energy-dispersive X-ray analysis showed that O/Si atom ratio in the deposited nanoclusters was approximately one. Si nanowires nucleated at the tube wall and grew towards the tube center. The growth time of the nanowires was kept at 5 h for each experimental run. The as-grown Si nanowires are a brown spongelike product extending from the wall to the center of the tube. A transmission electron microscope (TEM) image of a typical sample taken from the sponge-like product shows that most nanowires have primarily smooth and uniform wire-like structures while a few possess bends and kinks [Fig. 1(a)]. The product consists almost entirely of

nanowires. The diameters of the nanowires are in the range of 6}28 nm with a distribution peak at 15 nm. Their lengths extend up to a few millimeters. The diameters remain nearly constant throughout the length of the nanowires. The growth rate along the axial direction of the nanowires is up to 8 lm/min. Energy dispersive X-ray analysis in the TEM and Rutherford backscattering spectroscopy con"rm that the nanowires consist of only Si and oxygen. Selected area electron di!raction pattern taken from the sample shows strong di!raction rings of crystalline cubic Si [inset of Fig. 1(a)]. High-resolution TEM image reveals that each nanowire consists of a single-crystalline core and an amorphous sheath [Fig. 1(b)]. The axis of most Si nanowires is approximately along the [2 1 1] direction. X-ray di!raction evaluation con"rms the existence of crystalline Si. Neither axial screw dislocations nor metal balls were found. The growth tip

Y.F. Zhang et al. / Journal of Crystal Growth 212 (2000) 115}118

of each nanowire is hemispherical in shape and terminated by a SiO layer, which is slightly thin ner than the SiO sheath along the length of the  nanowires. The above observations show that the Si nanowires grown from SiO vapor are similar to those produced by the laser ablation of a pure Si target [1}4] or a mixed target of Si and SiO [7}9].  The similarity of the two products of nanowires, together with the successful synthesis of Si nanowires from the SiO vapor, provides strong support for the validity of the oxide-assisted growth model of semiconductor nanowires proposed recently by us [7}11]. During sublimation, the SiO molecules combined into nanoclusters in the vapor phase so as to decrease the free energy (nanoclusters are generated normally in inert gases at sub-atmospheric vapor pressure) [12,13]. The nanoclusters in the vapor subsequently deposited on the tube wall where the temperature was lower than 9503C. The melting temperature of the SiO nanocluster was reduced due to it nanoscale size and was possibly comparable to the temperature (920}9503C) at which Si nanowires grew [12]. The nanoclusters were `liquid-likea and coalesced into a viscous matrix of SiO at the growth tip of Si nanowires. Subsequent disproportionation resulted in the precipitation of Si and thus the formation of Si nanowires, which composed of a crystalline Si core and an oxide sheath. The growth of Si nanowires from SiO is proposed to proceed as follows: (1) thermal sublimation: SiO powderPSiO vapor; (2) deposition: SiO vaporPSiO viscous deposit; (3) disproportionation: SiO viscous deposit PSi#SiO mixture;  (4) phase separation (incorporation of Si atoms into the crystal lattice): Si#SiO mixturePSi  crystalline core#SiO amorphous sheath.  Fig. 2 shows the dependence of the yield of Si nanowires on the sublimation temperature when the gas pressure was kept at 4;10 Pa and the corresponding Arrhenius plot of the weight loss of SiO due to sublimation. Evidently, the production of Si nanowires became appreciable at 11003C and increased sharply with increasing sublimation temperature of SiO. The SiO weight loss also increased quickly with increasing temperature, and was more

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Fig. 2. Yield of Si nanowires and the weight loss of SiO versus the reciprocal sublimation temperature when the chamber pressure was kept at 4;10 Pa.

than one order of magnitude higher than the yield of Si nanowires. Thus, less than 10% of the sublimated SiO vapor contributed to the production of Si nanowires. Fig. 2 shows -1that the weight loss (=) of SiO powders follows closely the Arrhenius relation, i.e. ="= exp(!E /k ¹ ), where = is   a constant. The activation or vaporization energy, E was found to be 2 eV, which is lower than that of Si [for Si(1 1 1) surface, E +4.3 eV] [14]. When the sublimation temperature increased, the partial pressure of SiO vapor in the Ar gas would rise due to the elevated sublimation rate of SiO. Therefore, consistent with the proposed growth mechanism, the yield increase of Si nanowires with increasing sublimation temperature can be attributed to the increase of SiO vapor pressure and thus increased SiO deposit. Fig. 3 shows the pressure dependence of the yield of Si nanowires and the weight loss of SiO when the sublimation temperature was kept at 13003C. It appears that 655 Pa is a threshold pressure for Si nanowires growth. The yield of Si nanowires increased with increasing pressure, while the SiO weight loss decreased slowly. The yield increase of Si nanowires with increasing pressure may be attributed to faster SiO transport.

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Acknowledgements The authors wish to thank Dr. H.Y. Peng for her help in TEM observations and Professor F.H. Lee for useful discussions. This work is supported in part by the Research Grants Council of Hong Kong (project No. 9040459) and the Strategic Research Grant of the City University of Hong Kong. References

Fig. 3. Pressure dependence of the yield of Si nanowires and the weight loss of SiO when the sublimation temperature was kept at 13003C.

In summary, bulk quantity of high-purity Si nanowires has been synthesized by thermal sublimation of SiO powders. The diameter of the Si nanowires grown in an Ar atmosphere ranged from 6 to 28 nm with lengths of a few millimeters. The characteristics of Si nanowires synthesized from SiO are similar to those obtained from laser ablation of a pure Si target or a mixed target of Si and SiO . This similarity and the successful synthesis of  Si nanowires from SiO vapor rendered strong support for the recently proposed oxide-assisted growth mechanism of Si nanowires. The yield of Si nanowires increased with increasing sublimation temperature of SiO and increasing pressure of the argon carrier gas.

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