ARTICLE IN PRESS
Journal of Crystal Growth 257 (2003) 69–74
H2-assisted control growth of Si nanowires X.Q. Yan, D.F. Liu, L.J. Ci, J.X. Wang, Z.P. Zhou, H.J. Yuan, L. Song, Y. Gao, L.F. Liu, W.Y. Zhou, G. Wang, S.S. Xie* Institute of Physics A05 group, Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 603-65, Beijing 100080, People’s Republic of China Received 19 April 2003; accepted 23 May 2003 Communicated by D.W. Shaw
Abstract Large-scale desired silicon nanowires without amorphous silicon oxide sheath have been synthesized by thermal chemical vapor deposition using SiH4 gas at 650 C in a flow mixture of H2 and N2, compared with the short and thick Si nanowires with amorphous SiOx coating obtained in N2. Scanning electron microscopy (SEM), Energy dispersive Xray spectrometry (EDX) analysis, and high-resolution transmission electron microscopy (HRTEM) have been employed to characterize the Si nanowires. The effects of H2 gas on the catalytic particle size and on the formation of Si nanowires are discussed in detail. Photoluminescence (PL) characteristics further demonstrate the large differences between the H2-assisted grown Si nanowires and the Si nanowires grown in N2. r 2003 Elsevier B.V. All rights reserved. PACS: 81.15. Gh; 81.05. Ys Keywords: A3. Chemical vapor deposition processes; B1. Nanomaterials; B2. Semiconducting silicon
1. Introduction Silicon nanowires exhibit many exceptional and useful electronic, optical, and chemical properties and will be a promising candidate for application in nanodevices [1–10]. To date, Si nanowires have been prepared by employing different techniques, such as excimer laser ablation [1–3], physical thermal evaporation [4–6,11], chemical vapor deposition (CVD) [7,8,12,13], and other methods *Corresponding author. Tel.: +86-1082649078; fax: +861082640215. E-mail address:
[email protected] (S.S. Xie).
[9,14]. However, the Si nanowires are often sheathed with an amorphous silicon oxide shell, as well as with abundant defects such as microtwins, stacking faults, and grain boundaries, which seriously influence the future applications. For example, Zhou et al. [10] have reported that silicon oxide sheath is responsible for the decrease of the chemical sensitivity of Si nanowires to the NH3 gas and water vapor. Therefore, the fabrication of defect-free single crystal Si nanowires without amorphous silicon oxide sheath is a very important and challenging issue. Wang et al. [2,3] reported that silicon oxide played a crucial role in enhancing the formation
0022-0248/03/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0022-0248(03)01412-X
ARTICLE IN PRESS 70
X.Q. Yan et al. / Journal of Crystal Growth 257 (2003) 69–74
and growth of the silicon nanowires by laser ablation of Si powder sources mixed with SiO2, and an oxide-assisted growth model was proposed. In our work, we control the growth of Si nanowires by H2-assisting using chemical vapor deposition of silane. During the same growth time, Si nanowires synthesized in the mixture of H2 and N2, without an amorphous silicon oxide coating, are much thinner and longer than the Si nanowires sheathed with an amorphous SiOx shell grown in N2. Although the mixture of SiH4, H2 and other inert gases was often used to grow an epitaxial silicon thin film, the effects of H2 gas on the formation of Si nanowires are not discussed in detail. Different from the deposition of thin film, the effects of dilute H2 gas on the catalytic particle size and the growth of Si nanowires as well as the properties of resulting products in CVD process are investigated. For example, the PL related to the amorphous SiOx coating was observed for Si nanowires synthesized in N2, but PL at the visible wavelength range was not detected for the Si nanowires grown by H2-assisting due to the absence of an amorphous silicon oxide sheath, indicating that oxide layer or defects on the interface between Si and SiOx is responsible to PL. On the other hand, the disappearance of quantum size effect resulting from the large diameter of Si nanowires compared to the critical Bohr radius of about 5 nm cannot contribute to PL at infrared range.
2. Experimental procedure The substrates used in our experiments were 2–4 O cm n-type Si (1 0 0) wafers with an oxide layer of about 50 nm thickness. They were ultrasonically stirred for 30 min in acetone solution to clean their surfaces. Then the cleaned substrates were deposited with Au–Pd film for about 0.5 min under 101 Torr at 100 V and 20 mA by using ion sputter films deposition system (Hitachi, E-1010). The thickness of Au–Pd alloy film was approximately estimated as 5 nm. The substrates were placed in a tube furnace that has been described elsewhere [15]. Prior to deposition, the Au–Pd alloy coated substrates were pretreated in a flow mixture of H2 and N2 at 650 C for 1 h in order to
break the Au–Pd alloy films into discrete islands and to remove the residual oxygen out of our reaction system. Subsequently, 5 sccm silane was introduced to deposit Si nanowires for 2 h, and the carrying gases flowing through the quartz tube were 100 sccm N2, 100 sccm N2 mixed with 5 sccm H2 and 10 sccm H2, respectively. The total pressure was all maintained at about 160 Torr. And then the samples were cooled down to room temperature in N2. The light-yellow color products covering Si substrates were examined by a field-emission scanning electron microscope (SEM; Philips XL 30 FEG), and energy-dispersive X-ray (EDX) spectra attached to SEM. A high-resolution transmission electron microscope (HRTEM; JEOL 2010F at 200 kV) was used to characterize the structures of silicon nanowires. Photoluminescence (PL) measurements were carried out on a microRaman spectrometer (JY T64000 France) at room temperature. The 514.5 nm emission from an argon ion laser was used to excite the luminescence.
3. Results and discussion Fig. 1 shows the morphologies of the nanowires synthesized by chemical vapor deposition of silane at 650 C in 2 h in the different carrier gases. Nanowires obtained in N2 are smoothly curved with some straight sections, which have diameters of 80–110 nm and a length of 1 mm in Fig. 1a. However, during the same growth time of 2 h most H2-assisted grown nanowires are thin with a diameter of 40–50 nm and a length of more than 5 mm, while some nanowires were coarsened due to a certain amount of nanoparticles grown on their surfaces (see Fig. 1b and c). What is more, the length of H2-assisted grown nanowires increases accompanied with an increase of H2 flow in the carrier gases from 5 to 10 sccm. On the other hand, the EDX spectra taken from the nanowires grown in N2 show the presence of silicon and oxygen (84.41 and 15.59 at%, respectively). In contrast, the oxygen content of 9.36 at% (the oxygen may come from the oxide layer on the Si substrate) in the H2-assisted grown Si nanowires grown in N2 mixed with 10 sccm H2 is much less than that in the Si nanowires grown in N2.
ARTICLE IN PRESS X.Q. Yan et al. / Journal of Crystal Growth 257 (2003) 69–74
(a)
71
(a)
(b)
(b)
(c)
Fig. 1. SEM images of the Si nanowires synthesized in 2 h using different carrier gases: (a) N2, (b) N2 mixed with 5 sccm H2, (c) N2 mixed with 10 sccm H2. The scale bar is 5 mm.
TEM was typically employed to characterize the microstrucures of Si nanowires produced in 100 sccm N2 and 100 sccm N2 mixed with 10 sccm H2, respectively. Fig. 2a represents a low magnification image of Si nanowire produced in N2 with a diameter of about 100 nm and a catalytic particle on the top end, where the catalytic particle has
Fig. 2. (a) Representative TEM image of the Si nanowire produced in N2, the inset shows the high-magnification image of the catalytic nanoparticle on the tip of the nanowire. (b) HRTEM image of the Si nanowire sheathed with SiOx shell. The nanowire was seen from the /1 1 1% S zone axis, which is determined by the selected-area electron diffraction pattern shown in the inset. The growth direction of the Si nanowire, indicated by the arrow, is /211S:
been encapsulated by amorphous silicon oxide layer (see the inset in Fig. 2a). Furthermore, an HRTEM image reveals that Si nanowire synthesized in N2 consists of a crystalline Si core and an amorphous silicon oxide sheath with a thickness of more than 10 nm (Fig. 2b). A typical selected-area electron diffraction pattern (the inset in Fig. 2b) of the nanowire can be indexed for the /1 1 1% S zone
ARTICLE IN PRESS X.Q. Yan et al. / Journal of Crystal Growth 257 (2003) 69–74
72
(a)
(b)
(c)
axis of single-crystal Si and suggests that the nanowire growth occurs along the /2 1 1S direction. Fig. 3a shows a TEM image of the Si nanowires synthesized in the mixture of 10 sccm H2 and 100 sccm N2, which are straight and thin with a diameter of about 40–50 nm, and a few of the nanoparticles do exist on the surfaces of these nanowires (Fig. 3a, upper left inset), which accords with that of SEM observation. Moreover, different from the encapsulated catalytic particle, the catalytic nanoparticle on the tip of the nanowire produced in H2/N2 is still naked (Fig. 3a, lower right inset). Compared with the Si nanowires sheathed with an amorphous SiOx layer, no amorphous coating was found on the Si nanowires synthesized in the mixture of H2 and N2 in the present study. A typical HRTEM image of this kind of Si nanowire is shown in Fig. 3b. At the same time, some stripes of amorphous hydrogenated Si were observed. Fig. 3c shows an image of Si nanoparticles grown on the surface of Si nanowire synthesized in H2/N2, whose crystal lattice is almost consistent with that of the Si nanowire. This suggests that the formation of the particles was simultaneous with the growth of the nanowire. In short, from the above SEM, TEM and HRTEM studies we can conclude that the H2 added to the carrier gas of N2 in the reaction chamber actually affects the growth of Si nanowires. Since catalytic nanoparticles can be seen at the tips of the nanowires, the growth mechanism of Si nanowires is coincident with the vapor–liquid– solid (VLS) model. For discussing the effects of H2 added to the carrier gas on the growth of Si nanowires, the following aspects are addressed according to the VLS growth mechanism: First, silicon atoms from silane’ decomposition, not from the Si substrates owing to the presence of oxide layers about 50 nm in thickness, react with
Fig. 3. (a) TEM image of the Si nanowires synthesized in N2 mixed with 10 sccm H2. High-magnification images of the catalytic nanoparticle on the tip of the nanowire and some Si nanoparticles grown on the surface of the nanowire are shown in the lower right and upper left insets, respectively. (b) Typical HRTEM image of the Si nanowire. The scale bar is 5 nm. (c) HRTEM image of the Si nanoparticles on the nanowire.
ARTICLE IN PRESS X.Q. Yan et al. / Journal of Crystal Growth 257 (2003) 69–74
Au–Pd alloy islands on the Si substrates to form silicide/AuPd catalytic droplets, which will act as the initial nucleation centers for the nanowires growth and define the diameter of the nanowires. When H2 was added to the carrier gas in the process chamber, the chemical reaction of silane decomposition ½SiH4 ðgÞ-SiðsÞ þ 2H2 ðgÞ was suppressed. With the decrease of concentration of Si absorbed in Au–Pd alloy islands, the size of catalytic droplets became smaller than that of the droplets formed in N2, which would result in the growth of thinner nanowires compared with the nanowires grown in N2. This point has been confirmed by the fact that the size of catalytic nanoparticles on the tip of the nanowires grown in H2/N2 shown in Fig. 3a is smaller than that of the particles shown in Fig. 2a. Subsequently, when the concentration of Si absorbed in the catalytic droplets is supersaturated, the Si atoms will diffuse from one side of the droplet to the opposite side, giving rise to the formation of Si nanowires directly on the surface of catalytic droplets. Although it is difficult to establish the direct effect of H-atoms solely on the growth rate of Si nanowires, it is found that during the same growth time the Si nanowires synthesized in H2/N2 are obviously longer than that synthesized in N2 (see Fig. 1). Bootsman [16] reported that silane decomposition at the vapor/liquid (V/L) interface is the rate-limiting step in the wire growth from silane. In our studies, we find that the catalytic droplets formed in N2 are about more than 100 nm. In this case, the diffusion process of Si atoms in the catalytic droplets may affect the growth of the nanowires [17]. In addition, the pile and oxidation of redundant Si atoms of the large catalytic droplets formed a lot of islands of SiOx and led to the droplets being gradually encapsulated by the amorphous silicon oxide layer (see the inset in Fig. 2a). This process decreased the active area of the droplets and resulted in slowing the growth of Si nanowires. In the end, the activity of large catalytic droplets was completely lost and the short and thick nanowires were obtained in N2. Here, we believe that the presence of oxygen is due to the leakage of our vacuum system; in the meanwhile it acts as the oxygen source during the growth of silicon oxide sheath. However, when
73
the H2 was added in the carrier gases, the following reaction may occur: SiOx ðsÞ þ H2 -SiðsÞ þ xH2 OðgÞ so that the catalytic droplets were kept active all the time (the naked catalytic particle shown in the lower right inset in Fig. 3a). What is more, the added hydrogen may etch the silicon oxide covering the nanowires and Si nanowires as well as the catalyst nanoparticles. Once Si atoms are absorbed at the V/L interface of the smaller catalytic droplets, they will diffuse more rapidly to the liquid/solid (L/S) interface to participate, which leads to the increased growth rate of the nanowires without SiOx coating, though the lower decomposition rate of silane in H2/N2 than in N2. Moreover, the activity of catalytic nanoparticles increases with the increase of H2 flow, which is supported by the evidence that the nanowires grow longer in N2 mixed with 10 sccm H2 than that in N2 mixed with 5 sccm H2 shown in Fig. 1. In addition, the H2 added to N2 suppressed the decomposition of silane and a large quantity of undecomposed siliane molecules interacted with the dangling bonds in the surfaces of Si nanowires by several possible reactions [18,19]: SiH4 ðgÞ þ SiðsÞ-HðgÞ þ H3 Si SiðsÞ; HðgÞ þ H2SiðsÞH2 ðgÞ þ 2SiðsÞ; H2SiðsÞ þ H2SiðsÞ"Si2SiðsÞ þ H2 : Therefore, the reaction of undecomposed siliane with the dangling bonds and the cross-linking reaction between two adjacent Si–H bonds resulted in the growth of some silicon nanoparticles on the surfaces of Si nanowires. The above differences observed by HRTEM between the samples in silicon oxide sheath were also confirmed by using photoluminescence (PL) measurements. The typical room temperature PL spectra of the samples synthesized in N2 and N2 mixed with 10 sccm H2 are shown in Fig. 4 with two curves. The observed PL for Si nanowires synthesized in N2 is related to the amorphous coatings of silicon oxide (curve (a) in Fig. 4). The luminescence peak at 560 nm is attributed to the non-bridging oxygen hole center (O3Si–O) [20,21], and the luminescence band at about
ARTICLE IN PRESS X.Q. Yan et al. / Journal of Crystal Growth 257 (2003) 69–74
74
Acknowledgements
Intensity (a.u.)
(a)
56 0nm
The authors gratefully thank X.A. Yang and J.L. Jin for their assistance in SEM and HRTEM work. This work is supported in part by the National Natural Science Foundation of China.
References (b) 550
600
650
700
75 0
Wavelength (nm)
Fig. 4. Room-temperature PL spectra of Si nanowires synthesized using (a) N2 and (b) N2 mixed with 10 sccm H2 as the carrier gases.
650 nm is associated with the interface between the silicon core and the silicon oxide sheath [6]. However, no PL was detected for the Si nanowires synthesized in H2/N2. There are two causes for the absence of PL: no amorphous silicon oxide sheath on the Si nanowires synthesized in H2/N2 and the disappearance of quantum size effect resulting from the large diameter of Si nanowires (compared to the critical Bohr radius about 5 nm [22]).
4. Conclusion In conclusion, large-scale Si nanowires with different morphology, size and chemical composition have been synthesized by thermal chemical vapor deposition using SiH4 at 650 C in different carrier gases. Si nanowires with diameters of 80–110 nm and a length of 1 mm have been obtained in N2, and are sheathed with an amorphous silicon oxide shell. Si nanowires produced in the mixture of H2 and N2, without amorphous SiOx coating, have a diameter of 40– 50 nm and a length of more than 5 mm. The effects of H2 on the catalytic particle size and on the formation of the single-crystal silicon nanowires are discussed in detail. It is possible that these silicon nanowires may offer more opportunities for both fundamental researches and technological applications.
[1] A.M. Morles, C.M. Lieber, Science 279 (1998) 208. [2] N. Wang, Y.F. Zhang, Y.H. Tang, C.S. Lee, S.T. Lee, Phys. Rev. B 73 (1998) 3902. [3] N. Wang, Y.H. Tang, Y.F. Zhang, C.S. Lee, S.T. Lee, Phys. Rev. B 58 (1998) R16024. [4] D.P. Yu, Z.G. Bai, Y. Ding, Q.L. Hang, H.Z. Zhang, J.J. Wang, Y.H. Zou, W. Qian, G.C. Xiong, H.T. Zhou, S.Q. Feng, Appl. Phys. Lett. 72 (1998) 3458. [5] N. Wang, Y.H. Tang, Y.F. Zhang, C.S. Lee, I. Bello, S.T. Lee, Chem. Phys. Lett. 299 (1999) 237. [6] C.P. Li, X.H. Sun, N.B. Wong, C.S. Lee, S.T. Lee, B.K. Teo, Chem. Phys. Lett. 365 (2002) 22. [7] S.W. Chung, J.Y. Yu, J.R. Heath, Appl. Phys. Lett. 76 (2000) 2068. [8] J.Y. Yu, S.W. Chung, J.R. Heath, J. Phys. Chem. B 104 (2000) 11864. [9] J.D. Holmes, K.P. Johnston, R.C. Doty, B.A. Korgel, Science 287 (2000) 1471. [10] X.T. Zhou, J.Q. Hu, C.P. Li, D.D. Ma, C.S. Lee, S.T. Lee, Chem. Phys. Lett. 369 (2003) 220. [11] X.H. Fan, L. Xu, C.P. Li, Y.F. Zheng, C.S. Lee, S.T. Lee, Chem. Phys. Lett. 334 (2001) 229. [12] J. Westwater, D.P. Gosain, S. Tomiya, S. Usui, H. Ruda, J. Vac. Sci. Technol. B 15 (1997) 554. [13] N. Ozaki, Y. Ohno, S. Takeda, Appl. Phys. Lett. 73 (1998) 3700. [14] M.K. Sunkara, S. Sharma, R. Miranda, G. Lian, E.C. Dickey, Appl. Phys. Lett. 79 (2001) 1546. [15] Z.Q. Liu, S.S. Xie, W.Y. Zhou, L.F. Sun, Y.B. Li, D.S. Tang, X.P. Zou, C.Y. Wang, G. Wang, J. Crystal Growth 224 (2001) 230. [16] G.A. Bootsma, H.J. Gassen, J. Crystal Growth 10 (1971) 223. [17] Z.Q. Liu, L.F. Sun, D.S. Tang, W.Y. Zhou, Y.B. Li, X.P. Zou, G. Wang, Chin, Phys. 9 (2000) 774. [18] R. Robertson, A. Gallagher, J. Chem. Phys. 85 (1986) 3623. [19] M. Tao, Thin Solid Films 223 (1993) 201. [20] F.L. Galeener, D.B. Kerwin, A.J. Miller, J.C. Mikkeksen, Solid State Commun. 82 (1992) 271. [21] L.X. Yi, J. Heitmann, R. Scholz, M. Zacharias, Appl. Phys. Lett. 81 (2002) 4248. [22] G.D. Sanders, Y.C. Chang, Phys. Rev. B 45 (1992) 9202.
