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Nanowire growth on Si wafers by oxygen implantation and annealing
Applied Surface Science 252 (2006) 5572–5574 www.elsevier.com/locate/apsusc
Nanowire growth on Si wafers by oxygen implantation and annealing Elder A. de Vasconcelos a,*, Fa´bio R.P. dos Santos a, Eronides F. da Silva Jr. a, Henri Boudinov b a
Departamento de Fı´sica, Universidade Federal de Pernambuco, Cidade Universita´ria, Recife, PE 50670-901, Brazil b Instituto de Fı´sica, Universidade Federal do Rio Grande do Sul, 91501-970, Porto Alegre, RS, Brazil Available online 14 February 2006
Abstract We report on nanowire formation on oxygen implanted Si wafers. In this method, a Si wafer is first oxygen-implanted and then annealed at high temperatures in Ar ambient to promote growth of nanowires with high aspect ratio. Their lengths range from several micrometers to thousands of micrometers and their diameters range from tens of nanometers to a few microns. # 2006 Elsevier B.V. All rights reserved. PACS: 61.46. w; 73.21.Hb Keywords: Silicon nanowires; Ion implantation; Stress
1. Introduction One-dimensional structures, such as nanotubes and nanowires have great potential for novel applications due to their mechanical, optical, and electrical properties. Carbon nanotubes [1] and nanowires from Si and a variety of other materials have been prepared by different methods [2]. One common method of Si nanowire growth is the vapor–liquid–solid (VLS) method [3], in which a catalyst metal droplet acts as a site for vapor-phase adsorption of Si atoms. Continued adsorption of Si atoms results in supersaturation of the liquid alloy, leading to nucleation of the solid semiconductor and nanowire growth at the liquid metal/semiconductor interface. Variations of the VLS method using different metal catalysts or different sources of Si vapor have been demonstrated [4,5]. Another common method is the so-called oxide-assisted growth, proposed by Lee and coworkers [6], in which no metal catalyst is used. Instead of metals, oxides are used to induce the nucleation and growth of nanowires. This method has been used to fabricate nanowires from various materials, including Si, Ge, and compound semiconductor nanowires. In particular, for Si nanowire
* Corresponding author. Tel.: +55 81 2126 8450; fax: +55 81 3271 0359. E-mail address: [email protected] (E.A. de Vasconcelos). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.12.140
growth, they propose that SixO (x > 1) vapor decomposes into Si nanoparticles, which act as the nuclei of silicon nanowires covered by shells of silicon oxide. Recently, Prokes and Arnold [7] reported growth of nanowires by a process which does not require any siliconbased vapor or metal catalyst. They reported growth of silicon nanowires from silicon substrates containing a native oxide layer and annealed in an Ar/H2 mixture. They suggested a stress-driven mechanism, in which stress plays a key role. Upon heating, due to the difference in thermal expansion coefficients, the native oxide layer is under tensile stress and the underlying Si region is under compressive stress. The compressive stress is relieved locally by stress cracks. The stress gradient leads to silicon atom flux to these crack regions, leading to silicon wire growth. They reported growth of Si nanowires up to 90 mm long with diameters in the 20–30 nm range. In this work, we report an alternate method to grow Si nanowires, which also does not require any silicon-based vapor or metal catalyst. In this method, a Si wafer is first oxygenimplanted and then annealed in Ar ambient to promote the growth of the wires. We observed growth of Si wires with very large aspect ratio, some more than 1 mm long, with diameters ranging from tens of nanometers to a few microns. We will present basic structural and chemical properties of the wires and discuss possible growth mechanisms.
E.A. de Vasconcelos et al. / Applied Surface Science 252 (2006) 5572–5574
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2. Experimental We used (1 0 0) oriented, n-type Si wafers. The wafers were tilted at 78 and implanted using 185 keV oxygen ions from a gaseous source. The substrate temperature during implantation was kept at 590 8C. We used different doses, from 1016 to 1018 cm 2. Using these implantation parameters, the projected implantation depth (depth at maximum oxygen concentration) is 300–350 nm and the width of the concentration distribution is 80–100 nm. After implantation, the wafers were cleaned in 5 min steps in metal–oxide–semiconductor (MOS) grade acetone, methanol and HF 10% with a 2 min rinse in dionized water after each step. After cleaning, the wafers were placed on a quartz boat and inserted into the quartz tube of a Thermco MB-80 furnace set at 1100 8C. The annealing was performed in Ar for 4 h. During the heat up cycle the tube was continuously flushed with nitrogen, and the samples were not inside it. The samples were analyzed by scanning electron microscopy (SEM) and by their energy dispersive X-ray spectrum. 3. Results and discussion Fig. 2. Low-magnification image of wires grown along a crack.
Figs. 1 and 2 show low magnification images of wafers after wire growth. The energy dispersive X-ray spectra indicated that the wires consisted of silicon and a small amount of oxygen. One can see bundles of wires with varying diameters protruding from the surface. The dark regions shown in Fig. 1 are depressions which appeared after annealing. The wires tend to appear near these depressions, as shown in Fig. 3, along scratches or at the edges of the wafers. The wires have very large aspect ratio and some are more than 1000 mm long, with diameters ranging from tens of nanometers to a few microns. The trend was that the shortest the wire, the smallest the diameter. Fig. 4 shows a region which contains a nanowire with a diameter of approximately 60 nm. The fact that the wires tend to appear near depressions, along scratches or at the edges of the wafers is consistent with a stressdriven mechanism, as proposed by Prokes and Arnold [7]. However, they did not observe growth of wires in Ar gas only.
They observed growth only when a mixture of Ar and H2 was flowing and suggested that the role of hydrogen was to enhance the diffusion kinetics of the silicon atoms accumulating near stress cracks. In contrast, we performed the annealing in Ar only. Moreover, they did not observe wire growth if the native oxide was stripped by a dilute HF solution. In our case, we dipped the samples in HF 10% before loading the wafers into the furnace. We would like to suggest that the results reported here can be explained by the emission of Si atoms, leading to stress-driven growth. The emission of Si atoms during oxidation was studied in detail by first-principles calculation with ultrasoft pseudopotentials by Kageshima et al. [8], who derived a universal theory of Si oxidation rate based on the behavior of the emitted Si atoms. They showed that Si emission occurs because the
Fig. 1. Low-magnification image of wires with varying diameters and large aspect ratio grown on the surface of an oxygen-implanted Si wafer.
Fig. 3. View of wires grown near a depression.
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E.A. de Vasconcelos et al. / Applied Surface Science 252 (2006) 5572–5574
Si nanowire growth involving stress mechanisms looks promising. However, our work and a previous work [7] show that this growth method is very sensitive to surface condition, annealing temperatures and gas composition. Further investigation needs to be done to disclose the details of the growth mechanism and to improve the degree of control on the location of wire growth, on wire diameter, on yield, etc. 4. Conclusions
Fig. 4. Image of sub-micron wires. The arrow indicates a particular wire with a diameter of approximately 60 nm.
accumulation of the strain due to the large volume expansion cannot be released efficiently without emission of Si atoms. In their model, few Si atoms are emitted into the substrate and become self-interstitials. When the oxide layer is thin, most of the emitted Si atoms diffuse through the oxide to the surface and are emitted as SiO molecules or grown as an oxide layer. We propose that, during Ar annealing, the oxygen atoms in the implanted region may precipitate and create SiO2 clusters and Si interstitials, thus generating strain and emission of Si atoms. Therefore, the presence of the oxygen-rich region formed close to the surface after ion implantation might compensate the absence of the hydrogen and the native oxide layer still leading to a stress-driven growth mechanism, similar to that reported by Prokes and Arnold [7]. However, we cannot rule out the possibility that some sort of oxide-assisted growth might occur simultaneously. Decomposition of non-stoichiometric silicon oxide near the surface and its reaction with Si atoms at the surface, as in the oxide-assisted growth model, is not unlikely in our experiments. We further suggest that a growth enhancement due to incorporation of both mechanisms may explain why the wires that we observed are so long.
We observed growth of Si nanowires when a Si wafer is first oxygen-implanted and then annealed at high temperatures in Ar. The Si wires had large aspect ratio, and some were more than 1 mm long, with diameters ranging from tens of nanometers to a few microns. We propose that the oxygen atoms in the implanted region may precipitate and create SiO2 clusters and Si interstitials, thus generating strain and emission of Si atoms, leading to stress-driven growth. Further work needs to be done to optimize the growth process and improve repeatability and the degree of control on wire parameters. Acknowledgements This work was supported by grants CNPq/NanoSemiMat 550.015/01-9 and CAPES-PRODOC 188/03. We thank Mr. George C. Nascimento from ITEP-PE for his participation in the microscopy work. References [1] V.N. Popov, Mater. Sci. Eng. R 43 (2004) 61. [2] C.N. Rao, F.L. Deepak, G. Gundiah, A. Govindaraj, Prog. Solid State Chem. 31 (2003) 5. [3] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 8. [4] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [5] T. Ono, H. Saitoh, M. Esaki, Appl. Phys. Lett. 70 (1997) 1852. [6] R.Q. Zhang, Y. Lifschitz, S.T. Lee, Adv. Mater. 15 (2003) 635. [7] S.M. Prokes, S. Arnold, Appl. Phys. Lett. 86 (2005) 193105. [8] H. Kageshima, K. Shiraishi, M. Uematsu, Jpn. J. Appl. Phys. 38 (1999) L971.
Nanowire growth on Si wafers by oxygen implantation and annealing Elder A. de Vasconcelos a,*, Fa´bio R.P. dos Santos a, Eronides F. da Silva Jr. a, Henri Boudinov b a
Departamento de Fı´sica, Universidade Federal de Pernambuco, Cidade Universita´ria, Recife, PE 50670-901, Brazil b Instituto de Fı´sica, Universidade Federal do Rio Grande do Sul, 91501-970, Porto Alegre, RS, Brazil Available online 14 February 2006
Abstract We report on nanowire formation on oxygen implanted Si wafers. In this method, a Si wafer is first oxygen-implanted and then annealed at high temperatures in Ar ambient to promote growth of nanowires with high aspect ratio. Their lengths range from several micrometers to thousands of micrometers and their diameters range from tens of nanometers to a few microns. # 2006 Elsevier B.V. All rights reserved. PACS: 61.46. w; 73.21.Hb Keywords: Silicon nanowires; Ion implantation; Stress
1. Introduction One-dimensional structures, such as nanotubes and nanowires have great potential for novel applications due to their mechanical, optical, and electrical properties. Carbon nanotubes [1] and nanowires from Si and a variety of other materials have been prepared by different methods [2]. One common method of Si nanowire growth is the vapor–liquid–solid (VLS) method [3], in which a catalyst metal droplet acts as a site for vapor-phase adsorption of Si atoms. Continued adsorption of Si atoms results in supersaturation of the liquid alloy, leading to nucleation of the solid semiconductor and nanowire growth at the liquid metal/semiconductor interface. Variations of the VLS method using different metal catalysts or different sources of Si vapor have been demonstrated [4,5]. Another common method is the so-called oxide-assisted growth, proposed by Lee and coworkers [6], in which no metal catalyst is used. Instead of metals, oxides are used to induce the nucleation and growth of nanowires. This method has been used to fabricate nanowires from various materials, including Si, Ge, and compound semiconductor nanowires. In particular, for Si nanowire
* Corresponding author. Tel.: +55 81 2126 8450; fax: +55 81 3271 0359. E-mail address: [email protected] (E.A. de Vasconcelos). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.12.140
growth, they propose that SixO (x > 1) vapor decomposes into Si nanoparticles, which act as the nuclei of silicon nanowires covered by shells of silicon oxide. Recently, Prokes and Arnold [7] reported growth of nanowires by a process which does not require any siliconbased vapor or metal catalyst. They reported growth of silicon nanowires from silicon substrates containing a native oxide layer and annealed in an Ar/H2 mixture. They suggested a stress-driven mechanism, in which stress plays a key role. Upon heating, due to the difference in thermal expansion coefficients, the native oxide layer is under tensile stress and the underlying Si region is under compressive stress. The compressive stress is relieved locally by stress cracks. The stress gradient leads to silicon atom flux to these crack regions, leading to silicon wire growth. They reported growth of Si nanowires up to 90 mm long with diameters in the 20–30 nm range. In this work, we report an alternate method to grow Si nanowires, which also does not require any silicon-based vapor or metal catalyst. In this method, a Si wafer is first oxygenimplanted and then annealed in Ar ambient to promote the growth of the wires. We observed growth of Si wires with very large aspect ratio, some more than 1 mm long, with diameters ranging from tens of nanometers to a few microns. We will present basic structural and chemical properties of the wires and discuss possible growth mechanisms.
E.A. de Vasconcelos et al. / Applied Surface Science 252 (2006) 5572–5574
5573
2. Experimental We used (1 0 0) oriented, n-type Si wafers. The wafers were tilted at 78 and implanted using 185 keV oxygen ions from a gaseous source. The substrate temperature during implantation was kept at 590 8C. We used different doses, from 1016 to 1018 cm 2. Using these implantation parameters, the projected implantation depth (depth at maximum oxygen concentration) is 300–350 nm and the width of the concentration distribution is 80–100 nm. After implantation, the wafers were cleaned in 5 min steps in metal–oxide–semiconductor (MOS) grade acetone, methanol and HF 10% with a 2 min rinse in dionized water after each step. After cleaning, the wafers were placed on a quartz boat and inserted into the quartz tube of a Thermco MB-80 furnace set at 1100 8C. The annealing was performed in Ar for 4 h. During the heat up cycle the tube was continuously flushed with nitrogen, and the samples were not inside it. The samples were analyzed by scanning electron microscopy (SEM) and by their energy dispersive X-ray spectrum. 3. Results and discussion Fig. 2. Low-magnification image of wires grown along a crack.
Figs. 1 and 2 show low magnification images of wafers after wire growth. The energy dispersive X-ray spectra indicated that the wires consisted of silicon and a small amount of oxygen. One can see bundles of wires with varying diameters protruding from the surface. The dark regions shown in Fig. 1 are depressions which appeared after annealing. The wires tend to appear near these depressions, as shown in Fig. 3, along scratches or at the edges of the wafers. The wires have very large aspect ratio and some are more than 1000 mm long, with diameters ranging from tens of nanometers to a few microns. The trend was that the shortest the wire, the smallest the diameter. Fig. 4 shows a region which contains a nanowire with a diameter of approximately 60 nm. The fact that the wires tend to appear near depressions, along scratches or at the edges of the wafers is consistent with a stressdriven mechanism, as proposed by Prokes and Arnold [7]. However, they did not observe growth of wires in Ar gas only.
They observed growth only when a mixture of Ar and H2 was flowing and suggested that the role of hydrogen was to enhance the diffusion kinetics of the silicon atoms accumulating near stress cracks. In contrast, we performed the annealing in Ar only. Moreover, they did not observe wire growth if the native oxide was stripped by a dilute HF solution. In our case, we dipped the samples in HF 10% before loading the wafers into the furnace. We would like to suggest that the results reported here can be explained by the emission of Si atoms, leading to stress-driven growth. The emission of Si atoms during oxidation was studied in detail by first-principles calculation with ultrasoft pseudopotentials by Kageshima et al. [8], who derived a universal theory of Si oxidation rate based on the behavior of the emitted Si atoms. They showed that Si emission occurs because the
Fig. 1. Low-magnification image of wires with varying diameters and large aspect ratio grown on the surface of an oxygen-implanted Si wafer.
Fig. 3. View of wires grown near a depression.
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E.A. de Vasconcelos et al. / Applied Surface Science 252 (2006) 5572–5574
Si nanowire growth involving stress mechanisms looks promising. However, our work and a previous work [7] show that this growth method is very sensitive to surface condition, annealing temperatures and gas composition. Further investigation needs to be done to disclose the details of the growth mechanism and to improve the degree of control on the location of wire growth, on wire diameter, on yield, etc. 4. Conclusions
Fig. 4. Image of sub-micron wires. The arrow indicates a particular wire with a diameter of approximately 60 nm.
accumulation of the strain due to the large volume expansion cannot be released efficiently without emission of Si atoms. In their model, few Si atoms are emitted into the substrate and become self-interstitials. When the oxide layer is thin, most of the emitted Si atoms diffuse through the oxide to the surface and are emitted as SiO molecules or grown as an oxide layer. We propose that, during Ar annealing, the oxygen atoms in the implanted region may precipitate and create SiO2 clusters and Si interstitials, thus generating strain and emission of Si atoms. Therefore, the presence of the oxygen-rich region formed close to the surface after ion implantation might compensate the absence of the hydrogen and the native oxide layer still leading to a stress-driven growth mechanism, similar to that reported by Prokes and Arnold [7]. However, we cannot rule out the possibility that some sort of oxide-assisted growth might occur simultaneously. Decomposition of non-stoichiometric silicon oxide near the surface and its reaction with Si atoms at the surface, as in the oxide-assisted growth model, is not unlikely in our experiments. We further suggest that a growth enhancement due to incorporation of both mechanisms may explain why the wires that we observed are so long.
We observed growth of Si nanowires when a Si wafer is first oxygen-implanted and then annealed at high temperatures in Ar. The Si wires had large aspect ratio, and some were more than 1 mm long, with diameters ranging from tens of nanometers to a few microns. We propose that the oxygen atoms in the implanted region may precipitate and create SiO2 clusters and Si interstitials, thus generating strain and emission of Si atoms, leading to stress-driven growth. Further work needs to be done to optimize the growth process and improve repeatability and the degree of control on wire parameters. Acknowledgements This work was supported by grants CNPq/NanoSemiMat 550.015/01-9 and CAPES-PRODOC 188/03. We thank Mr. George C. Nascimento from ITEP-PE for his participation in the microscopy work. References [1] V.N. Popov, Mater. Sci. Eng. R 43 (2004) 61. [2] C.N. Rao, F.L. Deepak, G. Gundiah, A. Govindaraj, Prog. Solid State Chem. 31 (2003) 5. [3] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 8. [4] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [5] T. Ono, H. Saitoh, M. Esaki, Appl. Phys. Lett. 70 (1997) 1852. [6] R.Q. Zhang, Y. Lifschitz, S.T. Lee, Adv. Mater. 15 (2003) 635. [7] S.M. Prokes, S. Arnold, Appl. Phys. Lett. 86 (2005) 193105. [8] H. Kageshima, K. Shiraishi, M. Uematsu, Jpn. J. Appl. Phys. 38 (1999) L971.