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Surface morphology of GaN nanorods grown by catalyst-free hydride vapor phase epitaxy
Applied Surface Science 256 (2009) 1078–1081
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface morphology of GaN nanorods grown by catalyst-free hydride vapor phase epitaxy Yuri Sohn, Chinkyo Kim * Department of Physics and Research Institute of Basic Sciences, Kyung Hee University, 1 Hoegi-dong, Dongdaemun-gu, Seoul 130-701, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 6 June 2009
GaN nanorods were grown on c-plane sapphire substrates by using catalyst-free hydride vapor phase epitaxy (HVPE). The effects of substrate temperature, Ga boat temperature, and Ga pretreatment on the surface morphology of GaN nanorods were investigated. From the dependence of a radial and axial growth rate on the substrate temperature, the kinetically limited process was found to be a rate determining step in the growth of GaN nanorods in HVPE. In addition, the activation energy of the growth along the both axial and radial directions were estimated. The dependence of a Ga boat temperature and the Ga pretreatment effect revealed that the density of nanorods were dependent on the flux of Ga species on the substrate. ß 2009 Elsevier B.V. All rights reserved.
PACS: 81.15.Kk 68.55. a 68.37.Hk Keywords: GaN Nanorods Hydride vapor phase epitaxy
1. Introduction GaN is one of the most promising candidate materials for visible and UV wavelength optoelectronic devices and high power devices due to its wide and direct bandgap. Growth of high quality GaN films is, however, very challenging because of a large mismatch of thermal expansion coefficients and lattice parameters between GaN and its foreign substrates. As a consequence, residual strains and a high density of dislocations result in wafer bending and crack, and they affect electrical and optical properties in a detrimental way. Although broad area GaN films typically contain a high density of extended defects, one-dimensional (1D) nanostructures of GaN are known to be almost defect-free and reveal improved optical properties compared to that of broad area films in device application [1]. 1D structures such as nanorods or nanowires are considered to be promising configurations for nanoscale optoelectronic devices. Several attempts to grow GaN nanorods have been reported by using chemical vapor transport [2–4], laser ablation [5], metal organic chemical vapor deposition [6–9], molecular beam epitaxy [10–12]. Usually, these methods utilize metal catalysts, buffer layers or pre-patterned substrates. Catalyst-free, vapor-phase-epitaxial growth of vertically aligned GaN nanorods without using pre-patterned substrates was accomplished so far mainly by using HVPE [13–15]. Due to parasitic gas reactions
* Corresponding author. Tel.: +82 2 961 0379; fax: +82 2 957 8408. E-mail address: [email protected] (C. Kim). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.05.129
within a reactor, the reproducible growth of GaN nanorods by using HVPE was, however, limited. Consequently the dependence of growth behavior and surface morphology of GaN nanorods grown by using HVPE on various growth parameters have not been much investigated yet. We recently reported a technique for the reproducible growth of GaN nanorods on an air-cooled substrate by using HVPE without any patterned substrates or catalyst [15,16]. In this paper, utilizing this reproducible growth technique to grow GaN nanorods, we investigated the surface morphology dependence on various growth parameters and were able to estimate the kinetic barriers of nanorod growth along both the radial and axial directions. 2. Experimental procedure GaN nanorods were grown by utilizing a horizontal HVPE system. The temperature of Ga metal loaded in quartz boats was maintained at 850 C and HCl was flown over Ga. NH3gas was used for N source. 40 sccm of HCl and 2 slm of NH3 were supplied during the growth time of 20 min except for the case of the growth after Ga pretreatment described in the last section of this paper. c-Plane sapphire (0 0 0 1) was used as a substrate and 10 slm of N2 gas was used as a carrier gas. The temperature of a growth zone, in which the substrate was placed, was maintained around 850 C. On the other hand, the substrate temperature itself was independently lowered further by using an external-cooling-air system [15]. To measure the temperature of a substrate itself more accurately, a thermocouple was contacted on the front side of the substrate, not on the back side of a substrate holder as shown in the previous
Y. Sohn, C. Kim / Applied Surface Science 256 (2009) 1078–1081
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Fig. 1. 20 -Tilted SEM images of GaN nanorods grown at substrate temperature of (a) 735 C, (b) 745 C, (c) 755 C, and (d) 765 C. (e)–(h) Are the plan-view images of (a)–(d), respectively.
work. Since the thermocouple touched the back side of a substrate holder in the previous work, substrate temperatures were underestimated in the previously reported work [15]. The size of GaN nanorods was measured by using scanning electron microscopy (SEM) images. 3. Results and discussion 3.1. Substrate temperature Fig. 1 (a)–(h) shows SEM images of GaN nanorods grown at substrate temperatures ranging from 735 to 765 C. At 735 C, approximately 30-nm-high GaN nanorods started to be formed with the aspect ratio of unity as shown in Fig. 1(a). The length and the diameter of nanorods gradually increased with the substrate temperature. This implies that the growth of GaN nanorods along both axial and radial directions in HVPE is not a mass-transportlimited, but a kinetically limited process. The activation energies of nanorod growth along the both directions were calculated from the logarithmic plot of the growth rate as a function of an inversegrowth-temperature as shown in Fig. 2. For axial and radial
Fig. 2. Both axial and radial growth rates were plotted as a function of an inverse substrate temperature. For the axial growth, only one activation energy was observed, but for the radial growth two different activation energies depending on the growth temperature were found.
growth, adatoms should successfully diffuse and be incorporated into each corresponding surface. For diffusion and incorporation process, kinetic barriers exist and their values usually depend on the crystallographic orientations. Our measured activation energies stand for the energies required to get over these barriers for each axial and radial growth directions. Note that the activation energy of the axial growth was estimated to be 6:8 1:2 eV, but for the radial growth two activation energies were observed depending on the growth temperature. This transitional behavior in the activation energy along the radial growth implies that there are two different regimes in the radial growth depending on the substrate temperature. At higher temperature the activation energy for the radial growth was the same with that of the axial growth, but the activation energy at lower temperature was much smaller. This implies that from the viewpoint of the kinetic barriers the radial growth is not distinguishable with the axial growth at higher temperature, but that at lower temperature the atomic species feel higher kinetic barrier along the axial direction. 3.2. Ga boat temperature The growth rate of GaN films in HVPE is mainly controlled by a Ga flux, which is determined by a HCl flow rate and Ga boat temperature [17]. On the other hand, the temperature of a Ga boat was found to influence on the growth of nanorods in a different way. Fig. 3 (a)–(h) are GaN nanorods grown at different Ga boat temperatures. Since HCl to GaCl conversion at 800 C is incomplete at this flow rate, more GaCl is thought to be available to the substrate at higher Ga boat temperature [18]. The density of GaN nanorods grown at different Ga boat temperatures decreased monotonically, but the growth rate along the axial direction was not monotonic with the Ga boat temperature. On the other hand, the diameter of nanorods increased monotonically with the Ga boat temperature. Thus, these behaviors can be explained if at a higher Ga flux more species get together to form a larger nanograins in diameter in such a way that the overall density decreased. Another possible scenario to explain the decreased density with Ga temperature is that the exposure of a sapphire substrate to a higher flux of Ga might limit the number of available nucleation sites. A similar behavior was observed in the case of nanorod
1080
Y. Sohn, C. Kim / Applied Surface Science 256 (2009) 1078–1081
Fig. 3. GaN nanorods grown at the substrate temperature of 760 C with a different Ga boat temperature of (a) 810 C, (b) 830 C, (c) 850 C, and (d) 870 C. (e)–(h) Are planview images of (a)–(d), respectively. A few multipods in (h) are circled in white.
growth on a Ga pretreated substrate, which will be discussed in the following section. 3.3. Ga pretreatment effect Another important factor to consider in epitaxy is the initial condition of a substrate. It is well known that the properties of subsequently grown epitaxial layers are significantly depend on the pretreatment conditions of a substrate. In this part the effect of the surface pretreatment with Ga is presented. Fig. 4 shows SEM images of GaN nanorods grown with different Ga pretreatment (Ga + HCl) conditions. Substrates were exposed to GaCl flow for different amount of time ranging from 0 to 40 min. After this pretreatment, GaN nanorods were grown with 60 sccm of HCl. Compared with Fig. 1(d), the nanorods grown on pretreated substrates (shown in Fig. 4(b)–(d)) are well-separated from one another. One interesting feature is that the density of nanorods remarkably and monotonically decreased with the duration of Ga pretreatment. This result is consistent with the case of the
increased temperature of Ga boat. As mentioned above, it is likely that the increased species of GaCl changed the surface state of sapphire in such a way that available nucleation sites reduced. In order for GaN grains to be nucleated, Ga and N should be stroichiometrically available on sapphire substrate. When sapphire substrate is, however, over-exposed to Ga flux, it is likely that Ga atoms aggregate and form nano-droplets. Then, since the area covered by Ga droplets is no longer available for nucleation, the effective number of nucleation sites is decreased. One great difference in the morphology of nanorods shown in Fig. 4 from others in the previous SEM images is that there is virtually no multipod in the sample grown after Ga pretreatment. Not like molecular beam epitaxy, the mixed growth of GaN multipods and vertically aligned nanorods is quite commonly observed in catalyst-free growth of GaN nanostructures in vapor phase epitaxy. For either device application for a specific purpose or characterization of an individual rod, it is of great advantage to grow these two kinds separately. This result on the surface treatment effect implies that the surface
Fig. 4. GaN nanorods grown at 765 C with different Ga pretreatment conditions; (a) no pretreatment, (b) 15 min, (c) 30 min and (d) 40 min of Ga pretreatment with 40 sccm of HCl. (e)–(h) Are plan-view of SEM image of (a)–(d), respectively.
Y. Sohn, C. Kim / Applied Surface Science 256 (2009) 1078–1081
pretreatment can be usefully applied to selectively grow vertically aligned nanorods. 4. Conclusion We investigated the effect of substrate temperature, Ga boat temperature, a Ga pretreatment on the surface morphology of GaN nanorods. From the dependence of both radial and axial growth rates on the substrate temperature, the growth of GaN nanorods in HVPE was found to be a kinetically limited process. In addition, the activation energy of growth along both axial and radial directions for nanorods were estimated. Unlike the axial growth, the radial growth exhibited two different activation energies depending on the substrate temperature. At higher temperature, both axial and radial growth have the same activation energy, but at lower temperature the activation energy of the axial growth had a much smaller value. As for the growth at different temperatures of Ga, the density of GaN nanorods decreased monotonically with the Ga temperature. The radial growth increased with the Ga temperature, but not the axial growth. It was inferred that increased Ga species played a role in modifying the initial surface state of the substrate in such a way that larger nanograins in a lower density were formed. A Ga pretreatment significantly reduced the density of nanorods. In addition, the Ga pretreatment suppressed the growth of multipod, and a selective growth of vertically aligned nanorods was successfully accomplished.
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Acknowledgement This research was supported by the Kyung Hee University Research Fund in 2007 (KHU-20071603). References [1] H.M. Kim, Y.H. Cho, H. Lee, S.I. Kim, S.R. Ryu, D.Y. Kim, T.W. Kang, K.S. Chung, Nano. Lett. 4 (2004) 1059. [2] G. Cheng, L. Zhang, Y. Zhu, G. Fei, L. Li, C. Mo, Y. Mao, Appl. Phys. Lett. 75 (1999) 2455. [3] C. Tang, S. Fan, H. Dang, P. Li, Y. Liu, Appl. Phys. Lett. 77 (2000) 1961. [4] J. Kim, H. So, J. Park, J. Kim, J. Kim, C. Lee, S. Lyu, Appl. Phys. Lett. 80 (2002) 3548. [5] X. Duan, C. Lieber, J. Am. Chem. Soc. 122 (2000) 188. [6] G. Wang, A. Talin, D. Werder, J. Creighton, E. Lai, R. Anderson, I. Arslan, Nanotechnology 17 (2007) 5773. [7] J. Su, G. Cui, M. Gherasimova, H. Tsukamoto, J. Han, D. Ciuparu, S. Lim, L. Pfefferle, Appl. Phys. Lett. 86 (2005) 013105. [8] F. Qian, Y. Li, S. Gradecak, D. Wang, C. Barrelet, C. Lieber, Nano Lett. 4 (2004) 1975. [9] T. Kuykendall, P. Pauzauskie, S. Lee, Y. Zhang, J. Goldberger, P. Yang, Nano Lett. 3 (2003) 1063. [10] K. Kusakabe, A. Kikuchi, K. Kishino, J. Cryst. Growth 237 (2002) 988. [11] Y. Kim, J. Lee, S. Lee, J. Oh, H. Lee, Appl. Phys. A80 (2005) 1635. [12] R. Calarco, M. Marso, T. Richter, A. Aykanat, R. Meijers, A. Hart, T. Stoica, H. Luth, Nano Lett. 5 (2005) 981. [13] G. Seryogin, I. Shalish, W. Moberlychan, V. Narayanamurti, Nanotechnology 16 (2005) 2342. [14] H. Kim, D. Kim, D. Kim, T. Kang, Appl. Phys. Lett. 81 (2005) 2193. [15] Y. Sohn, S. Lee, H. Choe, C. Kim, J. Kor. Phys. Soc. 53 (2008) 908. [16] Y. Sohn, C. Kim, J. Kor. Phys. Soc. 53 (2008) 1393. [17] P. Kempisty, S. Krukowski, J. Cryst. Growth 310 (2008) 900. [18] V.S. Ban, J. Electrochem. Soc. 118 (1971) 1473.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface morphology of GaN nanorods grown by catalyst-free hydride vapor phase epitaxy Yuri Sohn, Chinkyo Kim * Department of Physics and Research Institute of Basic Sciences, Kyung Hee University, 1 Hoegi-dong, Dongdaemun-gu, Seoul 130-701, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 6 June 2009
GaN nanorods were grown on c-plane sapphire substrates by using catalyst-free hydride vapor phase epitaxy (HVPE). The effects of substrate temperature, Ga boat temperature, and Ga pretreatment on the surface morphology of GaN nanorods were investigated. From the dependence of a radial and axial growth rate on the substrate temperature, the kinetically limited process was found to be a rate determining step in the growth of GaN nanorods in HVPE. In addition, the activation energy of the growth along the both axial and radial directions were estimated. The dependence of a Ga boat temperature and the Ga pretreatment effect revealed that the density of nanorods were dependent on the flux of Ga species on the substrate. ß 2009 Elsevier B.V. All rights reserved.
PACS: 81.15.Kk 68.55. a 68.37.Hk Keywords: GaN Nanorods Hydride vapor phase epitaxy
1. Introduction GaN is one of the most promising candidate materials for visible and UV wavelength optoelectronic devices and high power devices due to its wide and direct bandgap. Growth of high quality GaN films is, however, very challenging because of a large mismatch of thermal expansion coefficients and lattice parameters between GaN and its foreign substrates. As a consequence, residual strains and a high density of dislocations result in wafer bending and crack, and they affect electrical and optical properties in a detrimental way. Although broad area GaN films typically contain a high density of extended defects, one-dimensional (1D) nanostructures of GaN are known to be almost defect-free and reveal improved optical properties compared to that of broad area films in device application [1]. 1D structures such as nanorods or nanowires are considered to be promising configurations for nanoscale optoelectronic devices. Several attempts to grow GaN nanorods have been reported by using chemical vapor transport [2–4], laser ablation [5], metal organic chemical vapor deposition [6–9], molecular beam epitaxy [10–12]. Usually, these methods utilize metal catalysts, buffer layers or pre-patterned substrates. Catalyst-free, vapor-phase-epitaxial growth of vertically aligned GaN nanorods without using pre-patterned substrates was accomplished so far mainly by using HVPE [13–15]. Due to parasitic gas reactions
* Corresponding author. Tel.: +82 2 961 0379; fax: +82 2 957 8408. E-mail address: [email protected] (C. Kim). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.05.129
within a reactor, the reproducible growth of GaN nanorods by using HVPE was, however, limited. Consequently the dependence of growth behavior and surface morphology of GaN nanorods grown by using HVPE on various growth parameters have not been much investigated yet. We recently reported a technique for the reproducible growth of GaN nanorods on an air-cooled substrate by using HVPE without any patterned substrates or catalyst [15,16]. In this paper, utilizing this reproducible growth technique to grow GaN nanorods, we investigated the surface morphology dependence on various growth parameters and were able to estimate the kinetic barriers of nanorod growth along both the radial and axial directions. 2. Experimental procedure GaN nanorods were grown by utilizing a horizontal HVPE system. The temperature of Ga metal loaded in quartz boats was maintained at 850 C and HCl was flown over Ga. NH3gas was used for N source. 40 sccm of HCl and 2 slm of NH3 were supplied during the growth time of 20 min except for the case of the growth after Ga pretreatment described in the last section of this paper. c-Plane sapphire (0 0 0 1) was used as a substrate and 10 slm of N2 gas was used as a carrier gas. The temperature of a growth zone, in which the substrate was placed, was maintained around 850 C. On the other hand, the substrate temperature itself was independently lowered further by using an external-cooling-air system [15]. To measure the temperature of a substrate itself more accurately, a thermocouple was contacted on the front side of the substrate, not on the back side of a substrate holder as shown in the previous
Y. Sohn, C. Kim / Applied Surface Science 256 (2009) 1078–1081
1079
Fig. 1. 20 -Tilted SEM images of GaN nanorods grown at substrate temperature of (a) 735 C, (b) 745 C, (c) 755 C, and (d) 765 C. (e)–(h) Are the plan-view images of (a)–(d), respectively.
work. Since the thermocouple touched the back side of a substrate holder in the previous work, substrate temperatures were underestimated in the previously reported work [15]. The size of GaN nanorods was measured by using scanning electron microscopy (SEM) images. 3. Results and discussion 3.1. Substrate temperature Fig. 1 (a)–(h) shows SEM images of GaN nanorods grown at substrate temperatures ranging from 735 to 765 C. At 735 C, approximately 30-nm-high GaN nanorods started to be formed with the aspect ratio of unity as shown in Fig. 1(a). The length and the diameter of nanorods gradually increased with the substrate temperature. This implies that the growth of GaN nanorods along both axial and radial directions in HVPE is not a mass-transportlimited, but a kinetically limited process. The activation energies of nanorod growth along the both directions were calculated from the logarithmic plot of the growth rate as a function of an inversegrowth-temperature as shown in Fig. 2. For axial and radial
Fig. 2. Both axial and radial growth rates were plotted as a function of an inverse substrate temperature. For the axial growth, only one activation energy was observed, but for the radial growth two different activation energies depending on the growth temperature were found.
growth, adatoms should successfully diffuse and be incorporated into each corresponding surface. For diffusion and incorporation process, kinetic barriers exist and their values usually depend on the crystallographic orientations. Our measured activation energies stand for the energies required to get over these barriers for each axial and radial growth directions. Note that the activation energy of the axial growth was estimated to be 6:8 1:2 eV, but for the radial growth two activation energies were observed depending on the growth temperature. This transitional behavior in the activation energy along the radial growth implies that there are two different regimes in the radial growth depending on the substrate temperature. At higher temperature the activation energy for the radial growth was the same with that of the axial growth, but the activation energy at lower temperature was much smaller. This implies that from the viewpoint of the kinetic barriers the radial growth is not distinguishable with the axial growth at higher temperature, but that at lower temperature the atomic species feel higher kinetic barrier along the axial direction. 3.2. Ga boat temperature The growth rate of GaN films in HVPE is mainly controlled by a Ga flux, which is determined by a HCl flow rate and Ga boat temperature [17]. On the other hand, the temperature of a Ga boat was found to influence on the growth of nanorods in a different way. Fig. 3 (a)–(h) are GaN nanorods grown at different Ga boat temperatures. Since HCl to GaCl conversion at 800 C is incomplete at this flow rate, more GaCl is thought to be available to the substrate at higher Ga boat temperature [18]. The density of GaN nanorods grown at different Ga boat temperatures decreased monotonically, but the growth rate along the axial direction was not monotonic with the Ga boat temperature. On the other hand, the diameter of nanorods increased monotonically with the Ga boat temperature. Thus, these behaviors can be explained if at a higher Ga flux more species get together to form a larger nanograins in diameter in such a way that the overall density decreased. Another possible scenario to explain the decreased density with Ga temperature is that the exposure of a sapphire substrate to a higher flux of Ga might limit the number of available nucleation sites. A similar behavior was observed in the case of nanorod
1080
Y. Sohn, C. Kim / Applied Surface Science 256 (2009) 1078–1081
Fig. 3. GaN nanorods grown at the substrate temperature of 760 C with a different Ga boat temperature of (a) 810 C, (b) 830 C, (c) 850 C, and (d) 870 C. (e)–(h) Are planview images of (a)–(d), respectively. A few multipods in (h) are circled in white.
growth on a Ga pretreated substrate, which will be discussed in the following section. 3.3. Ga pretreatment effect Another important factor to consider in epitaxy is the initial condition of a substrate. It is well known that the properties of subsequently grown epitaxial layers are significantly depend on the pretreatment conditions of a substrate. In this part the effect of the surface pretreatment with Ga is presented. Fig. 4 shows SEM images of GaN nanorods grown with different Ga pretreatment (Ga + HCl) conditions. Substrates were exposed to GaCl flow for different amount of time ranging from 0 to 40 min. After this pretreatment, GaN nanorods were grown with 60 sccm of HCl. Compared with Fig. 1(d), the nanorods grown on pretreated substrates (shown in Fig. 4(b)–(d)) are well-separated from one another. One interesting feature is that the density of nanorods remarkably and monotonically decreased with the duration of Ga pretreatment. This result is consistent with the case of the
increased temperature of Ga boat. As mentioned above, it is likely that the increased species of GaCl changed the surface state of sapphire in such a way that available nucleation sites reduced. In order for GaN grains to be nucleated, Ga and N should be stroichiometrically available on sapphire substrate. When sapphire substrate is, however, over-exposed to Ga flux, it is likely that Ga atoms aggregate and form nano-droplets. Then, since the area covered by Ga droplets is no longer available for nucleation, the effective number of nucleation sites is decreased. One great difference in the morphology of nanorods shown in Fig. 4 from others in the previous SEM images is that there is virtually no multipod in the sample grown after Ga pretreatment. Not like molecular beam epitaxy, the mixed growth of GaN multipods and vertically aligned nanorods is quite commonly observed in catalyst-free growth of GaN nanostructures in vapor phase epitaxy. For either device application for a specific purpose or characterization of an individual rod, it is of great advantage to grow these two kinds separately. This result on the surface treatment effect implies that the surface
Fig. 4. GaN nanorods grown at 765 C with different Ga pretreatment conditions; (a) no pretreatment, (b) 15 min, (c) 30 min and (d) 40 min of Ga pretreatment with 40 sccm of HCl. (e)–(h) Are plan-view of SEM image of (a)–(d), respectively.
Y. Sohn, C. Kim / Applied Surface Science 256 (2009) 1078–1081
pretreatment can be usefully applied to selectively grow vertically aligned nanorods. 4. Conclusion We investigated the effect of substrate temperature, Ga boat temperature, a Ga pretreatment on the surface morphology of GaN nanorods. From the dependence of both radial and axial growth rates on the substrate temperature, the growth of GaN nanorods in HVPE was found to be a kinetically limited process. In addition, the activation energy of growth along both axial and radial directions for nanorods were estimated. Unlike the axial growth, the radial growth exhibited two different activation energies depending on the substrate temperature. At higher temperature, both axial and radial growth have the same activation energy, but at lower temperature the activation energy of the axial growth had a much smaller value. As for the growth at different temperatures of Ga, the density of GaN nanorods decreased monotonically with the Ga temperature. The radial growth increased with the Ga temperature, but not the axial growth. It was inferred that increased Ga species played a role in modifying the initial surface state of the substrate in such a way that larger nanograins in a lower density were formed. A Ga pretreatment significantly reduced the density of nanorods. In addition, the Ga pretreatment suppressed the growth of multipod, and a selective growth of vertically aligned nanorods was successfully accomplished.
1081
Acknowledgement This research was supported by the Kyung Hee University Research Fund in 2007 (KHU-20071603). References [1] H.M. Kim, Y.H. Cho, H. Lee, S.I. Kim, S.R. Ryu, D.Y. Kim, T.W. Kang, K.S. Chung, Nano. Lett. 4 (2004) 1059. [2] G. Cheng, L. Zhang, Y. Zhu, G. Fei, L. Li, C. Mo, Y. Mao, Appl. Phys. Lett. 75 (1999) 2455. [3] C. Tang, S. Fan, H. Dang, P. Li, Y. Liu, Appl. Phys. Lett. 77 (2000) 1961. [4] J. Kim, H. So, J. Park, J. Kim, J. Kim, C. Lee, S. Lyu, Appl. Phys. Lett. 80 (2002) 3548. [5] X. Duan, C. Lieber, J. Am. Chem. Soc. 122 (2000) 188. [6] G. Wang, A. Talin, D. Werder, J. Creighton, E. Lai, R. Anderson, I. Arslan, Nanotechnology 17 (2007) 5773. [7] J. Su, G. Cui, M. Gherasimova, H. Tsukamoto, J. Han, D. Ciuparu, S. Lim, L. Pfefferle, Appl. Phys. Lett. 86 (2005) 013105. [8] F. Qian, Y. Li, S. Gradecak, D. Wang, C. Barrelet, C. Lieber, Nano Lett. 4 (2004) 1975. [9] T. Kuykendall, P. Pauzauskie, S. Lee, Y. Zhang, J. Goldberger, P. Yang, Nano Lett. 3 (2003) 1063. [10] K. Kusakabe, A. Kikuchi, K. Kishino, J. Cryst. Growth 237 (2002) 988. [11] Y. Kim, J. Lee, S. Lee, J. Oh, H. Lee, Appl. Phys. A80 (2005) 1635. [12] R. Calarco, M. Marso, T. Richter, A. Aykanat, R. Meijers, A. Hart, T. Stoica, H. Luth, Nano Lett. 5 (2005) 981. [13] G. Seryogin, I. Shalish, W. Moberlychan, V. Narayanamurti, Nanotechnology 16 (2005) 2342. [14] H. Kim, D. Kim, D. Kim, T. Kang, Appl. Phys. Lett. 81 (2005) 2193. [15] Y. Sohn, S. Lee, H. Choe, C. Kim, J. Kor. Phys. Soc. 53 (2008) 908. [16] Y. Sohn, C. Kim, J. Kor. Phys. Soc. 53 (2008) 1393. [17] P. Kempisty, S. Krukowski, J. Cryst. Growth 310 (2008) 900. [18] V.S. Ban, J. Electrochem. Soc. 118 (1971) 1473.