- Email: [email protected]
Mechanism of catalyst-free growth of GaAs nanowires by selective area MOVPE
ARTICLE IN PRESS
Journal of Crystal Growth 298 (2007) 616–619 www.elsevier.com/locate/jcrysgro
Mechanism of catalyst-free growth of GaAs nanowires by selective area MOVPE Keitaro Ikejiri, Jinichiro Noborisaka, Shinjiroh Hara, Junichi Motohisa, Takashi Fukui Graduate School of Information Science and Technology and Research Center for Integrated Quantum Electronics, Hokkaido University, North 14 West 9, Sapporo 060-8628, Japan Available online 19 December 2006
Abstract We studied the growth mechanism of GaAs nanowires in selective-area metalorganic vapor phase epitaxy (MOVPE) by investigating the dependences on substrate orientations and growth conditions. The nanowire structures were formed only on GaAs (1 1 1)B substrate under high temperature (750 1C) and low arsine partial pressure conditions. Structures selectively grown on substrates with various orientations always exhibited specific low-index facets such as {1 1 0}, {1 1 1}A, and {1 1 1}B. It was also found that the appearance of these facets depended strongly on the growth conditions. Furthermore, we have observed a considerable lateral growth on the sidewalls of the nanowires when the growth temperature was lowered and arsine partial pressure was increased, indicating that the growth mode could be changed by the growth conditions. These results demonstrate that the growth mechanism of GaAs nanowires by SA-MOVPE is neither catalyst nor oxide assisted but by the formation of facets during growth. r 2006 Elsevier B.V. All rights reserved. PACS: 61.46.+w; 81.07.b; 81.10.h; 81.15.Gh; 81.16.c Keywords: A1. Nanostructures; A3. Metalorganic vapor phase epitaxy; A3. Selective epitaxy; B2. Semiconducting gallium arsenide
1. Introduction Recently, semiconductor nanowires have attracted much attention for their unique physical and electrical properties as well as their possible applications to nanoscale devices [1–3]. For instance, they are expected to exhibit onedimensional transport properties and two-dimensional quantum confinement effects because of their small diameters on the order of nanometers. So far, most semiconductor nanowires have been grown by catalystassisted vapor–liquid–solid (VLS) mechanism [4,5] to realize highly anisotropic shape elongated in the growth direction. In VLS mechanism, constituent materials are supplied from the vapor and incorporated into the droplet of metal catalyst. It solidifies and forms nanometer-sized whisker after the liquid undergoes supersaturation of constituent materials. In most cases, Au [4–7] is used for the growth of semiconductor nanowires, and some Corresponding author.
E-mail address: [email protected] (S. Hara). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.10.179
report on the VLS growth of InN [8] and InP [9] nanowires where In acts as the catalyst. Oxide-assisted mechanism is also reported for the growth of nanowires using laser ablation using GaAs and gallium oxide (Ga2O3) as source targets [10]. We have fabricated semiconductor nanowires by using selective-area metalorganic vapor phase epitaxy (SA-MOVPE) [11–13]. GaAs nanowires are surrounded by f1¯ 1 0g vertical sidewall facets on the opening area of partially masked GaAs (1 1 1)B substrates. The underlying mechanism we believe is in the formation of facet during growth. Some, however, claim that it is not possible to rule out the catalyst- or oxide-assisted mechanism for the preferential growth in one direction. Thus, nanowire growth modes and their mechanisms in SA-MOVPE are still controversial. In this paper, we investigated the mechanism of growth of nanowires by SA-MOVPE. We studied the dependence of selective-area growth mode on the substrate orientation and growth conditions and found that the nanowires were formed only in a limited combination of growth conditions
ARTICLE IN PRESS K. Ikejiri et al. / Journal of Crystal Growth 298 (2007) 616–619
617
and substrate orientations, which identifies clearly that the formation of nanowires solely relied on the formation of facets during SA-MOVPE. 2. Experimental procedures GaAs substrates partially covered with SiO2 were used for SA-MOVPE. GaAs (1 1 1)B, (0 0 1), (1 1 0) and (3 1 1)B were used as substrates to investigate substrate orientation dependence. After SiO2 was deposited by plasma sputtering on each substrates, periodic opening in SiO2 mask were formed by electron-beam (EB) lithography and wetchemical etching. The size of the mask opening was 100 or 500 nm, which directly determines the size of grown structures. GaAs was grown using a horizontal lowpressure MOVPE system at the working pressure of 0.1 atm. The source materials were trimethylgallium (TMG) and arsine (AsH3). The growth time was 40 min. Two kinds of growth conditions were used in this experiment to investigate growth condition dependence. In the growth condition A, growth temperature, TG, and partial pressure of AsH3, [AsH3], were set to be 750 1C and 5.0 104 atm, respectively. We also refer to this condition as high TG and low [AsH3] condition, which is a standard condition for the growth of GaAs nanowires on (1 1 1)B substrates. For the growth condition B, GaAs were grown at 600 1C and 1.0 103 atm, respectively, and will be referred to as low TG and high [AsH3] conditions. The partial pressure of TMG was 2.7 106 atm for both conditions. The grown structures were characterized by secondary electron microscopy (SEM).
Fig. 1. Bird’s-eye and top view (inset) SEM images of typical GaAs nanowires on GaAs (1 1 1)B substrate grown by SA-MOVPE. The nanowire diameter and height is approximately 100 nm and 5 mm, respectively.
3. Results and discussion Fig. 1 show a typical SEM image of GaAs nanowires grown on (1 1 1)B substrates with the growth condition A. Array of uniformly sized nanowires were formed. The diameter of the nanowires is 100 nm and is the same as the mask-opening size. Note that, as we reported previously, nanowires have hexagonal cross-section. The sidewalls of the nanowires are confirmed to be f1¯ 1 0g facets vertical to (1 1 1)B based on the relative directions of the sidewalls and substrates. Results of the selective growth carried out on the substrates with different orientations and under different growth conditions are summarized in Fig. 2. The nanowire structures were formed only on (1 1 1)B substrate with growth condition A. These experimental data clearly indicate that the mechanism of nanowire growth in SA-MOVPE is neither catalyst nor oxide assisted but is the formation of facet sidewalls that have slower growth rates than that on (1 1 1)B. We will examine these in detail below. Firstly, we focus on the dependence of the substrate orientation. In the case of nanowires in VLS mechanism, the nanowires tend to grow in some particular direction, for instance, in the [1 1 1]B direction for III–V compound semiconductor nanowires. Therefore, the nanowires are
Fig. 2. Bird’s-eye and top view of SEM images of GaAs grown on (1 1 1) B, (0 0 1), (3 1 1)B, and (1 1 0) substrates. The images in (a) summarize the structure grown under high TG and low [AsH3] condition, whereas there in (b) under low TG and high [AsH3] condition.
vertical or tilted to the substrates if the substrate is (1 1 1)B or (0 0 1), respectively. For GaAs nanowires by oxideassisted mechanism, the preferential growth direction is also in the [1 1 1]B. In any case, we could expect to obtain
ARTICLE IN PRESS 618
K. Ikejiri et al. / Journal of Crystal Growth 298 (2007) 616–619
nanowires in the particular (namely, in the [1 1 1]B) direction independent of the orientation of the substrates if the catalysts or oxides would play a role. This clearly is not our case, identifying the absence of catalyst- or oxideassisted mechanisms in SA-MOVPE. As we mentioned earlier, the nanowire structures in Fig. 1 formed hexagonal pillar structures having six f1¯ 1 0g vertical surfaces as a sidewall and a (1 1 1)B top surface. In the case of Fig. 2(a), thickness of the nanowires is much greater (500 nm) due to the different size of the mask opening, but they also have similar hexagonal pillar structures. This means the underlying mechanism for the formation of pillar structure does not depend on the size of the mask opening. On the other hand, grown structures on (0 0 1) and (3 1 1)B substrates exhibited pyramidal-like structures and surrounded clear crystallographic facets. Theses facets are assigned to be {1 1 0}, (1 1 1)A and (1 1 1)B surfaces from the orientation of the substrates and the angle between each facet and substrates, as schematically shown in Fig. 3. We obtained almost the same index facets on both (0 0 1) and (3 1 1)B. Note, generally in selective-area or patterned-area growth, that a facet appears because the growth stops or the growth rate is quite slow in that particular orientation of the surface. In the case of (0 0 1) or (311)B, the growth rate on {1 1 0}, (1 1 1)A, and (1 1 1)B is much lower than that on (0 0 1) and (3 1 1)B. In addition, very little growth occurred and flat (1 1 0) surface is obtained in the case of SA-MOVPE on (1 1 0) surface. Thus, the results indicate clearly that f1¯ 1 0g facets preferentially appear during GaAs growth on any substrate orientation in the present growth condition A (high T and low [AsH3] condition). Therefore, it is concluded that the nanowires on (1 1 1)B are obtained with faceted f1¯ 1 0g surfaces. Formation of the facets is further confirmed by the dependence on growth conditions. In (1 1 1)B, we can see the growth is considerably suppressed for condition B. It is well known that, under this condition where effective coverage of As is high, As overlayer attaches onto the Asterminated (1 1 1)B surface to form stable As trimmers [14]. Because they prevent growth from proceeding on the (1 1 1)B surface, the growth rate becomes low. Similar suppression of the growth in (1 1 1)B surface was also observed on the structures grown on (0 0 1) and (3 1 1)B partially masked substrates, in which the areas of (1 1 1)B facet were enlarged, as schematically shown in Fig. 3. We also observed the growth rate on (1 1 1)A facet became faster, and as a result, (1 1 1)A facets disappeared from the growth structures, as shown in Fig. 2(b). On the (1 1 0) substrate, we can see the growth towards the [1 1 0] direction was enhanced for condition B. These enhancements of the growth on the (1 1 1)A and {1 1 0} surfaces can be explained by the increase of available Ga sites at high As coverage, or the low TG and high [AsH3]. These results on growth condition dependence indicate that the growth mechanism of GaAs nanowires by SA-MOVPE is attributable to the formation of specific low-index
Fig. 3. Schematic illustrations of grown structures on (1 1 1)B, (0 0 1) and (3 1 1)B under growth condition A (high TG and low [AsH3] condition, left) and growth condition B (low TG and high [AsH3] condition, right).
facets and those growth rates depend strongly on the growth conditions. Finally, we grew GaAs with condition A for 40 min, and successively grew with condition B for 20 min. Results of growth are summarized in Fig. 4. The height and the diameter of GaAs nanowires were 1.3 and 0.5 mm, respectively, after the first GaAs growth. However, their diameter became 1.3 mm after the second GaAs growth, while their height was maintained. This result further confirms that GaAs grew only on (1 1 1)B under the high TG and low [AsH3], and only on f1¯ 1 0g under the low TG and high [AsH3], and the importance of the formation of facets during SA-MOVPE. This controllability of the growth mode is promising for the formation nanowires with of complex structures, such as core-shell heterostructures or pn junctions. 4. Summary We investigated the SA-MOVPE growth of GaAs nanowires by studying its dependence on the substrate orientations and growth conditions. The nanowire structures were formed only on (1 1 1)B substrate with
ARTICLE IN PRESS K. Ikejiri et al. / Journal of Crystal Growth 298 (2007) 616–619
619
Fig. 4. SEM images of selectively grown GaAs on (1 1 1)B with one step (a) and (b) two step growth conditions.
particular set of growth conditions. It was also found that the grown structures were always composed of specific lowindex facets and that the growth rate of these facets depended strongly on the growth conditions for any substrate orientation. These results clearly show that the growth mechanism of nanowires by SA-MOVPE is neither catalyst- nor oxide-assisted one, but relies solely on the formation of the facets during growth. Acknowledgments The authors would like to thank K. Tomioka for stimulating discussions and A. Koike for supporting MOVPE experiments. This work was financially supported by a Grantin-aid for Scientific Research From the MEXT in Japan. References [1] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897.
[2] Y. Huang, X. Duan, Y. Cui, L.J. Lauhon, K.-H. Kim, C.M. Lieber, Science 294 (2001) 1313. [3] H.T. Ng, J. Han, T. Yamada, P. Nguyen, Y.P. Chen, M. Meyyappan, Nano Lett. 4 (2004) 1247. [4] K. Hiruma, G.P. Morgan, T. Katsuyama, K. Ogawa, M. Koguchi, H. Kobayashi, Appl. Phys. Lett. 59 (1991) 431. [5] K. Hiruma, H. Yazawa, T. Katuyama, K. Ogawa, K. Haraguchi, M. Koguchi, H. Kakibayashi, J. Appl. Phys. 77 (1995) 447. [6] B.J. Ohlsson, M.T. Bjo¨rk, M.H. Magnusson, K. Deppert, L. Samuelson, L.R. Wellenberg, Appl. Phys. Lett. 79 (2001) 3335. [7] Z.H. Wu, X. Mei, D. Kim, M. Blumin, H.E. Ruda, J.Q. Liu, K.L. Kavanagh, Appl. Phys. Lett. 83 (2003) 3368. [8] H. Parala, A. Devi, F. Hipler, E. Maile, A.H.W. Becker, R.A. Fischer, J. Crystal Growth 231 (2001) 68. [9] C.J. Novotny, P.K.L. Yu, Appl. Phys. Lett. 87 (2005) 203111. [10] W.S. Shi, Y.F. Zheng, N. Wang, C.S. Lee, S.T. Leeb, Appl. Phys. Lett. 78 (2001) 3304. [11] J. Motohisa, J. Noborisaka, J. Takeda, M. Inari, T. Fukui, J. Crystal Growth 272 (2004) 180. [12] J. Noborisaka, J. Motohisa, T. Fukui, Appl. Phys. Lett. 86 (2005) 213102. [13] J. Noborisaka, J. Motohisa, S. Hara, T. Fukui, Appl. Phys. Lett. 87 (2005) 093109. [14] T. Hayakawa, M. Morishima, Appl. Phys. Lett. 59 (1991) 3321.
Journal of Crystal Growth 298 (2007) 616–619 www.elsevier.com/locate/jcrysgro
Mechanism of catalyst-free growth of GaAs nanowires by selective area MOVPE Keitaro Ikejiri, Jinichiro Noborisaka, Shinjiroh Hara, Junichi Motohisa, Takashi Fukui Graduate School of Information Science and Technology and Research Center for Integrated Quantum Electronics, Hokkaido University, North 14 West 9, Sapporo 060-8628, Japan Available online 19 December 2006
Abstract We studied the growth mechanism of GaAs nanowires in selective-area metalorganic vapor phase epitaxy (MOVPE) by investigating the dependences on substrate orientations and growth conditions. The nanowire structures were formed only on GaAs (1 1 1)B substrate under high temperature (750 1C) and low arsine partial pressure conditions. Structures selectively grown on substrates with various orientations always exhibited specific low-index facets such as {1 1 0}, {1 1 1}A, and {1 1 1}B. It was also found that the appearance of these facets depended strongly on the growth conditions. Furthermore, we have observed a considerable lateral growth on the sidewalls of the nanowires when the growth temperature was lowered and arsine partial pressure was increased, indicating that the growth mode could be changed by the growth conditions. These results demonstrate that the growth mechanism of GaAs nanowires by SA-MOVPE is neither catalyst nor oxide assisted but by the formation of facets during growth. r 2006 Elsevier B.V. All rights reserved. PACS: 61.46.+w; 81.07.b; 81.10.h; 81.15.Gh; 81.16.c Keywords: A1. Nanostructures; A3. Metalorganic vapor phase epitaxy; A3. Selective epitaxy; B2. Semiconducting gallium arsenide
1. Introduction Recently, semiconductor nanowires have attracted much attention for their unique physical and electrical properties as well as their possible applications to nanoscale devices [1–3]. For instance, they are expected to exhibit onedimensional transport properties and two-dimensional quantum confinement effects because of their small diameters on the order of nanometers. So far, most semiconductor nanowires have been grown by catalystassisted vapor–liquid–solid (VLS) mechanism [4,5] to realize highly anisotropic shape elongated in the growth direction. In VLS mechanism, constituent materials are supplied from the vapor and incorporated into the droplet of metal catalyst. It solidifies and forms nanometer-sized whisker after the liquid undergoes supersaturation of constituent materials. In most cases, Au [4–7] is used for the growth of semiconductor nanowires, and some Corresponding author.
E-mail address: [email protected] (S. Hara). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.10.179
report on the VLS growth of InN [8] and InP [9] nanowires where In acts as the catalyst. Oxide-assisted mechanism is also reported for the growth of nanowires using laser ablation using GaAs and gallium oxide (Ga2O3) as source targets [10]. We have fabricated semiconductor nanowires by using selective-area metalorganic vapor phase epitaxy (SA-MOVPE) [11–13]. GaAs nanowires are surrounded by f1¯ 1 0g vertical sidewall facets on the opening area of partially masked GaAs (1 1 1)B substrates. The underlying mechanism we believe is in the formation of facet during growth. Some, however, claim that it is not possible to rule out the catalyst- or oxide-assisted mechanism for the preferential growth in one direction. Thus, nanowire growth modes and their mechanisms in SA-MOVPE are still controversial. In this paper, we investigated the mechanism of growth of nanowires by SA-MOVPE. We studied the dependence of selective-area growth mode on the substrate orientation and growth conditions and found that the nanowires were formed only in a limited combination of growth conditions
ARTICLE IN PRESS K. Ikejiri et al. / Journal of Crystal Growth 298 (2007) 616–619
617
and substrate orientations, which identifies clearly that the formation of nanowires solely relied on the formation of facets during SA-MOVPE. 2. Experimental procedures GaAs substrates partially covered with SiO2 were used for SA-MOVPE. GaAs (1 1 1)B, (0 0 1), (1 1 0) and (3 1 1)B were used as substrates to investigate substrate orientation dependence. After SiO2 was deposited by plasma sputtering on each substrates, periodic opening in SiO2 mask were formed by electron-beam (EB) lithography and wetchemical etching. The size of the mask opening was 100 or 500 nm, which directly determines the size of grown structures. GaAs was grown using a horizontal lowpressure MOVPE system at the working pressure of 0.1 atm. The source materials were trimethylgallium (TMG) and arsine (AsH3). The growth time was 40 min. Two kinds of growth conditions were used in this experiment to investigate growth condition dependence. In the growth condition A, growth temperature, TG, and partial pressure of AsH3, [AsH3], were set to be 750 1C and 5.0 104 atm, respectively. We also refer to this condition as high TG and low [AsH3] condition, which is a standard condition for the growth of GaAs nanowires on (1 1 1)B substrates. For the growth condition B, GaAs were grown at 600 1C and 1.0 103 atm, respectively, and will be referred to as low TG and high [AsH3] conditions. The partial pressure of TMG was 2.7 106 atm for both conditions. The grown structures were characterized by secondary electron microscopy (SEM).
Fig. 1. Bird’s-eye and top view (inset) SEM images of typical GaAs nanowires on GaAs (1 1 1)B substrate grown by SA-MOVPE. The nanowire diameter and height is approximately 100 nm and 5 mm, respectively.
3. Results and discussion Fig. 1 show a typical SEM image of GaAs nanowires grown on (1 1 1)B substrates with the growth condition A. Array of uniformly sized nanowires were formed. The diameter of the nanowires is 100 nm and is the same as the mask-opening size. Note that, as we reported previously, nanowires have hexagonal cross-section. The sidewalls of the nanowires are confirmed to be f1¯ 1 0g facets vertical to (1 1 1)B based on the relative directions of the sidewalls and substrates. Results of the selective growth carried out on the substrates with different orientations and under different growth conditions are summarized in Fig. 2. The nanowire structures were formed only on (1 1 1)B substrate with growth condition A. These experimental data clearly indicate that the mechanism of nanowire growth in SA-MOVPE is neither catalyst nor oxide assisted but is the formation of facet sidewalls that have slower growth rates than that on (1 1 1)B. We will examine these in detail below. Firstly, we focus on the dependence of the substrate orientation. In the case of nanowires in VLS mechanism, the nanowires tend to grow in some particular direction, for instance, in the [1 1 1]B direction for III–V compound semiconductor nanowires. Therefore, the nanowires are
Fig. 2. Bird’s-eye and top view of SEM images of GaAs grown on (1 1 1) B, (0 0 1), (3 1 1)B, and (1 1 0) substrates. The images in (a) summarize the structure grown under high TG and low [AsH3] condition, whereas there in (b) under low TG and high [AsH3] condition.
vertical or tilted to the substrates if the substrate is (1 1 1)B or (0 0 1), respectively. For GaAs nanowires by oxideassisted mechanism, the preferential growth direction is also in the [1 1 1]B. In any case, we could expect to obtain
ARTICLE IN PRESS 618
K. Ikejiri et al. / Journal of Crystal Growth 298 (2007) 616–619
nanowires in the particular (namely, in the [1 1 1]B) direction independent of the orientation of the substrates if the catalysts or oxides would play a role. This clearly is not our case, identifying the absence of catalyst- or oxideassisted mechanisms in SA-MOVPE. As we mentioned earlier, the nanowire structures in Fig. 1 formed hexagonal pillar structures having six f1¯ 1 0g vertical surfaces as a sidewall and a (1 1 1)B top surface. In the case of Fig. 2(a), thickness of the nanowires is much greater (500 nm) due to the different size of the mask opening, but they also have similar hexagonal pillar structures. This means the underlying mechanism for the formation of pillar structure does not depend on the size of the mask opening. On the other hand, grown structures on (0 0 1) and (3 1 1)B substrates exhibited pyramidal-like structures and surrounded clear crystallographic facets. Theses facets are assigned to be {1 1 0}, (1 1 1)A and (1 1 1)B surfaces from the orientation of the substrates and the angle between each facet and substrates, as schematically shown in Fig. 3. We obtained almost the same index facets on both (0 0 1) and (3 1 1)B. Note, generally in selective-area or patterned-area growth, that a facet appears because the growth stops or the growth rate is quite slow in that particular orientation of the surface. In the case of (0 0 1) or (311)B, the growth rate on {1 1 0}, (1 1 1)A, and (1 1 1)B is much lower than that on (0 0 1) and (3 1 1)B. In addition, very little growth occurred and flat (1 1 0) surface is obtained in the case of SA-MOVPE on (1 1 0) surface. Thus, the results indicate clearly that f1¯ 1 0g facets preferentially appear during GaAs growth on any substrate orientation in the present growth condition A (high T and low [AsH3] condition). Therefore, it is concluded that the nanowires on (1 1 1)B are obtained with faceted f1¯ 1 0g surfaces. Formation of the facets is further confirmed by the dependence on growth conditions. In (1 1 1)B, we can see the growth is considerably suppressed for condition B. It is well known that, under this condition where effective coverage of As is high, As overlayer attaches onto the Asterminated (1 1 1)B surface to form stable As trimmers [14]. Because they prevent growth from proceeding on the (1 1 1)B surface, the growth rate becomes low. Similar suppression of the growth in (1 1 1)B surface was also observed on the structures grown on (0 0 1) and (3 1 1)B partially masked substrates, in which the areas of (1 1 1)B facet were enlarged, as schematically shown in Fig. 3. We also observed the growth rate on (1 1 1)A facet became faster, and as a result, (1 1 1)A facets disappeared from the growth structures, as shown in Fig. 2(b). On the (1 1 0) substrate, we can see the growth towards the [1 1 0] direction was enhanced for condition B. These enhancements of the growth on the (1 1 1)A and {1 1 0} surfaces can be explained by the increase of available Ga sites at high As coverage, or the low TG and high [AsH3]. These results on growth condition dependence indicate that the growth mechanism of GaAs nanowires by SA-MOVPE is attributable to the formation of specific low-index
Fig. 3. Schematic illustrations of grown structures on (1 1 1)B, (0 0 1) and (3 1 1)B under growth condition A (high TG and low [AsH3] condition, left) and growth condition B (low TG and high [AsH3] condition, right).
facets and those growth rates depend strongly on the growth conditions. Finally, we grew GaAs with condition A for 40 min, and successively grew with condition B for 20 min. Results of growth are summarized in Fig. 4. The height and the diameter of GaAs nanowires were 1.3 and 0.5 mm, respectively, after the first GaAs growth. However, their diameter became 1.3 mm after the second GaAs growth, while their height was maintained. This result further confirms that GaAs grew only on (1 1 1)B under the high TG and low [AsH3], and only on f1¯ 1 0g under the low TG and high [AsH3], and the importance of the formation of facets during SA-MOVPE. This controllability of the growth mode is promising for the formation nanowires with of complex structures, such as core-shell heterostructures or pn junctions. 4. Summary We investigated the SA-MOVPE growth of GaAs nanowires by studying its dependence on the substrate orientations and growth conditions. The nanowire structures were formed only on (1 1 1)B substrate with
ARTICLE IN PRESS K. Ikejiri et al. / Journal of Crystal Growth 298 (2007) 616–619
619
Fig. 4. SEM images of selectively grown GaAs on (1 1 1)B with one step (a) and (b) two step growth conditions.
particular set of growth conditions. It was also found that the grown structures were always composed of specific lowindex facets and that the growth rate of these facets depended strongly on the growth conditions for any substrate orientation. These results clearly show that the growth mechanism of nanowires by SA-MOVPE is neither catalyst- nor oxide-assisted one, but relies solely on the formation of the facets during growth. Acknowledgments The authors would like to thank K. Tomioka for stimulating discussions and A. Koike for supporting MOVPE experiments. This work was financially supported by a Grantin-aid for Scientific Research From the MEXT in Japan. References [1] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897.
[2] Y. Huang, X. Duan, Y. Cui, L.J. Lauhon, K.-H. Kim, C.M. Lieber, Science 294 (2001) 1313. [3] H.T. Ng, J. Han, T. Yamada, P. Nguyen, Y.P. Chen, M. Meyyappan, Nano Lett. 4 (2004) 1247. [4] K. Hiruma, G.P. Morgan, T. Katsuyama, K. Ogawa, M. Koguchi, H. Kobayashi, Appl. Phys. Lett. 59 (1991) 431. [5] K. Hiruma, H. Yazawa, T. Katuyama, K. Ogawa, K. Haraguchi, M. Koguchi, H. Kakibayashi, J. Appl. Phys. 77 (1995) 447. [6] B.J. Ohlsson, M.T. Bjo¨rk, M.H. Magnusson, K. Deppert, L. Samuelson, L.R. Wellenberg, Appl. Phys. Lett. 79 (2001) 3335. [7] Z.H. Wu, X. Mei, D. Kim, M. Blumin, H.E. Ruda, J.Q. Liu, K.L. Kavanagh, Appl. Phys. Lett. 83 (2003) 3368. [8] H. Parala, A. Devi, F. Hipler, E. Maile, A.H.W. Becker, R.A. Fischer, J. Crystal Growth 231 (2001) 68. [9] C.J. Novotny, P.K.L. Yu, Appl. Phys. Lett. 87 (2005) 203111. [10] W.S. Shi, Y.F. Zheng, N. Wang, C.S. Lee, S.T. Leeb, Appl. Phys. Lett. 78 (2001) 3304. [11] J. Motohisa, J. Noborisaka, J. Takeda, M. Inari, T. Fukui, J. Crystal Growth 272 (2004) 180. [12] J. Noborisaka, J. Motohisa, T. Fukui, Appl. Phys. Lett. 86 (2005) 213102. [13] J. Noborisaka, J. Motohisa, S. Hara, T. Fukui, Appl. Phys. Lett. 87 (2005) 093109. [14] T. Hayakawa, M. Morishima, Appl. Phys. Lett. 59 (1991) 3321.