- Email: [email protected]
Growth and characterization of defect free GaAs nanowires
ARTICLE IN PRESS
Journal of Crystal Growth 287 (2006) 504–508 www.elsevier.com/locate/jcrysgro
Growth and characterization of defect free GaAs nanowires Brent A. Wacasera,, Knut Depperta, Lisa S. Karlssonb, Lars Samuelsona, Werner Seiferta a
Solid State Physics, Lund University, Box 118, Lund S-211 00, Sweden Materials Chemistry/nCHREM, Lund University, Box 124, Lund S-211 00, Sweden
b
Available online 5 January 2006
Abstract ¯ direction (B wires) and Most III–V compound semiconductor nanowires seeded by metal particles grow preferentially in a h1¯ 1¯ 1iB most commonly with many stacking faults perpendicular to the growth direction. If growth proceeds in an alternate direction, defect-free growth has been observed. We present experimental results for the growth of GaAs nanowires in a previously uninvestigated growth ¯ direction, a h1 1 1iA direction (A wires). One novelty is that a {1 1 1}A growth plane, like a f1¯ 1¯ 1gB, is a close packed plane where the stacking sequence can be interrupted forming stacking faults, but unlike the B wires the A wires lack stacking faults. It is also observed that, when grown under equivalent conditions, the growth rate of the A wires is approximately twice that of the B wires. Additionally, B ¯ and three f1¯ 1¯ 2g side facets. A wires, on the other hand, have only three major side wires have a hexagonal cross section with three f1 1 2g ¯ type, giving them a triangular cross section. facets which are of the f1 1 2g r 2005 Elsevier B.V. All rights reserved. PACS: 61.50.A; 61.72; 81.05.E; 81.07 Keywords: A1. Defects; A1. Interfaces; A1. Nanostructures; A3. Organometallic vapor phase epitaxy; B2. Semiconducting III–V materials
1. Introduction Semiconducting nanowires are being investigated as contacts for integrated circuits and as nanowires containing devices incorporated within their crystalline structure [1–3]. Studying the electrical and optical properties of nanowires also allows us to experiment physically with the theories of one-dimensional quantum confinement. These and numerous other application, along with the high level of control achieved in the accuracy of the diameter, length, composition, and even the positioning of nanowires, have prompted an increasing interest in nanowire research [4,5]. One of the final difficulties to overcome in growth of ‘perfect’ nanowires is that most nanowires grown to date have been shown to have abundant stacking faults [6–8]. Defects of this nature have been shown to negatively affect the electrical and optical properties of semiconductors [9]. A better understanding of the formation of these defects is needed if we hope to eliminate them from the wires. Corresponding author. Tel.: +46 46 222 95 86; fax: +46 46 222 36 37.
E-mail address: [email protected] (B.A. Wacaser). 0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.11.075
Growth of semiconductor wires, or whiskers, seeded from metal particles was first presented in the 1960s and 1970s [6,7]. The wires were generally observed to grow preferentially in h1 1 1i directions, although deviations from these directions were also reported [6,7]. In III–V semiconductors, the preferential growth direction is com¯ monly a h1¯ 1¯ 1iB direction (B wires). When grown in a ¯ h1¯ 1¯ 1iB direction the wires exhibit a high density of stacking faults perpendicular to the growth direction. Recently, though a few publications report growth of defect free III–V semiconducting wires in alternative directions, showing that the defect density is somehow related to the growth direction [10–13]. We supplement these reports by presenting data on the growth of GaAs wires in h1 1 1iA directions (A wires) which lack stacking faults, have a higher growth rate than the B wires, and have a triangular cross section. Crystallographically the h1 1 1iA growth direction is ¯ closely related to the preferential h1¯ 1¯ 1iB wire growth direction. The growth plane in both directions is a close packed {1 1 1} plane. These face centered cubic planes and ¯ the corresponding (0 0 0 1)A and ð0 0 0 1ÞB close packed
ARTICLE IN PRESS B.A. Wacaser et al. / Journal of Crystal Growth 287 (2006) 504–508
hexagonal planes are the stacking planes where stacking faults can form. The only major difference is that the A surface is terminated by the group III element Ga and the B surface is terminated by the group V element As. The close crystallographic relationship between these two directions makes the differences especially the stacking fault related differences observed in A and B wires even more interesting. 2. Experiment ¯ Epi-ready ð1¯ 1¯ 1ÞB, (0 0 1), and (1 1 1)A GaAs substrates were first prepared by placing the sample in a bath of 37% HCl mixed 1:1 with de-ionized H2O for 1 min, followed by blowing off with N2. Gold aerosol particles with a typical diameter of 40 nm were then deposited on the various substrates. The deposition density, set at 1 or 2 particles per square micron, was achieved by deposition from an aerosol evaporation generator as described previously [14]. After particle deposition, some of the samples were coated with poly lysine by placing a small drop on the substrate, waiting 1 min and blowing off the residue with nitrogen. The substrates were then transferred into a metal-organic vapor phase epitaxy (MOVPE) system. In a standard growth procedure, the samples were heated to growth temperature in an AsH3/H2 atmosphere (6 l/min H2 at 10 kPa, AsH3 partial pressure 5 104). Trimethylgallium (TMG) was then introduced at a partial pressure of 1.25 105 for 3 min. The substrates where then cooled to room temperature in the AsH3/H2 atmosphere. Growth conditions were varied staying within the growth window previously published [15]. A pre-growth anneal was performed on some samples, by heating the samples to temperatures 100–200 1C above the growth temperature in the AsH3/H2 atmosphere for 5–10 min prior to cooling to growth temperatures. The growth was characterized with a JSM 6400 F scanning electron microscope (SEM) and a JEOL 3000F transmission electron microscope (TEM). For SEM studies, samples where mounted perpendicular to the beam and then tilted in the microscope to determine growth angles and wire lengths. Crystallographic directions were determined by comparing the known substrate orientation and the f1 1¯ 0g cleave planes on the edge of the samples with the wire growth directions. Samples for TEM were prepared by placing a Cu TEM grids, with a lacey carbon film, on a substrate with nanowires and scraping the nanowires off onto the grid. 3. Results and discussions ¯ As previously published [13,15], growth on ð1¯ 1¯ 1ÞB GaAs substrates under a large variety of conditions results ¯ in wire growth perpendicular to the substrate in the ½1¯ 1¯ 1B direction (see Fig. 1(a)). Under similar conditions on (0 0 1) substrates, wires did not grow perpendicular to the surface, but grew at an angle of 54.71 to the perpendicular in four
505
different h1 1 1i directions (see Fig. 1(b)). Annealing prior to growth also influenced the preferential growth directions. When the (0 0 1) substrates with Au particles were annealed prior to growth, nearly all growth was either in ¯ directions. When the anneal step was the ½1 1¯ 1B or ½1 1 1B skipped, a small percentage of the wires (see Table 1) also ¯ directions. grew in [1 1 1]A or ½1 1¯ 1A Results similar to growth on (0 0 1) substrates were realized on the (1 1 1)A substrates. When the (1 1 1)A substrates where annealed prior to growth the wires grew at an angle of 70.51 to the perpendicular in the ½1¯ 1 1, ½1 1¯ 1 or ¯ ½1 1 1B directions. Without the anneal some wires also grew perpendicular to the substrate in the [1 1 1]A direction (see Fig. 1(c)). At lower growth temperatures a higher percentage of the wires grew in the A directions than in the B directions (see Table 1). The preparation of a substrate by poly lysine has been observed to increase the percentage of wires that grow in a direction perpendicular to the substrate [13,15]. We observed the same tendency on the (1 1 1)A substrates (see Table 1 and Fig. 1(d)). Properties that were not dependent on sample substrate orientation were the growth rate ratios for different directions, wire kinking, and the side facet orientation. The growth rate of A wires was, in all cases approximately twice that of B wires. Wires that start in one crystallographic orientation and then change direction to continue to grow in another direction are said to be ‘kinked’. Kinked wires could be found on all the samples, but at very low densities (o0.1% of the wires). There was no observed preference for kinking of A wires as opposed to B wires. There are three major side facets perpendicular to the growth direction exhibited on the A wires. The three side facets have a ¯ type crystallographic orientation such that the cross f1 1 2g section is an equilateral triangle which can be seen when viewing the wires from the growth direction in the ¯ SEM (see Fig. 2(a)). The B wires also exhibit three f1 1 2g ¯ ¯ type side facets, but in addition they exhibit three f1 1 2g type side facets. This gives the B wires a hexagonal cross section when viewed from the growth direction (see Fig. 2(b)). The TEM analysis revealed wires with and without stacking faults (see Figs. 3(a) and (b), respectively). We were unable to prepare samples with only A wires, so there was always a significant percentage of B wires on the A samples. On the other hand samples with predominantly B wires were prepared. On the mixed samples, the ratio of wires with to wires without stacking faults corresponded to the ratio of A to B wires. On samples with predominantly B wires, virtually all wires were observed to exhibit stacking faults. We thus could conclude that the A wires were the ones without stacking faults. Another confirmation that the A wires lack stacking faults is that the longest wires on the mixed samples lacked stacking faults. Due to the faster growth rate of the A wires, we were able to determine that these longer defect free wires were all A wires.
ARTICLE IN PRESS B.A. Wacaser et al. / Journal of Crystal Growth 287 (2006) 504–508
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Fig. 1. SEM images of GaAs nanowires grown on various GaAs substrates imaged with the e-beam perpendicular to the substrate surface. Wires growing perpendicular to the substrate appear as white dots. Projections of wires grown in other directions appear as white lines with the slight tapering of the wires ¯ ½1 2¯ 1 and ½2¯ 1 1 indicating the growth direction. (a) (1 1 1)A substrate, dots indicate growth in the [1 1 1]A direction and the lines extending in the ½1¯ 1 2, ¯ ¯ ¯ directions correspond to wires grown in the ½1 1 1B, ½1 1¯ 1B and ½1¯ 1 1B directions, respectively. (b) ð1¯ 1¯ 1ÞB substrate, all wires grew in the ½1¯ 1¯ 1B ¯ direction. (c) (0 0 1) substrate, projections parallel the [1 1 0] direction correspond to wires grown in the [1 1 1]A and ½1¯ 11A direction. The others grew in the ½1¯ 1 1B and ½1 1¯ 1B directions. (d) Poly lysine prepared (1 1 1)A substrate, wires grow in similar directions as in (a), but with a higher density of A wires.
Table 1 Growth statistics for various substrate pretreatments, orientations, and growth temperatures Pre-treatment
Substrate orientation
Growth temperature (1C)
Percent A (%)
Growth rate ratio A/B
HCl HCl HCl HCl HCl HCl+580 1C HCl+580 1C HCl+580 1C HCl+poly lysine
(0 0 1) (1 1 1)A (1 1 1)A (1 1 1)A ¯ ð1¯ 1¯ 1ÞB (0 0 1) (1 1 1)A ¯ ð1¯ 1¯ 1ÞB (1 1 1)A
460 450 460 470 460 460 460 460 470
12 60 50 42 0 0 0 0 74
2.370.5 2.270.2 1.570.4 2.270.6 NA NA NA NA 1.970.4
The pretreatments were preformed as described in the text with 580 1C signifying a pre-growth anneal at that temperature. The percent A and growth rate ratio A/B refer to the percentage or ratio of A wires compared to B wires on the same sample.
4. Conclusion The preferential growth conditions of GaAs nanowires ¯ direction are such that the wires usually grow in a h1¯ 1¯ 1iB
with many stacking faults. We were able to grow GaAs wires in h1 1 1iA directions. These A wires had a growth rate of almost twice that of the B wires with no observable ¯ type side stacking faults. The A wires only have three f1 1 2g
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Fig. 3. TEM images of GaAs wires with a dark gold particle at the end of each wire. 10 nm scale bar. (a) Stacking fault free wire grown in the [1 1 1]A ¯ wire with stacking faults as evidenced by the dark direction. (b) ½1¯ 1¯ 1B lines perpendicular to the growth direction.
Acknowledgements
Fig. 2. SEM images of GaAs wires images from the top. The directions labeled on the image are related to a f1¯ 1 0g cleavage plane at the edge of the sample and to B wires grown on this cleavage plane. (a) Wires grown ¯ type side facets and thus have a on a [1 1 1]A substrate exhibit three f1 1 2g ¯ substrate exhibit triangular cross section. (b) Wires grown on a ½1¯ 1¯ 1B ¯ type side facets and thus have a three f1¯ 1¯ 2g type and three f1 1 2g hexagonal cross section.
facets whereas the B wires have in addition to these three f1¯ 1¯ 2g type side facets. We have experimentally found a way to grow wires without stacking faults and have also observed that these wires have different side facets and growth rates. The lack of stacking faults and the different side facets are especially distinctive for the h1 1 1iA direction because of it is close crystallographic relationship to the h1 1 1iB direction. We hope that these results coupled with theoretical modeling and an in-depth study of the growth mechanisms of these wires will one day lead to a greater understanding and control of epitaxial growth in general and especially in the formation of stacking faults.
This work was carried out within the Nanometer Structure Consortium in Lund and was supported by Grants from the Swedish Research Council (VR), the Swedish Foundation for Strategic Research (SSF), the Office of Naval Research (ONR) and the Knut and Alice Wallenberg Foundation. We would like to give special thanks to M. Larsson, M. Lexholm and H. Nilsson for their valuable contributions in the initial growth and identification of A wires during a class research project. References [1] L. Samuelson, Mat. Today 6 (2003) 22. [2] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, Y.Q. Yan, Adv. Mater. 15 (2003) 353. [3] C.N.R. Rao, F.L. Deepak, G. Gundiah, A. Govindaraj, Prog. Solid State Chem. 31 (2003) 5. [4] L. Samuelson, C. Thelander, M.T. Bjo¨rk, M. Borgstro¨m, K. Deppert, K.A. Dick, A.E. Hansen, T. Ma˚rtensson, N. Panev, A.I. Persson, W. Seifert, N. Sko¨ld, M.W. Larsson, L.R. Wallenberg, Physica E 25 (2004) 313. [5] L. Samuelson, M.T. Bjo¨rk, K. Deppert, M. Larsson, B.J. Ohlsson, N. Panev, A.I. Persson, N. Sko¨ld, C. Thelander, L.R. Wallenberg, Physica E 21 (2004) 560. [6] R.S. Wagner, VLS mechanism of crystal growth, in: A.P. Levitt (Ed.), Whisker Technology, Wiley, New York, 1970, p. 47.
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[7] E.I. Givargizov, Growth of whiskers by the vapor-liquid-solid mechanism, in: E. Kaldis (Ed.), Current Topics in Materials Science, North-Holland, Amsterdam, 1978, p. 79. [8] A. Mikkelsen, N. Sko¨ld, L. Ouattara, M. Borgstro¨m, J.N. Andersen, L. Samuelson, W. Seifert, E. Lundgren, Nat. Mater. 3 (2004) 519. [9] E. Weiss, E. Kedar, N. Mainzer, J. Crystal Growth 132 (1993) 191. [10] M.T. Bjo¨rk, B.J. Ohlsson, T. Sass, A.I. Persson, C. Thelander, M.H. Magnusson, K. Deppert, L.R. Wallenberg, L. Samuelson, Appl. Phys. Lett. 80 (2002) 1058. [11] Z.H. Wu, X. Mei, D. Kim, M. Blumin, H.E. Ruda, J.Q. Liu, K.L. Kavanagh, Appl. Phys. Lett. 83 (2003) 3368.
[12] U. Krishnamachari, M. Borgstro¨m, B.J. Ohlsson, N. Panev, L. Samuelson, W. Seifert, M.W. Larsson, L.R. Wallenberg, Appl. Phys. Lett. 85 (2004) 2077. [13] W. Seifert, M. Borgstro¨m, K. Deppert, K.A. Dick, J. Johansson, M.W. Larsson, T. Ma˚rtensson, N. Sko¨ld, C.P.T. Svensson, B.A. Wacaser, L.R. Wallenberg, L. Samuelson, J. Crystal Growth 272 (2004) 211. [14] M.H. Magnusson, K. Deppert, J.-O. Malm, J.-O. Bovin, L. Samuelson, J. Nanopart. Res. 1 (1999) 243. [15] M. Borgstro¨m, K. Deppert, L. Samuelson, W. Seifert, J. Crystal Growth 260 (2004) 18.
Journal of Crystal Growth 287 (2006) 504–508 www.elsevier.com/locate/jcrysgro
Growth and characterization of defect free GaAs nanowires Brent A. Wacasera,, Knut Depperta, Lisa S. Karlssonb, Lars Samuelsona, Werner Seiferta a
Solid State Physics, Lund University, Box 118, Lund S-211 00, Sweden Materials Chemistry/nCHREM, Lund University, Box 124, Lund S-211 00, Sweden
b
Available online 5 January 2006
Abstract ¯ direction (B wires) and Most III–V compound semiconductor nanowires seeded by metal particles grow preferentially in a h1¯ 1¯ 1iB most commonly with many stacking faults perpendicular to the growth direction. If growth proceeds in an alternate direction, defect-free growth has been observed. We present experimental results for the growth of GaAs nanowires in a previously uninvestigated growth ¯ direction, a h1 1 1iA direction (A wires). One novelty is that a {1 1 1}A growth plane, like a f1¯ 1¯ 1gB, is a close packed plane where the stacking sequence can be interrupted forming stacking faults, but unlike the B wires the A wires lack stacking faults. It is also observed that, when grown under equivalent conditions, the growth rate of the A wires is approximately twice that of the B wires. Additionally, B ¯ and three f1¯ 1¯ 2g side facets. A wires, on the other hand, have only three major side wires have a hexagonal cross section with three f1 1 2g ¯ type, giving them a triangular cross section. facets which are of the f1 1 2g r 2005 Elsevier B.V. All rights reserved. PACS: 61.50.A; 61.72; 81.05.E; 81.07 Keywords: A1. Defects; A1. Interfaces; A1. Nanostructures; A3. Organometallic vapor phase epitaxy; B2. Semiconducting III–V materials
1. Introduction Semiconducting nanowires are being investigated as contacts for integrated circuits and as nanowires containing devices incorporated within their crystalline structure [1–3]. Studying the electrical and optical properties of nanowires also allows us to experiment physically with the theories of one-dimensional quantum confinement. These and numerous other application, along with the high level of control achieved in the accuracy of the diameter, length, composition, and even the positioning of nanowires, have prompted an increasing interest in nanowire research [4,5]. One of the final difficulties to overcome in growth of ‘perfect’ nanowires is that most nanowires grown to date have been shown to have abundant stacking faults [6–8]. Defects of this nature have been shown to negatively affect the electrical and optical properties of semiconductors [9]. A better understanding of the formation of these defects is needed if we hope to eliminate them from the wires. Corresponding author. Tel.: +46 46 222 95 86; fax: +46 46 222 36 37.
E-mail address: [email protected] (B.A. Wacaser). 0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.11.075
Growth of semiconductor wires, or whiskers, seeded from metal particles was first presented in the 1960s and 1970s [6,7]. The wires were generally observed to grow preferentially in h1 1 1i directions, although deviations from these directions were also reported [6,7]. In III–V semiconductors, the preferential growth direction is com¯ monly a h1¯ 1¯ 1iB direction (B wires). When grown in a ¯ h1¯ 1¯ 1iB direction the wires exhibit a high density of stacking faults perpendicular to the growth direction. Recently, though a few publications report growth of defect free III–V semiconducting wires in alternative directions, showing that the defect density is somehow related to the growth direction [10–13]. We supplement these reports by presenting data on the growth of GaAs wires in h1 1 1iA directions (A wires) which lack stacking faults, have a higher growth rate than the B wires, and have a triangular cross section. Crystallographically the h1 1 1iA growth direction is ¯ closely related to the preferential h1¯ 1¯ 1iB wire growth direction. The growth plane in both directions is a close packed {1 1 1} plane. These face centered cubic planes and ¯ the corresponding (0 0 0 1)A and ð0 0 0 1ÞB close packed
ARTICLE IN PRESS B.A. Wacaser et al. / Journal of Crystal Growth 287 (2006) 504–508
hexagonal planes are the stacking planes where stacking faults can form. The only major difference is that the A surface is terminated by the group III element Ga and the B surface is terminated by the group V element As. The close crystallographic relationship between these two directions makes the differences especially the stacking fault related differences observed in A and B wires even more interesting. 2. Experiment ¯ Epi-ready ð1¯ 1¯ 1ÞB, (0 0 1), and (1 1 1)A GaAs substrates were first prepared by placing the sample in a bath of 37% HCl mixed 1:1 with de-ionized H2O for 1 min, followed by blowing off with N2. Gold aerosol particles with a typical diameter of 40 nm were then deposited on the various substrates. The deposition density, set at 1 or 2 particles per square micron, was achieved by deposition from an aerosol evaporation generator as described previously [14]. After particle deposition, some of the samples were coated with poly lysine by placing a small drop on the substrate, waiting 1 min and blowing off the residue with nitrogen. The substrates were then transferred into a metal-organic vapor phase epitaxy (MOVPE) system. In a standard growth procedure, the samples were heated to growth temperature in an AsH3/H2 atmosphere (6 l/min H2 at 10 kPa, AsH3 partial pressure 5 104). Trimethylgallium (TMG) was then introduced at a partial pressure of 1.25 105 for 3 min. The substrates where then cooled to room temperature in the AsH3/H2 atmosphere. Growth conditions were varied staying within the growth window previously published [15]. A pre-growth anneal was performed on some samples, by heating the samples to temperatures 100–200 1C above the growth temperature in the AsH3/H2 atmosphere for 5–10 min prior to cooling to growth temperatures. The growth was characterized with a JSM 6400 F scanning electron microscope (SEM) and a JEOL 3000F transmission electron microscope (TEM). For SEM studies, samples where mounted perpendicular to the beam and then tilted in the microscope to determine growth angles and wire lengths. Crystallographic directions were determined by comparing the known substrate orientation and the f1 1¯ 0g cleave planes on the edge of the samples with the wire growth directions. Samples for TEM were prepared by placing a Cu TEM grids, with a lacey carbon film, on a substrate with nanowires and scraping the nanowires off onto the grid. 3. Results and discussions ¯ As previously published [13,15], growth on ð1¯ 1¯ 1ÞB GaAs substrates under a large variety of conditions results ¯ in wire growth perpendicular to the substrate in the ½1¯ 1¯ 1B direction (see Fig. 1(a)). Under similar conditions on (0 0 1) substrates, wires did not grow perpendicular to the surface, but grew at an angle of 54.71 to the perpendicular in four
505
different h1 1 1i directions (see Fig. 1(b)). Annealing prior to growth also influenced the preferential growth directions. When the (0 0 1) substrates with Au particles were annealed prior to growth, nearly all growth was either in ¯ directions. When the anneal step was the ½1 1¯ 1B or ½1 1 1B skipped, a small percentage of the wires (see Table 1) also ¯ directions. grew in [1 1 1]A or ½1 1¯ 1A Results similar to growth on (0 0 1) substrates were realized on the (1 1 1)A substrates. When the (1 1 1)A substrates where annealed prior to growth the wires grew at an angle of 70.51 to the perpendicular in the ½1¯ 1 1, ½1 1¯ 1 or ¯ ½1 1 1B directions. Without the anneal some wires also grew perpendicular to the substrate in the [1 1 1]A direction (see Fig. 1(c)). At lower growth temperatures a higher percentage of the wires grew in the A directions than in the B directions (see Table 1). The preparation of a substrate by poly lysine has been observed to increase the percentage of wires that grow in a direction perpendicular to the substrate [13,15]. We observed the same tendency on the (1 1 1)A substrates (see Table 1 and Fig. 1(d)). Properties that were not dependent on sample substrate orientation were the growth rate ratios for different directions, wire kinking, and the side facet orientation. The growth rate of A wires was, in all cases approximately twice that of B wires. Wires that start in one crystallographic orientation and then change direction to continue to grow in another direction are said to be ‘kinked’. Kinked wires could be found on all the samples, but at very low densities (o0.1% of the wires). There was no observed preference for kinking of A wires as opposed to B wires. There are three major side facets perpendicular to the growth direction exhibited on the A wires. The three side facets have a ¯ type crystallographic orientation such that the cross f1 1 2g section is an equilateral triangle which can be seen when viewing the wires from the growth direction in the ¯ SEM (see Fig. 2(a)). The B wires also exhibit three f1 1 2g ¯ ¯ type side facets, but in addition they exhibit three f1 1 2g type side facets. This gives the B wires a hexagonal cross section when viewed from the growth direction (see Fig. 2(b)). The TEM analysis revealed wires with and without stacking faults (see Figs. 3(a) and (b), respectively). We were unable to prepare samples with only A wires, so there was always a significant percentage of B wires on the A samples. On the other hand samples with predominantly B wires were prepared. On the mixed samples, the ratio of wires with to wires without stacking faults corresponded to the ratio of A to B wires. On samples with predominantly B wires, virtually all wires were observed to exhibit stacking faults. We thus could conclude that the A wires were the ones without stacking faults. Another confirmation that the A wires lack stacking faults is that the longest wires on the mixed samples lacked stacking faults. Due to the faster growth rate of the A wires, we were able to determine that these longer defect free wires were all A wires.
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Fig. 1. SEM images of GaAs nanowires grown on various GaAs substrates imaged with the e-beam perpendicular to the substrate surface. Wires growing perpendicular to the substrate appear as white dots. Projections of wires grown in other directions appear as white lines with the slight tapering of the wires ¯ ½1 2¯ 1 and ½2¯ 1 1 indicating the growth direction. (a) (1 1 1)A substrate, dots indicate growth in the [1 1 1]A direction and the lines extending in the ½1¯ 1 2, ¯ ¯ ¯ directions correspond to wires grown in the ½1 1 1B, ½1 1¯ 1B and ½1¯ 1 1B directions, respectively. (b) ð1¯ 1¯ 1ÞB substrate, all wires grew in the ½1¯ 1¯ 1B ¯ direction. (c) (0 0 1) substrate, projections parallel the [1 1 0] direction correspond to wires grown in the [1 1 1]A and ½1¯ 11A direction. The others grew in the ½1¯ 1 1B and ½1 1¯ 1B directions. (d) Poly lysine prepared (1 1 1)A substrate, wires grow in similar directions as in (a), but with a higher density of A wires.
Table 1 Growth statistics for various substrate pretreatments, orientations, and growth temperatures Pre-treatment
Substrate orientation
Growth temperature (1C)
Percent A (%)
Growth rate ratio A/B
HCl HCl HCl HCl HCl HCl+580 1C HCl+580 1C HCl+580 1C HCl+poly lysine
(0 0 1) (1 1 1)A (1 1 1)A (1 1 1)A ¯ ð1¯ 1¯ 1ÞB (0 0 1) (1 1 1)A ¯ ð1¯ 1¯ 1ÞB (1 1 1)A
460 450 460 470 460 460 460 460 470
12 60 50 42 0 0 0 0 74
2.370.5 2.270.2 1.570.4 2.270.6 NA NA NA NA 1.970.4
The pretreatments were preformed as described in the text with 580 1C signifying a pre-growth anneal at that temperature. The percent A and growth rate ratio A/B refer to the percentage or ratio of A wires compared to B wires on the same sample.
4. Conclusion The preferential growth conditions of GaAs nanowires ¯ direction are such that the wires usually grow in a h1¯ 1¯ 1iB
with many stacking faults. We were able to grow GaAs wires in h1 1 1iA directions. These A wires had a growth rate of almost twice that of the B wires with no observable ¯ type side stacking faults. The A wires only have three f1 1 2g
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Fig. 3. TEM images of GaAs wires with a dark gold particle at the end of each wire. 10 nm scale bar. (a) Stacking fault free wire grown in the [1 1 1]A ¯ wire with stacking faults as evidenced by the dark direction. (b) ½1¯ 1¯ 1B lines perpendicular to the growth direction.
Acknowledgements
Fig. 2. SEM images of GaAs wires images from the top. The directions labeled on the image are related to a f1¯ 1 0g cleavage plane at the edge of the sample and to B wires grown on this cleavage plane. (a) Wires grown ¯ type side facets and thus have a on a [1 1 1]A substrate exhibit three f1 1 2g ¯ substrate exhibit triangular cross section. (b) Wires grown on a ½1¯ 1¯ 1B ¯ type side facets and thus have a three f1¯ 1¯ 2g type and three f1 1 2g hexagonal cross section.
facets whereas the B wires have in addition to these three f1¯ 1¯ 2g type side facets. We have experimentally found a way to grow wires without stacking faults and have also observed that these wires have different side facets and growth rates. The lack of stacking faults and the different side facets are especially distinctive for the h1 1 1iA direction because of it is close crystallographic relationship to the h1 1 1iB direction. We hope that these results coupled with theoretical modeling and an in-depth study of the growth mechanisms of these wires will one day lead to a greater understanding and control of epitaxial growth in general and especially in the formation of stacking faults.
This work was carried out within the Nanometer Structure Consortium in Lund and was supported by Grants from the Swedish Research Council (VR), the Swedish Foundation for Strategic Research (SSF), the Office of Naval Research (ONR) and the Knut and Alice Wallenberg Foundation. We would like to give special thanks to M. Larsson, M. Lexholm and H. Nilsson for their valuable contributions in the initial growth and identification of A wires during a class research project. References [1] L. Samuelson, Mat. Today 6 (2003) 22. [2] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, Y.Q. Yan, Adv. Mater. 15 (2003) 353. [3] C.N.R. Rao, F.L. Deepak, G. Gundiah, A. Govindaraj, Prog. Solid State Chem. 31 (2003) 5. [4] L. Samuelson, C. Thelander, M.T. Bjo¨rk, M. Borgstro¨m, K. Deppert, K.A. Dick, A.E. Hansen, T. Ma˚rtensson, N. Panev, A.I. Persson, W. Seifert, N. Sko¨ld, M.W. Larsson, L.R. Wallenberg, Physica E 25 (2004) 313. [5] L. Samuelson, M.T. Bjo¨rk, K. Deppert, M. Larsson, B.J. Ohlsson, N. Panev, A.I. Persson, N. Sko¨ld, C. Thelander, L.R. Wallenberg, Physica E 21 (2004) 560. [6] R.S. Wagner, VLS mechanism of crystal growth, in: A.P. Levitt (Ed.), Whisker Technology, Wiley, New York, 1970, p. 47.
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