Article pubs.acs.org/JPCC
Thermodynamic Stability of Gold-Assisted InAs Nanowire Growth Marcus Tornberg,† Kimberly A. Dick,†,‡ and Sebastian Lehmann*,† †
Solid State Physics, Lund University, Box 118, 22100 Lund, Sweden Centre for Analysis and Synthesis, Lund University, Box 124, 22100 Lund, Sweden
‡
S Supporting Information *
ABSTRACT: Growth of III−V semiconductor nanowires is generally assisted by a liquid particle in order to get a highly anisotropic crystallization. The thermodynamic stability of the particle is therefore of importance for control and understanding of the nanowire growth process. In this report we explore the particle stability by manipulating its properties, specifically its surface tension and volume, by accumulating indium in the particle during nanowire growth. We demonstrate a droplet displacement, from the top to one of the nanowire side facets, when exceeding the stability limit for a gold particle wetting an [0001]̅ -oriented InAs nanowire. This particle displacement is attributed to a lowered surface tension and a truncation of the top facet. In addition, our results indicate reversibility of the displacement, showing that the (1̅1̅1̅/0001̅) facet is the most favorable for a droplet to wet during common growth conditions. The stability condition for InAs growth is determined experimentally, and the understanding developed can easily be applied to other III−V nanowires.
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INTRODUCTION Particle-assisted, specifically gold-assisted, growth of III−V semiconductor nanowires has been shown to predominantly occur along the ⟨1̅1̅1̅/0001̅⟩-type directions for cubic/ hexagonal crystal structures.1−5 It is generally accepted that growth occurs in this orientation as it represents the most energetically favorable case for common growth conditions.6−8 This preferred nucleation event is primarily determined by the dynamics of the liquid particle during the vapor−liquid−solid (VLS) growth of semiconductor nanowires.9−12 Fundamental understanding of the particle−nanowire interaction is therefore important in order to increase control of nanowire growth for events such as altering of the growth direction or nucleation of growth assisted by particles consisting of materials other than gold. Nebol’sin and Shchetinin proposed in 2003 an expression for the stability regime for a particle residing at the top facet of a nanowire, using interfacial energies and the wetting angle of a particle (an extension of Young’s equation).13 This stability regime is predicting a mechanical limit for the thermodynamics of the particle−nanowire system. The limit can be considered the minimum criterion for growth as it addresses the criterion for the particle being located at the top facet of a nanowire rather than the nanowire growth. Nanowires whose particle are no longer favored to be on the top nanowire facet will be referred to as destabilized. In this work we will refer to a basic thermo-geometrical model of the interfacial energies of interest as shown schematically in Figure 1. The figure displays the relation between the surface tension (γlv) and the interfacial energies (γls,101̅0, γsv,101̅0), connected by the contact angle (α + 90°). Indices are used to identify the phases of interest involved at a given interface: vapor (v), liquid (l), and solid (s). It has previously been suggested that in order for the particle to © 2017 American Chemical Society
Figure 1. Schematic of the interfacial energies (γ) and their relative orientation for nanowires grown in the [0001̅] direction. The illustration displays the interfacial energies of the nanowire and the surface tension of the liquid particle with indices corresponding to the phases involved at the interfaces; vapor (v), liquid (l), and solid (s).
remain in its steady state on a facet, forces connected to the interfacial energies should be in balance, both horizontally and vertically.13,14 Based on this model, the stability limit for having a particle on top facet may be reached or exceeded by either an increase in particle volume (contact angle) or a reduction of the surface tension. In order for the argument of thermodynamic stability to be experimentally investigated and verified, knowledge of the interfacial energies of the system is crucial and necessary for any conclusive work. However, since the interfacial energies are dependent on the material system, atomic ordering, and ambient conditions, absolute values for interfacial energies must be theoretically predicted for the surface reconstructions of interest.6 Theoretical calculations have been reported for the Received: June 22, 2017 Revised: August 31, 2017 Published: September 6, 2017 21678
DOI: 10.1021/acs.jpcc.7b06138 J. Phys. Chem. C 2017, 121, 21678−21684
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The Journal of Physical Chemistry C vapor−solid surface energies (γsv) of the low-index facets, e.g., {111}, {110̅ }, or {112̅}.6,15,16 With increasing availability of high-purity hexagonal structured (wurtzite) nanowires, comparative calculations also have been carried out for the corresponding hexagonal facets.17−20 The theoretical predictions of the surface energy vary depending on the calculation method used, ranging from 25 to 57 meV/Å2 for wurtzite InAs γvs,101̅0. Nevertheless, most calculations agree on the finding that the surface energies of {1120̅ }/{1010̅ }-type facets are lower than the corresponding zincblende facets ({11̅0}/{112̅}).18−20 This theoretical prediction was strongly supported by experiments investigating vapor−solid growth of nanowire sidewalls for nanowires comprised of both wurtzite and zincblende in 2013.21 As of now theoretical calculations are widely accepted and used for validating observed changes in nanowire growth, even if experimental reports are limited.4,21 The primary goal of this study is to obtain fundamental understanding of the stability of nanowire growth as well as to investigate the validity of basic thermodynamic models of the interfacial energies. We here demonstrate a stability limit for a liquid Au−In particle to wet the top facet of a [0001]̅ -oriented InAs nanowire for varying group V-limited growth conditions. Having experimental grounds for the stability conditions of nanowire growth can indicate whether or not the calculated surface energies are reasonable for the steady-state system. In addition, it may be possible to evaluate if the interfacial energies are the main factor determining successful growth and its preferred direction. As an extension, the interfacial energies could be an important engineering parameter for controlling the growth direction of particle-assisted nanowires. Finally, using the same model it could also be possible to predict the experimental limit for other indium-based nanowires as, e.g., Au−InP, or for alternatives to gold as assisting particle as in Ag−InAs.
Figure 2. General growth scheme along with an exemplary nanowire describing the set values of the molar fraction of arsine (solid line) and TMIn (dashed line) for the different growth steps. The process flow including substrate annealing (gray), nanowire nucleation (purple), and stable wurtzite InAs stem-growth (blue) conditions prior to the indium accumulation in the particle (red) is given as a color underlay to the growth scheme.
energetic stability limit of the particle on the top (0001̅) facet, the nominal V/III ratio was then decreased to accumulate indium in the particle. Accumulation was realized by lowering the molar fraction of AsH3 (χAsH3 = 1.3e−5 to 0) during a 5 min step while keeping the supply of TMIn constant at χTMIn = 1.8e−6. Following this strategy, we aimed to create a buildup of indium in the particle thus changing the particle size and composition as well as its interfacial energies. Figure 2 schematically displays the material supply sequence for the nanowire growth experiments including annealing, nucleation, stem growth, and at last the accumulation of indium within the particle increasing its size. In order to investigate the reversibility of our process, validating the thermodynamic approach, a second set of experiments was performed where nanowire growth was resumed following the accumulation step. This was performed by addition of 5 min growth after accumulation of indium in the particle at conditions and flows previously used for stem growth. The nanowire morphology and crystal structure were investigated by scanning electron microscopy (SEM) in a Hitachi SU8010 microscope and transmission electron microscopy (TEM) with both a JEOL 3000F setup and a Hitachi HF3300S microscope. SEM images are acquired at a 30° angle normal to the substrate surface unless stated otherwise. All instruments are equipped with field emission guns (FEG), with the Hitachi and JEOL setups using cold and Schottky FEGs, respectively, operated at 15 kV for SEM and 300 kV for TEM. For the TEM imaging, nanowires were broken off from the growth substrate and transferred mechanically onto lacey carbon-coated copper grids. Size analysis of the droplets/particles was carried out by measuring the actual ex situ particle diameter and the nanowire width.24 The particles were assumed to be a spherical cap, and the contact angle can then be calculated through geometric relations, a procedure presented as Supporting Information (SI I).
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EXPERIMENTAL DETAILS In this study, InAs nanowires were grown assisted by gold particles in an AIXTRON 3 × 2” close-coupled showerhead (CCS) metal−organic vapor phase epitaxy (MOVPE) system. The growths were performed at a global reactor pressure of 100 mbar and a total carrier gas flow of 8 standard liters per minute. Gold aerosol particles of nominal diameters between 15 and 90 nm (single diameters for different substrates), with a nominal density of 1 μm−2,22were deposited onto epi-ready ⟨1̅1̅1̅⟩-type oriented InAs substrates provided by WaferTechⓇ. Prior to setting the growth temperature to 460 °C, all particle covered substrates were annealed for 7 min at a set temperature of 550 °C in an arsine (AsH3)/hydrogen (H2) environment with an AsH3 molar fraction (χAsH3) of 2.3e−3, to remove the epi-ready oxides. Upon reaching the growth temperature, the nanowire growth was initiated with a nucleation step by introducing trimethyl-indium (TMIn) and changing χAsH3 to 1.2e−4. The 4 min nucleation step at a nominal V/III-ratio of 65 was followed by 15 min of “stem” growth at lower χAsH3 resulting in a nominal V/III ratio of 12 (conditions previously found to yield wurtzite crystal structure21,23). As a result of the initial growth, the nanowires showed a diameter-dependent length behavior ranging from 1850 to 750 nm. By using such a stem, we ensure that we start with a particle located on the vertical growth front (top facet of the nanowire) and that it is the most energetically favorable particle position at the stem growth conditions. In order to determine the 21679
DOI: 10.1021/acs.jpcc.7b06138 J. Phys. Chem. C 2017, 121, 21678−21684
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Figure 3. Lowering the molar fraction of AsH3 (to values displayed in the figures) after growth of the nanowire stem makes the particle expand as shown in the scanning electron micrographs (a−d). In addition to expanding, the particle may be displaced from the (0001̅)- to the {101̅0}-type facets as seen in the top view micrograph (e), as a result of limiting the AsH3 supply. Bright-field TEM image of a nanowire with displaced particle shows the nanowire decomposition at the particle−nanowire interface of micrograph (f). The truncation (extended red line) of the top facet shown in (g) was present for all destabilized nanowires. The facet is labeled by the accompanying diffraction pattern as a {101̅1̅}-type facet. The nanowires of the figures are approximately 50 nm in diameter, and the scale bars of (a−f) are 100 nm.
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RESULTS AND DISCUSSION Starting from gold-assisted InAs nanowires, accumulation of indium in the particle resulted in changes of the particle− nanowire morphology depending on the amount of AsH3 supplied. Enlargement of the particle occurred under the limited AsH3 supply as demonstrated in Figure 3a, b, an observation attributed to the intended indium accumulation. As accumulation of indium in the particle proceeds, the particle is eventually observed to displace from the top to one of the side facets as seen in Figure 3c, d. The particle displacement from the top (0001̅) facet to one of the {101̅0}-type facets is an effect of exceeding the stability limit for a particle wetting the top facet. No preferred side facet can be observed when looking at the full ensemble of nanowires with displaced particles, an expected result due to the sixfold, cross-sectional symmetry of the [0001̅]-oriented InAs nanowire. In addition, the particles wetting one of the side facets are much larger than the nanowire itself and most often wet more than one of the side facets once it has gotten displaced as can be seen in Figure 3e with inset. Wetting of several nanowire side facets has previously been observed and reported for intersected nanowires by Dalacu et al.25 Once the particle is located at the sidewall facet(s), it positions itself somewhere along the nanowire. This observation was also reported for self-seeded InAs nanowires by Madsen,26 who, by supplying indium in absence of arsenic during cool down, observed particles positioned along the nanowire side facets. More recently, similar results were demonstrated through annealing of selfseeded InAs in the absence of material supply by Potts et al.27 These results indicate that this displacement by accumulation may not be exclusive to the Au−InAs nanowire system but perhaps also In−InAs. Decomposition and Its Consequences. Apart from the indium accumulation leading to particle displacement, the
limited AsH3 supply does also lead to decomposition at the particle−nanowire interface as seen in Figure 3f. If substantial decomposition occurs at the interface between the particle and the {101̅0}-type facet, the top part of the nanowire will “fall off” and position itself around the particle as observed for the kinked nanowire shown in Figure 3d. Although it is an extreme case, it is still considered as a destabilization of a particle wetting the (0001̅) facet. All nanowires whose particle has been displaced from the top facet display an oblique facet which truncates the top, (0001)̅ , facet. The {101̅1̅}-type truncation facet can be observed in both SEM and TEM images of Figure 3 c, f, g. This type of growth front truncation is an observation in agreement with the oblique facet reported by Kelrich et al., who showed particle migration from top to the side facet of InP nanowires.12 In addition, they suggested that the oblique facet was part of the unpinning of the particle wetting the (0001̅) facet. Similar oblique facets have previously been demonstrated for in situ growth of different material systems such as Si, GaP, and GaAs, proving the possibility of formation of a truncation during growth.28−30 In the case of III−V nanowires, the truncation was reported for growth of zincblende under arsenic-limited conditions. In this work, we do not have growth under the conditions for indium accumulation but instead decomposition as the nanowire length decreases with decreasing AsH3 supply as seen in Figure 3a−d. Additional decomposition statistics are provided as Supporting Information (SI II). As the nanowires in our study do not grow, we do not expect truncation to be formed similarly to the in situ growth studies. It could be that the indium supply from both nanowire decomposition and vapor creates a transient particle condition favoring a truncation to compensate for the arsenic deficit in the particle. We further speculate that the supposed addition of arsenic in the particle leaves to the vapor as the ambient is highly arsine limited and thus the arsenic content/chemical potential of the vapor is 21680
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Figure 4. Graph (a) showing stable (squares) and destabilized (triangles) nanowires depending on the particle to nanowire diameter ratio (ϕP/ ϕNW). Due to variations in original particle density and diameter, one substrate can show both stable and unstable particles at the top facet (circle). The table in (b) shows nanowire diameters and diameter ratios together with the color-coded morphology for selected nanowires for an average nanowire diameter of 80 nm exposed to χAsH3 = 1.26e−6.
aerosol deposition. We observe that thinner nanowires within the same substrate have reached the critical particle size while the thicker have not, as presented in the table of Figure 4b. The diameter dependence is related to the rate of volume change during indium accumulation. A constant, absolute supply of indium will introduce variations in the change of particle volume since the indium addition relative to the volume depends on the original particle size. In other words, smaller particles receive a larger relative change in volume from the addition of an absolute amount of indium, in contrast to a larger particle under the same conditions. As a result, the smaller particles reach their stability limit faster than larger particles for the same condition of indium accumulation. Statistics of the morphology with respect to AsH3 supply and nanowire thickness can be found as Supporting Information (SI III). Stability Limit of Vertical Nanowire Growth. Having determined that there is an experimental limit to the particle wetting the (0001̅) facet, we next investigate the theoretical limit of the Au−InAs nanowires using the Nebol’sin− Shchetinin criterion.13 Taking into account that our InAs nanowires are nontapered, the stability criterion is presented below for a nanowire with vertical side facets:
suspected to be low. This is in contrast to the truncation formed during in situ growth of gold-assisted GaAs, which was suggested to be related to the arsenic needed for nucleation of one zincblende bilayer.30 Stability Conditions. We observe particle displacement as a consequence of the particle size increasing beyond the stability limit for a particle wetting the (0001̅) facet. The stability limit has been determined by correlating particle size to nanowire morphology for low AsH3 supply. This provides an upper limit of particle size for a particle to be stable while wetting the top, (0001̅), facet. To evaluate the particle size, the particle diameter (ϕP) was measured and correlated with the nanowire diameter (ϕNW). A minimum of 50 nanowires from each sample considering different average particle diameter and arsine molar fraction was measured post growth in order to get the average diameter ratio. Here we present the average diameter ratio in Figure 4 for varying χAsH3 together with the color-coded particle−nanowire morphology. The graph shows data points for substrates only containing nanowires whose particles are stable at the top, (0001̅), facet as squares. Data points for conditions where close to all nanowires had their particle at the sidewall (“destabilized”) are displayed as triangles. Circles show conditions where both stable and destabilized nanowires were present on the same substrate. Starting from particles larger than 1.5 times the nanowire diameter, a gradual transition from stable to destabilized particles on the (0001̅) facet is observed. This ex situ measurement corresponds to a contact angle of 138° for a spherical cap. In addition, we did not find any case for a diameter ratio larger than 1.7 (144°) where the particle is located at the top facet. Furthermore, the diameter ratio for the stability limit is not significantly dependent on the nanowire diameter, meaning that if a curvature dependence is present in our experiment it is small or canceled out. This sets a critical size of the liquid particle, where a smaller particle, with respect to the nanowire diameter, is thermodynamically favored to wet the top, (0001̅), facet. A larger particle than the critical size will instead favor wetting of one of the {1010̅ }-type side facets, according to our results. Coexisting nanowire morphologies with particles on the top (0001̅)- and side {101̅0}-type facets on a sample can be explained by a spread of the original particle diameters from the
γsv,10 1̅ 0 γlv
≤ sin(α) + cos(α) (1)
By computing the relation of interfacial energies of our system, we can evaluate the stability conditions for having a particle located at the (0001̅) facet according to the Nebol’sin− Shchetinin criterion. Here, we consider the surface tension and contact angle of the liquid particle as a function of indium content. Given that the surface tension of pure indium31 is lower than the reported values for pure gold,32 the surface tension of the liquid Au−In alloy is expected to decrease with indium enrichment. More specifically, the surface tension is approximated as a linear combination of the particle constituents, gold and indium. This approximation is based on interpretation of the results of Novakovic et al., who investigated the surface tension of Au−In liquid particles as a function of composition.32 With the assumption of having nanowires of constant radius, we show the ratio γsv,1010̅ /γlv for various contact angles in the graph of Figure 5 (black).13 21681
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additional growth experiments were conducted. The initial growth conditions were identical to conditions where the particle becomes displaced, occupying one of the nanowire sidewalls as seen for example in Figure 3c. After that step, we returned to growth conditions previously providing stable [0001̅]-oriented wurtzite (stem) growth for 5 min. As a first observation of the 5 min of growth, the particles have recovered their original size due to the increased AsH3 supply. In addition to the decreased particle size, no particle could be found midway on the side facets after being exposed to the wurtzite growth conditions. Instead, as shown in Figure 6, the particles
Figure 5. Illustration of the Nebol’sin−Shchetinin criterion of eq 1 including estimations of interfacial energy ratio (dashed lines) and the trigonometrical expression (gray) related to the Au−InAs nanowire geometry. The dashed lines represent two of the reported theoretical values for the solid−vapor interface. The theory predicts stable nanowires for contact angles less than 135°, while larger particles may or may not be stable depending on the theoretical value used. A schematic illustration of the contact angle (α + 90°) is displayed in the figure inset.
Figure 6. Falsely colored SEM image of 70 nm diameter nanowires which have undergone a particle displacement step followed by an additional process step of stable wurtzite growth, identical to the stem growth conditions. Returning to stable growth results in movement of the particle toward the substrate or toward the nanowire top.
In addition, we further assume that variations in γsv,101̅0 with the chemical potential of arsenic (μAs,v) in the vapor are small in comparison to the change in surface tension of the droplet (γlv), allowing us to neglect the influence of the changes in the gas phase. Small variations in surface energy with arsenic pressure are assumed to hold for zincblende InAs, which maintains the same surface structure for a wide range of arsenic pressures/ chemical potentials.6,33 However, due to the metastable nature of wurtzite InAs along with the relatively new availability of the high-purity wurtzite InAs phase in the form of nanowires,4,34,35 almost no data is available, and so we assume a similar insensitivity to arsenic pressure. Finally, we assume that the curvature dependencies of interfacial energies are minimal as the diameters of our structures are larger than 20 nm and thus above the critical size for significant dependence.36,37 Calculations and justifications of the interfacial energies of interest are presented as Supporting Information (SI IV). As a result of the large interval of reported theoretical values for the solid−vapor interface energy (25−57 meV/Å2), we present two stability regions in the Nebol’sin−Shchetinin diagram as dashed lines in Figure 5.6,18,19 There exists a region, below 135° (blue), where all reported values predict a nanowire with the particle located at the (0001)̅ facet. For particles with a contact angle exceeding 135°, the different theoretical values provide different results for the stability. We can, however, say that our experimental limit is at the lower end (138°−144°) of the theoretical uncertainty interval given (>135°). The interval where any theoretical value predicts stability (green) for having a particle on the (0001̅ ) facet exceeds beyond the experimentally obtained contact angle of this study. As a result we conclude that the particle is experimentally stable on top of the nanowire until the contact angle of 138°−144°(or corresponding γvs,101̅0) is reached for a droplet wetting a nontruncated (0001̅) facet. However, if a truncation occurs as a result of a dynamic droplet, it may temporarily increase the displacement probability of the particle. Regrowth. In order to further validate whether the thermodynamic description applies for our experiments,
are located at either the top or the bottom of the nanowire. Although the conditions are identical compared to growth prior to particle displacement, the particles that migrate upward do not resume growth in the original [0001̅]-direction. Instead, nonvertical growth is observed at the topmost part of the nanowire sidewall as if the particle has tried to reach the top facet. Considering that the growth conditions were the same as the initial vertical wurtzite growth, the most obvious difference is the presence of the truncation formed during particle displacement. We speculate that the truncation makes the particle wet both the top facet and the truncated facet which could explain why we observe the nonvertical growth at the top part of the nanowire. This observation may be an indication that the truncation could have a key role in the particle displacement. While the particle does not stabilize at the top (0001̅), the particles that migrate downward end up at the (11̅ 1̅ )̅ surface of the substrate. The observation of particle migration toward the (0001̅)/(1̅1̅1̅) facet validates the thermodynamic description of the system to some extent as the particle tries to regain an identical position as before displacement from (0001)̅ . In other words, this result indicates an energy minimum of the droplet−nanowire system when the particle wets the (0001̅)/(1̅1̅1̅) facet for common growth conditions. Displacement Mechanism. The thermodynamic driving force for the particle displacement is attributed to the reduction of the surface tension of the droplet. Reduction of the surface tension, causes the vertical balancing component of γlv to decrease far below γvs,101̅0, resulting in a net force downward. Consequently, the particle gets “pulled down” toward one of the side facets in order to find its new local energy minimum, i.e., wetting one of the {101̅0}-type facets. However, we suggest that this is only one part of the displacement mechanism. For the particle to migrate it has to either wet the sidewall directly or, more likely, transition through an intermediate step such as a truncated {101̅1̅}-type facet. Given the observation of 21682
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The Journal of Physical Chemistry C a truncation for investigated nanowires and its “boundary effect” when resuming growth, we propose the truncation to be a key to displacement of the particle. We assume that the truncated facet, formed during conditions for indium accumulation, is wetted by the droplet simultaneously with the (0001̅) facet, similar to the droplet oscillations shown by in situ growth experiments.28−30 For particles with experimental contact angles lower than the displacement onset of 138°, we consider the particle to remain on the (0001)̅ facet, assuming any truncation being temporary or small enough not to accommodate the entire particle. As the contact angle increases so do the resulting forces of the interfacial energies, eventually displacing the particle from the two facets to one of the side facets before stabilizing at its new local energy minimum along the nanowire sidewall. The displacement of the particle is suggested to originate from the reduction of the particle surface tension in combination with changed interface area between liquid and solid. We can unfortunately not separate the effect of surface tension from contact angle of the particle as we rely on ex situ measurements and do not have a direct measure of either γlv or γvs,101̅0. Nevertheless, we can investigate their effect on the model of interest and particle wetting on a nanowire since the properties are coupled.
ORCID
Marcus Tornberg: 0000-0002-6285-9932 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge financial support from the Knut and Alice Wallenberg Foundation (KAW) and NanoLund.
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(1) Hiruma, K.; Yazawa, M.; Katsuyama, T.; Ogawa, K.; Haraguchi, K.; Koguchi, M.; Kakibayashi, H. Growth and Optical Properties of Nanometer-Scale GaAs and InAs Whiskers. J. Appl. Phys. 1995, 77, 447−462. (2) Dubrovskii, V. G.; Sibirev, N. V.; Harmand, J. C.; Glas, F. Growth Kinetics and Crystal Structure of Semiconductor Nanowires. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78, 235301. (3) Krogstrup, P.; Yamasaki, J.; Sørensen, C. B.; Johnson, E.; Pennington, R.; Aagesen, M.; Tanaka, N.; Nygård, J.; Wagner, J. B. Junctions in Axial III−V Heterostructure Nanowires Obtained via an Interchange of Group III Elements. Nano Lett. 2009, 9, 3689−3693. (4) Joyce, H. J.; Wong-Leung, J.; Hoe Tan, H.; Jagadish, C.; Gao, Q. Phase Perfection in Zinc Blende and Wurtzite III−V Nanowires Using Basic Growth Parameters. Nano Lett. 2010, 10, 908−915. (5) Ghalamestani, S. G.; Johansson, S.; Borg, B. M.; Dick, K. A.; Wernersson, L. E. Highly Controlled InAs Nanowires on Si(111) Wafers by MOVPE. Phys. Status Solidi Curr. Top. Solid State Phys. 2012, 9, 206−209. (6) Pehlke, E.; Moll, N.; Kley, A.; Scheffler, M. Shape and Stability of Quantum Dots. Appl. Phys. A: Mater. Sci. Process. 1997, 65, 525−534. (7) Fonseka, H. A.; Caroff, P.; Wong-Leung, J.; Ameruddin, A. S.; Tan, H. H.; Jagadish, C. Nanowires Grown on InP (100): Growth Directions, Facets, Crystal Structures, and Relative Yield Control. ACS Nano 2014, 8, 6945−6954. (8) Fortuna, S. a.; Li, X. Metal-Catalyzed Semiconductor Nanowires: A Review on the Control of Growth Directions. Semicond. Sci. Technol. 2010, 25, 024005. (9) Wagner, R. S.; Ellis, W. C. Vapor-Liquid-Solid Mechanism of Single Crystal Growth. Appl. Phys. Lett. 1964, 4, 89−90. (10) Dick, K. A.; Kodambaka, S.; Reuter, M. C.; Deppert, K.; Samuelson, L.; Seifert, W.; Wallenberg, L. R.; Ross, F. M. The Morphology of Axial and Branched Nanowire Heterostructures. Nano Lett. 2007, 7, 1817−1822. (11) Wang, J.; Plissard, S. R.; Verheijen, M. A.; Feiner, L. F.; Cavalli, A.; Bakkers, E. P. A. M. Reversible Switching of InP Nanowire Growth Direction by Catalyst Engineering. Nano Lett. 2013, 13, 3802−3806. (12) Kelrich, A.; Sorias, O.; Calahorra, Y.; Kauffmann, Y.; Gladstone, R.; Cohen, S.; Orenstein, M.; Ritter, D. InP Nanoflag Growth from a Nanowire Template by in Situ Catalyst Manipulation. Nano Lett. 2016, 16, 2837−2844. (13) Nebol’sin, V. A.; Shchetinin, A. A. Role of Surface Energy in the Vapor-Liquid-Solid Growth of Silicon. Inorg. Mater. 2003, 39, 899− 903. (14) Schwarz, K. W.; Tersoff, J. From Droplets to Nanowires: Dynamics of Vapor−Liquid−Solid Growth. Phys. Rev. Lett. 2009, 102, 1−4. (15) Cahn, J.; Hanneman, R. (111) Surface Tensions of III-V Compounds and Their Relationship to Spontaneous Bending of Thin Crystals. Surf. Sci. 1964, 1, 387−398. (16) Oshcherin, B. On Surface Energies of ANB(8-N) Semiconducting Compounds. Phys. Status Solidi Appl. Mater. Sci. 1976, 34, K181. (17) Leitsmann, R.; Bechstedt, F. Surface Influence on Stability and Structure of Hexagon-Shaped III−V Semiconductor Nanorods. J. Appl. Phys. 2007, 102, 1−9. (18) Galicka, M.; Bukała, M.; Buczko, R.; Kacman, P. Modelling the Structure of GaAs and InAs Nanowires. J. Phys.: Condens. Matter 2008, 20, 454226.
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CONCLUSIONS We have demonstrated the stability limit for having an Au−In particle wetting the top facet of epitaxially grown ⟨1̅1̅1̅/0001̅⟩oriented InAs nanowires. Through accumulation of indium in the liquid alloy, the particle expanded until it reached the stability limit for having a particle on the top facet. Particle displacement to one of the {101̅0}-type facets occurs when the particle reaches a critical contact angle of 138°−144°. Theoretical estimations for a gold particle wetting the (0001̅) InAs facet suggest the stability limit to occur from a contact angle of 135° to at least 165°, an interval which encloses our experimental result. The displacement is attributed to a decrease of the particle’s surface tension and volume expansion in conjunction with formation of a truncation of the nanowire top facet. The truncation is suggested to aid displacement as the particle may wet both the top facet and the truncation. In addition, we demonstrated that the particle migrates toward one of the horizontal facets, either the (0001)̅ or a (11̅ 1̅ )̅ facet, when standard growth conditions are resumed for nanowires with particles located at a {101̅0}-type side facet. This type of reversibility verifies the thermodynamic description of the particle stability on a nanowire. Finally, knowing the stability limit for vertical nanowire growth may open new ways of investigating the growth process or altering the growth direction in a controlled fashion.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06138. Data on the diameter dependence of particle migration; the (11̅ 1̅ /̅ 0001)̅ decomposition; calculations and justifications for contact angle and surface tension (γlv) (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 21683
DOI: 10.1021/acs.jpcc.7b06138 J. Phys. Chem. C 2017, 121, 21678−21684
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DOI: 10.1021/acs.jpcc.7b06138 J. Phys. Chem. C 2017, 121, 21678−21684
