Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Aluminum-Catalyzed Growth of Silicon Nanowires in High-Energy Growth Directions Mel F. Hainey, Jr.,*,† Xiaotian Zhang,† Ke Wang,‡ and Joan M. Redwing*,†,‡ Department of Materials Science and Engineering and ‡Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
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ABSTRACT: Silicon nanowires grown by metal-mediated techniques, such as vapor− liquid−solid growth, typically exhibit a predominant growth direction; however, growth in the and other high-energy directions is also desirable due to their predicted superior transport properties compared to those of wires. In the case of aluminum-catalyzed silicon nanowire growth via chemical vapor deposition (CVD), wire growth has been previously demonstrated; however, the conditions promoting growth over growth are not fully understood. In this report, we demonstrate that variations in precursor partial pressure within the CVD reactor play a significant role in determining the wire growth direction in this process. In the case of growth on Si(110) substrates, the preferential wire growth direction changes from to along the reactor tube length, corresponding to a reduction in the SiH4 gas-phase concentration due to gas-phase depletion as predicted from computational fluid dynamics simulations. While the change in growth direction occurs without a substantial reduction in the wire growth rate, significant changes occur in the shape of the aluminum-catalyst tip, suggesting a change in growth mechanism arising from possible changes in catalyst supersaturation and/or nanowire sidewall termination. Finally, the identified growth window for wires is used to demonstrate wire growth on Si(100) substrates. KEYWORDS: silicon, nanowire, aluminum, ⟨110⟩ growth direction, CVD, silane, gas-phase chemistry
1. INTRODUCTION Silicon nanowires are attractive materials for nanoscale electronic,1,2 optoelectronic,3 sensing,4 and photovoltaic applications.5 Considerable efforts have been devoted to understanding the growth of silicon nanowires using the metal-catalyzed vapor−liquid−solid (VLS) process, which commonly produces oriented nanowires.6−8 Gold is the most common catalyst used but is expensive and acts as a deep-level trap in silicon.9 In comparison, aluminum is earthabundant, has a relatively low eutectic temperature with silicon (577 °C),10 and is a shallow p-type dopant, making it an attractive alternative catalyst material. Initial studies of Al-catalyzed silicon nanowire growth using chemical vapor deposition (CVD) have focused on growth of wires under a variety of conditions, and a growth window of pressure and temperature has been defined under reduced pressure conditions (∼1−700 Torr).11 Additional research has focused on the effects of hydrogen and precursor partial pressure on nanowire growth and morphology.12,13 However, silicon nanowire growth in directions such as and is also desirable. Prior theoretical studies have predicted that silicon nanowires grown in the direction will have superior hole mobility relative to or wires14 and that both and wires are predicted to show increased conductivity relative to wires.15 Note that increased hole mobility in germanium nanowires relative to wires has been observed experimentally as well.16 Wires grown in the direction would also allow for direct vertical integration with CMOS electronics. Direct © XXXX American Chemical Society
growth of wires in these directions has proven difficult, however, since , the close-packed direction, is the lowest-energy growth direction for silicon. Preferential growth of wires in other directions requires the development of processes that can overcome the energetic differences. We previously demonstrated the Al-catalyzed growth of oriented silicon nanowires on Si(110) substrates using a SiH4 precursor in H2 carrier gas, which was found to occur under conditions of high H2 partial pressures and subeutectic preannealing and growth temperatures.17 Subeutectic growth conditions were initially believed to be important to limit the Al droplet supersaturation by keeping the droplet in the solid phase. At temperatures at or above the eutectic, the preferential growth direction changed to even for growth from a Si(110) surface. However, this explanation cannot explain several behaviors observed for wire growth under these conditions. The growth rate of the silicon nanowires was ∼200 nm/min,17 comparable to growth rates for wires at similar SiH4 partial pressures11,13 and much faster than any previous reports for growth from a solid catalyst.18 Additionally, earlier reports on Al-catalyzed nanowire growth had described that was the preferential growth direction observed at high H2 partial pressures and 550 °C growth temperature.11 Thus, further study is needed to Received: June 2, 2018 Accepted: September 28, 2018 Published: September 28, 2018 A
DOI: 10.1021/acsanm.8b00925 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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ACS Applied Nano Materials Table 1. Reactions Included in Kinetic Model of SiH4 Pyrolysis and Si Deposition log A G-1 G-2 S-1 S-2
gas-phase reactions SiH4 ⇒ SiH2 + H2 SiH2 + H2 ⇒ SiH4 surface reactions SiH4 ⇒ Si + 2H2 SiH2 ⇒ Si + H2
(cm3/mol−1s−1)
(1/s) 12.3
12.12 A 0.0537 1
Ea (kcal/mol) 52.2 5.5 Ea (kcal/mol) 18.68 0
ref 24 25 ref 26 26
positioned in the same location as previously reported,17 roughly 22 cm from the reactor outlet in the center of the 10 cm uniform hot zone. Nanowire growth was performed at 300−500 Torr reactor pressure and 2.5 Torr SiH4 partial pressure in a H2 carrier gas with a total flow rate of 100 sccm, using a 550 °C preanneal for 1 h and 550 °C growth for 30 min before cooling. The SiH4 mass flow controller used had a programmable range of 1−100 sccm, limiting the minimum SiH4 partial pressure values to ∼1 Torr. Cross section scanning electron microscopy (SEM) was performed using Zeiss Leo 1530 and Zeiss Merlin field emission scanning electron microscopes. Transmission electron microscopy (TEM) and energy-dispersive spectroscopy (EDS) were performed on an FEI Titan3 G2* aberration-corrected TEM at 200 and 80 keV accelerating voltages with a SuperX EDS detector attachment. TEM specimens were prepared by sonicating samples in IPA for ∼1 min, followed by dropping the solution onto a lacey carbon grid. Computational fluid dynamics simulations of the thermal-fluid environment, SiH4 gas-phase chemistry, and Si thin film deposition rate along the length of the reactor tube were performed to gain insight into the conditions required to achieve oriented nanowire growth. The commercial software COMSOL Multiphysics 5.3 was used to perform the computational simulations. The transport and chemistry model of the silicon nanowire deposition process is based on the solution of the coupled partial differential equations for the conservation of momentum, energy, mass, and individual species using the modules “Non-Isothermal Flow” and “Heavy Species Transports” built into the software. A 2D-axisymmetric model was used due to the axial symmetry of the reactor to reduce the computational complexity of the simulations. The transport model used “Non-Isothermal Flow”, which couples both gas laminar flow and heat transfer in fluids based on the low pressure and low flow rates used in the reactor tube. Considering H2 as the main carrier gas, a compressible form of Naiver−Stokes and continuity equations were applied to maintain mass and momentum conservation in the simulations. The experimentally measured temperature profile in the reactor tube as a function of position was used to set the thermal profile of the reactor wall in the simulation. Conduction, convection, and radiation were considered in the heat transfer model. The properties of the mixture gases were calculated using the ideal gas law and relevant thermodynamic properties. The values for the Lennard−Jones parameters (potential characteristic length and potential energy minimum) for H2, SiH4, and SiH2 were provided by the software. The solid material used in the reactor wall was quartz, and Si deposition was assumed to occur on the surface of the reactor tube and Si substrate during the growth. The pyrolysis of SiH4 gas has been studied both theoretically and experimentally under various conditions.24,25,27,28 Although the actual gas-phase decomposition mechanism is complicated and includes high-order silanes (such as Si2H6 and Si3H8),25 the main limiting reaction is known as the homogeneous pyrolysis of silane (SiH4) into silylene (SiH2) and hydrogen as shown in G-1 in Table 1 and the corresponding reverse reaction (G-2)24,29 with Arrhenius parameters from the literature.24,25 For simplicity, the surface reaction mechanisms are considered to consist of the irreversible adsorption of SiH4 (S-1) and SiH2 (S-2), leading to Si deposition. Arrhenius parameters for the sticking coefficient of SiH4 were adapted from the work of Moffat and Jensen26 and the sticking coefficient of SiH2 on the Si surface was assumed to be unity due to highly reactive open shell molecule of SiH2.26
determine how similar growth conditions can give rise to different observed preferential growth directions. In this report, we observe that under nominally identical temperature and reactor pressure conditions, the silicon nanowire growth direction is dependent on position on the Si(110) substrate within the reactor tube, changing from a preferential wire direction of to moving downstream from the gas inlet. Wires grown in the direction have structures that combine aspects of vapor− solid−solid (VSS) growth, such as a faceted final catalyst tip structure and preferential growth directions corresponding to low droplet supersaturation, with the higher axial growth rates more typical of vapor−liquid−solid growth. Distinctive oxide precipitation was also observed postgrowth from the aluminum-catalyst droplet for wires, which was not observed for oriented wires. The change in growth direction correlates with variations in the local gas-phase SiH4 precursor concentration along the length of the reactor, with growth occurring in regions of high SiH4 concentration and growth occurring downstream once the SiH4 concentration has become depleted. Computational fluid dynamics simulations of gas-phase concentrations and predicted Si thin film deposition rate as a function of position in the reactor support this observation, suggesting that SiH4 depletion occurs in the region where the growth direction switches to . The reduction in SiH4 concentration and corresponding changes in gas-phase chemistry are believed to impact both the supersaturation of silicon in the catalyst droplet and also the nanowire sidewall termination, which has previously been shown to influence nanowire growth direction in Au-catalyzed Si and Ge nanowire growth19−23 and catalyst droplet morphology in Al-catalyzed Si nanowire growth.12,17 Finally, the flexibility of the SiH4-depleted growth regime is demonstrated by growing nanowires in the high-energy growth direction.
2. EXPERIMENTAL AND MODELING Silicon nanowire growth was carried out in a hot wall horizontal quartz tube reactor that consisted of a 24 mm inner diameter outer quartz tube and 18 mm inner diameter inner liner tube situated in the center of a single-zone tube furnace that had a central constant temperature zone approximately 10 cm in length. The inner line tube was primarily used to allow for easy tube replacement after excess silicon deposition. In order to ensure an epitaxial relationship between nanowires and the substrate surface, silicon substrates with (110) and (100) orientations were prepared by etching in 10:1 buffered oxide etch for 2 min to remove native oxide from the substrate surface. Samples were rinsed with DI water, full hydrogen termination was confirmed visually, and samples were dried with nitrogen before being loaded into an electron beam evaporator (Semicore) for aluminum deposition. Once base pressures below 1 × 10−6 Torr were reached, 5 or 10 nm of aluminum was evaporated at a rate of 1.5 Å/s. After unloading, samples were stored in ambient nitrogen until being used for growth. The silicon substrates were prepared in ∼3 cm long by 0.5 cm wide segments for these experiments, with the rear of the sample B
DOI: 10.1021/acsanm.8b00925 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Figure 1. Effect of reactor temperature and H2 partial pressure on Si nanowire growth on Si(110) substrates. (a,b) Schematic and summary of the Al-catalyzed low-pressure CVD growth process. (c) At subeutectic growth temperatures (e.g., 550 °C), preferential growth is in the direction on Si(110) substrates. (d) Increasing the growth temperature to the Al−Si eutectic point (575 °C) leads to a change to the preferential orientation, with a characteristic ∼35.3° angle between the surface and wires. (e) At 550 °C but lower hydrogen partial pressures (98 Torr), wire growth is suppressed by heavy a-Si vapor−solid deposition. (f) At 550 °C and higher hydrogen partial pressures (298 Torr), SiH4 decomposition is slowed, resulting in a reduced a-Si deposition and promotion of epitaxial growth. (g,h) Characteristic wire tip structure and selected area diffraction pattern (taken along axis). Figures 1e,g,h adapted with permission from ref 17. Copyright The Minerals, Metals and Materials Society 2015.
3. RESULTS AND DISCUSSION
were performed to investigate the origin of the differences in growth direction. In the next set of experiments, larger silicon substrates (∼3 cm long × ∼ 0.5 cm wide) were used with identical growth conditions to span a greater distance in the reactor tube with the front of the samples being closer to the gas inlet in the reactor than in the previous growths. As shown in Figure 2a,
Figure 1a,b summarizes the influence of the hydrogen partial pressure and growth temperature on the growth directions of Si nanowires on Si(110) substrates grown in our horizontal CVD system. As seen in Figure 1c,d, growth temperatures must remain below the Al−Si eutectic temperature to promote vertical, large-diameter (∼100−230 nm) wire growth from Si(110) substrates (Figure 1c). When growth temperatures reach or exceed the eutectic temperature, the preferential growth direction changes from to (Figure 1d), as seen by the angle (∼35.3°) of the wires relative to the substrate surface. Note that and wires have approximately the same diameter distribution. In Figure 1e,f, the effect of hydrogen partial pressure on epitaxial wire growth can also be observed. Wires grown at low hydrogen partial pressures (Figure 1e) tend to have kinks and highly irregular morphologies. This behavior has been previously explained as arising from amorphous silicon that deposits simultaneously with wire growth at reduced hydrogen partial pressures.12 In comparison, wires grown at ∼300 Torr in hydrogen do not show such irregular morphology, growing in well-defined crystallographic directions (Figure 1f). By increasing the hydrogen partial pressure, the SiH4 decomposition rate can be reduced, minimizing vapor−solid amorphous silicon deposition.12 Wires grown in the direction grow vertically relative to the substrate surface and have a distinctive angled tip structure (Figure 1g) that enables straightforward identification, which was further confirmed by selected area electron diffraction (Figure 1h) after removal of the wires from the substrate surface. The results demonstrate that wire growth can be promoted through the use of subeutectic nanowire growth temperatures and high hydrogen partial pressures.17 However, given that previous reports11,13 on silicon nanowire growth under nominally similar conditions produced preferential wire growth, further experiments
Figure 2. (a) Photo of sample showing visible change in sample surface corresponding to change in wire growth direction from (b) to (c) growth. The change in growth direction also corresponds to a change in the shape of the aluminum-catalyst tip for (d,e) and (f,g) wires.
after growth using the above-mentioned wire growth conditions, the front part of the sample appears dark brown and transitions to a gray color further back on the sample. Cross section images of the different regions show that wires grown in the brown region (marked “b”) are uniformly oriented (Figure 2b) based on their orientation relative to the substrate surface and grow in high density. Meanwhile, in the gray region (marked “c”), growth is the preferential C
DOI: 10.1021/acsanm.8b00925 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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ACS Applied Nano Materials growth direction (Figure 2c) and remains so for the rest of the sample. The growth rate of the nanowires decreases slightly with position along the sample length, changing from ∼290 nm/min for wires closer to the gas inlet to ∼260 nm/ min for wires further downstream. The growth regime is also observed to be associated with a unique aluminum-catalyst droplet shape compared to Si wires grown in the standard VLS regime. As shown in Figure 2d,e, catalyst droplets on wires appear grooved and crown-shaped, similar to previous observations of wire growth.11,12 In comparison, catalyst tips on wires appear well-faceted and crystalline (Figure 2f,g). Initially, these result suggested that vapor−solid−solid (VSS) growth is occurring in the case of growth regions as suggested by prior reports.30,31 However, the prior studies, which occurred under UHV growth conditions, also reported growth rates on the order of a few nm/min, much lower than the growth rates (∼260−290 nm/min) observed in this work.30 Even for studies of VSS growth of wires under LPCVD conditions (reactor pressure ∼40 Torr), reported growth rates were in the range of ∼1−3 nm/min, orders of magnitude below the rates observed in this and earlier reports of growth.18,32 These significant differences in growth rate suggest that VSS growth is likely not the reason for the preferential growth observed in this work. Note given the combination of temperature, surface and gas chemistry, and nanoscale droplet size, the actual state of the aluminum catalyst during growth remains unclear, as catalyst droplets can undergo a wide variety of phase transformations depending on supersaturation conditions even under isothermal conditions.33 Another distinguishing feature of the wires is the unusual oxide precipitation behavior observed on the surface of the catalyst tip upon cooling from growth temperature. The oxide likely is formed from oxygen present in the initial Al film due to exposure to ambient air; however, its morphology and precipitation behavior appear different than those of previously observed oxides on the Al catalysts of wires. Previous atom probe analysis by Eichfeld et al. of Al-catalyzed Si nanowires revealed that, for the crown-shaped tips similar to those observed in Figure 2, nonuniform oxide coating was present on the outer surface of the catalyst tip.34 The bulbous regions corresponded to Al-rich areas, while grooves between the bulbous areas corresponded to oxide-rich areas. In comparison, for wires grown in the directions, the catalyst droplets appear to eject significant quantities of oxide material from their tip in random fashion (Figure 3a,b). As shown in Figure 3c−e, EDS analysis of the ejected material reveals that while the outer surface of the material contains silicon, most of the material is an aluminum-rich oxide. This nonuniform oxide precipitation, which was observed for a majority of the wires grown in high-energy growth directions in this study, suggests that the droplet exists in a different state than for wire growth. Note that beneath the region of ejected oxide material (Figure 3b), the droplets maintain the faceted shape shown in Figure 2f−g. The presence of two distinct nanowire growth directions under nominally identical pressure and temperature conditions suggests that additional factors must be influencing nanowire growth. We previously observed a change from bulbous to faceted nanowire tip structure for Al-catalyzed Si nanowires grown under different hydrogen partial pressures.12 The changes in catalyst morphology were attributed to changes
Figure 3. Oxide precipitation from catalyst tip for Si nanowires. (a) SEM images of wires showing randomly ejected oxide from catalyst droplets. (b−f) TEM and EDS analysis of the wire and precipitated oxide showing that the catalyst tip is primarily aluminum, while the ejected particle is an aluminum oxide with a surface coating of silicon oxide.
in the extent of hydrogen surface termination on the wire sidewalls. At higher H2 partial pressures, the extent of hydrogen surface termination was increased, promoting Al diffusion and a bulbous tip structure. In comparison, decreasing hydrogen partial pressure was associated with reduced hydrogen surface termination, blocking Al diffusion and leading to faceted tip formation. Importantly, changes in sidewall termination have been found to promote changes in preferential growth directions as well. Previous studies of Si nanowire growth employing in situ infrared spectroscopy found that increased hydrogen termination of the nanowire sidewalls promoted kinking of wires grown vertically on Si(111) substrates into the direction, and the preferential sidewall facets changed from close-packed {111} to lower-density {110} and {110} corresponding with increasing hydrogen surface concentration.21 This is particularly important for wires, which have been previously reported to have large {111} facets comprising the majority of their sidewall surfaces.35 Furthermore, initiation of nanowire growth under high hydrogen termination conditions promoted direct growth of wires in the “kinked” direction,21,36 suggestive of the direct growth observed of Si(110) substrates observed in this manuscript. Additionally, previous reports on germanium nanowires19,31,37 have demonstrated that precursor partial pressure can influence catalyst droplet behavior at subeutectic temperatures. With increasing Ge flux, nanowires grown originally in the direction were observed to switch to a preferential growth direction, with the metal catalyst droplet shifting completely to one of the two {111} facets at the catalyst/wire interface. Therefore, to investigate whether growth could be associated with changes in growth chemistry, computational fluid dynamics simulations were carried out to model the gasphase concentration of SiH4 and SiH2 and predict the Si thin film deposition rate as a function of position in the reactor tube to provide additional insights into the factors impacting the nanowire growth direction. Following the reactor schematic in Figure 1a, the simulation conditions included 300 Torr reactor pressure and 2.5 Torr SiH4 partial pressure in a H2 carrier gas with a total gas flow rate of 100 sccm. The experimentally measured thermal profile along the length of the reactor was D
DOI: 10.1021/acsanm.8b00925 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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contrast, a reduction in H2 partial pressure suppressed Al diffusion, resulting in the faceted tip shape. In this case, the changes in tip shape were attributed to differences in the extent of H-termination of the nanowire sidewall, with increased Htermination promoting Al diffusion. It is likely that similar changes in sidewall termination associated with variations in the gas-phase concentrations of SiH4, SiH2, H2, and other species along the length of the reactor tube are responsible for both the change in tip shape and growth direction observed in this study, although it is difficult to directly correlate it with the extent of H-termination in this case. Nevertheless, it is clear that upstream in the reactor tube where the SiH4 concentration is highest, the Al catalyst can wet and diffuse down the nanowire sidewalls, which causes the wires to kink away from toward the direction. Under the conditions of our reactor chamber, it was not possible to controllably switch from to growth by simply controlling the inlet SiH4 partial pressure. For example, a similar position dependence was observed when the inlet SiH4 partial pressure was increased from 2.5 to 5 Torr, although the yield of nanowires was reduced. Given the reactor design and the complex chemistry of SiH4 decomposition, changes in depletion behavior, particularly at lower SiH4 partial pressures, could not be observed in this system. In comparison, in manuscripts in which controllable nanowire growth directions during growth were demonstrated, precursor fluxes were often able to be changed by orders of magnitude,37,38 or for III−V and II−VI wire growth, they were often able to be changed by controllable supersaturation of catalyst droplets with one element (such as indium or Zn) before the introduction of the other component and subsequent III−V or II−VI precipitation.39,40 It is also difficult to directly connect the gas-phase depletion and reaction with catalyst supersaturation and surface chemistry. Further study on the species present during nanowire growth in proximity to the nanowire growth front is necessary to fully understand the factors promoting highenergy growth in this case. In particular, while the reactor ambient may be H2, previous studies have shown that H2 alone may not significantly affect preferential growth directions.21 Instead, the local chemical environment surrounding the catalyst droplet, containing a variety of SiHx species and radicals that can more readily react with the droplet and nanowire sidewalls, may have a much stronger influence on the final nanowire growth direction than H2 partial pressure alone. Nevertheless, the correlation between SiH4 precursor depletion and growth direction provides a plausible explanation for the to transition, as similar transitions have been observed in the other nanowire system, even though the growth environment likely includes other factors that can influence preferential growth direction but are more difficult to quantify. Along with promoting growth, the SiH4-depleted VLS regime enables growth of silicon nanowires in other highenergy growth directions, such as . This can be seen in Figure 5, where vertical wire growth from Si(100) substrates is observed (Figure 5a), and the orientation is confirmed through TEM. The wires were grown under identical conditions to the wires from Si(110) substrates and retain the characteristic tip structures seen for wires, suggesting that they are being formed by the same mechanism. In comparison, wires grown from these substrates under VLS conditions grow exclusively in the direction, regardless
used to define the wall temperatures for a furnace set point at 550 °C. The temperature was relatively uniform in the center of the reactor (30−40 cm away from inlet) and decreased moving toward the inlet or outlet. As mentioned previously, the substrate was placed ∼34 cm downstream from the gas inlet in the constant temperature region of the tube furnace. The gas-phase concentrations of SiH4 and SiH2 as a function of position in the reactor tube are shown in Figure 4a and b,
Figure 4. Two dimensional cross-sectional plots of (a) SiH4 and (b) SiH2 gas-phase mole fractions along gas flow direction. (c) Silicon thin film deposition rate as a function of position.
respectively. In both cases, there is a significant decrease in gasphase concentration along the length of the reactor tube where the substrate is located due to Si thin film deposition on the reactor wall surfaces, which is plotted as a function of position in Figure 4c. Based on these modeling results, an explanation for the observed change in preferential growth direction can be developed. In the case of Al-catalyzed wire growth, as the SiH4 flows through the reactor, it decomposes and deposits silicon on the reactor sidewalls and substrate or is dissolved in the Al film. Further downstream from the reactor inlet, the SiH4 concentration is reduced, resulting in decreased droplet supersaturation relative to wires in the upstream portion of the sample. The reduced precursor partial pressure due to gasphase depletion acts in the same manner as the low precursor flux in the Ge nanowire case,19,37 reducing catalyst droplet supersaturation and changing the preferential growth direction. In addition, both the growth rate and density of the wires are slightly reduced compared to that of the wires, consistent with a reduced supply of silicon, which provides further support for the gas-phase depletion hypothesis. Given the change in catalyst droplet shape that is also observed, it is likely that the change in H2/SiH4 gas-phase chemistry also leads to changes in surface termination of the nanowire sidewalls as well that impact the growth direction. For Al-catalyzed Si nanowire growth, our previous study demonstrated that increased H2 partial pressure during growth results in wetting and diffusion of Al down the nanowire sidewalls, giving rise to the bulbous tip shape.12 In E
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the SiH4-depleted VLS regime and should enable fabrication and characterization of wires grown in a variety of high-energy growth directions.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (M.F.H.) *E-mail:
[email protected] (J.M.R.) ORCID
Mel F. Hainey, Jr.: 0000-0003-3878-099X Joan M. Redwing: 0000-0002-7906-452X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the financial support of the Alfred P. Sloan Foundation and the National Science Foundation under Grant No. PFI:AIR-TT 1414236. The authors would also like to thank Dr. Jennifer Gray of Penn State MCL for her assistance with analysis of TEM data.
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REFERENCES
(1) Appenzeller, J.; Knoch, J.; Bjork, M. T.; Riel, H.; Schmid, H.; Riess, W. Toward Nanowire Electronics. IEEE Trans. Electron Devices 2008, 55, 2827−2845. (2) Kempa, T. J.; Cahoon, J. F.; Kim, S.-K.; Day, R. W.; Bell, D. C.; Park, H.-G.; Lieber, C. M. Coaxial Multishell Nanowires with HighQuality Electronic Interfaces and Tunable Optical Cavities for Ultrathin Photovoltaics. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 1407−1412. (3) Wang, X.; Zhuang, X.; Yang, S.; Chen, Y.; Zhang, Q.; Zhu, X.; Zhou, H.; Guo, P.; Liang, J.; Huang, Y.; Pan, A.; Duan, X. High Gain Submicrometer Optical Amplifier at Near-Infrared Communication Band. Phys. Rev. Lett. 2015, 115, 027403. (4) Wang, Y.; Wang, T.; Da, P.; Xu, M.; Wu, H.; Zheng, G. Silicon Nanowires for Biosensing, Energy Storage, and Conversion. Adv. Mater. 2013, 25, 5177−5195. (5) Kempa, T. J.; Day, R. W.; Kim, S.-K.; Park, H.-G.; Lieber, C. M. Semiconductor Nanowires: A Platform for Exploring Limits and Concepts for Nano-Enabled Solar Cells. Energy Environ. Sci. 2013, 6, 719−733. (6) Wagner, R. S.; Ellis, W. C. Vapor-Liquid-Solid Mechanism of Single Crystal Growth. Appl. Phys. Lett. 1964, 4, 89−90. (7) Schmidt, V.; Wittemann, J. V.; Senz, S.; Gösele, U. Silicon Nanowires: A Review on Aspects of Their Growth and Their Electrical Properties. Adv. Mater. 2009, 21, 2681−2702. (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) Schmidt, V.; Wittemann, J. V.; Gö s ele, U. Growth, Thermodynamics, and Electrical Properties of Silicon Nanowires. Chem. Rev. 2010, 110, 361−388. (10) Murray, J.; McAlister, A. The Al-Si (Aluminum-Silicon) System. Bull. Alloy Phase Diagrams 1984, 5, 74−84. (11) Ke, Y.; Weng, X.; Redwing, J. M.; Eichfeld, C. M.; Swisher, T. R.; Mohney, S. E.; Habib, Y. M. Fabrication and Electrical Properties of Si Nanowires Synthesized by Al Catalyzed Vapor-Liquid-Solid Growth. Nano Lett. 2009, 9, 4494−4499. (12) Ke, Y.; Hainey, M. F.; Won, D.; Weng, X.; Eichfeld, S. M.; Redwing, J. M. Carrier Gas Effects on Aluminum-Catalyzed Nanowire Growth. Nanotechnology 2016, 27, 135605. (13) Kohen, D.; Cayron, C.; De Vito, E.; Tileli, V.; Faucherand, P.; Morin, C.; Brioude, A.; Perraud, S. Aluminum Catalyzed Growth of Silicon Nanowires : Al Atom Location and the Influence of Silicon Precursor Pressure on the Morphology. J. Cryst. Growth 2012, 341, 12−18.
Figure 5. (a) wires grown vertically from a Si(100) substrate under silane-depleted VLS growth conditions. (b,c) TEM image and diffraction pattern on [100] zone axis indicating wire growth direction. (d,e) Faceted tips of wires, indicating silane-depleted VLS growth.
of substrate orientation. These experiments demonstrate the versatility of the SiH4-depleted VLS regime. Growth of appreciable quantities of wires in the and other directions should allow for detailed characterization of the effects of orientation on nanowire properties.
4. SUMMARY AND CONCLUSIONS In conclusion, two growth regimes for Al-catalyzed Si nanowire growth on Si(110) have been demonstrated using conditions previously attributed to growth. Toward the inlet of the reactor where the SiH4 concentration is high, the nanowires grow in a preferred direction via a VLS growth mechanism. As SiH4 becomes depleted from the gas phase, the preferential growth direction switches to . In this regime, the catalyst droplet remains highly faceted, similar to VSS growth, but growth rates remain comparable to VLS growth. Oxide precipitation from the Al catalyst tips is also observed in the growth regime, and the tips take on morphologies that vary from wire to wire, in comparison to the relatively uniform tip morphologies observed for nanowires, further suggesting that the growth regime is fundamentally different. Based on previous research and the modeling presented in this study, it is likely that, along with reactor temperature and pressure, the changing reaction environment corresponding with SiH4 depletion strongly influences preferential nanowire growth direction. In particular, the changing droplet morphology and growth direction are strongly suggestive of changing nanowire sidewall hydrogen surface termination that promotes either or nanowire growth. Finally, growth using the “SiH4-depleted” VLS regime was demonstrated in the higher-energy direction from Si(100) substrates. Vertically oriented wires are observed in quantities sufficient for further characterization. This result demonstrates the flexibility of F
DOI: 10.1021/acsanm.8b00925 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsanm.8b00925 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
