Droplet Dynamics in Controlled InAs Nanowire Interconnections

May 1, 2013 - demonstrate unprecedented control of the position of InAs nanowires using selective-area vapor−liquid−solid epitaxy. (VLS) on an InP...
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Letter pubs.acs.org/NanoLett

Droplet Dynamics in Controlled InAs Nanowire Interconnections Dan Dalacu,* Alicia Kam, D. Guy Austing, and Philip J. Poole National Research Council of Canada, Ottawa, Canada, K1A 0R6 S Supporting Information *

ABSTRACT: Semiconductor nanowires offer a versatile platform for the fabrication of new nanoelectronic and nanophotonic devices. These devices will require a high level of control of the nanowire position in relation to both other components of the device and to other nanowires. We demonstrate unprecedented control of the position of InAs nanowires using selective-area vapor−liquid−solid epitaxy (VLS) on an InP ridge template. The high level of control allows us to design structures which connect individual nanowires through coalescence of their catalyst particles. The interconnection process acts as a perturbation to the geometry of the nanowire system that can contribute to the understanding of droplet dynamics in VLS growth. Postgrowth imaging reveals a complex sequence of droplet configurations, including predicted geometries that have not previously been observed. KEYWORDS: Nanowire, VLS, selective-area epitaxy, droplet configuration

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orientations other than (111)B.12 However, as in sequential seeding, there are typically several possible growth directions, limiting the yield of directed interconnections. One potentially promising route entails altering the orientation of the nanowire during growth by initiating a kink through an appropriate choice of materials13,14 or through growth conditions.15,16 Here, we demonstrate a technique to create controlled interconnections between individual InAs nanowires using an architecture suitable for scale-up. The technique is similar in spirit to that described in ref 17 where trenches etched in a (110) wafer were used expose vertical {111} facets for nanowire growth. Rather than etching {111}B facets into the InP substrate we use selective-area epitaxy on a (001) substrate to create ridges with {111}B sidewalls.18,19 These sidewalls are then used as the initial growth surface for the nanowires. In this geometry Au catalysts can be positioned on the {111}B facets using lithography and metal lift-off for VLS growth of nanowires oriented at an inclination of 35.3° with respect to the substrate plane. Parallel ridges with aligned Au catalysts deposited on opposing {111}B facets are used to controllably connect pairs of nanowires as shown schematically in Figure 1. Using selective-area epitaxy to form a template for the nanowire growth rather than etching the substrate17 allows additional functionality to be realized. For example, if the substrate is semi-insulating and the grown ridges are electrically conducting, then electrical contacts on each ridge, simplified through the patterning of large pads in the selective-area growth step, allow self-aligned leads to the nanowires to be made.

emiconductor nanowires offer a bottom-up approach for the fabrication of nanoscale versions of conventional electronic and optoelectronic devices, including transistors, light-emitting diodes (LEDs), and detectors,1,2 and show promise as building blocks in future spintronic devices for quantum computation.3−5 Implementation of these devices will require a scalable approach where the position of the nanowires are controlled in relation to each other and to other components on the chip, such as electrical gates1,4 or optical cavities.3,6 Devices have been made by removing the nanowires from their original substrate and randomly placing them flat on another. The positions of individual nanowires are then determined using optical or electron microscopy before patterning for device fabrication. This makes it very difficult to fabricate the complex network of nanowires that would be required for more advanced quantum devices. There are a number of techniques whereby nanowires have been organized on surfaces, including electric-field-directed, fluidic-flow-directed, and Langmuir−Blodgett assembly; see Lieber and Wang7 and references therein. Ideally, one would like to design a direct nanowire to nanowire interconnection in a deterministic manner and do so in a geometry amenable to electrical measurements. Nanowires produced via vapor−liquid−solid (VLS) epitaxy8 are typically grown on (111)B wafers with the nanowires oriented perpendicular to the substrate, a geometry which is not conducive to direct nanowire to nanowire interconnection. One notable method for the formation of directly interconnecting nanowires relies on sequential seeding of catalyst particles producing secondary nanowires branching off the initial nanowire.9−11 Techniques still need to be developed to control the location and direction of these branches. Alternatively, nonparallel nanowires can be grown using substrates with Published XXXX by the American Chemical Society

Received: March 5, 2013 Revised: April 16, 2013

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Figure 1. Schematic of the template used to grow position-controlled interconnected nanowires. Selective-area epitaxy is used to grow InP ridges with {111}B sidewalls on a (001) InP substrate. Au catalysts deposited on the {111}B sidewalls are used to grow InAs nanowires via VLS epitaxy.

To make useful devices using this process, it is critical to understand what happens as two nanowires collide. There are two possible collision geometries that can occur between the growing nanowires, tip-to-tip collision where the Au particles at the top of each nanowire make contact, and tip-to-sidewall collision where one nanowire has grown faster than the other. The later results in a collision between the tip of the slow growing nanowire and the sidewall of the fast growing one. Postgrowth imaging at different stages of the interconnection process reveals the occurrence of both collision geometries. In particular, the tip-to-tip collision of a pair of nanowires offers a unique opportunity to observe catalyst droplet dynamics in VLS growth20−22 not possible otherwise. To fabricate the interconnected nanowires, two epitaxial growths are required, the first for the InP ridge template and the second for the InAs nanowires. Both growths employ selective-area techniques where a patterned 20 nm thick SiO2 mask is used to restrict growth to areas of exposed semiconductor. The mask for the selective-area epitaxy of the InP ridge template consists of rectangular openings 1 μm wide and 50 μm long which are in registry with alignment marks etched into the substrate. Growth on such a patterned substrate proceeds by material migration off developing sidewall facets on to the (001) top apex where incorporation takes place.23 The resulting growth geometry, when viewed in cross-section, is a trapezoid where the dimension of the top apex decreases as material is deposited, eventually disappearing to produce a triangular ridge. The orientation of the ridge sidewall is dependent on the orientation of the rectangular openings, and in this study, the stripes are oriented along the [110] direction to produces ridges with {111}B sidewalls18,19 as shown in the scanning electron microscopy (SEM) images in Figure 2a,b. The mask for the selective-area VLS growth of the nanowires is deposited directly after the ridge template growth, conformally covering the InP ridges with 20 nm of oxide and increasing the oxide thickness on the (001) surface to 40 nm. The mask consists of ∼100 nm circular openings which are positioned on the {111}B sidewalls of the ridges using the etched alignment marks. A self-aligned lift-off process24 is then used to deposit 10 nm of Au exclusively in the wet-etched openings prior to stripping the electron-beam resist. The template, now ready for VLS growth of the nanowires, is shown in Figure 2c,d. The nanowire growth proceeds as in selective-area VLS growth on (111)B substrates24 and similar to conventional VLS growth on (111)B substrates.12,25 The InAs nanowires grow perpendicular to the (111)B surface with the diameter dictated by the size of the gold catalyst, as shown in Figure 2e,f. The growth conditions used produce untapered nanowires 2 μm

Figure 2. (a, b) SEM images viewed at 45° of the InP ridge template grown using selective-area epitaxy. For [110] oriented ridges, the sidewalls are {111}B facets. Scale bars are 1 μm. (c, d) A SiO2 coated ridge showing the self-aligned catalyst in the oxide opening. Scale bars are 1 μm and 100 nm, respectively. (e, f) InAs nanowire grown on the (11̅1)B sidewall viewed at 45° and in cross-section, respectively. The nanowires are 2 μm long and are untapered. They are oriented perpendicular to the (11̅1)B sidewall of the ridge, making an angle of 90° − ϕ = 35.3° with respect to the (001) substrate plane. Scale bars are 500 nm.

long. The process is completely selective as mentioned above, with no growth observed on the oxide-coated surfaces (the unpatterned sections of the ridge and the (001) substrate). The only substrate growth observed occurs on the annulus of exposed InP surrounding the Au catalyst, resulting in a small pedestal at the base of the nanowire. The {111}B facets make an angle of ϕ = 54.7° with respect to the (001) substrate, Figure 2f, so that the nanowires, growing perpendicular to a {111}B plane, are inclined at 35.3° with respect to the substrate plane. To connect pairs of nanowires, pairs of InP ridges are used with Au catalysts deposited on opposing {111}B facets. Figure 3 shows an array of InP ridges with Au catalysts deposited on both sidewalls of each ridge at 10 μm intervals along the length of the ridges. The ridges are pitched at 4 μm, designed to produce intersecting nanowires for 2 μm long nanowires with bases located midpoint on the {111}B facets of ridges. The yield of the process is extremely high, with only 2 of the 100 Au catalysts shown failing to produce proper InAs nanowires. Not all of the nanowire pairs, however, are connected. Approximately half of the pairs have one or both of the nanowires shorter than the 2 μm length required to intersect. In a previous study of selective-area VLS growth,24 we found the nanowire growth rate to be highly dependent on the Au catalyst size, and we developed a model that correctly predicted the increase in growth rate with decreasing catalyst B

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substrate. Similar sidewall crawling nanowires have also been observed in Si nanowires grown on the buried oxide layer of a SOI substrate.28 There, the Si nanowire was observed to turn 180° and grow back upon itself after colliding with an undercut region of the top Si layer of the SOI structure. Although the result is the same, the mechanism for producing sidewall crawling nanowires in this study is distinct and is captured in the SEM images in Figure 4. After

Figure 3. (a) SEM image viewed at 45° of 100 InAs nanowire pairs grown on an array of InP ridges pitched at 4 μm. The scale bar is 10 μm. (b) A nanowire pair right at the onset of connection, scale bars are 1 μm (top panel) and 50 nm (bottom panel). (c) A nanowire pair where postconnection growth has occurred, viewed at 45° (upper panel) and in plan-view (lower panel). From the plan-view image, three {211}-oriented sidewalls of the nanowire are clearly visible. The scale bar is 100 nm.

diameter. We make the assumption that the observed variation in nanowire length is dominated by a variation in Au catalyst diameter, as supported by our previous study.24 Our model for the diameter-dependent nanowire growth rate predicts an increase in nanowire height from 2 to 2.5 μm for a reduction in Au particle size of 5 nm. We estimate the control of the Au particle size to be of the order ±2 nm,26 and therefore the observed variation on nanowire length is expected. In fact, this variation in nanowire length can be used advantageously to study the different stages in the growth of intersecting nanowire pairs using a single growth. In Figure 3b,c we show two examples of the ideal scenario where both nanowires have the same diameter and therefore have equal growth rates. In this case, the two Au catalysts on each nanowire meet at the midpoint between the InP ridges where they coalesce to form a single Au particle as shown in Figure 3b. Interestingly, postintersection growth proceeds by the newly formed catalyst growing backward down one of the nanowires, as shown in Figure 3c. This growth mode is reminiscent of the growth of heterostructure nanowires (e.g., InAs on GaAs or InP on GaP) where the interface energies, γ, between the two semiconductors and the Au catalyst determine whether the nanowire grows axially or kinks.14,27 There, growth of a semiconductor A on a semiconductor B with interface energy γA−B > [γAu−B − γAu−A] results in island growth in analogy with planar epitaxy. Subsequent growth of the island forces the Au catalyst off the (111)B top facet and onto the nanowire sidewall where it continues to grow, now toward the

Figure 4. (a−e) SEM images viewed in cross-section (left panel) and plan view (right panel) of the different stages in the growth of intersecting nanowires highlighting the different configurations of the Au catalyst. See Figure S1 in Supporting Information for additional SEM images. The scale bar is 100 nm.

coalescence, the Au catalyst has two interfaces with {111}B InAs facets, one with each nanowire. Growth continues at these two interfaces until they eventually meet and form a (001) facet underneath the Au particle (Figure 4b). Continued growth consumes the Au-{111}B interfaces and pushes the Au catalyst up until it becomes unpinned from the facet edge and migrates onto the sidewall of one of the nanowires (Figure 4c). Subsequent growth results in the complete elimination of the {111}B surfaces and the development of a triangular joint (Figure 4d) reminiscent of kinked Si nanowires produced through modulation of growth conditions.15 The reconfiguration of the Au catalyst during the joint growth is striking. It has reformed to cover the five sidewalls of the nanowire visible in the plan view and cross-section SEM images, Figure 4d. We assume the remaining sidewall is also covered, so that the Au is in the form of an annulus around the nanowire. Once the joint is completed, growth of the crawling nanowire commences, presumably via material incorporation at the (111)A surface that develops at the corner of the triangular joint. We note that once growth commences the Au catalyst reconfigures again, elongating in the growth direction and C

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Figure 5. (a) Typical droplet configuration observed in nanowire growth. (b) Schematic of the annular configuration initiated through a decrease in Rs/V1/3 l . (c) Possible configurations of the liquid droplet during sidewall crawling nanowire growth.

it one has to perturb the geometry of the system while it is in its normal growth mode.31 The growth of intersecting nanowires studied here, with the concomitant decrease in Rs/Vl1/3, provides the perturbation required to initiate the observed transformation in the droplet configuration. As mentioned above, with the completion of the triangular joint at the intersection of the nanowire pair, the liquid catalyst reconfigures, and nanowire growth resumes. The same arguments can be applied to the peculiar configuration of the liquid catalyst driving this sidewall crawling nanowire growth. A single sidewall facet is not large enough to support the liquid catalyst pinned at the facet edges. Instead, the liquid catalyst is pinned at the two adjacent facet edges, shown schematically in Figure 5c, with a corresponding droplet configuration that covers three nanowire sidewall facets as observed in Figure 4e. Finally, as alluded to earlier the ideal scenario of equal length nanowires does always occur, and we observe many instances where one nanowire is longer than its pair. In Figure 6 we show a nanowire pair where one nanowire, the one with the smaller diameter (the primary nanowire), reaches the intersection point first and the nanowire with the larger diameter (the secondary nanowire) collides with the sidewall of the primary nanowire. After colliding, the secondary nanowire continues to grow, but along the sidewall facets of the primary nanowire, (i.e., as a sidewall crawling nanowire). This growth mode is similar to that mentioned above for Si nanowires28 with the distinction than the secondary nanowire does not collide normal to the primary nanowire. As a result, the secondary nanowire always grows down the primary nanowire, preferentially choosing the oblique angle as the new growth direction, even though this is in the A direction and not the normally observed B. Interestingly, the growth rate of the secondary crawling nanowire increases with respect to the primary nanowire and is now the faster growing one of the pair. This behavior can be understood by considering the growth mechanisms involved in selective-area VLS growth. The dominant material collection modes for 2 μm long nanowires are (i) direct impingement of trimethylindium (TMI) on the sidewall of the nanowire and (ii) impingement of TMI from scattering off the SiO2 mask.24 Both of these collection modes are followed by cracking of the TMI to form metallic indium and subsequent diffusion to the Au/InAs growth interface where it incorporates. It is expected that material collected by the left-hand branch of the nanowire pair, Figure 6b, will be shared evenly between the two Au catalysts, but a large portion of the material collected by the right-hand branch will be taken up by the Au particle growing

covering three of the six {211}-oriented sidewalls so that the crawling nanowire covers the top half of the underlying nanowire. Figure 5a depicts schematically the typical droplet configuration observed in nanowire growth. For a liquid droplet to remain pinned at the edges of a nanowire on which it sits places conditions on the relevant interface energies at the three-phase boundary: γsv (solid−vapor), γsl (solid−liquid), and γlv (liquid− vapor). In particular, the droplet remains pinned only for contact angles, θ, in the range θY ≤ θ ≤ θY + π/2 where

cos(θY ) =

γsv − γsl γlv

(1)

and θY is the Young angle.22 This limited range of contact angles places constraints on the volume of the liquid droplet, Vl that can be supported by a nanowire of a given radius, Rs. A through a reduction in nanowire decrease in the ratio Rs/V1/3 l radius or an increase in the volume of liquid can result in a spontaneous rearrangement of the droplet to a lower energy configuration.20−22 In the case of intersecting nanowires studied here, the volume of the Au catalyst doubles upon coalescence. At the same time, the area of the Au-InAs interface is reduced as the Au-{111}B interfaces disappear and are replaced by the Au(001) interface. These changes in the geometry of the system and corresponding decrease in Rs/V1/3 cause the contact angle l to increase, from θ < 90° for a catalyst on a single nanowire (Figure 4a) to θ > 100° for a catalyst sitting atop a nanowire pair (Figure 4b). At some stage during the growth of the intersection, the ratio Rs/V1/3 reaches a critical value, and the l contact angle passes outside the stable range causing the observed depinning of the catalyst. Similar Rs/V1/3 l -mediated depinning has recently been observed during the growth of hybrid group III−V/group IV nanowires29,30 and Si nanoarises when wires.30 In the former, a decrease in Rs/V1/3 l growing Si on GaP while in the latter the catalyst volume is increased through deposition of additional Au during a growth interrupt. According to Roper et al.,22 there are conditions where the lowest energy configuration of the catalyst is not a spherical cap sitting atop the nanowire, but an annulus surrounding the nanowire as in Figure 5b. Which configuration will prevail depends on the relevant interface energies and the value of Rs/ 1/3 tends to V1/3 l . In particular, they find that a decrease in Rs/Vl increase the stability of the annular configuration with respect to the typical droplet configuration. The annular configuration, however, does not support steady state growth, and to observe D

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a complex sequence of droplet configurations including a previously unobserved annular configuration predicted to be the lowest state of the system under certain conditions. The growth technique developed can be adapted for the fabrication of complex architectures based on networks of interconnected nanowires. Methods. Three distinct processing steps were employed in the fabrication of the interconnected nanowires: (1) alignment mark etching, (2) ridge template growth, and (3) nanowire growth. For fabricating the alignment marks, the (001) InP substrate was coated with a 100 nm SiO2 mask which was patterned with 1 μm wide crosses using electron-beam lithography and buffered HF wet etching. The pattern was transferred to the InP substrate using Cl2 chemistry in an inductively coupled plasma etcher to produce 500 nm deep trenches. The InP ridge template and the InAs nanowires were grown by chemical beam epitaxy with trimethylindium and precracked PH3 and AsH3. For the ridge template, the sample temperature was first ramped to 550 °C for native oxide removal and then lowered to the growth temperature of 485 °C. The growth time was 50 min, calculated to produce a completed ridge in a 1 μm wide opening23 for the growth rate used. For the InAs nanowire growth, the sample was ramped directly to the growth temperature of 450 °C. The nanowires were grown by first growing an InP section for 5 min and then grading to InAs by switching from PH3 to AsH3 over a 30 s interval. This sequence was found to yield more reproducible growths as compared to growing InAs directly on the ridge sidewall. The total nanowire growth time was 16 min.

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Figure 6. (a) SEM image viewed at 85° from normal of a pair of connected nanowires grown on InP ridges pitched at 3.5 μm. The scale bar is 1 μm. (b) Close-up of the intersection. The scale bar is 100 nm.

ASSOCIATED CONTENT

S Supporting Information *

Additional SEM images showing various configurations of the Au catalyst during the nanowire interconnection process. This material is available free of charge via the Internet at http:// pubs.acs.org.

down that right-hand branch. We cannot, however, exclude the possibility that the catalyzed growth rate on {111}A surfaces may exceed that of {111}B surfaces.32 The growth of the sidewall crawling nanowires are equivalent for both the tip-to-tip and the tip-to-sidewall collision cases with the exception that in the tip-to-tip case the Au particle has twice the volume. This manifests itself in a change in shape of the Au catalyst with the larger volume resulting in a Au particle that is elongated along the length of the nanowire. Although less elongated, the catalyst on the sidewall nanowire arising from the tip-to-sidewall collision is still sufficiently large that it is pinned across three sidewall facets of the primary nanowire. The technique we have discussed presents a number of opportunities. The interconnected nanowire pairs can be grown with barrier layers for an InAs/InP nanowire quantum dot system complete with a scalable contacting architecture inherent to the growth process. The tip-to-tip connection is ideal for double quantum dot devices, whereas the tip-tosidewall connection can be engineered to have the dot of the primary nanowire grown after the connection point. The device architecture can be modified to include electrostatic gating through the addition of a third InP ridge positioned under the nanowire pair. In summary, we have demonstrate unprecedented control of the position of InAs nanowires grown on a nanotemplate of InP ridges. The nanotemplate approach is used to controllably interconnect pairs of InAs nanowires to study droplet dynamics in VLS nanowire growth. The perturbation in the dropletnanowire geometry arising from the intersection process reveals

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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