Letter pubs.acs.org/NanoLett
Directed Branch Growth in Aligned Nanowire Arrays Allan L. Beaudry,*,† Joshua M. LaForge,† Ryan T. Tucker,† Jason B. Sorge,† Nicholas L. Adamski,† Peng Li,‡ Michael T. Taschuk,† and Michael J. Brett†,‡ †
Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada NRC National Institute for Nanotechnology, Edmonton, Alberta T6G 2M9, Canada
‡
S Supporting Information *
ABSTRACT: Branch growth is directed along two, three, or four inplane directions in vertically aligned nanowire arrays using vapor− liquid−solid glancing angle deposition (VLS-GLAD) flux engineering. In this work, a dynamically controlled collimated vapor flux guides branch placement during the self-catalyzed epitaxial growth of branched indium tin oxide nanowire arrays. The flux is positioned to grow branches on select nanowire facets, enabling fabrication of aligned nanotree arrays with L-, T-, or X-branching. In addition, a flux motion algorithm is designed to selectively elongate branches along one in-plane axis. Nanotrees are found to be aligned across large areas by X-ray diffraction pole figure analysis and through branch length and orientation measurements collected over 140 μm2 from scanning electron microscopy images for each array. The pathway to guided assembly of nanowire architectures with controlled interconnectivity in three-dimensions using VLS-GLAD is discussed. KEYWORDS: Branched nanowire, nanotrees, epitaxy, vapor−liquid−solid, glancing angle deposition, indium tin oxide
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tions.31,33 However, these interconnections are typically randomly positioned in both height and azimuthal orientation. Though randomly structured NW networks may be sufficient for some applications, such as structured electrodes in organic photovoltaic51,52 and water splitting devices,53,54 improved control over directing branch growth in both height and azimuthal orientation is required to meet the challenge of guided NW assembly into designed 3D nanoelectronic device architectures.28,38,55 Control over branch placement in height and orientation has been recently demonstrated in branched indium tin oxide (ITO) NW arrays using a flux engineering technique named vapor−liquid−solid glancing angle deposition (VLSGLAD).44,46,56 Growth in VLS-GLAD occurs via a similar mechanism to regular VLS NW growth; a catalyst liquid droplet is used to collect vapor and precipitate crystalline material at the liquid−solid interface.57,58 GLAD uses a collimated and dynamically directed vapor flux that can be arbitrarily oriented relative to the substrate during growth (via substrate motion) to structure thin films.59,60 VLS-GLAD’s distinguishing feature is the use of a ballistic vapor flux to guide the catalyzed VLS growth of crystalline NWs.61 ITO nanotrees are formed in a single deposition step because of the unique self-catalyzed VLS growth mechanism of ITO branches and trunks.62−68 ITO has a cubic bixbyite crystal
ver the past decade significant progress toward controlling the growth and properties of nanowires (NWs) has been made.1−6 These advancements have fuelled optimism for NW applications in transistors,7−14 photovoltaic devices,15−19 and biological sensors.20−25 Techniques capable of guiding the growth of NW arrays into 3D interconnected device architectures may reduce reliance on traditional topdown fabrication methods and enhance NW device functionality. Design targets for 3D NW networks will increase in complexity as device architectures mature. Therefore, reliable and scalable techniques for aligning and joining NWs will be required. NWs can be three-dimensionally shaped by kinking23,24,26,27 and branch growth.28−43 Branched NWs, or nanotrees, consist of NW “branches” grown epitaxially on the sidewall of vertical NW “trunks”. Advances in branched NW morphology include designed branch placement,32,38,44 hyperbranching control,29,34,45 branch diameter oscillations,46 and helical nanotree growth.35,37 In addition, branched NWs have recently enabled the formation of NW crosses designed to explore potential configurations for Majorana fermion exchange.42 Branched NWs are candidate building block materials for the directed self-assembly of interconnected NW architectures.31,33,41,45,47−50 NW interconnectivity can be designed by strategic patterning of liquid catalyst droplets prior to epitaxial trunk growth on a lattice matched substrate. By placing trunks in patterns that resemble the system’s crystal symmetry (i.e., square lattice for a cubic crystal system), branch growth can be directed toward adjacent trunks, thereby forming connec© 2014 American Chemical Society
Received: November 25, 2013 Revised: February 19, 2014 Published: March 14, 2014 1797
dx.doi.org/10.1021/nl404377v | Nano Lett. 2014, 14, 1797−1803
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Figure 1. The vapor flux is deposited at oblique angles (α) relative to substrate normal and offset by an angle (ϕ) from the [100] direction of the single cubic crystal YSZ substrate. SHIM images of ITO nanotrees grown on YSZ with α = 85° and (a−d) ϕ = 45° (resulting in L-shaped nanotrees) and (e−h) ϕ = 0° (resulting in T-shaped nanotrees). (a) and (e) are oblique images. (b), (c), (f), and (g) are plan view images. (d) and (h) are cross-sectional images. Red arrows depict vapor flux orientation, and black arrows indicate crystal directions of YSZ substrate. All scale bars are 200 nm.
structure69,70 and ITO nanotrees are single crystals with four branch growth directions per trunk.44 Growth of branches shadowed from the vapor flux during VLS-GLAD processes is inhibited.44 Shadowing between neighboring NWs ensures that only a limited length near NW tips receive flux to promote branch growth. These features, combined with vapor flux motion, allow branches to be grown at select heights and orientations as the trunks grow vertically.44
In VLS-GLAD processes, vertical growth rates depend strongly on the in-plane flux capture cross section due to the use of an obliquely incident flux. We have previously engineered the azimuthal flux distribution to promote the growth of select in-plane nanotree crystal orientations44 and have shown this orientation selection to be an evolutionary process.71 As a result, competitive growth can be designed to align nanotree crystals and branches in-plane without requiring substrate epitaxy. However, directed branch growth control is 1798
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Deposition was performed in a custom GLAD system (Kurt J Lesker, AXXIS) that uses two stepper motors to control substrate motion. Substrate motion enables control over vapor flux angle relative to substrate normal (α) and azimuthal position of the substrate with respect to the vapor source (ϕ)59,60 (See Figure 1a). Substrate motion is used during deposition to change the vapor source’s orientation with respect to the growing NW arrays. NW arrays in this work were grown to a nominal thickness of 300 nm at a temperature of 300 °C, deposition angle of 85°, and nominal flux rate of 1 nm/s unless otherwise stated. Flux motion was either fixed position (constant ϕ), constant rotation (isotropic motion: 1 rotation per 10 nm of nominal material deposited), or serial bideposition (SBD). NW arrays grown with SBD motion required two stages: the first stage was grown at standard conditions with continuous substrate rotation to a nominal thickness of 300 nm, the second stage was grown to a nominal thickness of 150 nm at a reduced flux rate of 0.1 nm/s as the deposition angle was gradually reduced from 85° to 70°. During the second stage, ϕ was alternated between two azimuthal positions separated by 180° (28 s at fixed ϕ, 2 s transition). Branch placement was characterized using image analysis of plan view SEM images. The length and orientation of the longest branch per trunk sidewall was measured using ImageJ software78 from five plan view SEM images taken with a Hitachi-S4800 at 20 000 times magnification (corresponding a total area of 140 μm2 for each NW array) using the same methodology as in previous work.44 At least ∼1400 branches were measured in each array. Sample measurements from SEM images can be seen in the Supporting Information. A Bruker D8 Discover with HiStar area detector was used to acquire pole figures of the (400), (222), and (440) XRD peaks. GADDS (Bruker) software was used to process pole figure data. Transmission electron microscopy (TEM) analysis was performed on a JEOL JEM-ARM200F spherical aberration corrected STEM/TEM, equipped with a cold-field emission gun (c-FEG) and operated at 200 kV accelerating voltage. Cross-sectional TEM samples were prepared using a JEOL EM09100IS Ion Slicer with 6 kV argon ion beam for initial polishing and 1 kV for final polishing. Scanning helium ion microscopy (SHIM) (Zeiss ORION Plus) was used to obtain images of nanotree arrays on YSZ substrates to overcome charging effects that obscure SEM images. SHIM images from multiple perspectives are shown for ITO nanotree arrays grown on YSZ at 1 nm/s and α = 85° in Figure 1 with (a−d) ϕ = 45° and (e−h) ϕ = 0°. The nanotrees appear to be vertically and azimuthally aligned. As expected, branches grow preferentially on the side of the trunks facing the flux. The resulting films are composed primarily of L-shaped nanotrees (Figure 1b) for ϕ = 45° and T-shaped nanotrees for ϕ = 0°. Plan view images show a representative L-shaped nanotree for ϕ = 45° (Figure 1c) and a T-shaped nanotree for ϕ = 0° (Figure 1g). However, some nanotrees in the films do not adopt the representative shape. For example, under close inspection, a few L-shaped nanotrees can be observed in Figure 1f. These imperfections are likely a result of stochastic nucleation, which may result in nearby nanotrees shadowing facets that would otherwise be exposed to flux. Variations in self-shadowing environments could potentially be alleviated by prepatterning seeds to grow periodic arrays of nanotrees. In addition, T-shaped nanotrees with short branches facing away from the flux are present in Figure 1b, which may be a result of adatom migration around the trunk catalyzing back facing
limited by the dependence of in-plane NW alignment on the azimuthal flux position. Additionally, competitive growth results in a preferentially aligned nanotree ensemble but not a truly self-similar and precisely aligned array of nanostructures. In this work, branch placement is decoupled from in-plane NW alignment through epitaxial VLS growth of branched ITO NWs on lattice matched yttria-stabilized zirconia (YSZ) substrates (lattice mismatch less than 2% between aIn2O3 = 1.0118 nm and 2aYSZ = 1.026 nm).72−75 Epitaxial growth of ITO NWs on YSZ enforces both in-plane and out-of-plane crystal alignment.48,76,77 Therefore, the flux’s azimuthal configuration can be engineered to grow branches on select facets of the cubic NWs without affecting in-plane NW alignment. Prior to this work, branch growth in epitaxially aligned ITO NW arrays was equal in all four directions due to the isotropic nature of the vapor source. Herein, we demonstrate branch growth along select directions in epitaxially aligned nanotree arrays using VLS-GLAD. Controlling the azimuthal flux position with respect to the YSZ substrate’s in-plane [100] crystal direction (ϕ) enables branch growth aligned along two, three, or four in-plane directions. The result is nanotree arrays composed of L-, T-, or X-shaped branched NWs (when viewed from above). Controlled branching anisotropy provides an approach for the formation of directional interconnects between adjacent NWs. Thus, VLS-GLAD may enable the directed self-assembly of NW networks with designed conductive pathways required for the fabrication of new 3D NW device architectures. The nanotree arrays were found to be aligned across large areas by using X-ray diffraction (XRD) pole figure analysis and by measurements of branch length and orientation from plan view scanning electron microscopy (SEM) images spanning an area of 140 μm2 for each NW array. In addition, flux motion algorithms designed to exploit diffusion nonidealities in the VLS-GLAD process enables the elongation of branches selectively along one axis of the substrate. Strategies to guide the assembly of these shaped branched NW building blocks into 3D interconnected architectures are discussed. Electron beam evaporation of ITO source material (3−12 mm pieces of In2O3:SnO2, 91:9% mol; 99.99% purity; Materion Inc.) was performed at high vacuum (base pressure
