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Capillarity-Driven Welding of Semiconductor Nanowires for Crystalline and Electrically Ohmic Junctions Thomas A. Celano, David John Hill, Xing Zhang, Christopher W. Pinion, Joseph D Christesen, Cory J. Flynn, James R. McBride, and James F. Cahoon Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b02361 • Publication Date (Web): 26 Jul 2016 Downloaded from http://pubs.acs.org on July 29, 2016
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Capillarity-Driven Welding of Semiconductor Nanowires for Crystalline and Electrically Ohmic Junctions Thomas A. Celano1,†, David J. Hill1,†, Xing Zhang1, Christopher W. Pinion1, Joseph D. Christesen1, Cory J. Flynn1, James R. McBride2, and James F. Cahoon1* 1
Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-
3290, USA, 2Vanderbilt Institute of Nanoscale Science and Engineering, Vanderbilt University, Nashville, Tennessee 37235, USA * J.F.C. (
[email protected]) † Equal contribution
Semiconductor nanowires (NWs) have been demonstrated as a potential platform for a wide-range of technologies, yet a method to interconnect functionally-encoded NWs has remained a challenge. Here, we report a simple capillarity-driven and self-limited welding process that forms mechanically robust and Ohmic inter-NW connections. The process occurs at the point-of-contact between two NWs at temperatures 400-600 °C below the bulk melting point of the semiconductor. It can be explained by capillarity-driven surface diffusion, inducing a localized geometrical rearrangement that reduces spatial curvature. The resulting weld is comprised of two fused NWs separated by a single, Ohmic grain boundary. We expect the welding mechanism to be generic for all types of NWs and to enable the development of complex interconnected networks for neuromorphic computation, battery and solar cell electrodes, and bioelectronic scaffolds. Keywords: silicon nanowires, capillarity-driven welding, percolation network, Ohmic junction, 1 ACS Paragon Plus Environment
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surface diffusion, finite-element simulation
Nanowires (NWs) composed of semiconductors and metals have been widely developed for applications in electronics, photonics, energy, and biology.1,2 These applications are often demonstrated using small, proof-of-concept systems; however, further advancements will require integration on a massive scale, connecting millions of NWs into device structures. To enable these applications, substantial effort has been focused on NW directed-assembly3 and selfassembly methods, which include flow-alignment,4 mechanical transfer printing,5 dip coating,6 electric-field assisted placement,7 and top-down patterning strategies.8 These methods generally rely on conventional lithographic fabrication to form the electrical connections between NWs. Here, we instead demonstrate a process to synthesize electrical connections between semiconductor NWs on a growth wafer by a capillarity-induced welding process. For metallic NWs, electrically-active networks have been developed as a class of transparent and conductive thin films with silver9 (Ag), copper10 (Cu), and gold11 (Au) NWs, as well as carbon nanotubes.12 Ohmic connections between these components have been formed through a variety of techniques,13 including cold welding,14 plasmonic welding,15 thermal annealing,16 mechanical pressure,17 diffusion bonding,18 and electron-beam induced welding.19 In comparison to metallic NWs, however, semiconductor NWs could offer a wide range of more advanced functionality by encoding field-effect transistors,20 p-n junctions,21 and memory bits22 within the individual NWs of the network.23 Flexible meshes of silicon (Si) and germanium (Ge) NWs have been fabricated,24-28 but the structures are electrically-inactive or exhibit high resistances and non-linear current-voltage (I-V) curves27,28 as a result of insulating oxide layers or organic capping ligands between the
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NWs. The formation of electrically-active connections between semiconductor NWs grown by a vapor-liquid-solid (VLS) mechanism has been limited to junctions formed either by electrical biasing individual wires,29 patterning NWs to intersect during the VLS process,30-32 or using a multi-step VLS process to create branched nanowires.33-35 These strategies are generally limited to a low number of NWs and interconnection points. Here, we demonstrate a complementary method to join distinct semiconductor NWs with crystalline and Ohmic junctions after VLS growth. In this process, as illustrated in Figure 1a, NWs are first grown by the VLS mechanism using Au catalysts and then collapsed using liquid capillary forces to yield multiple inter-NW points-of-contact on each NW.36 Finally, the NWs are welded by a capillarity-driven surface diffusion process for ~4 minutes at temperatures 400-600 °C below the bulk melting point (see Methods in the Supporting Information for all experimental details), yielding inter-NW junctions over the entire growth substrate (see the SEM image in Figure S1). For the Si and Ge NWs used in this study, the VLS process was performed using a twostep growth procedure that avoids vapor-solid overcoating on the wire surface.22 NWs were typically doped with phosphorus (P) to create degenerately-doped n-type Si. The collapse process was performed with liquid nitrogen to avoid formation of an oxide layer on the wire surface prior to welding. After completion of the welding process, the NW networks were exposed to ambient conditions, causing formation of a 2-3 nm native oxide on the surface. Figure 1b shows a high-resolution transmission electron microscopy (HR-TEM) image and an energydispersive x-ray spectroscopy (EDS) elemental map of oxygen (O) for two welded junctions (denoted by arrows in the TEM image and labeled 1 and 2 in the EDS map) formed between three crossed NWs. The welding process reshapes the points-of-contact between NWs, and a clear junction is observed.
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A TEM image and three separate EDS elemental maps of Si, O, and P are displayed in Figure 1c for a single, welded junction. The images highlight several features. First, the TEM image shows an increase in the NW volume in the vicinity of the junction, and the Si map shows that the junction region contains approximately twice the Si content of the intersecting NWs. Second, the P map shows that the P dopants are uniformly distributed throughout the wire and junction without any segregation at surfaces or interfaces. Third, the O map shows that a native oxide layer has formed uniformly on the surface of the wires and the junction but not at the interface between the two wires, as evidenced by the O signal at the junction being equivalent in amplitude to the signal along the individual NWs. A combined Si and O map, and corresponding elemental line scan, of a second welded junction (Figure 1d) further confirm the absence of an oxide layer at the interface between the wires. In addition, elemental line scans (Figure S2) of the NWs in Figure 1b show similar behavior, indicating that the intersection points labeled 1 and 2 in Figure 1b are welded whereas the intersection points labeled 3 and 4 are not. The images in Figure 1 suggest that the two wires have locally fused to form a compositionally-uniform and oxygen-free interface. HR-TEM images of a second crossed junction (Figure 2a) corroborate this observation. Lattice-resolved images (Figure S3) indicate that both NWs in Figure 2a grew in the [112] direction, and the junction consists of the two crystal structures overlaid (depicted schematically in the lower left panel of Figure 2a) without any polycrystalline or amorphous material. The junction formed by this welding process is morphologically similar to the junctions observed in crossed Ag NWs as a result of plasmonic heating15 or electroformation.37 Additional TEM images for two adjacent, welded NWs (Figure 2b) show that the process fuses the two wires, leaving a single zig-zag grain boundary. We examined the diameter and temperature dependence of the welding process, as shown
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in Figure 3a-c. The temperature required for welding shows a strong dependence on diameter, and the qualitative results from multiple NW diameters and temperatures are summarized in Figure 3a. Three regimes were assigned from qualitative analysis of SEM images: absence of welding, optimal welding, and structural instability. Optimal welding consisted of NWs that formed obvious junctions at the NW intersections, without substantial reductions in the NW diameter, and exhibited no other structural changes outside the junction region. Structural instability consisted of NWs that exhibited substantial reductions in diameter and obvious structural changes outside the junction region, as indicated by additional SEM images in Figure S4. As shown by the SEM images in Figure 3b, NWs with diameters of ~30, ~50, and ~100 nm welded at temperatures of ~840, ~840, and ~875 °C, respectively. For each diameter, temperatures ~25 °C higher resulted in structural instability at the junction whereas temperatures ~25 °C lower resulted in no noticeable morphological change, as exemplified by the SEM images in Figure 3c for NWs ~80 nm in diameter. We also examined the morphology of the weld as a function of the process time. Although 4 minutes was typically used, longer weld times up to 120 minutes resulted in no additional change to the junction or NW morphology for the optimal weld conditions (Figure S5), indicating that the junction formation is a self-limited process. In addition, no welding was observed for NWs that had been exposed to ambient conditions prior to the weld process, an observation we attribute to formation of a thin native oxide layer that inhibits the welding mechanism (Figure S6). The welding process was also performed on Ge nanowires (Figure 3d), and a substantially lower temperature of 600 °C was necessary to create high-quality junctions. The results on Si and Ge NWs suggests that the weld process is successful with high-quality, oxide-free NW surfaces at temperatures ~400-600 °C below the
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bulk melting temperature of the NW material. The strong dependence of the optimal weld temperature on diameter suggests that the weld results from a physical process that depends on surface curvature. Capillarity-induced surface diffusion, a process in which a change in curvature creates a free-energy gradient that drives atoms away from areas of high curvature, is well-known to occur in spherical and cylindrical nanostructures.38 The effects of this phenomenon include the blunting of W tips39 and necking or sintering of nanoparticles.40 To understand the possible role of this phenomenon for the welding of NWs, we modeled the diffusion process for realistic NW geometries.38 The surface diffusion flux, Js, is related to the local curvature, K, by: Js = − (Ds(T)γΩ2/3/kBT)∇sK ,
(1)
where K is (1/R1+1/R2), R1 and R2 are the principle radii of curvature for a two-dimensional surface, Ds(T) is the temperature-dependent surface diffusion coefficient, γ is the surface tension (1.6 N/m), Ω is the atomic volume of 0.020 nm3, kB is the Boltzmann constant, T is temperature, and ∇sK is the surface gradient of the curvature. The velocity of the surface normal, vs, which describes the deformation of the NW junction as an expansion (positive velocity perpendicular to the surface, increasing diameter) or contraction (negative velocity perpendicular to the surface, decreasing diameter), can be calculated simply as: vs = Ω2/3Js .
(2)
We performed three-dimensional simulations of the welding process (Figure 4) using a surface-evolving simulation41 combined with finite-element calculations of vs (see Methods in the Supporting Information). As shown in the initial simulation image (labeled I in Figure 4a and 4b) for a 100 nm diameter NW at a T of 850 °C and corresponding D of 1.8 x 10-12 m2/s,42 a large surface diffusional flux toward the junction area is predicted, producing a vs as high as 6 ACS Paragon Plus Environment
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~112 nm/s. The process drives the system to reduce curvature by forming the welded geometry, as shown in the second and third simulation snapshots in Figure 4a with maximum vs of ~10 and ~3 nm/s, respectively. The weld eliminates the high curvature interface between the two wires, as shown by the cross-sectional profiles in Figure 4b. The dramatic reduction of vs as the weld evolves explains the apparent self-limited nature of the mechanism over the time scale (typically 4 minutes) of the process. However, the welding mechanism is self-limited only if the initial formation of the weld at the point of intersection between the two NWs occurs on a time scale (minutes) much shorter than structural instability (hours to days). This condition is fulfilled for the optimal weld conditions at each diameter but not for the higher temperature conditions that exhibit structural instability. As indicated by the heatmap of vs in Figure 4c, the CVD temperature and NW diameter strongly influence the magnitude of vs at the initial stage of the weld. The higher curvature in smaller-diameter crossed junctions causes a higher vs, and this effect gives rise to the diameterdependence of the process (cf. Figure 3a). In addition, the presence of three regimes—no welding, optimal welding, and structural instability—over a relatively narrow temperature range is explained by the strong temperature dependence of vs. At higher temperatures, vs remains significant even for the welded geometry (Figure S7), causing further structural evolution that is consistent with the instability observed at higher temperatures. This temperature dependence originates from the Arrhenius dependence of Ds(T), which for Si varies by one order of magnitude from 800 to 900 °C.42 The welded junctions predicted by this model are consistent with the local sintering of Ag NWs observed upon thermal annealing, and, in addition, the structural instability is consistent with the onset of Plateau–Rayleigh instability43 and the spheroidization of Ag NWs upon extended annealing.16 Furthermore, the capillarity-driven
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mechanism explains the absence of any morphological change in regions far from the junction that have nearly uniform curvature, for which vs is approximately zero because there is negligible change in curvature (i.e. ∇sK is zero). We probed the electrical transport properties of a single junction by fabricating electrodes (Figure 5a) on two crossed NWs degenerately-doped at an encoded P doping level of 2.5 x 1020 cm−3.44 The I-V curve (Figure 5b) collected using a four-point-probe configuration is linear and yields a junction resistance of 69 ± 4 kΩ (see Table S1 in the Supporting Information for details of resistance measurements). This junction resistance indicates that the welding process forms relatively low-resistance and Ohmic connections between NWs. The resistance of the junction is approximately equal to the resistance associated with a ~30 µm length of a 100 nm diameter NW, using the measured NW resistivity of ~0.0018 Ω·cm. The ratio of junction resistance to wire resistance is comparable to the ratio observed in Ag NWs,45 and for lower Si doping levels and thus higher resistivity wires, the junction resistance is expected to be negligible compared to the NW resistance. In conclusion, we have demonstrated the first instance of capillarity-driven welding of semiconductor NWs to create crystalline and electrically Ohmic interconnects between NWs. Because of the capillarity-driven surface-diffusion mechanism, we expect this welding process to be generic for all types of semiconductor NWs. Controllable synthetic doping distinguishes semiconductor NWs from metallic analogs and future research should focus on exploiting this distinction to generate interconnected NWs with both electronic and material junctions. We expect that welded semiconductor NWs can be developed into large-area networks and will find application as solar energy and battery electrodes,46 bioelectronic scaffolds,47 and a potential platform for neuromorphic computation.23
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Acknowledgements This work was primarily supported by the National Science Foundation (NSF) under award DMR-1308695. We also acknowledge the support of personnel (C.J.F) by the UNC Energy 11 ACS Paragon Plus Environment
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Frontier Research Center (EFRC) “Center for Solar Fuels,” an EFRC funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DESC0001011. C.W.P. and D.J.H. acknowledge individual NSF graduate research fellowships. J.F.C. acknowledges a Packard Fellowship for Science and Engineering and a Sloan Research Fellowship. This work made use of instrumentation at the Chapel Hill Analytical and Nanofabrication Laboratory (CHANL), a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), which is supported by the NSF (ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure (NNCI). Supporting Information Methods, Figures S1-S7, Table S1, and details on the calculation of junction resistance. Author Information Corresponding Author *Email:
[email protected] Notes The authors declare no competing financial interests.
Figure Legends
Figure 1. Growth and welding of VLS-grown semiconductor NWs. (a) Schematic illustration of the three-step process—NW growth, collapse, and welding—to create conductive NW junctions. (b) Images of two welded NW junctions (indicated by arrows and labeled 1 and 2) as shown by a TEM image (upper) and EDS map of O (lower). Overlapped NWs labeled 3 and 4 are not welded; scale bars, 100 nm. (c) TEM image (upper left) and EDS maps of Si (upper right; red), P (lower left; green) and O (lower right; blue) for a single, welded junction; scale bars, 50 12 ACS Paragon Plus Environment
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nm. (d) Upper: overlaid Si (red) and O (blue) EDS maps of a single junction; scale bar, 10 nm. Lower: EDS line scan of Si (red) and O (blue) signals corresponding to the vertical average of the EDS signal in the white dashed boxed region of the upper image.
Figure 2. TEM imaging of crystalline NW junctions. (a) Upper left: low-magnification image of a NW junction; scale bar, 20 nm. Lower-left: schematic of the superimposed crystallographic directions of the two welded NWs shown in the upper panel. Parallel lines denote the {111} lattice planes observed in the HR-TEM images (see also Figure S3). Right-hand panels: HRTEM images of the regions 1 and 2 denoted by the blue and red boxes, respectively, in the upperleft panel; scale bars, 20 nm. Arrows and labels denote the crystallographic directions of the NWs. (b) Left: TEM image of two parallel, welded NWs; scale bar, 100 nm. Right: higher magnification lattice-resolved image of the region denoted by the red dashed box in left-hand panel; scale bar, 5 nm.
Figure 3. Diameter-dependence and generality of NW welding. (a) Qualitative diagram indicating the temperatures and average diameters for which the NWs successfully welded (green circles) or exhibited structural instability (red triangles) or no obvious structural change (blue squares), as determined by a qualitative survey and analysis of the growth substrate by SEM. Shaded regions and dashed boundaries denote the approximate threshold between different regimes. (b) Representative SEM images of NWs ~100 (upper), ~50 (middle), and ~30 nm (lower) in diameter that have been welded at temperatures of 875, 840, and 840 °C, respectively; scale bars, 100 nm. (c) Representative SEM images of wires ~80 nm in diameter in which the junctions exhibit instability (upper), a weld (middle), or no change (bottom) at temperatures of
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875, 850, and 820 °C, respectively; scale bars, 100 nm. (d) SEM image of two ~100 nm diameter Ge NWs welded at a temperature of 600 °C; scale bar, 200 nm.
Figure 4. Modeling of the capillarity-driven welding mechanism. (a) Three-dimensional simulation of the welding process showing initial (upper), intermediate (middle), and final (lower) steps in the junction formation process for 100 nm diameter NWs at a temperature of 850 °C. The color axis reflects the velocity of the surface normal, vs, with positive (red) and negative (blue) values indicating increasing and decreasing diameters, respectively; scale bars, 100 nm. (b) Cross-sectional profiles from three-dimensional simulations calculated in the plane that longitudinally bisects the upper NW and passes through the center of the junction; scale bars, 50 nm. Upper, middle, and lower images labeled I, II, and III correspond to the initial, intermediate, and final steps, respectively, in panel a. (c) Maximum vs calculated at the initial welding step (denoted I in panels a and b) as a function of temperature and diameter. The color axis reflects a logarithmic scale.
Figure 5. Electrical transport properties of a single, welded Si junction. (a) Left: SEM image of electrodes, labeled 1-4, on two NWs with a single welded junction; scale bar, 2 µm. Right: SEM image of the junction and the two electrodes adjacent to the junction; scale bar, 1 µm. (b) Four-point probe I-V measurement of the junction shown in panel a collected by sourcing current on electrodes 1 and 4 and measuring the voltage across electrodes 2 and 3.
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