Guided VLS Growth of Epitaxial Lateral Si Nanowires - American

Jul 30, 2013 - The vapor−liquid−solid (VLS) technique is a proven method for growing nanowires (NWs) in a wide variety of semiconductor material s...
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Guided VLS Growth of Epitaxial Lateral Si Nanowires Somilkumar J. Rathi,† David J. Smith,‡ and Jeff Drucker*,‡ †

Materials Science and Engineering, School for Engineering of Matter, Transport, and Energy, Arizona State University, Tempe, Arizona 85287-6106, United States ‡ Department of Physics, Arizona State University, Tempe, Arizona 85287-1504, United States S Supporting Information *

ABSTRACT: Using the Au-seeded vapor−liquid−solid technique, epitaxial single-crystal Si nanowires (NWs) can be grown laterally along Si(111) substrates that have been miscut toward [112]̅ . The ratio of lateral-to-vertical NWs increases as the miscut angle increases and as disilane pressure and substrate temperature decrease. By exploiting these trends, conditions can be identified whereby all of the deposited Au seeds form lateral NWs. Growth is guided along the nanofaceted substrate via a mechanism that involves pinning of the trijunction at the liquid/solid interface of the growing nanowire. KEYWORDS: Epitaxy, guided growth, miscut, in-plane or lateral Si nanowires, nanofaceted substrate, VLS

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reflectometry system that is capable of detecting the onset of vertical Si NW growth.14 Wires were grown on 0.4 cm × 3.3 cm Si strips that were cleaved from n-type, on-axis, or miscut Si(111) (2.5°, 4°, or 10° tilted toward [112̅]) wafers with resistivity ρ = 0.01−0.05 Ω cm. The long axis of the strips was along ⟨112⟩. The substrates were resistively heated with direct current. Substrate temperature was measured using an infrared pyrometer (Omega IR 2C) with an accuracy of ±10 °C. Clean, oxide-free surfaces were obtained using standard UHV methods. The samples were degassed at T = 650 °C followed by flash desorption of the native oxide at T = 1250 °C. Au was deposited onto freshly cleaned Si substrates from an alumina-coated W basket. The Au deposition rate was calibrated using Rutherford backscattering spectrometry (RBS) and monitored during deposition using a quartz crystal microbalance. In order to identify any size-dependent behavior, we followed a previously described7 two-step Au deposition procedure to form a bimodal distribution of Au island sizes. Au was deposited at a 3/4 ML/min rate (1 ML = 7.84 × 1014 atoms/cm2, which is the atomic density of bulk-terminated Si(111)). Three ML was deposited while the substrate was heated to T = 550 °C, and then T was abruptly reduced to 450 °C while a further 1 ML was deposited. This procedure resulted in an Au island ensemble with 107 cm−2 islands with ∼160 nm diameter and 109 cm−2 islands with ∼60 nm diameter. Immediately following Au deposition, the substrate was transferred under vacuum to the NW growth chamber, and nanowires were grown using undiluted disilane (Si2H6) as the precursor gas. Growths were carried out at substrate temper-

he vapor−liquid−solid (VLS) technique is a proven method for growing nanowires (NWs) in a wide variety of semiconductor material systems.1,2 Typically, NWs grow normal to the substrate upon which the catalytic seed particles have been deposited, rendering their integration into planar device technology problematic. Usually, these so-called “vertical” NWs must be removed from the growth substrate for assembly into a planar device architecture. An alternate approach would be to directly assemble the NWs on a substrate by identifying conditions where they follow a lateral, rather than vertical, growth trajectory. This strategy has proven useful for growth of lateral III−V and ZnO NWs.3−6 The morphology of Si NWs grown via the VLS technique using UHV CVD, the method employed here, can be affected by processing conditions.7−10 Growth direction can be controlled by surface chemistry,11 and the diameter can be varied along the NW length via growth interruption.12 Here, we demonstrate that single-crystal, epitaxial Si NWs can be grown directly onto Si(111) substrates that have been misoriented toward the [112̅] direction. Our experimental observations indicate that the nanofaceted “hill and valley” surface morphology13 formed when Au is deposited onto the misoriented substrate is crucial for lateral NW growth in this system. For the range of growth conditions explored, lateral NWs were never found growing along on-axis Si(111) substrates. In contrast, we found that the fraction of NWs that grew laterally increased as the misorientation away from (111) increased. By varying seed diameter, precursor pressure, and substrate temperature, we have identified growth conditions for which all NWs catalyzed by deposited seeds grow parallel to the substrate. All the Si NWs were grown using the VLS method in a home-built, multichamber ultrahigh-vacuum chemical vapor deposition (UHV CVD) system with a base pressure of 2 × 10 −10 Torr. The system is equipped with an optical © 2013 American Chemical Society

Received: May 29, 2013 Revised: July 3, 2013 Published: July 30, 2013 3878

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atures of 400, 450, and 500 °C with disilane pressures in 0.05− 1.0 mTorr range, with a fixed flow rate of 0.2 sccm (sccm denotes cubic centimeter per minute at STP). Si NW morphology was characterized using field-emission scanning electron microscopy (FESEM) (Hitachi S-4700 operated at 15 kV) and atomic force microscopy (AFM) in tapping mode. Both plan-view and cross-sectional transmission electron microscopy (TEM) were used to confirm the crystallinity and epitaxial relationship to the substrate of the lateral Si NWs. Micrographs were acquired using a JEM4000EX operated at 400 keV with a structural resolution of ∼1.7 Å. Several aspects of Au epitaxy on Si(111) are relevant to the growth of lateral Si NWs. The Au/Si(111) system follows the Stranski−Krastanov growth mode.15,16 3D Au islands form atop a 1 ML thick √3 × √3-R30° Au/Si(111) layer (hereafter √3 layer). Previously, we have found that this √3 layer dewets when exposed to disilane at pressures above a substrate temperature-dependent minimum pressure, Pmin(T). Pmin(400 °C) = 0.7 mTorr and Pmin(500 °C) = 10 mTorr.17 The dewet Au agglomerates to form small (sub-10−20 nm diameter) islands that can then serve as seeds for VLS growth of small diameter Si NWs. Au growth onto heated Si(111) miscut toward [112̅] produces a nanofaceted surface with (111) and (hhk) planes forming a “hill and valley” structure.13 The hilltops and valley bottoms are aligned along ⟨110⟩ directions on the surface, perpendicular to the miscut direction. At an Au coverage of 1 ML, identical to that in the √3 layer between the 3D Au/Si(111) islands, the hill and valley structure is comprised of (111) and (331) planes.13 We find a significant decrease in Pmin(T) for disilane-induced dewetting of Au from the miscut surfaces investigated here. That is, the Au layer between the 3D islands dewets and agglomerates to form small islands at a pressure much less than Pmin(T) for disilaneinduced dewetting of the √3 Au/Si(111) layer. For example, growth at 400 °C and 5 × 10−5 Torr disilane on 10° miscut Si(111) can produce a high density of small NWs that form via this disilane-induced dewetting mechanism. This represents an order of magnitude depression in Pmin(T) compared to growth atop on-axis Si(111) but with much slower dewetting kinetics. These small islands then seed VLS growth of both lateral and vertical NWs. While further work is required to conclusively identify the mechanism for this phenomenon, we speculate that the observed behavior may be related to either the high edge density on the nanofaceted substrate or a significant reduction in Pmin(T) for dewetting from the higher energy (331) surface. Figure 1 shows a scanning electron micrograph of a NW ensemble formed by heating 160 nm diameter Au seeds

deposited onto a Si(111) substrate misoriented by 10° toward [112̅] to 450 °C and exposing them to 0.2 mTorr of disilane. Note that the lateral NWs are all parallel and that about half grow to the left, along [11̅0], while the other half grow to the right, along [11̅0]. Since the lateral NWs show no preference for growth along opposite ⟨110⟩ directions, we believe that neither temperature gradients nor electromigration-related effects play a role in determining their growth trajectory. The Si strips are clamped between Ta electrodes that are cooler than the hotter center region of the resistively heated sample. Thus, current flow, and any temperature gradient, are along the ⟨112⟩ direction, which is perpendicular to the lateral NW growth direction. Note also that the lateral NWs are not all the same length, which is discussed in more detail below. We observed growth of lateral NWs on 2.5°, 4°, and 10° miscut substrates. All lateral NWs grown on these substrates incorporate a (111) plane that is inclined by the miscut angle to the average orientation of the nanofaceted surface. Very few lateral NWs grew on 2.5° miscut substrates, but their morphology is similar to those that grew on 4° miscut substrates. Figure 2 displays the morphology typical for lateral Si NWs grown on 4° miscut surfaces. Figure 2a is an amplitudemode AFM image of a lateral NW grown by exposing a ∼160 nm diameter Au seed to 0.2 mTorr of disilane while the substrate was heated to 450 °C. Figure 2d is a SEM image of a lateral NW grown under identical conditions except for the substrate temperature of 400 °C. The linear contrast features running along ⟨110⟩ from the top to bottom of each image are the hilltops and valley bottoms of the nanofaceted substrate. The (111) plane is visible as the flat region running the entire length of the NWs. The Au seed is clearly visible near the top of each NW. Note that, except for a region near the “base”, the NW grows perpendicular to the miscut direction and is guided by the nanofaceted hill and valley structure of the substrate. Figure 2b displays a higher resolution height mode AFM image of the base of the NW shown in Figure 2a. As growth initiates, the NW forms a ramp as it grows along [112]̅ , perpendicular to the substrate facets. As shown in the height profiles of Figure 2c, which were acquired along the indicated lines in Figure 2b, the base of the NW has a triangular cross section with the (111) plane atop the NW intersecting the substrate. Away from the base, the (111) plane atop the NW no longer intersects the substrate and the NW exhibits a polygonal cross section. Note that the intersection of the lateral NWs with the substrate and the edges of the (111) plane at the NW top are rough and that this roughness correlates well with that of the roughness observed along the substrate nanofacets. This roughness was observed in all lateral NWs grown on 4° miscut surfaces. As discussed below, this observation suggests that the liquid seeds are guided along the substrate nanofacets as the NWs grow. Figure 3 displays the lateral NW morphology typical for growth on 10° miscut surfaces. In contrast to the lateral NWs grown on 4° miscut substrates, these NWs grow with straight edges, as seen in Figures 3a,b. The straight edges at the intersection of the lateral NWs and the substrate reflect the straight edges of the substrate facets. Note also that these NWs follow the substrate facets for their entire length. In contrast to the lateral NWs grown on 4° miscut surfaces, they do not begin growing perpendicular to the substrate facets. Similar to the NWs grown on 4° miscut substrates, these NWs incorporate a (111) plane, as indicated on the AFM image shown in Figure 3b. Since this (111) plane remains in contact with the substrate

Figure 1. SEM image of a NW ensemble acquired 70° away from [111] toward [112̅] showing both lateral and vertical NWs. About half of the lateral NWs grow in opposite ⟨110⟩ directions. Note that the lateral NWs have different lengths. 3879

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Figure 2. Lateral NWs grown on 4° miscut substrates. (a) AFM image of lateral NW and (b) higher resolution AFM image of its base. (c) AFM line profiles along the lines indicated in (b) showing that the (111) plane at the top of the NW contacts the substrate at the base giving the NW a triangular cross section. Away from the base, the (111) plane does not make contact with the substrate and the cross section is polygonal. (d) SEM image of lateral NW acquired 70° away from (111) toward [110]. In (a), (b), and (d) the linear contrast features on the substrate running from top to bottom of the images is due to the “hill and valley” structure of the nanofaceted substrate. Note the rough edges of the NWs and that its length scale is the same as the edge roughness of the substrate nanofacets.

Figure 3. Lateral NWs grown on 10° miscut substrates. (a) SEM image of lateral NW acquired 70° away from (111) toward [112̅]. (b, d) AFM images of lateral NWs from the same sample but with different lengths. The NWs shown in (a), (b), and (d) have straight edges, mirroring the straight edges of the substrate nanofacets. Note that the steps at the NW base evident in (b) and (d) are continuations of substrate nanofacets onto the (111) plane of the lateral NW. Note also in (d) that the left-hand edge of the NW intersects a valley bottom on the substrate. (c) Cross-sectional TEM image of lateral NW acquired along ⟨110⟩ with inset atomic-resolution detail of the NW/substrate interface showing that the NW is epitaxial with the substrate and that the NW/substrate interface is Au-free although there are some Au nanoclusters along the NW and substrate surfaces.

10° substrates on the side of the NW that is bound by a (111) plane. In both Figures 3b,d, these steps at the NW base correspond exactly with the nanofacets on the substrate. That is, each step at the NW base is a continuation of a substrate nanofacet. These steps at the NW base make an angle of 60° with the direction of the hilltops of the substrate nanofacets, so they are also running along a ⟨110⟩-type direction.

as the NW grows, the NW cross section is triangular along its entire length. Figures 3b,d display the bases of two NWs grown on the same 10° miscut substrate at a temperature of 400 °C and a disilane pressure of 0.2 mTorr. These images highlight the role of the faceted substrate in lateral NW growth. First, note the steps at the NW base on the left-hand side of each NW. These steps are always found at the bases of the lateral NWs grown on 3880

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The side of the lateral NWs grown on 10° miscut substrates that is opposite to the one with the (111) plane appears planar. However, careful measurements on 15 NWs of the angle that it makes to the (111) plane indicate that it is not a low-index facet. Interestingly, the angle that this plane makes with the opposing (111) facet usually changes as the NW lengthens. After growing to an average length that is slightly greater than the NW width, the internal angle this plane makes to the (111) plane increases in 80% of the NWs measured from an average of 26° to an average of 33°. For 15% of the NWs measured, the internal angle decreases by an average of 3°, and in the remaining NWs, the internal angle between the planes bounding the NW does not change as it grows. This change in angle between the planes bounding the lateral NWs grown on 10° substrates translates to a change in their width. The change in width is quantized in units of the substrate nanofacet periodicity. As discussed in more detail below, we believe that the steps observed at the base of lateral NWs grown on 10° substrates along with the observation that their width is defined by the substrate periodicity point to the crucial role of substrate morphology in guiding lateral NW growth. Figure 3c shows a cross-sectional TEM image in the ⟨110⟩ projection of a lateral NW grown on a 10° miscut substrate at T = 450 °C and P = 0.2 mTorr. The inset atomic-resolution TEM image that details the NW/substrate interface indicates that the lateral NWs are perfectly epitaxial with the substrate. There are no defects evident at the NW/substrate interface. Additionally, the absence of any dark contrast at the NW/substrate interface suggests that it is Au-free. We interpret the dark contrast along the NW surface and the adjacent substrate to indicate a thin coating of Au nanocrystals, which we expect for growth at these conditions.8,9 We hypothesize that as the lateral NW grows along the substrate, the liquid AuSi droplet at its tip consumes Au from the planar layer in its path and subsequently redeposits the consumed Au onto its surface in the form of 2−4 atom thick, 3−5 nm diameter Au nanoclusters. If the amount of Au in the liquid droplet increased (or decreased) as the NW grew, the NW width would change along its length. Apart from the change in width observed in the very early stages of growth detailed above, NW widths are nearly constant along their lengths. Analysis of cross sections similar to those shown in Figures 2c and 3c indicates that the cross-sectional area of lateral NWs is about 10% of the cross-sectional area of vertical NWs grown from seeds of the same size. This difference is indicative of the dissimilarity in the morphology of lateral and vertical Si NWs. Vertical NWs grown at the conditions that produce a significant fraction of lateral NWs are oriented along ⟨111⟩,10 have approximately hexagonal cross sections, and are bound by facets with an average {112} orientation.18 In contrast, lateral NWs have the triangular or polygonal cross sections discussed above. An additional difference between vertical and lateral NWs is that vertical NWs have very narrow length distribution while the length of lateral NWs is continuously distributed from some maximum value down to zero. The broad length distribution of lateral NWs is evident from Figure 1 and also from Figures 3b,d, which show lateral NWs from the same growth. One is 600 nm long while the other is 375 nm long. Figure 4 quantifies this observation, comparing the lengths of vertical and lateral NWs grown at the indicated conditions. Substrate miscut, growth conditions, and the size of the deposited Au seeds all affect the fraction of Si NWs that grow laterally. Figure 5 shows the fraction of lateral NWs that grow

Figure 4. Length of vertical NWs vs NW diameter and length of lateral NWs vs width. The length distribution of vertical NWs is very narrow, but the length of lateral NWs is continuously distributed from a maximum value down to zero. The vertical NWs were grown atop on axis Si(111) at T = 500 °C and P = 0.2 mTorr. The lateral NWs were grown on a 10° miscut substrate at T = 450 °C and P = 0.2 mTorr. Both the lateral and vertical NW ensembles were grown from identically deposited Au seeds with a bimodal size distribution peaked near ∼60 and 160 nm formed by a two-step deposition procedure.

Figure 5. Percentage of lateral NWs at indicated growth conditions on Si(111) substrates miscut by 4° and 10° toward [112]̅ . The incidence of lateral NWs increases as miscut increases, substrate temperature decreases, and disilane pressure decreases. For growth at 0.05 mTorr and 400 °C on 10° miscut substrates, 100% of 150 nm diameter seeds produce lateral NWs.

from Au seeds deposited onto 4° and 10° miscut substrates at the indicated growth conditions. Au seeds deposited onto nominally on-axis Si(111) substrates produced only vertical NWs for any growth conditions. Growth on 2.5° miscut substrates yielded a small fraction of lateral NWs (about 1%). This result is not plotted in Figure 5. The fraction of lateral NWs that grow from deposited seeds increases as growth temperature decreases, disilane pressure decreases, and substrate miscut increases. By exploiting these trends, we have identified conditions that result in lateral NW growth from all of the deposited Au seeds. Growth on 10° miscut surfaces using 5 × 10−5 Torr of disilane pressure at T = 400 °C results in 100% lateral NW growth from 160 nm diameter Au seeds. For these growth conditions, the lateral NW growth rate is very slow, about 500 nm/h for the longest lateral NWs. During these very long growths, disilane induced dewetting of the Au layer on the nanofaceted hill and valley structure between the 3881

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with both seed size and substrate miscut may also be due to the ability of the liquid seed to dewet from the substrate. The Auinduced hill and valley morphology is composed of (111) and (331) nanofacets, and the density of (331) nanofacets increases with increasing miscut. Since the surface energy of the (331) planes is greater than that of the (111) planes, it is plausible that larger miscut angles would facilitate lateral NW growth. Similarly, a larger seed would encounter more (331) nanofacets and this could facilitate more effective lateral NW growth. If this wetting-related mechanism were responsible for lateral NW growth, it could explain why both lateral and vertical NWs are observed for most growth conditions on miscut substrates. An instability of the liquid AuSi seed similar to that responsible for kinking of a ⟨111⟩-oriented NW to an alternate ⟨111⟩ direction may be responsible for determining whether a nascent NW follows a lateral or vertical trajectory.10 Some fraction of the ⟨111⟩ oriented NWs experience this instability and kink, and the fraction depends on growth conditions. By analogy, some growth-condition-dependent fraction of nascent NWs experience some instability (perhaps related to failed edge formation as discussed in refs 20 and 21) and grow laterally, while others remain unperturbed and grow vertically. Those that experience this instability while growing atop a miscut substrate then find it energetically favorable to follow a lateral trajectory due to the higher energy substrate encountered than while growing atop an on-axis Si(111) substrate. Finally, we address the experimental observation that vertical NWs have narrow length distributions while lateral NWs have broad length distributions. We believe that this phenomenon is related to delayed growth of some lateral NWs. Since NWs must incorporate Si at a rate which is related to the liquid/ vapor surface area and the disilane decomposition rate, there is no reason to believe that lateral NWs have different growth rates. Thus, we believe that lateral NWs have the same growth rates and that the broad length distribution reflects a distribution of growth times. This distribution of growth times could result from a delay in growth of lateral NWs caused by the craters that form beneath the thermally deposited Au seeds.23,24 Figure S3 displays AFM images of these craters, which show that there are crater rims that extend above the substrate surface that may impede growth for some lateral NWs. As shown, the crater rims extend for different heights above the substrate surface, and this height can vary around the perimeter of the rim. It is plausible that the variation in height above the substrate of this crater lip may impede the growth of some lateral NWs and produce the broad length distribution that is observed. In summary, using the Au-seeded VLS technique, epitaxial single crystal Si NWs can be grown laterally along Si (111) substrates that are miscut toward [112̅]. The fraction of lateral NWs increases with increasing miscut, decreasing disilane pressure and decreasing substrate temperature. By exploiting these trends, we have identified growth conditions for which lateral NWs grow from 100% of the deposited seeds. These NWs are guided by the nanofaceted hill and valley structure of the substrate that forms during deposition of the Au seeds. These lateral NWs combine growth and assembly into a planar architecture into a single step, which may prove beneficial for some applications. Our results should also provide guidance for the growth of lateral NWs on nanofaceted substrates in other material systems.

deposited seeds forms small islands that catalyze growth of a high density (109/cm2) of vertical NWs. Our observations indicate that substrate morphology is a crucial factor in the growth of lateral NWs. First, lateral NWs never grow atop on-axis Si(111) substrates, and their incidence increases with increasing miscut toward [112̅]. Second, the lateral NWs are templated by the Au-induced hill and valley topography of the nanofaceted miscut substrates. Their growth is guided along the nanofacets, and their edge roughness reflects that of the substrate nanofacets. We attribute these behaviors to impact by the substrate nanofacets on stability of the trijunction at the liquid/solid interface of the growing NW. Trijunction stability during NW growth has been previously considered19 and included in a twodimensional model of NW growth that captures many experimentally observed morphologies, including lateral growth of “crawling” NWs.20,21 In this 2D model, lateral growth initiates when the liquid seed rolls off the top of a nascent NW due to failed edge formation at the transition from the tapered base20,22 to vertical side walls. Our experimental observations cannot conclusively identify the mechanism leading to lateral NW growth. However, if it is due to this failure of edge formation, we speculate it may be related to the steps observed at the bases of the lateral NWs shown in Figures 3b,d. An alternate possibility related to dewetting of the liquid AuSi droplet at the onset of growth is discussed below. Once lateral growth begins, it is guided by the substrate nanofacets, as is evident in Figures 2 and 3. As the liquid seed crawls along the substrate, its advancing contact line is unconstrained by any edges. In contrast, the trijunction is constrained by edges on the nanofaceted substrate normal to the growth direction. We believe that the trijunction pins more effectively at hilltops rather than at valley bottoms, as detailed in the Supporting Information. Our belief that the trijunction pins at hilltops is supported by Figure 3d. The left-hand edge of the depicted NW, which will form a (111) plane after further growth, clearly intersects the substrate at a valley bottom. This morphology would result if the trijunction pinned at a hilltop and the liquid/solid interface advanced away from the valley bottom adjacent to the NW. This scenario is schematically depicted in Supporting Information Figure S2. Analysis of SEM images similar to Figure 2d indicates that lateral NWs grown on 4° substrates also intersect the substrate at valley bottoms. The obvious similarities in the edge profiles of the lateral NWs and the roughness observed along the substrate nanofacets, as seen in Figure 2d, may also be attributed to trijunction pinning at substrate hilltops. In addition, it is likely that the change in lateral NW width observed at the beginning of growth on 10° substrates is due to droplet pinning at a hilltop adjacent to the one at which it was pinned at the onset of growth. We now discuss an alternate mechanism leading to lateral NW growth related to wetting of the liquid AuSi droplet at the onset of growth. We have previously detailed the role of disilane pressure in the wetting and spreading of liquid AuSi.8,9,17 Lower pressures facilitate wetting and spreading while higher pressures inhibit spreading and facilitate dewetting. Our observation that lower disilane pressures lead to a higher fraction of lateral NWs shown in Figure 5 may be related to these phenomena. That is, growth at lower disilane pressures may reduce the propensity for the liquid seed to dewet from the substrate allowing more effective growth of lateral NWs. In addition, the reason that the fraction of lateral NWs increases 3882

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ASSOCIATED CONTENT

S Supporting Information *

Trijunction pinning at hilltops; Figures S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*E-mail jeff[email protected] (J.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Eric Dailey contributed to the early stages of this work. This work was supported by the Department of Energy under Award DE-FG02-06ER46345.

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REFERENCES

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