The Bottom-up Growth of Edge Specific Graphene Nanoribbons

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The bottom-up growth of edge specific graphene nano-ribbons Meredith Nevius, Feng Wang, Claire Mathieu, Nicholas T Barrett, Alessandro Sala, Tevfik Onur Mentes, Andrea Locatelli, and Ed H. Conrad Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl502942z • Publication Date (Web): 25 Sep 2014 Downloaded from http://pubs.acs.org on October 1, 2014

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The bottom-up growth of edge specific graphene nano-ribbons M.S. Nevius,1 F. Wang,1 C. Mathieu,2 N. Barrett,2 A. Sala,3 T. O. Mentes,3 A. Locatelli,3 and E.H. Conrad1, ∗ 1

The Georgia Institute of Technology, Atlanta, Georgia 30332-0430, USA 2

CEA, IRAMIS/SPEC/LENSIS, F-91191 Gif-sur-Yvette, France 3

Elettra–Sincrotrone Trieste S.C.p.A., Basovizza, Trieste, Italy

Abstract The discovery of ballistic transport in graphene grown on SiC(0001) sidewall trenches has sparked an intense effort to uncover the origin of this exceptional conductivity.

How a ribbon’s edge termination, width, and topography influence its

transport is not yet understood. This work presents the first structural and electronic comparison of sidewall graphene grown with different edge terminations. We show that armchair and zig-zag terminated ribbons, grown from SiC, have very different topographies and interact differently with the substrate, properties that are critical to device architecture in sidewall ribbon electronics.

Keywords: Graphene, graphite, SiC, silicon carbide, graphite thin film, nanoribbons

∗

email: [email protected]

1

ACS Paragon Plus Environment

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Transport in graphene grown on the sidewalls of trenches etched in the SiC(0001) surface has shown remarkable ballistic transport with mobilities two orders of magnitude higher than theoretical limits for isolated 2D graphene (107 cm2 V−1 sec−1 ).[1] However, the physical origin of these high mobilities remains to be formulated. Current theoretical models of ribbon electronic structure, based on simple hydrogen terminations,[2–6] cannot be presumed to apply even with the highly ordered edges produced in the sidewall ribbon bottom-up growth process.[7, 8] This is because the unique set of boundary conditions in sidewall graphene ribbons necessitates a more complicated view of how edge effects influence their band structure. On one side, sidewall graphene bends and terminates by bonding to the SiC, while on the other side, it seamlessly connects to a semiconducting graphene buffer layer. In addition to the edge bonding, a gradient exists in the onsite potential as the doping of graphene changes from n-type on the terrace tops to p-type on the sidewall facets.[9] This raises questions of whether or not transport in a protected edge state is sensitive to either the exact ribbon edge termination or the method by which it is grown. In this work, we begin the groundwork needed to address the origin of sidewall ribbons’ high mobilities by providing detailed structural and electronic information on the differences between armchair (AC) and zig-zag (ZZ) edge sidewall graphene ribbons. While there has been significant work on growing sidewall graphene on SiC step edges running in the < 11¯20 > direction (producing AC edge graphene ribbons),[7, 10–17] and some electronic structure measurements of ribbons with the same orientation,[9, 13] there has been surprisingly little work on graphene from SiC step edges with other orientations.[14] This is in part due to the tendency of intrinsic steps or H2 -etched SiC steps to facet or bunch into step edges running in the < 11¯20 > direction.[17, 18] Understanding how to produce ZZ edge graphene on SiC is in fact a critical step if AC and ZZ edge sidewall graphene ribbon transport is to be compared with theoretical models. Using a combination of low energy electron microscopy (LEEM), microspot-low energy electron diffraction (µ-LEED), x-ray photoemission microscopy (XPEEM), and micro-angle resolved photoemission spectroscopy (µ-ARPES), we have compared the growth and electronic structure of AC and ZZ graphene ribbon arrays. As we demonstrate below, ZZ ribbons with linear π-bands do not grow on the sidewalls like AC ribbons but instead grow along a narrow strip on the (0001) (Si-face) surface near the step edges that are perpendicular to the < 11¯20 >. 2


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FIG. 1. Orientation of the SiC trenches relative to the graphene. (a) The graphene lattice relative to the Brilluion zone in (b). (c-d) Schematic of a pre-graphene growth trenches with (c) AC and (d) ZZ sidewalls. AFM images of graphitized SiC trenches with (e) AC edge graphene and (f) ZZ edge graphene. Dark areas are the trench bottoms.

The graphene ribbons are grown from lithographically patterned trenches in SiC.[8, 19] The graphene ribbon edge direction can be controlled because graphene grows epitaxially rotated 30◦ relative to the SiC < 10¯10 > direction on the SiC(0001) surface.[20] When a trench in SiC is oriented perpendicular to the < ¯1100 > direction, the graphene that grows has its AC edge parallel with the step edge [see Fig. 1(a) and (e)]. For convenience we will refer to these SiC step edges as AC edge steps or AC facet walls. Conversely, when a SiC trench is oriented parallel to the < 1¯100 > direction, the graphene that grows has its ZZ edge parallel with the step edge [see Fig. 1(d)]. We will similarly refer to these SiC steps as ZZ steps. For these studies, the trench separation is P = 600 nm, the trench top width is t = 200 nm (trench bottom P − t = 400 nm), and the trench depth is 30-35 nm. Once these trenches are heated to grow graphene, the vertical trench walls facet.[21] From ARPES it is known that vertical trench walls on AC steps facet into (1¯107) or (1¯106) facets (tilted 28◦ and 33◦ relative to the (0001) Si-face, respectively).[9] There are no accurate measurements of preferred facets on ZZ steps, although AFM[21] and STM[22] experiments have estimated the facet angle as ∼ 28 − 30◦ corresponding to a range of (11¯2n) surfaces with n = 10 − 12. 3


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We begin with an analysis of AC edge ribbons. Figure 2(a), which shows a µ-LEED √ √ image of an AC ribbon array. The LEED shows the typical (6 3×6 3)R30◦ pattern of a √ graphene grown on the Si-face.[20] Besides the 6 3 spots from the (0001) surface, there are multiple spots that move in kk as a function of k⊥ (E) that are associated with diffraction rods from the faceted walls of the trenches. The (00)Facet facet rod, which appears near the edge of the Ewald sphere, is shown in the left insert of Fig. 2(a). The facet spots are seen √ more clearly in the right insets of Fig. 2(a) near the two 6 3 spots that lie on a line between the graphene (10)G and (0¯1)G spots. The right panel shows that these spots move in the direction of the n ˆ AC normal as the electron energy changes as expected for tilted AC facet diffraction rods. This is seen more clearly in Fig. 2(b) that shows a k⊥ versus kk cut through √ the same 6 3 spots. The tilted rods are from graphene on the facet walls. The facet angles measured from the tilted rods was found to be 32 ± 2◦ and 34 ± 2◦ . While an accurate absolute facet angle measurement is difficult because of facet order and aberrations in the image, the relative angle between the two pairs of tilted rods of 2◦ is consistent with the (1¯107) or (1¯106) facets previously determined by ARPES.[9] The µ-LEED therefore demonstrates that a structural AC-edge layer has flowed over and down the faceted trench walls. To topographically and electronically quantify the AC sidewall graphene we have used a combination of Dark-field (DF) LEEM and PEEM. To make the data easier to interpret, we have produced a composite image [see Fig. 2(c)] made from a DF-LEEM using the (0¯1)G graphene rod from the Si-face (circled in red in Fig. 2(a)) and DF-LEEM using the (¯10)FG facet graphene rod (circled in blue in Fig. 2(a)) for contrast. In the composite DF-PEEM in Fig. 2(c), graphene from the Si-face is imaged on a red/white contrast scale (red is a higher graphene coverage) while the graphene corresponding to the facet rod is imaged on a blue/white contrast scale (blue is a higher graphene coverage). Most of the graphene on the Si-face resides on the top (not the bottom) of the trenches. The width of the graphene covered area on the trench tops is slightly smaller than the as-patterned pre-growth width. The small shrinkage of the trench top width is due to mass diffusion as the vertical facet walls transform into more stable crystal facets, consistent with post graphene growth AFM images. An important point from the DF-LEEM images is that very little graphene grows in the bottom of the trenches [see Fig. 2(c)]. We believe this is a result of the reactive ion 4


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FIG. 2. (a) µ-LEED of AC ribbons. The image has a slight aberration. The normal to the step edges, n ˆ AC is marked. Two facet spots that disperse with energy (i.e., in k⊥ ) are shown in the magnified boxes on the right. The expanded box on the left, around the (1¯1)SiC SiC spot, shows the energy dispersing spot corresponding to the (00)Facet rod from the AC facet. (b) A k⊥ vs. kk cut through the LEED facet spots along a direction between the (10)G and (0¯1)G rods. Four tilted graphene rods are shown corresponding to two different facet angles on both the left, (¯10)FG , and right (0¯1)FG , sides of the trenches. (c) A composite image made by overlaying a DF-LEEM image of (0¯1)G spot from the Si-face marked by the red circle in (a) (E = 60 eV) [red contrast scale] and a DF-LEEM image using the brightest (0¯1)FG facet spot (E = 60 eV) of the sidewall graphene marked in the blue circle in (a) [blue contrast scale]. Vertical dashed lines mark the expected boundaries of the 200nm patterned trench tops.

etching (RIE) used to produce the trenches. RIE etching leaves a slightly roughened trench bottom that could inhibit graphene nucleation. As we’ll show below with ZZ trenches, graphene growth in the RIE roughened trench bottom is similarly inhibited. The graphene imaged from the most intense (¯10)FG facet rod is shown in blue in Fig. 2(c). 5


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FIG. 3. (a) µ-ARPES constant-E cut through the graphene BZ using a 500nm illumination aperture; E = 37eV, hν = 44eV. A faint facet cone is marked near the Γ point. (b) A series of equal energy shifted (∆E = 0.05 eV) MDC cuts through the Dirac cone from the trench tops −1 (kx = −1.7˚ A ) marked in (a). ky is perpendicular to the Γ-K direction. Dashed black the is

the constant energy contour at EF . The red lines are a best fit linear extrapolations using the maximum in each MDC below for E − EF can be used as a boundary to define ZZ-edge graphene ribbons. At the intersection of SiC < 1¯100 > and < 11¯20 > step edges, a narrow AC-edge graphene 13


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growth front will nucleate from the < 1¯100 > step. This narrow front will continue to grow in the < 1¯100 >, producing a narrow ZZ-edge graphene ribbon. By using an appropriate choice of etched patterns in SiC, ZZ-AC ribbon intersections can be formed.

Experimental Methods The substrates used in these studies were n-doped n = 2×1018 cm−2 4H-SiC. The trench arrays were prepared by first producing a negative ZEP mask by e-beam lithography. The patterned SiC substrate is then reactive ion etched with a SF6 −O2 −Ar plasma to produced up to 30nm deep trenches depending on etching time. After removing the mask material, the graphene was grown on the trench arrays by a two step selective growth method in a controlled silicon sublimation furnace.[8, 28] The arrays are first annealed at 1100 ◦ C to stabilize the trench facets. The samples are then graphitized by heating to 1525 ◦ C for 1 min. The samples were transported in air before introduction into the UHV analysis chamber. Prior to measurements the graphene films were thermally annealed at 500 ◦ C in UHV. The photoemission microscopy measurements were carried out using the spectroscopic photoemission and low energy electron microscope (SPELEEM) at the Elettra Synchrotron Light Laboratory.[29] This instrument combines low energy electron microscopy[30] with energyfiltered X-ray photoemission microscopy.[31] In the SPELEEM, the electron kinetic energy is controlled by biasing the sample with a negative potential. This bias is referred to as start voltage, Vstart . The kinetic energy of the electrons scattered (or emitted) by the sample is equal to Ekin = Vstart − δWi−s , the latter being the difference in work function between the instrument and the specimen. In our measurements, LEEM was used in both brightand dark-field modes, respectively, utilizing the first or secondary order diffraction beam for imaging. The microscope lateral resolution approaches a few tens of nanometers in XPEEM; energy resolution is better than 0.3 eV. Along with imaging, the SPELEEM allows diffraction operation mode. Depending whether the beamline photons or low energy electrons are used as probe, µ-ARPES or LEED measurements can be also carried out. The probed area is set by using a field limiting or illumination aperture. We have used a 2um diameter field limiting aperture for µ-ARPES, and 3 different illumination apertures for µ-LEED (500 nm, 1 um, 5 um). Part of the ARPES measurements were made utilizing the DF-PEEM method.[27] 14


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ACKNOWLEDGMENTS

This research was supported by the National Science Foundation under Grant No. DMR1005880. Additional support came from the W.M. Keck Foundation.

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The Bottom-up Growth of Edge Specific Graphene Nanoribbons

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