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Atomic Layer Epitaxy of h-BN(0001) multilayers on Co(0001), and Molecular Beam Epitaxy growth of Graphene on h-BN(0001)/Co(0001) Marcus Sky Driver, John D. Beatty, Opeyemi Olanipekun, Kimberly Reid, Ashutosh Rath, Paul Voyles, and Jeffry A. Kelber Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03653 • Publication Date (Web): 03 Mar 2016 Downloaded from http://pubs.acs.org on March 8, 2016

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Atomic Layer Epitaxy of h-BN(0001) Multilayers on Co(0001), and Molecular Beam Epitaxy growth of Graphene on h-BN(0001)/Co(0001) M. Sky Driver1δ, John D. Beatty1δ, Opeyemi Olanipekun1, Kimberly Reid1, Ashutosh Rath2, Paul M. Voyles2, Jeffry A. Kelber1* 1

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Department of Chemistry, University of North Texas, Denton, TX, 76203 Department of Materials Science and Engineering, University of Wisconsin, Madison, WI 53706

ABSTRACT

The direct growth of hexagonal boron nitride (h-BN) by industrially scalable methods is of broad interest for spintronic and nanoelectronic device applications. Such applications often require atomically precise control of film thickness and azimuthal registry between layers and substrate. We report the formation, by atomic layer epitaxy (ALE), of multilayer h-BN(0001) films (up to 7 monolayers) on Co(0001). The ALE process employs BCl3/NH3 cycles at 600 K substrate temperature.

X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction

(LEED) data show that this process yields an increase in h-BN average film thickness linearly proportional to the number of BCl3/NH3 cycles, with BN layers in azimuthal registry with each other and with the Co(0001) substrate. LEED diffraction spot profile data indicate an average BN domain size of at least 1900 Å. Optical microscopy data indicate the presence of some domains as large as ~ 20 microns. Transmission electron microscopy (TEM) and ambient exposure studies demonstrate macroscopic and microscopic continuity of the h-BN film, with the h-BN film highly conformal to the Co substrate. Photoemission data show that the h-BN(0001) film is p-type, with band bending near the Co/h-BN interface. Growth of graphene by molecular beam epitaxy (MBE) is observed on the surface of multilayer h-BN(0001) at temperatures of 800 K. LEED data indicate azimuthal graphene alignment with the h-BN and Co(0001) lattices,

δ

These authors contributed equally to this work *[email protected]


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with domain size similar to BN.

The evidence of multilayer BN and graphene azimuthal

alignment with the lattice of the Co(0001) substrate demonstrates that this procedure is suitable for scalable production of heterojunctions for spintronic applications.

I. INTRODUCTION The formation of macroscopic, highly ordered and azimuthally oriented h-BN(0001) multilayers is critical for a number of advanced electronic or spintronic applications1-5, and also for the use of BN as a dielectric substrate for the growth of other two-dimensional materials, including graphene6 and transition metal dichalcogenides.7 Of particular note is the predicted1,2 importance of the atomic-level control of the number of azimuthally-oriented h-BN(0001) monolayers (MLs) for spin injection and magnetic tunneling applications. Indeed, there is recent experimental corroboration for the thickness requirement, as a single layer of transferred BN is much less effective than a double layer for spin injection into graphene.8,9 The requirements for azimuthal alignment rule out using physical transfer—quite aside from the doubtful applicability of this method for industrial scale device development. Unfortunately, the popular h-BN(0001) growth method of borazine pyrolysis is largely selflimiting at about 1 ML.10,11 Sputter deposition12 and MBE13,14 can produce ordered multilayers, but not with the precise control of film thickness required for many applications. Atomic layer epitaxy (ALE) can in principle meet the above requirements. This is the first report of the use of ALE to grow large area, large domain multilayer azimuthally-oriented BN films suitable for proposed spintronic applications1,2 and for the subsequent formation of azimuthally oriented multilayer BN/graphene heterostructures. In a preliminary publication15, the use of a BCl3/NH3 deposition process—originally applied to the growth of polycrystalline films16—to produce highly oriented multilayer films macroscopic in area was demonstrated. Here, XPS and LEED data are presented demonstrating the linearity of h-BN(0001) film thickness with the number of ALE cycles, constancy of azimuthal orientation for growth on a


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Co(0001) substrate, and the macroscopic and microscopic continuity/conformality of such films. Conformal growth, crystallinity and thickness are confirmed by TEM and scanning TEM (STEM) imaging. XPS data indicate that the h-BN(0001) films are p-type, and that the interaction of such films with Co involves little or negligible interfacial charge transfer, orbital hybridization or covalent bond formation.

Finally, the ability to grow continuous, few layer graphene on

multilayer h-BN(0001) by MBE is demonstrated. The ability to grow multilayer h-BN sheets and graphene/BN heterojunctions by such scalable methods points the way toward numerous industrially practical device applications.

II. EXPERIMENTAL METHODS Co(0001) films were grown on clean 1.0 cm2 Al2O3(0001) by magnetron sputter deposition with an Ar plasma at a substrate temperature of 773 K, Ar pressure of 3.5 mTorr, and 25 W plasma power. The Al2O3(0001) substrate was cleaned prior to Co deposition by annealing in 1 x 10-7 Torr O2 at ~ 800 K. Co thicknesses measured by ellipsometry were ~ 250500 nm. Ex-situ AFM measurements yielded rms roughness values < 1 nm over different areas of the film. After deposition and AFM analysis, the Co substrate was transferred to the UHV growth and analysis system. The sample was annealed in oxygen and hydrogen to remove carbon and oxygen contamination, respectively, prior to BN growth while maintaining long-range order, as determined by XPS and LEED. BN and graphene film growth and analysis studies were carried out in a three chamber vacuum system equipped for atomic layer deposition (ALD), MBE, and surface analysis with XPS, LEED, and a residual gas analyzer (RGA). Sample transport between chambers occurred without sample exposure to ambient. XPS spectra were acquired with a non-monochromatic Al Kα x-ray source operated at 15 kV, 300 W, and a constant analyzer pass energy of 50 eV, with a 100 mm mean radius hemispherical analyzer and multichannel plate detector. Photoemission binding energies were referenced to a metallic Co 2p3/2 binding energy of 778.1 eV.17 Average


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film thicknesses and atomic compositions were obtained from XPS data by standard methods.18 XPS-derived BN film thickness values were converted into MLs by assuming a thickness of 3.3 Å/ML, in accord with Auger studies of graphene19 and STM studies of h-BN(0001) films deposited on Ru(0001).20 ALE was carried out using BCl3/NH3 cycles, adapted from a previously published procedure for growth of conformal multilayer polycrystalline BN films.16 Pressures in the ALE growth chamber were monitored by a baratron gauge and by a nude ion gauge out of line of site to the sample surface.

Exposures reported here have not been corrected for ion gauge

sensitivity or flux to the surface. The ALE growth chamber had a base pressure of 1.0 x 10-8 Torr. Research grade BCl3 and NH3 gases were used without further purification. The growth chamber was configured as a dynamic flow-through system, using a mechanical vacuum pump to maintain the desired pressure during ALE. The ALE chamber was pumped to < 1 x 10-5 Torr between cycles using a turbomolecular pump. After the desired number of cycles, the sample was then transferred under ultrahigh vacuum (UHV) to the analysis chamber for XPS and LEED characterization, and annealing to finish the chemical reaction. LEED was performed using a three-grid reverse view system attached to the UHV chamber. The average domain sizes of Co and of deposited BN and graphene films were obtained from the FWHM of LEED diffraction spots as previously described.15

Since the

coherence length of the LEED instrument was estimated at ~ 3000 Å, domains larger than this limit would not impact the observed FWHM of LEED diffraction peaks, and the domain sizes derived from LEED analysis should therefore be regarded as a conservative estimate of domain size. Briefly, the LEED images were digitized with commercial software, and the FWHM for specific LEED spots was measured in order to determine the angular divergence (β). The average domain size (R) was then determined according to21 (1) R = 0.5bλ/([1+∆E/E]β)


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In (1), b is the fidelity factor, generally estimated at 0.35, λ is the electron wavelength (1.52 Å ), and (∆E/E) is the energy spread for the electron gun, estimated at ~ 0.015. β is the angular divergence estimated from LEED diffraction beam spots FWHM values normalized to the sample–screen distance ( ~150 mm).

Previous measurements using this method for

determination of graphene domain sizes yielded results of ~ 1800 Å for films grown on Co3O4, which were subsequently confirmed by AFM measurements.22,23

This LEED methodology is

limited, however, in that there is a minimum measureable limit to β, leading to a point beyond which larger grain sizes will not yield narrower diffraction spots.

For this instrument, the

coherence limit is estimated at ~ 3000 Å. In order to determine the range of appropriate temperatures for BN ALE, desorption measurements were performed on deposited h-BN(0001) films using a quadrupole residual gas analyzer (RGA) attached to the UHV analysis chamber and mounted approximately 8 cm from the sample surface. Desorption measurements were carried out during annealing of deposited BN layers to monitor removal of HCl from the BN film. Annealing was performed with a resistive heater directly below the sample, and temperature was calibrated with a K-type thermocouple mounted directly on the sample. Annealing was performed at a heating rate of 10 K min-1 at 300 K – 1000 K. These measurements show that HCl evolution occurs at temperatures above ~ 520 K (Supporting Information, Fig. S1), indicating that ~550 K – 800 K is the approximate temperature window for this ALE process on Co. (Deposition at > 800 K resulted in significant Co etching.) ALE was therefore carried out at a growth temperature of 600 K to optimize growth rate while maintaining long range order in the film. Graphene deposition was carried out via MBE using a commercially available 4-pocket e-beam evaporator with carbon rod as the source and a substrate temperature of 800 K. The MBE chamber pressure was ≤ 1x10-9 Torr during deposition.

Final carbon thickness was

determined by XPS analysis directly following carbon MBE.


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TEM and STEM data were acquired on Co/BN/Co and Co/BN/Gr heterostructures with Ta passivation layers. Here, the Co overlayer and Ta passivation layers were deposited by MBE prior to exposure of the sample to ambient. Electron transparent cross-sectional TEM samples were prepared by in-situ lift out using a Zeiss Auriga focused ion beam (FIB). To minimize damage from implanted Ga, the samples were exposed to a Ga ion beam energy of 5 kV or less after lift-out, and final FIB thinning was performed at 2 kV.

STEM imaging was performed on

an FEI Titan STEM with a CEOS probe aberration corrector operated at 200kV, 24.5 mrad convergence angle.

High-angle annular dark-field images were collected with a detector

spanning 54 to 270 mrad in scattering angle. High resolution transmission electron microscopy (HRTEM) imaging was performed on a Tecnai TF30 transmission electron microscope operated at 300 kV. In order to provide additional insight into the BN nucleation process, some ex-situ optical microscopy measurements were acquired using an Olympus with Olympus BX51 microscope with 50x working objective.

Measurements were acquired using commercial software for

microscope mapping of surfaces.

III. RESULTS III-A. BCl3 / NH3 Reactions with and film growth on Co(0001). Co(0001) surface saturation by boron was studied by examining B 1s signal intensity (relative to the Co 2p3/2 intensity) vs BCl3 dose for 5 sequential 60 sec/250 mTorr exposures of BCl3 to a clean Co(0001) substrate.

Maximum boron saturation of the Co surface was

determined to occur at 300 seconds, corresponding to a total exposure of 7.5 x 107 Langmuir (L: 1 L = 10-6 Torr-sec) (Figure 1a). After boron saturation was achieved, NH3 was exposed to the B-saturated surface for sequential 60 sec exposures at 600 K/350 mTorr, with maximum N 1s


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signal intensity observed after two such exposures, corresponding to a total exposure of 4.2 x 107 L (Figure 1b). Further NH3 exposures under these conditions resulted in a decrease in both B and N surface coverage, as determined by XPS, suggesting some B desorption as BH3. These saturating exposures were therefore used in the ALE process, with a sample temperature of 600 K. Exposures of 7.5 x 107 L of BCl3 and 4.2 x 107 of NH3 produced controlled linear growth of BN, up to at least ~7 ML of BN (Fig. 2), with a B:N XPS-derived atomic ratio of 1.0 ±0.1. Cl remnant concentration (not shown), also determined by XPS, was generally observed to be < 2 at. %. LEED images acquired during the BN growth process (Fig. 3) were very similar to the images previously published15, but with substantially smaller FWHM, yielding an estimated BN domain size of ~ 1900 Å. The expected six-fold diffraction patterns were observed for both the clean Co(0001) surface and the BN film at various growth stages up to ~ 7 ML thickness. The LEED data therefore indicate that BN layers grow in azimuthal registry with each other and with the Co(0001) substrate. Close inspection of the LEED diffraction spots and corresponding line profiles in Fig. 3 a-d show that growth of BN corresponds to both a decrease in LEED diffraction spot FWHM and decrease in intensity above background. The decrease in spot size in going from clean Co(0001) (Fig. 3a) to BN/Co (Fig. 3b) indicates that BN domains are somewhat larger than the Co domains—a finding consistent with earlier results.15

The decreased

diffraction intensity upon BN deposition is expected due to the lower electron scattering cross section for B and N relative to Co.21 The substantially larger BN domain size determined here from the LEED data (Fig. 3), compared to the previous study15, indicates a substantial improvement in growth temperatures and exposure times. In order to study the conformality and continuity of the h-BN(0001) film, cross-sectional high resolution TEM and STEM images were acquired on Co/BN/Co samples with ~ 7 ML of BN and a top layer of Co and polycrystalline Ta deposited by MBE. A low magnification high-angle


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annular dark-field (HAADF) STEM image is shown in Fig. 4a. Fig. 4b shows the high resolution z- contrast STEM image of Co/BN/Co interface taken along the [1 1 -2 0] zone axis of sapphire substrate.

The h-BN(0001) film closely follows the large-scale gradual undulations of the

Co(0001) substrate, with uniform thickness, and with no evidence of pinholes or discontinuities on a nm-scale. Fig. 4c shows the HRTEM image of Co/BN/Co interface taken along the [1 1 -2 0] zone axis of sapphire substrate with lattice fringes in the BN layer. The thickness of the BN layer is about 1.28 nm, but the fringe spacing in the BN layer are not consistent with the expected 3.3 Å spacing. We speculate that this inconsistency arises due to roughness of the Co creating overlaps of offset BN layers in projection through the TEM sample. These results are consistent with XPS Co 2p spectra acquired for a 3 ML h-BN(0001) film on Co(0001) before and after exposure to ambient (Supporting Information, Fig. S2). The data, in agreement with previous results

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show no evidence, upon exposure to ambient, of Co oxide formation or other

reaction at the BN/Co interface. Therefore, the data in Figs 2-4, as well as the previously published LEED images and XPS results for ambient exposure15 demonstrate an ALE process capable of the deposition of highly ordered h-BN(0001) films at a rate linearly proportional to the number of BCl3/NH3 cycles, and also demonstrate that these films are uniform, continuous, and conformal over macroscopic length scales. Ex-situ optical microscopy images were acquired in order to provide additional insight into the BN nucleation process, especially at lower growth temperatures. A dark field image (Fig. 5) was acquired for a BN(0001)/Co(0001) sample grown near 500 K, and with an XPS-derived average thickness of ~ 6.5 ML. The image confirms the presence of large (roughly) triangular BN domains, oriented in the same direction on the Co(0001) surface. The image indicates the presence of BN domains on Co as large as 20 microns, and indicate that formation of large domains can occur under growth conditions as low as ~ 500 K by this method. III-B. BN electronic interactions with the Co(0001) surface


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XPS B 1s and N 1s spectra are displayed in Fig. 6 as a function of increasing h-BN(0001) film thicknesses on a Co(0001) substrate. The data show a monotonic shift toward smaller B 1s and N 1s binding energies, relative to the Fermi level, with increasing film thickness. This behavior is consistent with p-type doping of the BN, and in good agreement with recent firstprinciples calculations for multilayer h-BN(0001) on Co substrates.24

The general range of

binding energies observed in Fig. 6 is comparable to those reported for borazine-derived monolayer BN films on other transition metal surfaces.10,11,25

Disordered bilayer and

polycrystalline multilayer BN films have been prepared by very large exposures of borazine to a Ni(111) surface

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, but in that case, the trend in B 1s and N 1s binding energies with increasing

film thickness was the opposite of that shown in Fig. 6 The difference, ∆, in N 1s and B 1s binding energies shown in Fig. 6 (∆ = E(N 1s) – E(B 1s)) is 206.8±0.2 eV for the 1.7 ML film, and increases only slightly, to 207.1 ± 0.2 eV for the ~7 ML film. Similar results were obtained for numerous other samples with thicknesses between 1 and 7 ML (Supporting Information, Fig. S3). This value for ∆ is slightly but consistently smaller than the value of 208.0 ± 0.3 eV reported for monolayer films on Pt(111)

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, Ni(111)10, or Ru(0001)11.

This result, however, somewhat contradicts the results of earlier first principles calculations of hBN layers on Co(0001)

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that predicted a NCo charge transfer of ~ 0.15 electrons/N atom,

and a CoB charge transfer of 0.21 electrons/B atom. Such a transfer would have increased the N 1s binding energy and decreased the B 1s binding energy, opposite to what is observed in Fig. 4b. III.-C. Graphene growth by MBE on h-BN(0001)/Co(0001) Graphene was grown on a ~ 5 ML h-BN(0001)/Co(0001) substrate by exposure to the flux from a graphite rod target, at a substrate temperature of 800 K. The C 1s XPS spectrum is displayed in Fig. 7 for a resulting graphene film of 3 ML average thickness. The corresponding XPS survey scan (Supporting Information, Fig. S4) indicates the expected presence of B, N and


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C with no change in B:N composition upon MBE. A small O 1s impurity (Fig. S4) is typically present, prior to BN or C deposition, but decreases in intensity upon BN or C deposition. Corresponding LEED data are shown in Fig. 8 for the clean Co(0001) substrate (Fig. 8a), the subsequently deposited 5 ML h-BN(0001) film (Fig. 8b), and the 3 ML graphene film on BN (Fig. 8c). The XPS C 1s spectrum (Fig. 7) shows a C 1s peak binding energy of 284.7 ± 0.1 eV, in good agreement with the generally reported 284.5 eV value28 for sp2 C, corroborated by the presence of the expected ππ* shake up feature near 292 eV (Fig. 7a, arrow). LEED data for the Co(0001) substrate (Fig. 8a), h-BN(0001) film on Co(0001) (Fig. 8b), and the sp2 C film on h-BN(0001) (Fig. 8a-c, respectively) indicate that the Co, BN and graphene lattices are all in azimuthal registry. As expected, the LEED intensity for the BN and graphene films is less than for the Co films due to lower B, N, and C electron scattering cross section relative to Co.21 Estimates of graphene domain size yield a similar value to that of the BN films, ~ 1900 Å. expected for graphene.

The sp2 C film displays the 6-fold LEED pattern and lattice spacing The continuity of the graphene films was tested by XPS examination

before and after exposure to ambient (not shown). This exposure produced no observable evidence of B, N or Co oxidation, indicating that the graphene film is macroscopically continuous. Cross-sectional STEM micrographs of Co/BN/Gr//Ta samples are shown in Fig. 9a and Fig. 9b. The Co/BN interface is sharp, but there is no image contrast difference between BN and graphene. Fig. 9c shows a high resolution TEM image of Co/BN/Gr/Ta interface taken along the [1 1 -2 0] zone axis of sapphire substrate, showing lattice fringes in BN / Gr layer. There is no image contrast between the two light-element layers. Electron Energy Loss data (not shown) suggest higher N signal near the bottom of the ~1.25 nm thick combined BN/Gr layer and higher C near the top, but the data are inconclusive. The upper Gr/Ta interface is rougher, suggesting that the BN and Gr cannot be separated due to some small roughness viewed in projection by


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the STEM. This roughness may also be why the STEM-estimated film thickness is somewhat less than that estimated by XPS prior to Ta deposition. IV. DISCUSSION The data in Figs. 1-5 demonstrate that the ALE process employed here, based on BCl3/NH3 cycles at 600 K, produces highly oriented h-BN(0001) layers in azimuthal registry with each other and with the Co(0001) substrate. The films are conformal and macroscopically continuous, with the total h-BN(0001) film thickness linearly proportional to the number of BCl3/NH3 cycles. Thus, the films produced by this method meet the proposed requirements for spin filters and other spintronic applications.1,2 The evolution of B 1s and N 1s binding energies with BN film thickness (Fig. 6) is characteristic of p-type band bending, in general agreement with recent first-principles calculations

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for the Co/BN interface. Further, the XPS data show no evidence for strong

interfacial charge transfer or Co 3d-BN π orbital hybridization, as has been experimentally observed for Ni/BN interfaces.26 However, the data in Fig. 6 do show that the difference in N 1s and B 1s binding energies, ∆, is ~ 1 eV smaller than the values generally reported for BN monolayers produced by borazine pyrolysis on various transition metal surfaces.10,11,25 The reasons for this small variation in ∆ are not readily apparent, but one possible explanation is more effective screening of the N 1s core hole, due to the predicted 24 location of N atoms on atop Co sites, with B atoms located over hollow sites. Although such an effect would be limited to the first 1-2 ML, the effective sampling depth of XPS would yield an “average” ∆ that increases only gradually for the first few layers.

In any case, the absence of any strong

interfacial reaction or Co-BN interfacial chemical bond formation, combined with the maintenance of azimuthal registry, indicates that a number of predicted spintronic devices involving BN on ferromagnetic substrates can now be fabricated by this ALE process.


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The p-type BN band bending observed here is in direct contrast to the strong orbital hybridization and n-type doping reported for the ALD BN/Ru and CVD graphene/ALD BN/Ru interfaces.29,30

It is also in contrast to the strong Ni/BN hybridization recently reported for

borazine-derived films.26 It would therefore appear that the relatively weak orbital interactions at the BN/Co are well-suited for spintronics applications requiring vertical transport across the Co/BN interface.1 The demonstrated growth of graphene on h-BN(0001) (Figs. 7-9) is in general agreement with previous experiments on BN nanoflakes31 and on MBE BN14, and shows that few layer epitaxial graphene can be grown over macroscopic areas, and in azimuthal registry with the BN substrate.

The ability to fabricate macroscopic area graphene/h-BN(0001)

heterostructures with azimuthal registry is critical to certain spintronic applications.1 Moreover, the ability to fabricate these heterostructures by industrially practical and scalable methods enables the industrial development of a number of other devices, including graphene/BN tunneling transistors.4,5,32

IV. SUMMARY AND CONCLUSIONS XPS, LEED and STEM data demonstrate that multilayer h-BN(0001) films can be grown on Co(0001) by an ALE process using BCl3 and NH3 precursors at a growth temperature of 600 K. The data show that the h-BN(0001) film thickness is linearly proportional to the number of BCl3/NH3 cycles, and that the films are stoichiometric, in azimuthal registry with each other and the substrate, and also conformal and continuous over macroscopic areas. Few layer graphene growth on multilayer h-BN(0001)/Co(0001) by MBE is also demonstrated, at growth temperatures of 800 K. LEED data show that the graphene layers, BN layers, and Co(0001) substrate are in azimuthal registry with each other. The LEED data yield an estimate of average BN and graphene domain size of ~ 1900 Å. Since the coherence length of the LEED instrument


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used here was ~ 3000 Å, however, the LEED-determined domain size estimate should be regarded as conservative, consistent with optical microscopy results indicating some domains as large as 20 microns. Band bending is observed at the Co/BN interface, indicating that the BN film is p-type, but without strong Co/BN interfacial charge transfer or covalent bonding interactions.

These results provide a clear road map for the industrial-scale fabrication of

multilayer BN films and BN/heterostructures; addressing the key requirements of azimuthal orientation between layers and atomic-level control of BN film thickness.

ACKNOWLEDGMENT This work was supported by CSPIN, a MARCO/DARPA STARnet center, under task IDs 2381.001 and 2381.003, and by the National Science Foundation under Grant No. ECCS1508991.

Facilities and instrumentation for electron microscopy were supported by NSF

through the University of Wisconsin Materials Research Science and Engineering Center (DMR1121288). Peter Dowben is also acknowledged for stimulating discussions.


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Figures

Figure 1. XPS data of Co(0001) surfaces during the ALD process; (a) relative B1s intensity vs. BCl3 exposure at 250 mTorr, 600 K on a clean Co(0001) surface; (b) relative N 1s intensity vs NH3 exposure at 350 mTorr, 600 K on the B-covered Co(0001) surface. Both B1s and N 1s intensities were normalized to the total Co 2p3/2 XPS intensity.


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Figure 2. XPS-derived h-BN film thickness vs. the number of BCl3/NH3 cycles at 600 K. The XPS-derived B:N atomic ratio was in all cases 1.0 ±0.1.


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Fig. 3. LEED images and FWHM for BN growth on Co(0001). (a) and (b) LEED of Co(0001) and line scan showing diffraction spot profiles; (c) and (d) corresponding data for 5 ML BN(0001) on Co(0001). Beam enegry is 60 eV.


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Figure 4. (a) A low magnification STEM image of a ~7 ML h-BN(0001) film on a Co(0001) substrate., with protective Co and Ta over layers deposited by MBE. Scale bar = 50 nm; (b) High resolution z- contrast STEM image of Co/ BN/Co interface taken along the [1 1 -2 0] zone axis of sapphire substrate. Scale bar = 2 nm. (c) High resolution TEM image of Co/BN/Co interface taken along the [1 1 -2 0] zone axis of sapphire substrate. Scale bar= 2nm


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Figure 5. Dark field microscope image showing triangular BN crystals on Co(0001) substrate. image taken with Olympus BX51 microscope with 50x working objective. Measurement taken with Altµs™ Software used for microscope mapping of surfaces. Estimated BN coverage is ~6.5 layers at a deposition temperature of 500 K. Scale bar is 50 üm.


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Figure 6. Evolution of (a) B 1s and (b) N 1s XPS core levels for h-BN(0001) films of increasing thickness, deposited on Co(0001). Solid trace - 1.7 ML of BN with a B 1s peak binding energy (BE) of 191.9 eV and N 1s BE of 398.7 eV. Dashed trace - 3.8 ML of BN with a B 1s BE of 191.5 eV and a N 1s BE of 398.6 eV. . Open circles trace- 6.8 ML of BN with a B 1s BE of 191.2 eV and N 1s BE of 398.3 eV. The Cl 2p feature observed in (a) represents corresponds to < 1 at. % Cl impurity. (The estimated experimental uncertainty in all binding energies is ± 0.1 eV)


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Figure 7. (a) C 1s XPS spectrum of 3 ML of graphene deposited onto a 5 ML h-BN(0001) film on Co(0001). The C 1s peak binding energy at 284.7 ± 0.1 eV. The ππ* transition is marked by an arrow.


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Figure 8. Alignment of LEED patterns for (a) Co(0001); (b) 5 ML of h-BN(0001) on Co(0001); and (c) 3 ML of graphene on 5 ML h-BN(0001)/Co(0001).


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Figure 9. (a) A low magnification STEM images of a Co/BN/Gr/Ta heterostructures. Scale bar = 50 nm; (b) High resolution z- contrast STEM image of the Co/BN/Gr/Ta interface taken along the [1 1 -2 0] zone axis of sapphire substrate. Scale bar = 5 nm. (c) High resolution TEM image of Co/BN/Gr/Ta interface taken along the [1 1 -2 0] zone axis of sapphire substrate. Scale bar= 2 nm


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Atomic Layer Epitaxy of h-BN(0001) - ACS Publications - American

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