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Electrochemically prepared polycrystalline copper surface for the growth of hexagonal boron nitride Karthik Sridhara, Boris N. Feigelson, James A Wollmershauser, Jennifer K. Hite, Anindya Nath, Sandra C. Hangarter, Michael S Fuhrer, and D. Kurt Gaskill Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01665 • Publication Date (Web): 09 Feb 2017 Downloaded from http://pubs.acs.org on February 19, 2017
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Electrochemically prepared polycrystalline copper surface for the growth of hexagonal boron nitride Karthik Sridhara12, Boris N. Feigelson2*, James A. Wollmershauser2, Jennifer K. Hite2, Anindya Nath23, Sandra C. Hangarter2, Michael S. Fuhrer4, D. Kurt Gaskill2* 1
Department of Material Science and Engineering, University of Maryland, College Park, MD 20742, USA 2 U.S. Naval Research Laboratory, 4555 Overlook Ave., SW, Washington, DC 20375, USA 3 George Mason University, 4400 University Dr. Fairfax, Virginia, VA 22030, USA 4 School of Physics, Monash University, Clayton VIC 3800, Australia * To whom correspondence should be addressed.
Abstract: The controlled and reproducible growth of hexagonal boron nitride (h-BN) by chemical vapor deposition on polycrystalline copper foil substrates remains a challenge as typical growth surfaces contains microscopic ridges (height ~100 µm) arising from the foil manufacturing process. In this work, we report a method to prepare commercially coldrolled polycrystalline copper substrates for a greatly improved growth of h-BN by a combination of thermal annealing in a reducing environment and electrochemical polishing to create an excellent surface that enables control of BN nucleation sites. We report a RMS roughness of ~1.2 nm for the Cu substrate after electropolishing and a reduction of nucleation sites along with enlargement of h-BN crystals with this combined approach. We also assess the potential role of surface features that exist on the Cu surface as nucleation sites. The development of an electrochemical process to prepare 2D materials growth substrates and demonstration of greatly improved growth of 2D materials directly points more pragmatic large scale processing of 2D materials since such techniques are already utilized in large scale industrial processing.
Introduction Since the mechanical exfoliation of graphene experiments that led to gate controlled conductivity, there has been enormous scientific interest in the synthesis, properties and applications of twodimensional van der Waals crystals such as graphene, tungsten selenide (WSe2), molybdenum disulphide (MoS2) and hexagonal boron nitride (h-BN).1, 2 h-BN is potentially an excellent Page 1 of 20 ACS Paragon Plus Environment
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substrate and/or tunnel and gate dielectric material for future graphene devices because: 1) h-BN is structurally compatible with graphene’s hexagonal lattice (lattice mismatch of ~2%) and has surface roughness much lower than SiO2 3; 2) the insulating properties of h-BN 4-6 make it an ideal counterpart to graphene’s semi-metal electronic structure; and 3) the lack of dangling bonds on hBN results in minimal impact on electronic transport and, therefore, makes it suitable as a substrate to prepare graphene electronic devices.7 Use of 2D h-BN as a substrate and tunnel and gate dielectric8-11 for graphene-based transistors and metal-insulator-metal (MIM) heterostructure devices12 has been studied experimentally13 and theoretically10. h-BN can be synthesized by various methods such as by high temperature and high pressure processes6, combustion synthesis14, and chemical vapor deposition (CVD) 15. Individual atomic sheets of h-BN having crystal sizes ~10 µm2 can be drawn from the bulk using mechanical exfoliation.1, 16 However, of the traditional growth methods, CVD on transition metals such as nickel 17, 18, copper 19, 20 and also on Cu-Ni alloys21 is currently the most viable method suitable for large area (>10 cm2) growth of 2D h-BN. CVD growth of 2D h-BN is typically performed on polycrystalline copper (Cu) foils that are prepared using metallurgical cold rolling process 22, 23. In this process, the Cu metal stock is fed into rolling pins and comes out of the rolling pins in the form of a foil after undergoing plastic deformation. The process is usually performed at ambient temperature (below Cu’s recrystallization temperature) and the processed Cu foil often have rolling lines (ridges) imprinted from the roller pins, potential contaminants from the rolling pins, small highly deformed Cu grain sizes, and native oxide on the surface. Irregular ridges formed during the rolling process, along with the grain boundaries act as nucleation sites, potentially impeding the crystal continuity during growth of h-BN or other 2D materials 19, 24. Additionally, the native oxide adversely affects the catalytic properties of the Cu surface 25-27 that breakdown the precursor molecules. Thus, the poor quality of the cold-rolled Cu surface makes it unsuitable for controllable large area growth of good quality two-dimensional crystals. Improving the quality of Cu surface will lead to higher quality h-BN since CVD growth is surface driven26, 28-30; for this reason it is desired to have smoother surface morphology. Thus there is a need for an ultra-smooth, morphologically homogenous starting Cu surface to allow for deliberate control of h-BN quality. Various methods such as nitric acid dip 30, thermal annealing in reducing environment 30, 31, chemical mechanical polishing 28 and electropolishing 19, 29, 32 have been used to prepare Cu surfaces for CVD growth of 2D materials. But relating the morphology of Cu surface, especially with roughness, to the growth of the two-dimensional material remains unaddressed. Hence, this work explores a two-step thermal annealing and electrochemical polishing procedure to prepare the Cu foil surface for CVD growth of h-BN. Electrochemical polishing is an attractive technique to modify the surface of Cu foils since electro-polishing processes are well-known, can be well-controlled, and the technique is already used on an industrial scale. We find that the electrochemical polishing results in significant improvement to Page 2 of 20 ACS Paragon Plus Environment
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the Cu foil surface, yet, defects such as etch pits are still present. To mitigate etch pits, we draw upon prior work that showed two-additive electrolytes have yielded good morphology [32-34]. In this work, a unique two-additive electrolyte (50% H3PO4 + 25% Acetic Acid + 25% Ethylene Glycol) has been prepared to mitigate the issue of etch pits and to promote large areas (>100 µm2) of planarized Cu surface 33-35 suitable for large area growth of h-BN. This work also aims to establish the relationship of the Cu surface with the h-BN crystal size and density. Unique surface features, which appear to be h-BN nucleation sites, are also discussed.
Experimental Section Sample Preparation. Cu foils (Alfa Aesar 99.999%, 25 m thickness) are cut into ~20 mm × 10 mm rectangular pieces and pressed in an MTI 12T hydraulic press between two polished (using a 0.1 µm diamond slurry) hardened steel anvils under a pressure of about 0.35 GPa to induce plastic deformation and to ensure the foils are flat prior to treatment and growth. Before electropolishing the Cu foils were annealed at 1030°C for 5 hours under a flow of 180 standard cm3 min-1 (sccm) of N2 and 20 sccm of H2. Additional information on sample preparation is given in section 1 of supplementary information. Electropolishing. A setup as described in 36-38 was used with the Cu acting as the working electrode, platinum (~40 mm × 20 mm) as the counter electrode, and a Gamry Reference 3000 as a potentiostat. A potentiostat was used in place of a power supply since it enables the study of voltammetry (current-voltage) behavior by assessing the current-voltage and current-time relationship, and reproducibility of these relationships. Two different electrolyte recipes were used for electropolishing: 1) 85% concentrated phosphoric acid (H3PO4) electrolyte (standard recipe), and 2) 25% Acetic Acid + 25% Ethylene Glycol + 50% concentrated phosphoric acid by volume (two-additive recipe). The two-additives recipe was utilized to reduce etch pits which occur due to oxygen bubbling on the Cu surface using the standard recipe33-35, 39, 40. The electropolishing was performed at 1.8 V and 2.3 V for the standard and two-additive recipes, respectively. A much more detailed explanation of the electropolishing process is discussed in the supplementary information (Figures S.1-S.4) describing the choice of electropolishing potential. Growth of h-BN. CVD synthesized h-BN were grown on three sets of samples: (1) electropolished using the standard recipe, (2) electropolished using the two-additive recipe and (3) nonelectropolished control sample. All samples were thermally annealed in-situ the vertical CVD reactor at 1030°C for 5 hours under a flow of 180 standard cm3 min-1 (sccm) of N2 and 20 sccm of H2 prior h-BN growth. Samples using method (1) are designated Cu TA EP. Samples using method (2) are designated Cu TA EP-2. Control samples are designated Cu TA. Table 1 associates sample names with preparation procedure.
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The h-BN growth was performed at 1030°C with growth time limited to about 20 minutes 41 to grow separate h-BN crystals, so that the effect of electropolishing can be assessed and differentiated from the control sample. The precursor for CVD growth of h-BN is ammonia borane (H3NBH3) from BoroScience International, Inc. (99.9% purity). The precursor was sublimated upstream at temperatures of 60 - 90˚C and transported by 1.1 PSIG (816 Torr) in a gas mixture of 840 sccm of N2 and 20 sccm of H2 to the reactor. The Cu samples were supported by a perforated pyrolytic boron nitride crucible. Characterization methods. Tapping mode atomic force microscopy (AFM, Veeco D5000) with phase imaging was performed directly on Cu foils after growth to confirm the thickness of h-BN and to assess the lateral sizes of h-BN crystals. An AFM tip (Budget Sensors Tap150-G) with radius 1 µm valley-to-valley) and (2) microscopic features (
