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Wafer-scale single-crystal hexagonal boron nitride monolayers on Cu (111)

Abstract

Ultrathin two-dimensional (2D) semiconducting layered materials offer great potential for extending Moore’s law of the number of transistors in an integrated circuit1. One key challenge with 2D semiconductors is to avoid the formation of charge scattering and trap sites from adjacent dielectrics. An insulating van der Waals layer of hexagonal boron nitride (hBN) provides an excellent interface dielectric, efficiently reducing charge scattering2,3. Recent studies have shown the growth of single-crystal hBN films on molten gold surfaces4 or bulk copper foils5. However, the use of molten gold is not favoured by industry, owing to its high cost, cross-contamination and potential issues of process control and scalability. Copper foils might be suitable for roll-to-roll processes, but are unlikely to be compatible with advanced microelectronic fabrication on wafers. Thus, a reliable way of growing single-crystal hBN films directly on wafers would contribute to the broad adoption of 2D layered materials in industry. Previous attempts to grow hBN monolayers on Cu (111) metals have failed to achieve mono-orientation, resulting in unwanted grain boundaries when the layers merge into films6,7. Growing single-crystal hBN on such high-symmetry surface planes as Cu (111)5,8 is widely believed to be impossible, even in theory. Nonetheless, here we report the successful epitaxial growth of single-crystal hBN monolayers on a Cu (111) thin film across a two-inch c-plane sapphire wafer. This surprising result is corroborated by our first-principles calculations, suggesting that the epitaxial growth is enhanced by lateral docking of hBN to Cu (111) steps, ensuring the mono-orientation of hBN monolayers. The obtained single-crystal hBN, incorporated as an interface layer between molybdenum disulfide and hafnium dioxide in a bottom-gate configuration, enhanced the electrical performance of transistors. This reliable approach to producing wafer-scale single-crystal hBN paves the way to future 2D electronics.

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Data availability

All data needed to evaluate our conclusions are found in the main text and the Extended Data. Further data related to the paper are available from the corresponding authors on reasonable request.

References

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Acknowledgements

Tse-An Chen, C.-P.C., H.-S.P.W. and L.-J.L. acknowledge support from the Taiwan Semiconductor Manufacturing Company (TSMC). W.-H.C. acknowledges support from the Ministry of Science and Technology of Taiwan (grants MOST-108-2119-M-009-011-MY3 and MOST-107-2112-M-009-024-MY3) and from the CEFMS of the National Chiao Tung University, supported by the Ministry of Education of Taiwan. Y.Z. acknowledges financial support from the National Natural Science Foundation of China (grant 51861135201). Q.F. thanks the National Natural Science Foundation of China (grants 21688102 and 21825203) and the Strategic Priority Research Program of the Chinese Academy of Sciences (grant XDB17020000) for financial support. B.I.Y. acknowledges support from the US Department of Energy (grant DE-SC0012547) and a stimulating discussion with T. Ivanov (US Army Research Laboratory). Tse-An Chen and L.-J.L. acknowledge useful discussions with S. Brems at Imec.

Author information

Authors

Contributions

L.-J.L. and Tse-An Chen conceived the project. Tse-An Chen, C.-C.T. and C.-K.W. grew the hBN by CVD, performed the transfer of hBN, and carried out EBSD, Raman and AFM measurements. C.-P.C. performed first-principles calculations, and C.-P.C. and B.I.Y. carried out theoretical analysis. R.L. and Q.F. performed μ-LEED measurements. Y.Z. and S.P. performed STM experiments. C.-K.W., Tzu-Ang Chao and W.-C.C. fabricated the metal–insulator–metal device and the MoS2 FET. Tse-An Chen, C.-K.W. and Tzu-Ang Chao performed electrical measurements. L.-J.L., W.-H.C. and H.-S.P.W. supervised the project. All of the authors discussed the results and wrote the paper.

Corresponding authors

Correspondence to Boris I. Yakobson, Wen-Hao Chang or Lain-Jong Li.

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The authors declare no competing interests.

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Extended data figures and tables

Extended Data Fig. 1 Analysis of Cu (111) crystal orientation on c-sapphire substrates.

a, EBSD inverse pole figure (IPF) mapping (1 mm × 1 mm) of the Cu substrate annealed at 1,000 °C for 1 h. The normal direction (ND), transverse direction (TD) and rolling direction (RD) mappings, as indicated by the triangular colour map, show that Cu (111) is polycrystal. The line scan of misorientation on the RD map indicates an in-plane 60° rotation. b, IPF map of the Cu substrate annealed at 1,050 °C for 1 h; the film is characterized as single-crystal Cu (111), and no twinned grain is founded. c, d, XRD θ–2θ scans of the Cu (111)/c-sapphire substrate annealed at 1,000 °C for 1 h (c) and annealed at 1,050 °C for 1 h (d), revealing a Cu (111) peak at 2θ = 43.3°. e, f, XRD φ scans of the Cu (111)/c-sapphire substrate annealed at 1,000 °C for 1 h with an in-plane rotation of 60° (e) and annealed at 1,050 °C for 1 h with an in-plane rotation of 120° (f). Note that the sample annealed at 1,050 °C shows the signature of a single crystal without in-plane misorientation, because an hBN triangle has C3 symmetry and is symmetric after a 120° rotation.

Extended Data Fig. 2 Single-oriented hBN flakes in different Cu grains.

a, Optical micrograph of hBN molecules (triangles) grown on different Cu (111) grains (the black dashed line indicates the boundary between adjacent twin grains). Oppositely oriented hBN flakes are marked by red and blue circles. b, EBSD IPF maps of the area shown in a. The misorientation-of-line scan indicates that the twin grain is rotated by 60° (grain a to grain b). The RD map clearly shows the difference in in-plane orientation between grains a and b.

Extended Data Fig. 3 Statistical analysis of the orientation distribution of triangular hBN flakes.

a, Optical micrgraph of hBN grown on a Cu (111)/c-sapphire substrate at 1,050 °C. Misaligned hBN flakes are marked by red circles. b, Statistical analysis of the optical micrograph from a; more than 99.6% of the hBN flakes are aligned in one direction on Cu (111).

Extended Data Fig. 4 STM images from randomly selected locations.

ah, Images of a 1.5 × 1.5 cm2 hBN film grown on Cu (111)/c-sapphire, where the angle (θ) between the hBN lattice orientation (black arrow) and the horizontal line (black dashed line) in each image is 26.5° ± 1°, indicating that the hBN is a single-crystal film.

Extended Data Fig. 5 STM characterization of the atomic structure of monolayer hBN on Cu (111).

a, Large-scale STM image (Vtip = −1.008 V; Itip = 3.90 nA; T = 300 K) of hBN/Cu. b, Atomic-scale STM image (Vtip = −0.003 V; Itip = 46.50 nA; T = 300 K) of hBN/Cu. c, Typical STM image (Vtip = −0.003 V; Itip = 8.91 nA; T = 300 K) of hBN on Cu (111) with a relative rotation angle, θ, of approximately 3.3°, showing a moiré pattern with a period of 4.20 nm. d, Typical STM image (Vtip = −0.032 V; Itip = 18.51 nA; T = 300 K) of hBN on Cu (111) with a θ of roughly 1.5°, showing a moiré pattern with a period of 7.75 nm. The unit cell of the moiré pattern is highlighted by a black rhombus. e, Magnified STM image (Vtip = −0.039 V; Itip = 18.51 nA; T = 300 K) of hBN on Cu (111) with a θ of approximately 1.5°. f, Simulation of the moiré pattern for monolayer hBN on Cu (111) with a θ of roughly 1.5°. The unit cell of the moiré pattern for hBN/Cu (111) is highlighted by a black rhombus. The large-scale STM image in a shows a large-area flat terrace of hBN/Cu with a clean surface. The atomic-scale STM image in b reveals a honeycomb structure with a lattice constant of roughly 0.25 nm, which coincides well with the lattice parameters of hBN. Notably, in some typical regions of hBN/Cu (111), moiré patterns with different periods are observed. For instance, d and e show a moiré pattern with a period of around 7.75 nm, and c shows another with a period of roughly 4.20 nm. Such patterns arise from the lattice mismatch and/or relative rotation between hBN and the underlying Cu (111) substrate, and the moiré periods (D) correlate with the relative rotation angles (θ) between hBN and Cu (111) as23 $$D=(1+\delta )a/\sqrt{2(1+\delta )(1-\,\cos \theta )+{\delta }^{2}}$$, where δ is the lattice mismatch (roughly 2%) between hBN and the Cu (111) lattice24, and a is the lattice constant of hBN. Consequently, the θ for the moiré pattern with a D of around 7.75 nm (d, e) is calculated to be around 1.5°, and the simulated moiré pattern generated from monolayer hBN stacking on Cu (111) with a θ of roughly 1.5° (f) fits well with the STM result (d, e). The θ for the moiré pattern with a D of around 4.20 nm is calculated to be roughly 3.3° (c). gj, STM images showing the boundary between areas with and without moiré pattern. g, Typical STM image (Vtip = −0.003 V; Itip = 8.10 nA; T = 300 K) of hBN/Cu at the boundary site. h, Magnified STM image (Vtip = −0.003 V; ITip = 8.10 nA; T = 300 K) of the boundary in g (highlighted by the black square), showing that the hBN lattices present perfect coherence at the boundary site. i, Magnified STM image (Vtip = −0.003 V; Itip = 17.93 nA; T = 300 K) of hBN/Cu without moiré pattern in region 1 of g. j, Magnified STM image (Vtip = −0.003 V; ITip = 10.78 nA; T = 300 K) of hBN/Cu with moiré pattern in region 2 of g. The hBN atomic rows in region 1 and region 2 are along the same direction (black arrows). The STM image in g was captured at a typical boundary region, with moiré (region 2) and non-moiré (region 1) areas. The magnified atomic-resolution image at the boundary site in h shows that the hBN presents perfect lattice coherence at the patching boundary. The hBN atomic rows in the two adjacent regions are along the same direction (i, j). All images suggest that the hBN is aligned well and has mono-orientation, indicating epitaxial growth of hBN on Cu (111), and that the formation of moiré pattern does not affect the hBN orientation. We believe that the hBN completes its single-crystal growth at high temperatures, and that the strain associated with sample cooling after growth results in local moiré pattern.

Extended Data Fig. 6 Optical micrograph of hBN grown on Cu (111)/c-sapphire at different temperatures.

All as-grown hBN/Cu (111) was slowly oxidized at 150 °C in air in order to enhance the contrast between hBN and Cu. Black dashed lines indicate Cu twin grain boundaries. Regions with a red dashed outline are magnified in lower images. a, hBN grown at 995 °C, with hBN triangles aligned in the same Cu grain. b, hBN grown at 1,010 °C. Note that the hBN flakes grown at 995 °C and 1,010 °C are easily damaged after oxidation at 150 °C in air. c, d, All hBN triangles point in the same direction in the same Cu (111) grain after growth at 1,025 °C and 1,055 °C, and triangles seem unchanged after oxidation. e, After growth at high temperature (1,070 °C), all hBN flakes align in the same direction, but show different (stretched) shapes.

Extended Data Fig. 7 STM images of hBN/Cu (111).

ad, STM images of hBN/Cu steps without moiré pattern (ac) and with moiré pattern (d), showing that step edges are often observed in our Cu (111) crystals. e, Diagram showing that the meandering steps consist of segments of A and B types, and that BN kinetically nucleates at B-to-A corners while docking to stronger binding sites (B steps) with proper orientation. f, Atomic model of step edges on the top Cu (111) surface, showing A and B steps.

Extended Data Fig. 8 Calculated binding energy of six typical B6N7–Cu (111) configurations, taking into account docking to top-layer Cu step edges.

a, Atomic models of six configurations, showing the optimized in-plane distance, Di, away from A-step (red) or B-step (blue) edges. b, Binding energies as a function of Di (in units of $$\sqrt{3}a/3$$, where a is the lattice constant of Cu (111)), with the lowest-energy configurations marked with arrows. Plane-to-plane epitaxy corresponds to the absence of (or infinity distance away from) the step edge.

Extended Data Fig. 9 Comparison of planar epitaxy and step-edge docking of hBN on Cu (111).

a, Atomic models of fully docking configurations, NIBII (0°) and NIBIII (60°). b, Calculated binding-energy differences (left axis, black circles) and corresponding Boltzmann factors (right axis, red squares) between NIBII and NIBIII for stripes of BN on Cu (111) as a function of docking length (in number of hexagons) on the top-layer Cu step edge. c, d, Calculated energy of misfit B7N7–Cu (111) configurations as a function of small tilted angle along with top-layer Cu step-edge docking. c, Atomic models of B7N7–Cu (111) configurations docking to A steps or B steps at a tilt angle of θ. The aligned structures (θ = 0°) correspond to NIBII and NIBIII (the two lowest-energy structures docking in the vicinity of A-step and B-step edges). The lattice misfit between hBN and Cu (111) is 3.8% (ahBN = 2.5 Å; aCu(111) = 2.6 Å). d, Binding varies with the tilted angle along the step edge.

Extended Data Fig. 10 Diagrams and photographs illustrating hBN transfer.

a, A PMMA film is first spin-coated on the as-grown hBN/Cu (111)/sapphire as a protection layer. A TRT is then applied to the PMMA/hBN/Cu (111)/sapphire in order to avoid folding during the transfer process. b, Electrochemical delamination is carried out using an aqueous solution of NaOH (1 M) as the electrolyte, the Cu layer in the TRT/PMMA/hBN/Cu (111)/sapphire stack as the cathode, and a platinum foil as the anode, with an applied DC voltage of 4 V. During this process, the TRT/PMMA/hBN stacked film is detached from the Cu (111)/sapphire through the generation of hydrogen bubbles at the hBN/Cu interface. c, The TRT/PMMA/hBN stacked film is placed on the four-inch SiO2/Si substrate. d, TRT can be released by baking the TRT/PMMA/hBN/substrate on a hot-plate at 180 °C. e, The PMMA film is removed by immersing the sample in hot acetone for 40 min, leaving behind a two-inch monolayer hBN film on the four-inch SiO2/Si wafer.

Extended Data Fig. 11 FETs built with and without hBN as an interface dielectric in a bottom-gate configuration.

ad, Diagrams showing the various MoS2 FET device structures examined and their transfer characteristics (drain current, Id, versus gate voltage, Vg) at a driving voltage (Vds) of 0.1 V. Monolayer hBN and MoS2 are synthesized by CVD (see Methods). a, The hBN film is transferred onto a Si substrate with a 10-nm HfO2 dielectric layer, and topped with a single layer of MoS2. The metal contacts are processed by photolithography, and evaporated with Ti (5 nm) and Pd (50 nm) using an e-gun evaporator. bd, Top, diagrams showing a typical monolayer MoS2 FET on HfO2 (b), an MoS2 monolayer on a polycrystalline (PC) hBN monolayer (c), and an MoS2 monolayer on a single-crystal (SC) hBN monolayer (d). Bottom, Id versus Vg. The extracted two-probe electron mobility at room temperature in vacuum (μ) is 2.9 cm2 V s−1, 6.8 cm2 V s−1 and 11.8 cm2 V s−1, respectively. The mobility of MoS2 improves substantially (by about an order of magnitude) by replacing polycrystalline hBN with a single-crystal hBN. A suppressed current hysteresis is also observed when using single-crystal hBN as the buffer layer, and the subthreshold swing (SS) also improved from 111 mV dec−1 to 76 mV dec−1. The results indicate that single-crystal hBN can be used to enhance the electrical performance of 2D-based transistors. e, Diagram showing the hBN metal–insulator–metal (MIM) structure used to examine the quality of hBN. f, IV curves for MIM tunnel junctions with either a polycrystalline or a single-crystal hBN monolayer sandwiched between Pt/Ti (top) and Cu (bottom) electrodes. Electrical contacts were fabricated by photolithography and e-gun evaporation of Ti (5 nm) and Pt (40 nm) to form 100 × 100 μm2 pads on as-grown hBN/Cu/c-sapphire substrate. The device with single-crystal monolayer hBN exhibits a large breakdown voltage (of around 0.1 V), whereas the device with polycrystalline monolayer hBN shows direct tunnelling characteristics.

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Chen, TA., Chuu, CP., Tseng, CC. et al. Wafer-scale single-crystal hexagonal boron nitride monolayers on Cu (111). Nature 579, 219–223 (2020). https://doi.org/10.1038/s41586-020-2009-2

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