Nature | Vol 579 | 12 March 2020 | 219Article
Wafer-scale single-crystal hexagonal boron
nitride monolayers on Cu (111)
Tse-An Chen1,1 0, Chih-Piao Chuu1,1 0, Chien-Chih Tseng^2 , Chao-Kai Wen^2 , H.-S. Philip Wong^1 ,
Shuangyuan Pan^3 , Rongtan Li4,5, Tzu-Ang Chao1,2, Wei-Chen Chueh^2 , Yanfeng Zhang^3 ,
Qiang Fu^4 , Boris I. Yakobson6 ,7, 8 ✉, Wen-Hao Chang2,9 ✉ & Lain-Jong Li^1 ✉Ultrathin two-dimensional (2D) semiconducting layered materials offer great
potential for extending Moore’s law of the number of transistors in an integrated
circuit^1. 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 scattering^2 ,^3. Recent studies have shown the growth of single-crystal
hBN films on molten gold surfaces^4 or bulk copper foils^5. 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 films^6 ,^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.First, a single-crystal Cu (111) thin film on a wafer is needed. Single-crystal
Cu in thick foils can be achieved through recrystallization induced by
implanted seeds^5 ,^9. However, for the formation of Cu (111) thin film on a
wafer, the crystallinity relies strongly on the underlying substrate lattices.
Here we used a c-plane sapphire as the substrate, on which we sputtered
a 500-nm-thick polycrystalline Cu film and then carried out extensive
thermal annealing to achieve single-crystal Cu (111) films^10. One chal-
lenge is that Cu (111) tends to form twin grains separated by twin-grain
boundaries, through kinetic growth processes. Figure 1a illustrates the
atomic arrangements for the typical twinned Cu (111) structure. We find
that post-annealing at a high temperature (1,040–1,070 °C) in the pres-
ence of hydrogen is the key to removing the twin grains, consistent with
recent reports^10 ,^11. Figure 1b, c shows optical micrographs and electron
backscatter diffraction (EBSD) patterns for the Cu (111) thin films after
annealing at 1,000 °C and 1,050 °C. The EBSD results (see also Extended
Data Fig. 1a, b) confirm the coexistence of twinned Cu (111) polycrystals
with 0° and 60° in-plane misorientation for the Cu thin films annealed
at 1,000 °C. The in-plane misorientation was removed after annealing at
1,050 °C, producing single-crystal Cu (111). The X-ray diffraction results
(Extended Data Fig. 1c–f ) also consistently illustrate our success in obtain-
ing single-crystal Cu (111) thin films. Note that the Cu (111) is preferentially
formed with a thinner Cu film, but a sufficiently thick Cu film is necessary
to prevent Cu evaporation during subsequent hBN growth. Thus, there is
an optimal Cu thickness (around 500 nm) for single-crystal hBN growth.https://doi.org/10.1038/s41586-020-2009-2
Received: 23 June 2019
Accepted: 10 December 2019
Published online: 4 March 2020
Check for updates(^1) Corporate Research, Taiwan Semiconductor Manufacturing Company (TSMC), Hsinchu, Taiwan. (^2) Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan. (^3) Department of
Materials Science and Engineering, College of Engineering, Peking University, Beijing, China.^4 State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
Dalian, China.^5 University of Chinese Academy of Sciences, Beijing, China.^6 Department of Materials Science and Nanoengineering, Rice University, Houston, TX, USA.^7 Department of Chemistry,
Rice University, Houston, TX, USA.^8 Smalley-Curl Institute for Nanoscale Science and Technology, Rice University, Houston, TX, USA.^9 Center for Emergent Functional Matter Science (CEFMS),
National Chiao Tung University, Hsinchu, Taiwan.^10 These authors contributed equally: Tse-An Chen, Chih-Piao Chuu. ✉e-mail: [email protected]; [email protected]; [email protected]