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

Abstract

Ultrathin two-dimensional (2D) semiconducting layered materials offer a great potential to extend the Moore's Law (1). One key challenge for 2D semiconductors is to avoid the formation of charge scattering and trap sites from adjacent dielectrics. The insulating van der Waals layer, hexagonal boron nitride (hBN), is an excellent interface dielectric to 2D semiconductors, efficiently reducing charge scatterings (2, 3). Recent studies have shown the growth of single-crystal hBN films on molten Au surfaces (4) or bulk Cu foils (5). However, using molten Au is not favored in industry due to high cost, cross-contamination, and potential issues of process control and scalability. Cu foils may be suitable for roll-to-roll processes, but unlikely to be compatible with advanced microelectronic fabrication on Si wafers. Thus, only a reliable approach to grow single-crystal hBN on wafers can help realize the broad adoption of 2D layered materials in industry. Previous efforts on growing hBN triangular monolayers on Cu (111) metals have failed to achieve mono-orientation, resulting in unwanted grain boundaries when they merge as films (6,7). Growing singlecrystal hBN on such a high-symmetry surface planes (5,8) is commonly believed to be impossible even in theory. In stark contrast, we have successfully realized the epitaxial growth of single-crystal hBN monolayers on a Cu ( 111) thin film across a 2-inch c-plane sapphire wafer. This surprising result is corroborated by our first-principles calculations, suggesting that the epitaxy to the underlying Cu lattice is enhanced by the lateral docking to Cu (111) steps, to ensure the mono-orientation of hBN monolayers. The obtained singlecrystal hBN, incorporated as an interface layer between MoS2 and HfO2 in a bottom-gate configuration, has enhanced the electrical performance of transistors based on monolayer MoS2. This reliable approach of producing wafer-scale single-crystal hBN truly paves the way for developing futuristic 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 strongly relies on the underlying substrate lattices. Here we used a c-plane sapphire as the substrate, on which a 500-nm-thick polycrystalline Cu film was sputtered followed by extensive thermal annealing to achieve singlecrystal Cu (111) films (10). One challenge is that Cu (111) tends to form twin grains separated by twin grain boundaries, through kinetic growth processes. Fig. 1a illustrates the atomic arrangements for the typical twinned Cu (111) structure. We find that the post-annealing at a high temperature (1,040 -1,070 °C) in the presence of hydrogen is the key to removing the twin grains, consistent with recent reports (10,11). Figures 1b and 1c show the optical micrographs (OMs) and electron backscatter diffraction (EBSD) patterns for the Cu (111) thin films after annealing at 1,000 °...

RETRACTED: Artificial Intelligence Algorithm Enabled Industrial-Scale Graphene Characterization

“…Graphene and related two-dimensional (2D) materials are a family of exciting materials, which are as small as one to three atoms in vertical direction, but extremely large in horizontal space [1][2][3][4][5]. These systems have attracted significant attention for nanoelectronics and optoelectronics [6][7][8][9][10], non-von Neumann architecture computing [11,12], hybrid flexible and stretchable electronics [13][14][15], and many other applications. As a consequence of their extremely high market value, 2D materials are very attractive to many industries.…”

Large Single‐Crystal Cu Foils with High‐Index Facets by Strain‐Engineered Anomalous Grain Growth

“…[ 2–4 ] Furthermore, high‐index Cu surfaces, with special symmetries and surface states, provide a promising platform for the epitaxial growth of 2D materials, such as graphene and hexagonal boron nitride. [ 5–10 ] To obtain high‐index Cu single crystals, mechanical cutting of bulk Cu single crystals, [ 11,12 ] and epitaxial deposition of Cu films on single‐crystal inorganic substrates [ 3,13,14 ] are mainly employed, though these approaches remain hindered by the limited size and index choice.…”

The Epitaxy of 2D materials growth

“…Two dimensional (2D) materials are potentially the most promising materials for future device applications but in practice, waferscale single crystals of various 2D materials are needed to realize these applications 1,2,3,4 . Recently, the seamless coalescence of millions of unidirectionally aligned islands of a two-dimensional (2D) material epitaxially grown on a substrate has been successfully used to synthesize wafer-scale single crystals of graphene 5,6,7 , hexagonal boron nitride 8,9 and MoS 2 10 , This strategy is expected to be generalized to grow various 2D single crystals in the near future. Nevertheless, the unique behavior of 2D materials growth, different from that predicted by classical theory of epitaxy, necessitates the development of a general theory for the epitaxial growth of 2D materials 5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22 .…”

“…When grown on Cu(111) or Cu(110) surfaces, triangular hBN islands aligned along two opposite directions were found 13,14,15,28 while those grown on Cu(100) surface had four different orientations 14 . Recent works have shown that the unidirectional alignment hBN islands can be successfully achieved by using a Cu substrate with tailored step edges, thus enabling epitaxial growth of wafer-scale hBN single crystals 8,9 .…”

“…Cu(111) surfaces 8,9 , and centimeter scale single crystalline MoS 2 on a vicinal Au(111) surface 10 . Since high-index surfaces can be easily obtained by miscutting a single crystal, we believe that this could be a general strategy for the synthesis of various 2D materials in the future.…”