Robust epitaxy of single-crystal transition-metal dichalcogenides on lanthanum-passivated sapphire.

Two-dimensional (2D) transition-metal dichalcogenide (TMDC) semiconductors are promising materials for beyond-silicon electronics, but the growth of single-crystalline TMDCs has been limited to small wafer sizes in laboratory settings. We report the epitaxy of 150-millimeter single-crystalline TMDC wafers on lanthanum-passivated c-plane sapphire. The single atomic layer of lanthanum reduces the surface symmetry and increases the energy difference between antiparallel domains by as much as 200 times, leading to unidirectional domain alignment. We grew single-crystalline molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2), and tungsten diselenide (WSe2) by means of both chemical vapor deposition (CVD) and metal-organic CVD processes. Wafer-scale spectroscopies and device measurements demonstrate the exceptional quality and uniformity of 150-millimeter TMDCs, with average mobility of 110 and 131 square centimeters per volt per second for MoS2 and WSe2, respectively, at room temperature. Editor's summary: The epitaxial growth of wafer-scale (15-centimeter) single-crystalline transition-metal dichalcogenides on sapphire has been achieved by passivating the surface with lanthanum. Zou et al. showed that a single atomic layer of lanthanum reduced the surface symmetry and made the growth of antiparallel domains energetically unfavorable, which promoted unidirectional domain alignment (see the Perspective by Kim and Kang). Films of molybdenum disulfide transferred onto larger silicon wafers were used to fabricate wafer-scale arrays of field-effect transistors that had high carrier mobilities. —Phil Szuromi INTRODUCTION: Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) are envisioned as the channel materials to advance semiconductor technology to the atomic limit. However, the scalable production of single-crystal TMDCs, a prerequisite for industrial applications, has remained a formidable challenge. This is in part due to the lack of a substrate that enables universal and robust epitaxy of single-oriented TMDCs. RATIONALE: c-plane sapphire [Al2O3(0001)], which is widely used in the epitaxy of III-V semiconductors, is an ideal substrate considering its lattice symmetry, scalability, and cost. However, the top surface layer is nearly central-inversion symmetric, leading to antiparallel TMDC domains and mirror grain boundaries. Although engineered surface steps can break this degeneracy, these methods have not yet met the robustness and uniformity requirement for large-scale production. A more viable strategy toward large-scale production is to reduce the atomic symmetry of the Al2O3(0001) surface layer and greatly enlarge the formation energy difference between antiparallel domains. RESULTS: We show that the atomic central-inversion symmetry of the Al2O3(0001) surface can be broken by monolayer lanthanum (La) passivation, which enables epitaxy of single-crystal TMDCs [including molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2), and tungsten diselenide (WSe2)] up to 150 mm in diameter for the first time. Unlike previous approaches that only work for specific materials and growth processes, the La-Al2O3(0001) substrate provides a robust mechanism because of the reduced P1 surface symmetry and drastically increased energy difference between antiparallel domains by as much as 200 times. This enables reliable growth of single-crystal TMDCs by using a wide range of chemical vapor deposition (CVD) and metal-organic CVD (MOCVD) processes, paving the way for industrial-scale production of 2D semiconductors. Comprehensive characterizations and device measurements confirm the exceptional uniformity and quality of the TMDCs. The TMDCs can be easily peeled off the substrate and transferred on to silicon wafers. Field-effect transistors fabricated on n-type MoS2 and p-type WSe2 exhibited average mobilities of 110 and 131 cm2·V−1·s−1, respectively, at room temperature, setting a new standard for these materials. CONCLUSION: We have established a robust and universal pathway for growing wafer-scale single-crystal TMDCs by means of substrate engineering. The La-Al2O3(0001) substrate overcomes the limitations of previous works regarding narrow process window, material type, and large-area uniformity. Because the sapphire substrate can be readily scalable to 300 mm, we expect that this work will greatly accelerate the entrance of 2D semiconductors into mainstream silicon lines. Robust epitaxy of TMDCs on La-passivated sapphire.: Cartoon illustration of epitaxy of single-crystal TMDC domains on La-Al2O3(0001) substrate, enabled by a symmetry-breaking La surface monolayer. [ABSTRACT FROM AUTHOR]