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Non-epitaxial single-crystal 2D material growth by geometric confinement

Abstract

Two-dimensional (2D) materials and their heterostructures show a promising path for next-generation electronics1,2,3. Nevertheless, 2D-based electronics have not been commercialized, owing mainly to three critical challenges: i) precise kinetic control of layer-by-layer 2D material growth, ii) maintaining a single domain during the growth, and iii) wafer-scale controllability of layer numbers and crystallinity. Here we introduce a deterministic, confined-growth technique that can tackle these three issues simultaneously, thus obtaining wafer-scale single-domain 2D monolayer arrays and their heterostructures on arbitrary substrates. We geometrically confine the growth of the first set of nuclei by defining a selective growth area via patterning SiO2 masks on two-inch substrates. Owing to substantial reduction of the growth duration at the micrometre-scale SiO2 trenches, we obtain wafer-scale single-domain monolayer WSe2 arrays on the arbitrary substrates by filling the trenches via short growth of the first set of nuclei, before the second set of nuclei is introduced, thus without requiring epitaxial seeding. Further growth of transition metal dichalcogenides with the same principle yields the formation of single-domain MoS2/WSe2 heterostructures. Our achievement will lay a strong foundation for 2D materials to fit into industrial settings.

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Fig. 1: Schematic and synthesis mechanism of selective single-domain TMD arrays.
Fig. 2: Selective single-domain synthesis and layer-by-layer confined growth of WSe2.
Fig. 3: Electrical characterization of confined BL-WSe2.
Fig. 4: Confined TMD growth on a-HfO2 deposited on an Si wafer and valley-polarized interlayer excitations in ML-WSe2 and heterobilayer (MoS2/WSe2).

Data availability

The data that support the findings of this study are available from the corresponding author J.K. upon reasonable request.

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Acknowledgements

This material is based on work supported by DARPA (award no. 029584-00001)), NSF (grant no. CMMI-1825731), Intel, IARPA MicroE4AI program, MicroLink Devices, Inc., ROHM Co., and Samsung. Y.H. and K.C. were supported by nanomaterials development program through the National Research Foundation of Korea (NRF) (2022M3H4A1A04096496) funded by the Ministry of Science and ICT, Korea. S.C. and M.-H.J. acknowledge support from the Institute for Basic Science (IBS-R034-D1).

Author information

Authors and Affiliations

Authors

Contributions

K.S.K., S.-H.B. and J.K. conceived the idea and designed the experiments. K.S.K. performed confined TMD growth and coordinated the experiments and characterization. D.L. transferred confined TMDs for multiple characterizations. C.S.C. performed and analysed the TEM measurements. S.S., Ju-Hee Lee and J.-H.P. analysed the electrical characteristics. Y.H. and K.C. performed the DFT calculations. S.C. and M.-H.J. performed the ultrafast laser spectroscopy. K.S.K., D.L., C.S.C., H.K., J.S., S.L., J.S.K., K.H.K., J.M.S., Y.M., B.-I.P., Ju-Hee Lee, H.-S.P., H.S.K., G.Y.Y. and S.-H.B. performed material characterizations. K.S.K., J.-H.P., S.-H.B., and J.K. wrote the manuscript. All authors contributed to and commented on the analysis and discussion of the results.

Corresponding authors

Correspondence to Jin-Hong Park, Sang-Hoon Bae or Jeehwan Kim.

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Competing interests

The authors declare no competing interests.

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Nature thanks Deji Akinwande, Kian Ping Lo and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Growth selectivity on both c-Al2O3, a-HfO2 and a-SiO2 substrates.

ac, AFM images of WSe2 grown on c-Al2O3 (a), a-HfO2 (b), and a-SiO2 (c) under the same CVD process conditions. WSe2 only nucleates on the c-Al2O3 and a-HfO2 instead of a-SiO2 during 20 min of growth duration. This led to a successful selective confined growth of WSe2 on the exposed substrate surface of the micropatterned a-SiO2 trench arrays.

Extended Data Fig. 2 Lateral growth rate and secondary nucleation time for confined growth.

After an incubation time of 5 min, WSe2 was laterally grown up to 10 min, and secondary nucleation occurred after a confined monolayer was maintained for an additional 2 min. Scale bar, 2 μm.

Extended Data Fig. 3 Statistics of WSe2 single domains and multidomains grown in 10-μm-size trench patterns.

Approximately 25% were observed as multidomains (indicated as green and red) in the 10-μm-size trench patterns.

Extended Data Fig. 4 Statistics of WSe2 single domains grown in 2-μm-size trench patterns.

Each denoted top (red), bottom (blue), left (orange), and right (violet) region on the wafer includes 96 dies, whereas the denoted centre region (black) on the wafer includes 100 dies.

Extended Data Fig. 5 Characterization of confined single-domain WSe2.

a, AFM morphology of confined ML-WSe2. b, Root-mean-square (RMS) roughness in a cropped image of a confined ML-WSe2. c, Plan-view HAADF-STEM image of confined ML-WSe2. From plan-view STEM images, selenium vacancy was observed as the dominant point defect with a density of roughly less than 1% (~1.6 × 1013 cm−2). d,e, The atomic percentages of W 4f (d) and Se 3d (e) were 33.72% and 66.28%, respectively by XPS, confirming stoichiometric ratios, and the peak positions of W (4f5/2 and 4f7/2) and Se (3d3/2 and 3d5/2) were 34.8, 32.6, 55.7 and 55 eV, respectively. This result matches well with pure single-crystalline WSe232.

Extended Data Fig. 6 Layer-by-layer confined growth of TMDs.

a, SEM images of second set of nuclei formed on confined monolayer. b, The resulting growth of confined BL-WSe2.

Extended Data Fig. 7 Evaluation of the effect of trench edge on confined BL-WSe2.

a,b STEM images of BL-WSe2 grown at the edge (a) and centre (b) of SiO2 trench. The BL-WSe2 grown at the edge of the trench shows no difference from that at the centre, showing no obvious step bunching, distortions, or curling of BL-WSe2 due to possible strain that can be imposed by the edge.

Extended Data Fig. 8 Yield estimation and statistical analysis on FETs fabricated with confined BL-WSe2.

a, Schematic of wafer enabling to estimate a yield of 93.9% (200/213). Each denoted top region (red), bottom region (blue), left region (orange), and right region (violet) on the wafer includes 43 dies, and denoted centre region (black) on the wafer includes 41 dies. In each die, a centre-located FET was investigated for estimating yield. Coloured and ‘X’-marked dies denote good and defective dies, respectively. bf, Statistical analysis on 200 FETs fabricated with confined BL-WSe2, where field-effect mobility (b), on current (c), subthreshold slope (d), current on/off ratio (e), and threshold voltage (f) were extracted and investigated. g, Summary table for performance labels.

Extended Data Fig. 9 Transfer characteristics of confined BL-WSe2 FETs.

Each graph shows transfer characteristics for over 40 FETs; top region (red), bottom region (blue), left region (orange), right region (violet), and centre region (black) of FET arrays fabricated on 2-inch wafer. Grey curves denote transfer characteristics of defective FETs.

Extended Data Fig. 10 Confined heterobilayer growth of TMDs (MoS2/WSe2) and their characterization.

ac, Raman mapping of the E12g peak on the MoS2/WSe2 heterobilayer at each trench shows that ML-MoS2 is uniformly filled up (~94%) on top of ML-WSe2 at the wafer-scale (a). For the MoS2 layer of the heterobilayer, E12g (384.9 cm−1) and A1g (403.1 cm−1) peaks appeared in the Raman spectra (b). As shown in the representative photoluminescence spectra (c), two peaks associated with direct gaps of MoS2 and WSe2 were detected, confirming the ML-MoS2 formation. A slight shift of the peak from the original direct gap occurs due to a well-known interlayer exciton coupling33. d,e, The high crystalline quality of the MoS2 was further confirmed by ideal XPS spectra observed in narrow-scan data of Mo (3d3/2 and 3d5/2; d) and S (2p1/2 and 2p3/2; e), which showed no signs of defects or atomic mixing related Mo6+ peak at ~236 eV34.

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Kim, K.S., Lee, D., Chang, C.S. et al. Non-epitaxial single-crystal 2D material growth by geometric confinement. Nature (2023). https://doi.org/10.1038/s41586-022-05524-0

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