Introduction

The scalable synthesis of high-quality ultrathin 2D high-κ dielectrics is essential to deliver full theoretical performance of scaled 2D field-effect transistors (FET) for the miniaturization of 2D technologies without considerable device performance loss1,2,3,4,5. To largely reduce the short-channel effects and ensure the gate tunability for ultimate transistor scaling, serval fundamental material requirements of ultrathin 2D high-κ dielectrics need to be simultaneously addressed: (i) maintaining high dielectric constants of > 10 and breakdown field strength > 10 MV cm–1 at the equivalent oxide thickness (EOT) of <1 nm; (ii) large bandgap (> 4.0 eV) and band offset (> 1.0 eV) with the channel semiconductor to suppress the leakage current; (iii) high crystalline quality with low interface and bulk defect/trap density for achieving low subthreshold swing6,7,8,9,10. Traditional three-dimensional amorphous oxide dielectrics such as HfO2 and Al2O3 that work well in silicon technology still face the poor interface with deteriorated performance in 2D FETs, although the molecular seed layers growth and rapid thermal annealing passivation strategy have been used to improve the inhomogeneous islands formation and reduce high-density dangling bonds6,11,12. Single-crystalline layered insulators with excellent dangling-bond-free surface have been developed as gate dielectric for 2D electronic devices including hexagonal boron nitride (h-BN, εr = 5), CaF2 (εr = 8.4), Bi2SeO5 (εr = 15.6), LaOCl (εr = 10.8), VOCl (εr = 11.7), Bi2SiO5 (εr = 30) etc., which could largely reduce the interface carrier scattering, thus demonstrating intrinsic high carrier mobility and suppressing the gate hysteresis13,14,15,16,17,18,19,20. However, the specific 2D layered dielectrics simultaneously satisfying large dielectric constants and high breakdown field strength remain scarce21,22,23, which is also difficult to be scalable down to EOT below 1 nm with maintained dielectric properties. Therefore, the experimental exploration of single-crystalline high-κ dielectric candidates through facial scalable synthesis beyond 2D layered dielectrics has become the important projects for the development of 2D scaled electronic devices6,15.

Recently, although nonlayered 2D materials have significantly advanced the applications of magnetic, electronic, and optoelectronic devices24,25,26,27,28, the currently synthesized nanosheets still face the problems of small lateral size, large thickness, and poor surface quality and lack the strategy for scalable large-area production. Only few nonlayered 2D dielectrics have been reported, as large-area scalable synthesis of 2D nonlayered oxides is usually inhibited by three-dimensional growth in different directions, thus forming the inhomogeneous Volmer-Weber islands or thick nanosheets with high-density bulk and surface defects. Therefore, it is highly desirable to develop a general scalable growth strategy for single-crystalline high-quality 2D nonlayered oxide materials23,29,30. For instance, “single-crystalline” nonlayered m-ZrO2 nanosheets have been synthesized by the 2D template oxidation transformation of 2D ZrS2, demonstrating high dielectric constant of ≈ 19 and low breakdown strength of ≈ 7.22 MV cm−1 due to defective bulk and surface structures29. A large-scale single-crystalline SrTiO3 membrane has been grown by pulsed-laser-deposition technique23, exhibiting ultrahigh dielectric constant of ≈ 30 ~ 100 and low breakdown strength of ≈ 6.04 MV cm−1. The chemical exfoliation of 2D nonlayered Sr2Nb3O10 nanosheets show high dielectric constant of ≈ 24.6, large subthreshold swing, and gate hysteresis due to high-density surface dangling bonds30. These typical synthesis strategies for 2D nonlayered dielectrics not only demonstrate their great potential for the developing high-performance 2D electronic devices but also point out the limited synthesis strategy to achieve the scalable ultrathin nonlayered dielectrics with ultralow defects density in the bulk and surface, thus driving the application for ultimate transistor scaling even EOT below 1 nm. Meanwhile, systematic studies of gate hysteresis in 2D FETs based on nonlayered dielectrics are still lacking, which would be crucial for understanding and addressing high interfacial trap density caused by surface dangling bonds or vacancies23,29,30. Ternary nonlayered CaNb2O6 compounds have been shown interesting physical properties in the photocatalysts, scintillator, and ultraviolet photodetectors31,32,33. In particular, although bulk CaNb2O6 crystals exhibit high dielectric constants of 17–22 and proper bandgap of 4.2 eV33,34,35, scalable large-area of ultrathin 2D structures has not been synthesized and its dielectric properties still have not been explored at two-dimensional limits. Hence, the scalable CVD synthesis strategy of 2D CaNb2O6 with high-crystalline quality is highly demanded for its significant potential for future 2D electronic device applications.

Herein, we report a synthesis strategy of 2D edge-seeded heteroepitaxy of ultrathin 2D nonlayered CaNb2O6 nanosheets via ultraslow kinetics, demonstrating its potential as high-quality high-κ single-crystalline dielectrics with a large breakdown field strength of ~ 12 MV cm−1 and a large band gap of 4.26 eV. The thinnest CaNb2O6 nanosheets achieved a thickness of 2.9 nm (EOT = 0.7 nm), equivalent to two-unit cells, still demonstrated a high dielectric constant of ~ 16. The weak interfacial integration between the 2D CaNb2O6 and mica substrates makes it feasible to transfer these samples for integration into 2D FETs. Surface trap density in 2D CaNb2O6 due to dangling bonds and vacancies is significantly reduced by air-plasma treatment. The few-layer MoS2 FETs with the passivated 2D CaNb2O6 dielectric exhibit a high on/off ratio of over ~ 107, a steep subthreshold slope of 61 mV/dec, a negligible hysteresis and a low leakage current of 10−12 A. Our work showcases the universal 2D edge-seeded heteroepitaxy synthesis and electronic performance of the high-κ dielectric of 2D CaNb2O6, paving the way for scalable nonlayered 2D dielectrics in the emerging 2D electronic applications.

Results

In-plane epitaxial growth of 2D nonlayered CaNb2O6

A sandwich structure precursor is designed by stacking the Nb2O5, NaCl powders on the 0.5%-CaO/Al2O3 plates in ordered sequence (Fig. 1a, b), in which NaCl powders could simultaneously react with top and bottom layers and lower their sublimation temperatures by the formation of oxychloride36. This strategy is intended to control nucleation step and lateral growth kinetics of nonlayered CaNb2O6, which will be discussed later. Figure 1c shows atomic crystal structure of the CaNb2O6 unit cell, which belongs to an orthorhombic Pbcn space group, with the lattice constants of a = 14.94 Å, b = 5.73 Å, c = 5.22 Å, and the α = β = γ = 90° (Supplementary Table 1). The atoms arrangement along [100] direction exhibits ABCACB stacking order (A represents CaO6 layer while B and C represent NbO6 layers), showing strong three-dimensional bonding in all crystal-planes different from van der Waals layered structures (Supplementary Fig. 1). Ultrathin 2D CaNb2O6 single-crystalline nanosheets have been synthesized via atmospheric chemical vapor deposition (APCVD) on the fluorophlogopite substrate at 750–850 °C with our 2D-edge induced heteroepitaxial strategy (Fig. 1d and Supplementary Fig. 2). The comparative experiments with mixed precursor strategy are shown in Supplementary Fig. 3 and the detailed growth parameters could be found in the experimental methods section and Supplementary Figs. 4 and 5. Figure 1e shows the schematic diagram of our proposed spontaneous 2D-layered-edge epitaxy of ultrathin CaNb2O6 nanosheets synthesis mechanism. As three-layer sandwich precursors are heated to the sublimation temperatures, relatively small NbSe2 nuclei could be formed on the substrate after the Se gas molecules react with Nb2O5 clusters. The active layered edges of small NbSe2 nuclei would induce 2D-layered-edge epitaxy of 2D CaNb2O6 nuclei, which is due to the matched lattice constants in the vertical directions when the CaO clusters start to react with Nb2O5 molecules (Supplementary Fig. 6). Our proposed nucleation mechanism can be supported by the obtained in-plane CaNb2O6-NbSe2 heterostructures, which also indicates the formation of initial complex nucleus under the 2D-3D chemical bonding epitaxy principle (Supplementary Figs. 6 and 7). As shown in Supplementary Fig. 8, it can be assumed that CaNb2O6 nanosheets are induced by the cut-off edge of the Nb atom of the NbSe2 nuclei, where the a-axis and c-axis of NbSe2 correspond to the b-axis and a-axis of CaNb2O6 nanosheets, respectively. While the intrinsic instability of NbSe2 also results in the spontaneous decomposition, thus there is without any Se elements doping in the 2D CaNb2O6 products (Supplementary Figs. 6 and 9). Meantime, the amount of Se is also tuned to low levels for the pure growth of 2D CaNb2O6 nanosheets, demonstrating there are without obvious 2D NbSe2 in the final products and the Se cluster just plays a catalyst role to initiate 2D CaNb2O6 nucleation without doping effects. Eventually, in-plane lateral growth of CaNb2O6 nuclei is preferred under low-supply rate kinetic growth rather than vertical growth, which also drives the single-crystalline quality for nonlayered 2D CaNb2O6 by ultraslow kinetic atoms attachments from the active reaction edges, largely reducing the surface and bulk defects37,38.

Fig. 1: Heteroepitaxial nucleation and low-supply-rate kinetic control growth of 2D CaNb2O6 nanosheets.
figure 1

a Schematic synthesis diagram of 2D CaNb2O6 nanosheets by trace CaO precursor supply. b The sandwich structure precursor by stacking of Nb2O5, NaCl, and trace CaO layers, in which molten salt of NaCl could lower the sublimation temperature for top and bottom layers. (The arrows represent the volatilization direction of the powders). (c) Atomic crystal structure of non-layered CaNb2O6 unit-cell. d Optical microscopy (OM) image of 2D CaNb2O6 nanosheets on the mica substrate with uniform optical contrast. e The proposed chemical vapor deposition growth mechanism of ultrathin 2D CaNb2O6 nanosheets, including three steps of 2D edge formation in NbSe2 nucleus, edge-induced heteroepitaxial nucleation, and lateral epitaxy growth. f Atomic force microscope (AFM) image depicts the layer-by-layer growth mode of CaNb2O6 nanosheet, exhibiting a step size of approximately 1.5 nm. g The calculated surface energy results for different CaNb2O6 lattice planes and its corresponding atomic structure arrangements. h Raman spectrum and mapping image (inset, 904 cm−1) for 2D CaNb2O6 nanosheet on the mica substrate, the asterisks present the Raman peaks of CaNb2O6. i High-angle annular dark-field (HAADF) cross-section image of 2D CaNb2O6 nanosheet on mica and its energy dispersive X-ray spectrum (EDX) element mapping of Ca, Nb, and O, respectively. Scale bar: 20 nm. j Cross-sectional high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of CaNb2O6/mica interface. The inset serves as an enlarged view of their interface, emphasizing the clear visibility of the Van der Waals (vdWs) gap.

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Figure 1d shows the optical image of as-grown 2D CaNb2O6 nanosheets with uniform thickness and lateral size around 20 μm, exhibiting rectangular shapes with small aspect ratios. We also note that only ultrasmall 2D CaNb2O6 nanosheets were synthesized without the introduction of Se catalysts (Supplementary Fig. 10), well confirming our proposed growth mechanism. The approximate multiple-step thickness of 1.5 nm observed by atomic force microscope (AFM) is consistent with the unit cell distance (Fig. 1f). This evident step terrace structure reveals similar layer-by-layer growth mechanism as van der Waals layered materials due to slow kinetics, which does not mean that the synthesized CaNb2O6 nanosheets are with such uneven thickness (Supplementary Figs. 5−10). Density functional theory calculation is used to understand the surface energy of various crystal planes of 2D CaNb2O6 crystal, which could indicate the surface stability and anisotropy growth possibility (Fig. 1g). The approach used to compute surface energies in our calculation is the typical slab model, wherein a supercell of the crystal oriented to expose the relevant surface is generated, and atoms are removed from a portion of the supercell to create a vacuum. For a given slab model of a facet with Miller index (hkl), the surface energy \({\gamma }_{{hkl}}\) is given by the following expression39

$${\gamma }_{hkl}=\frac{{E}_{slab\,}^{hkl}-{E}_{bulk\,}^{hkl}\times {n}_{slab}}{2\times {A}_{slab}}$$
(1)

where, \({E}_{slab}^{hkl}\) is the total energy of the slab model, \({E}_{bulk}^{hkl}\) is the energy per atom of the bulk, \({n}_{{slab}}\) is the total number of atoms in the slab structure, \({A}_{{slab}}\) is the surface area of the slab structure, and the factor of 2 in the denominator accounts for the two surfaces in the slab model. The calculated surface energy values of selected facets are \({\gamma }_{100}\) = −152.6 meV Å−2, \({\gamma }_{010}\) = −34.8 meV Å−2, \({\gamma }_{001}\) = -64.9 meV Å-2, and \({\gamma }_{111}\) = -40.0 meV Å−2 (Fig. 1g). Therefore, the surface energy of (100) plane is substantially lowest among the other crystal facets, suggesting the (100) crystal plane would be the dominated planes. The Raman spectrum is attributed to Ag mode of ~ 904 cm−1 and B2g modes of 226 cm−1, 384 cm−1, respectively (Fig. 1h)40, where the uniform intensity distribution observed at 904 cm−1 confirms high-crystalline quality across the entire nanosheet. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) is conducted to investigate the cross-sectional CaNb2O6/mica interface (Fig. 1i, j, and Supplementary Fig. 11). The HAADF-STEM image of CaNb2O6/mica shows a distinct lamellar structure with different contrasts. Notably, the element mapping results for Ca and Nb exhibit excellent agreement with 2D CaNb2O6 sample location, while the oxygen mapping extends to the mica substrate (Fig. 1i). The atomic interface structure of CaNb2O6/mica is shown in Fig. 1j, in which both CaNb2O6 and mica exhibit similar lamellar structures with a clean interface. The perfect van der Waals gap exists at the 2D CaNb2O6/mica interface, providing further evidence for van der Waals epitaxy of 2D non-layered CaNb2O6 on the mica substrate. The ultrasmooth surface and low-density dangling bonds of layered mica substrate significantly facilitate the epitaxy of 2D CaNb2O6 nanosheets without the formation of chemical bonds at the interface, which benefits from the easy transfer of the as-grown 2D CaNb2O6 samples using PMMA-assisted wet transfer technique (Supplementary Figs. 12 and 13). Here, our 2D-layered-edge epitaxy and slow kinetic control strategy demonstrates that even complex three-dimensional oxides can be synthesized as large-area ultra-flat and ultra-thin two-dimensional structures comparable with layered materials through the employment of special CVD steps designs22.

Atomic structure of 2D CaNb2O6 single crystal

The controlled thickness of 2D CaNb2O6 could be achieved through optimizing the dynamic conditions and the growth temperature, allowing for a wide range of tunable thicknesses ranging from the thinnest of 2.9 nm to several tens of nanometers (Fig. 2a). The thinnest thickness of 2.9 nm is equivalent to two unit-cells, also exhibiting tens of micrometers in lateral dimensions41. The 2D CaNb2O6 nanosheets with varying thickness exhibit an atomically smooth surface with a low average roughness below 0.1 nm (Supplementary Fig. 14). The crystal anisotropy of 2D CaNb2O6 nanosheets significantly influences the physical properties such as dielectric constants41. Theoretical calculations for CaNb2O6 single crystals show that they exhibit anisotropic dielectric constants in the xx (a-axis), yy (b-axis), and zz (c-axis) crystal axis directions of 18.2, 15.96, and 21.8, respectively41. The characterization of the dielectric constant anisotropy behavior of the 2D CaNb2O6 nanosheets is of great importance in the evaluation of the dielectric, which can also guide the exploitation of the dielectric anisotropy to achieve best performance enhancement. The optical image of 2D CaNb2O6 nanosheets exhibits rectangular shapes, which is in accordance with in-plane lattice constants. Angle-resolved Raman spectroscopy measurements were used to understand the crystalline anisotropy of our grown samples (Fig. 2b)42,43, clearly illustrating the variation of Raman intensity with incident light angle (ranging from 0° to 360° with a step size of 15°). At 226 and 538 cm−1, the maximum Raman intensity occurs at angles of 0° and 180°, indicating the existence of a double symmetry at 180°. Similarly, the peaks at 384 and 904 cm−1 exhibit minimal intensity at approximately 0° and 180° (Supplementary Figs. 15 and 16). This angle-dependent Raman intensity can be attributed to the inherent anisotropy of 2D CaNb2O6 crystal, in which the short and long side edges could be assigned as the orientation of c-axis and b-axis, respectively.

Fig. 2: Structure control and characterization of as-grown 2D CaNb2O6 nanosheet.
figure 2

a Statistical analysis of the thickness distribution of 2D CaNb2O6 nanosheets. Top insets: OM images of CaNb2O6 nanosheets with different thicknesses. Below insets: corresponding AFM images. Scale bar: 2 μm. b The incident laser angle-dependent 2D mapping Raman spectrum of CaNb2O6 nanosheet. c Electron energy loss spectroscopy (EELS) of CaNb2O6 nanosheets transferred onto Cu grids. d HAADF image of CaNb2O6 nanosheet and EDX element mapping of Ca, Nb, and O, respectively. Scale bar: 250 nm. e Typical HAADF-STEM image of Pbcn CaNb2O6 nanosheet along [100] zone axis and its corresponding atomic sphere model, the yellow lines represent atomic separation. f, g Cross-sectional HAADF-STEM image of 2D CaNb2O6 nanosheet along [001] and [010] zone axis that corresponds to its atomic configuration, the orange box shows local enlarge image of (g). h, i The intensity of Nb and Ca atoms at the red and blue lines of (g).

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Aberration-corrected HAADF-STEM measurement was further used to characterize the atomic structure of 2D CaNb2O6 nanosheets. Electron energy loss spectroscopy (EELS) spectrum (Fig. 2c) demonstrates that the CVD-grown CaNb2O6 nanosheet comprises Ca, Nb, and O elements. Meanwhile, the energy dispersive X-ray spectrum (EDX) mapping images confirm the presence and uniform spatial distribution of Ca, Nb, and O elements (Fig. 2d). The atomic ratio of Ca, Nb, and O is estimated to be approximately 4.4: 8.8: 24.2, which is consistent with the stoichiometric ratio of CaNb2O6. Integrated differential phase contrast STEM (iDPC-STEM) technique has been used to image both light and heavy atoms in a thin sample at sub-Å resolution, which could clearly distinguish oxygen atoms here. Figure 2e depicts the HAADF-STEM image of 2D CaNb2O6 nanosheets, clearly showing the atomic structure arrangement in the (100) crystal plane with recognized oxygen atoms, which is consistent with the crystal structure simulated results. The lattice constants of 2D CaNb2O6 could be determined as b = 0.57 nm and c = 0.52 nm, respectively. The transferred 2D CaNb2O6 TEM sample shows the rectangular shapes in low magnification TEM images, which demonstrates the obvious orthorhombic atoms arrangements (Supplementary Fig. 17). The selected area electron diffraction (SAED) pattern confirms the single-crystal nature of orthorhombic CaNb2O6 flakes with the (002) and (020) crystal plane spots along the [100] zone axis (Supplementary Fig. 17). STEM image and EDX mapping of 2D CaNb2O6 nanosheet that viewed along the [001] zone axis demonstrates the alternating changes of CaO6 and NbO6 layers, showing a lattice constant of a = 1.5 nm (Fig. 2f and Supplementary Fig. 18). The atomic cross-sectional iDPC-STEM image along the [010] direction is presented in Fig. 2g, while the inset showcases an enlarged iDPC-STEM image with the corresponding atomic model. The intensity of Nb and Ca atoms at the (010) crystal plane along the red and blue lines are depicted in Fig. 2h, i, providing further evidence for the anisotropic crystal structure of the CaNb2O6 nanosheet. The cross-sectional HAADF-STEM atomic images and corresponding iDPC-STEM images of CaNb2O6 nanosheets at different magnifications are presented in Supplementary Fig. 19. Additionally, the fast Fourier transform diffraction pattern is consistent with our simulation data, showing no presence of amorphous rings, thus confirming the crystalline quality for whole nanosheet areas (Supplementary Fig. 19). The growth anisotropy and high-crystalline quality of 2D CaNb2O6 nanosheets were more definitively confirmed, providing a solid foundation for further dielectric properties.

Dielectric and leakage properties of 2D CaNb2O6 single crystal

Theoretical electronic band structure of CaNb2O6 is calculated by density functional theory calculation, which demonstrates the CaNb2O6 is an indirect band semiconductor with a large bandgap of 4.17 eV (Fig. 3a). Additionally, the density of states can be observed in Supplementary Fig. 20. The optical bandgap of 2D CaNb2O6 nanosheet was measured through micro-area UV-VIS absorption spectrum by transferring samples onto the quartz substrate, in which the bandgap is calculated about 4.26 eV well consistent with the theoretical bandgap (Fig. 3b)44. The output characteristic shows negligible currents up to the applied voltage of 10 V, indicating the insulating properties for our two-terminal metal-insulator-metal of 2D CaNb2O6 nanosheets devices (Fig. 3c). The dielectric properties of 2D CaNb2O6 nanosheets were systematically investigated for different thickness. Microwave impedance microscopy was used to calibrate thickness-dependent dielectric properties measurement of 2D CaNb2O6 nanosheets on SiO2 and mica substrate at f = 1.0 GHz45. The inset of Fig. 3d shows the schematic illustration of microwave impedance microscopy measurement based on tuning-fork (TF) AFM, in which the uniform microwave microscopy image contrast indicates spatial uniformity of the dielectric constant for the entire CaNb2O6 nanosheets. The dielectric constant of 2D CaNb2O6 nanosheets can be obtained by fitting the microwave signal, which remains stable at approximately 16 with the thickness range of 72 to 2.9 nm (Fig. 3d and Supplementary Fig. 21). The thin layer of 2D CaNb2O6 corresponding to an equivalent oxide thickness (EOT) from 17.5 down to 0.7 nm, still shows without the obvious degradation of the dielectric constant. This unique layer independent dielectric constants are highly required for the scaling-down FET devices, which is one of the best among 2D high-κ materials without size effect (Fig. 3e)22,29,46,47,48. To verify the dielectric properties of 2D CaNb2O6 single crystal, the capacitance-frequency (C-f) and capacitance-voltage (C-V) were performed using metal-insulator-metal (MIM) vertical devices with Cr/Au electrodes as shown the schematic structure and optical images of 2D CaNb2O6 devices in the inset of Fig. 3f and Supplementary Fig. 22. The relative dielectric constant could be calculated by the equation C = Aε0εr/d, where C is the capacitance, A is the electrode area, ε0 is the vacuum permittivity, εr is the relative permittivity, d is the thickness of CaNb2O6 nanosheet. Here, it is clearly seen that the permittivity remains a constant about 16 with the change of frequency and voltage for 14.3 nm CaNb2O6 nanosheet (Fig. 3f and Supplementary Fig. 22). As for the nanosheets with thicknesses of 25.5 nm, the dielectric constant remains at about 16 (Supplementary Fig. 22). The large bandgap and high-κ value of 2D CaNb2O6 nanosheets provides tremendous potential for their application as gate dielectrics.

Fig. 3: Bandgap, dielectric, and leakage properties of 2D insulating CaNb2O6 nanosheet.
figure 3

a Electronic band structure of CaNb2O6 along the high symmetry points. b Microregion ultraviolet-visible spectroscopy (UV-VIS) absorption spectrum of CaNb2O6 nanosheet transferred onto quartz glass substrate. The inset shows corresponding optical bandgap calculated from Taus plots, where α, h, and ν represent the absorption coefficient, Planck constant, and frequency respectively. c Current versus voltage characteristics of 2D insulating CaNb2O6 nanosheet. The inset shows the OM image of 2D CaNb2O6 device. Scale bar: 20 μm. d Thickness-dependent dielectric constant of 2D CaNb2O6 nanosheets measured by microwave impedance microscopy at f = 1.0 GHz. The error bar represents the deflection of the dielectric constant by repeating several measurements on the same samples. Inset: Schematic illustration of microwave impedance microscopy based on tuning-fork (TF) AFM, along with the corresponding AFM/microwave impedance microscopy signal mapping of CaNb2O6 nanosheet on the substrate. e Statistical comparison of thickness-dependent permittivity of 2D dielectrics22,29,46,47,48. f Typical frequency-dependent dielectric constant calculated from C-f characteristics of 2D CaNb2O6 nanosheet (14.3 nm) with a metal-insulator-metal (MIM) device under different bias. The inset shows the structure of Au/CaNb2O6/Au MIM device. g Current versus electric field characteristics of 2D CaNb2O6 nanosheets with different thicknesses by a metal-insulator-metal device measurement for extracting the breakdown field strength. h Comparison of dielectric constant and breakdown field between typical 2D dielectric materials and CaNb2O613,14,19,29,46,47,48,51,52.

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Dielectric breakdown strength is another key factor for evaluating gate dielectric material performance. Based on the MIM vertical devices in Fig. 3f, we measured the leakage current densities of 2D CaNb2O6 nanosheets from 24 down to 10.5 nm. The vertical conductivity measurement of 2D CaNb2O6 nanosheets by the macroscopic electrical measurements still exhibits a good insulating property (Fig. 3g and Supplementary Fig. 23), which demonstrates small leakage characteristics with a large breakdown field strength (~12 MV/cm). And the leakage current density of samples with different thicknesses is below the low power limit of 10−2 A/cm−2, indicating superior high breakdown strength performance that benefits from 2D edge-seeded heteroepitaxy via ultraslow kinetics with high crystalline quality for 2D CaNb2O6. Figure 3h shows the comparison of dielectric constants and breakdown field strength properties of different gate dielectric that applied to 2D FETs. Compared with the recently reported dielectrics, CaNb2O6 has a considerable dielectric constant, and its breakdown field strength is superior to much more other oxides, such as HfO2, ZrO2, Bi2SiO5, SrTiO3, and hBN, etc. The remarkable insulating and leakage properties observed in the 2D CaNb2O6 nanosheets indicate their potential as a dielectric material. The stable high dielectric constant of ~ 16, large bandgap of 4.26 eV, low leakage current density, and considerable breakdown field strength (~12 MV/cm) make 2D CaNb2O6 single crystal become a potential candidate for the gate-dielectric of 2D FETs.

Surface passivation of 2D nonlayered CaNb2O6 nanosheets

High-κ 2D oxide dielectric plays an important role in the development of electronic devices for integrated circuits in the post-Moore’s law era. To understand the electrostatic control characteristics of high-κ CaNb2O6 nanosheets, two kinds of back-gate field-effect transistors were fabricated for MoS2/SiO2 and MoS2/CaNb2O6/SiO2 configurations with same MoS2 nanoflakes channel (Fig. 4a, 4b). The transfer characteristic performance of both types devices is shown in Fig. 4c. The MoS2/CaNb2O6/SiO2 device exhibits smaller hysteresis voltage and subthreshold swing (0.285 V, 584 mV/dec) than that of MoS2/SiO2 devices (1.163 V, 843 mV/dec), indicating a much lower interfacial trap density at the 2D CaNb2O6 surface compared to SiO2 surface due to unique slow kinetic synthesized method (Fig. 4d)19. The negative shift of threshold voltage for MoS2/CaNb2O6/SiO2 transistors may result from the reduction in series capacitance and dielectrics doping effect. Furthermore, scanning Kelvin probe force microscopy (SKPM) images on the heterojunction region show a small potential difference (< 10 mV) in the MoS2 surface potential on the untreated CaNb2O6 dielectric, the SiO2 dielectric and the air-plasma treated CaNb2O6 dielectric (Supplementary Fig. 24), indicating that weak p-type doping may have occurred during the integration of MoS2/CaNb2O6/SiO2 transistors, resulting in a negative shift in the threshold voltage. Additional MoS2 FETs performance data with different thickness of CaNb2O6 shown in Supplementary Fig. 25 demonstrates similar phenomena. Moreover, back-gate MoS2/CaNb2O6/graphene FET was also fabricated to measure the transfer characteristic. The device exhibits a large on/off ratio over 107, a smaller hysteresis, and a subthreshold swing of approximately 145 mV/dec (Fig. 4e). It’s worth to note that the surface quality of 2D CaNb2O6 nanosheets is crucial for their effectiveness as gate dielectric, particularly due to the nonlayered crystal structures with high density of surface dangling bonds. The presence of oxygen and metal vacancies, as well as dangling bonds, can potentially act as interfacial traps and significantly deteriorate device performance. Therefore, it is crucial to passivate the surface traps in our 2D CaNb2O6 nanosheets, despite the presence of only a very small voltage hysteresis.

Fig. 4: Characterization and passivation of surface traps.
figure 4

a Schematic illustration of MoS2/SiO2 and MoS2/CaNb2O6/SiO2 devices. b OM image of the corresponding MoS2 field effect transistors (FETs), including the graphene electrode. The dashed lines represent the shape of the nanosheets. c Transfer characteristic curve of MoS2 back-gate FET using 2D CaNb2O6/SiO2/Si and SiO2/Si device configuration. d The extracted subthreshold swing and hysteresis voltage for two different devices. e Transfer characteristic curve of MoS2 back-gate FET using 2D CaNb2O6 dielectric and graphene electrode, showing a low SS ~ 145 mV/dec. f The height and scanning Kelvin probe force microscopy (SKPM) images for as-grown and air-plasma treated 2D CaNb2O6, respectively, and the work function changes for as-grown and air-plasma treated CaNb2O6. g Schematic diagram of band alignment of air-plasma treated 2D CaNb2O6 and monolayer MoS2. h, i The Ca K-edge and Nb L3-edge X-ray absorption near edge structure (XANES) spectra for as-grown 2D CaNb2O6 and air-plasma treated CaNb2O6, respectively.

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The air-plasma treatment was conducted under humid conditions to passivate the CaNb2O6 surface, in which the hydroxyl group (-OH) is possibly bonded to the CaNb2O6 surface49,50. Thus, the surface trap could be largely reduced by the formation of Nb-OH or Ca-OH occupying the oxygen vacancies and dangling bonds (Supplementary Figs. 26 and 27). The surface potential of as-grown CaNb2O6 and air-plasma-treated CaNb2O6 was investigated by SKPM, in which the samples were transferred onto gold substrates to understand the spatial passivation distribution. The SKPM image of air-plasma treated CaNb2O6 demonstrates uniform color with smaller potential contrast compared to gold substrate than that of as-grown CaNb2O6, indicating a high-density coverage of -OH passivated surface (Fig. 4f). The extracted work function of air-plasma treated CaNb2O6 shows nearly 140 meV higher than as-grown CaNb2O6, in which the work function of Au (5.1 eV) is used as the standard potential reference (Fig. 4f). Compared to the as-grown 2D CaNb2O6, the band offsets between air-plasma treated CaNb2O6 gate dielectric and MoS2 channel material that estimated from optical absorption measurements is more intrinsic (Fig. 4g), which demonstrate the conduction band offset of 1.18 eV and the valance band offset of 1.35 eV. The maintained conduction offset of 1.18 eV is sufficiently large to achieve reliable transistor performance with low leakage current, which could confine charge carriers within the MoS2 channel and reduce the Schottky leakage emission. Furthermore, the Ca K-edge and Nb L3-edge X-ray absorption near edge structure (XANES) spectra of as-grown 2D CaNb2O6 and air-plasma treated CaNb2O6 were measured by the total electron yield (TEY) mode, respectively, which could detect the surface chemical bonds due to the detection depth in the nanometer scale (Fig. 4h, i). The intensity of the blue line of Ca K-edge XANES of air-plasma-treated CaNb2O6 is lower than that of as-grown 2D CaNb2O6, in which the peak position does not show obvious shift. The similar phenomenon for the Nb L3-edge intensity change of air-plasma-treated CaNb2O6 is also observed. The relative intensity reduction of Ca K-edge and Nb L3-edge XANES white lines indicate the electrons are more attracted toward newly bonded oxygen atoms. Therefore, it could be concluded that the surface chemical environment of 2D CaNb2O6 has been modified by the passivation of hydroxyl group, which is consistent with the higher work function observed in CaNb2O6 treated with air-plasma and also indicates a reduced electron emission. Thus, the chemical environments and surface potential change at the surface confirm the reliable passivation by the air-plasma treatment.

Device performance of back-gate MoS2/CaNb2O6 FETs

To investigate the dielectric properties and surface passivation of CaNb2O6 nanosheets, we fabricated few-layer MoS2 FETs devices using surface-functionalized CaNb2O6 nanosheets as gate dielectrics. The schematic diagram of a back-gate MoS2 FET with 2D CaNb2O6 crystal as the gate dielectric is shown in Fig. 5a, which is fabricated by the precise transfer, the metal contact patterning and one-step deposition of source, drain, and back gate electrodes of Ni/Au layers. The device fabrication details could be found in the experimental methods. As shown in Fig. 5b, the transfer integration of exfoliated graphene layers and 2D passivated CaNb2O6 nanosheets act as the bottom gate electrode and bottom gate dielectric, respectively. Cross-sectional HAADF-STEM image and corresponding EDS mapping in Fig. 5c confirm the uniform distribution of the corresponding elements of Mo, S, Ca, Nb, and O in each layer, which is consistent with our deigned device structure. The interface structure of the back-gate MoS2 device is investigated by the cross-sectional bright-field (BF) STEM and HAADF-STEM images as shown in Fig. 5d. The few-layer MoS2 channel and 2D CaNb2O6 gate dielectric demonstrate clear layered structure with clean and flat surfaces, indicating the transfer process does not induce structural damages to these 2D materials. The van der Waals integration is beneficial for the effective gate control and device performance due to the formation of flat and well-defined interface. The ultrasmall van der Waals gap of <0.7 nm was observed between few-layer MoS2 channel and CaNb2O6 dielectrics, which could reduce the carrier tunneling probability and the gate leakage current (Fig. 5d).

Fig. 5: Back-gate MoS2 FETs with passivated 2D CaNb2O6 as the gate dielectric.
figure 5

a Schematic illustration of back-gate MoS2/CaNb2O6/graphene FET. b OM image of the corresponding MoS2 FET. c Cross-sectional HAADF-STEM image and corresponding EDX mapping of Mo, S, Si, C, Ca, Nb, and O, respectively. d Cross-sectional bright-field·(BF) STEM (left) and HAADF-STEM (right) images of MoS2/CaNb2O6/graphene FET device on the SiO2/Si substrate, which characterize the interface quality of as-fabricated MoS2/CaNb2O6/graphene FET. e Transfer characteristic curve of back-gate MoS2 FET at Vds = 0.2 V at room temperature, showing an SS about 61 mV/decade. The blue line represents the SS-fitted extension line. Inset: Output characteristic curve of back-gate MoS2/CaNb2O6/graphene FET with applied gate voltage from −2 V to 2 V. f Forward and backward sweep transfer (Ids-Vbg) curves of the MoS2 FET at Vds = 0.2, 0.4, 0.6, 0.8, 1.0 V. g Forward and backward sweep transfer curves of back-gate devices under different sweep speeds at Vds = 1 V. h Comparison of SS and current on/off ratio performance of FETs based on the 2D dielectrics6,13,16,21,22,23,29,30,48. i Comparison of hysteresis characteristics of the reported 2D insulated FETs19,22,23,46,47,51,55,56,57.

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The electrical characteristics of the 2D CaNb2O6 back-gate MoS2 transistors are shown in the Fig. 5e–g. Our back-gate MoS2 device has the channel length (L) and width (W) of 1.6 μm and 1.5 μm with the thickness of 1.8 nm. For the balance of low gate leakage currents and high capacitance tunability, a 15 nm 2D CaNb2O6 is chosen as the gate dielectric. The linear output curves (Ids-Vds) in the inset of Fig. 5e imply the ohmic contacts of devices, in which the Ids increase dramatically as the gate voltage was tuned from −2 V to 2 V. The transfer curve (Ids-Vbg) in Fig. 5e demonstrates a typical n-type behavior, with an on/off current ratio of 107, an electron mobility of 21.5 cm2 V−1 s−1, and a threshold voltage (Vth) of −1.07 V within the small Vbg range of ± 2 V. Additionally, a lowest subthreshold swing of 61 mV dec−1 is achieved for the back-gate modulated transistor, closing to the Boltzmann limit at room temperature and is superior compared to other nonlayered 2D dielectrics (Fig. 5e and Supplementary Fig. 28). The subthreshold swing of our FETs is dominated by the surface quality of 2D CaNb2O6 in terms of exfoliated MoS2 surface. The interfacial trap density (Dit) could be estimated using the following equation6:

$${{\rm{SS}}}=\,{{\mathrm{ln}}}\,\left(10\right)\frac{{k}_{B}T}{q}\left(1+\frac{q{D}_{{{\rm{it}}}}}{{C}_{{{\rm{G}}}}}\right)$$
(2)

in which SS is the extracted subthreshold swing from the transfer characteristic curve, kB is the Boltzmann constant, T is the absolute temperature, q is the elementary charge, CG is the capacitance of gate dielectric. Based on the exfoliated MoS2 FETs on the air-plasma treated CaNb2O6 nanosheets, the low interface trap density Dit could be estimated to the value of ~ 1.4 × 1011 cm−2 eV−1, much lower than the as-grown samples of 3.6 × 1012 cm−2 eV−1 (Fig. 5e and Supplementary Fig. 29). The reduction of interface trap density by an order of magnitude clearly reveals the high efficiency of air-plasma surface passivation. Figure 5f shows the typical transfer curves of the same device with the Vds from 0.2 V to 1 V, all of which demonstrate gate tunability for the MoS2/CaNb2O6 interface with a large on/off ratio > 107, small SS value of 61 mV dec−1 and negligible gate hysteresis (< 5 mV). In the meantime, the back-gate leakage current (Ig) of the device is reaching the detection limit of as low as ~ 10−13 A, indicating the great potential of high-quality CaNb2O6 as a high-κ dielectric for 2D electronics. Figure 5g also shows the transfer curves under different sweep rates of gate voltage from 0.02 to 0.2 V s−1. The similar transistors working performance is shown without the obvious hysteresis under the different scan rates of gate voltage, which reveals the perfect surface quality of passivated CaNb2O6 nanosheets. The gate hysteresis almost shows negligible change of <5 mV at different Vds bias, while the gate hysteresis also shows only slight increase from 10 to 25 mV under different sweep rate of gate voltage (Supplementary Fig. 30). Supplementary Fig. 31 shows the different devices performance, which strongly supports the reliability for our passivated CVD CaNb2O6 dielectric samples and device integration process. Furthermore, short-channel top-gate MoS2/CaNb2O6 FETs were fabricated to evaluate the gate controllability of CaNb2O6 dielectric material (Supplementary Fig. 32). While the channel length scaled down to 240 nm, the top-gate MoS2 FET also shows a large on/off ratio of >106 and a low DIBL about 79 mV/V, indicating that transistors still maintain good electrostatic regulation ability (Supplementary Fig. 32). As shown in Fig. 5h and Supplementary Table 2, our MoS2 FETs with CaNb2O6 gate dielectric exhibit considerable electrostatic modulation with a large on/off current ratio and a low subthreshold swing, which is competitive among the 2D-material-based transistors with representative high-κ dielectrics such as layered Bi2SeO5 and CaF213,21,22,51,52,53,54. It also shows the superior performance of subthreshold swing or on/ff current ratios compared to nonlayered Sr2Nb3O10, SrTiO3, and ZrO2, suggesting high-quality dielectric crystal with unique surface property23,29,30. The hysteresis window of different 2D gate dielectrics is summarized in Fig. 5i, indicating that the surface passivated 2D CaNb2O6 exhibits an ultra-low hysteresis characteristic, which suggests its considerable potential for various dielectric applications21,23,46,47,51,55,56,57. Due to the large capacitance and perfect interface of the back-gate CaNb2O6 dielectric, the steep subthreshold slope, low operation voltage, and ultra-small gate hysteresis of the device are promising for low-power-consumption transistor applications for future logic circuits.

Discussion

Scalable high-crystalline quality of ultrathin nonlayered 2D CaNb2O6 nanosheets are synthesized through our 2D edge-seeded heteroepitaxy and ultraslow kinetic growth strategy. The in-plane epitaxial nucleation of CaNb2O6 nuclei is initiated by the intermediate 2D NbSe2 edges, where ultrathin and uniform nanosheets are driven by a trace CaO precursor supply kinetics with slow atoms attachments for low-density surface and bulk defect that could exhibit the superior high breakdown strength. These high-quality 2D CaNb2O6 nanosheets demonstrate remarkable layer-independent dielectric constant of ~16 down to EOT = 0.7 nm, which also show large breakdown field strength of ~ 12 MV cm−1 and low leakage current density. Notably, air-plasma treatment effectively passivates the surface with hydroxyl groups, significantly reducing surface trap density of 2D CaNb2O6 to ~ 1.4 × 1011 cm−2 eV−1. Employing these high-κ CaNb2O6 nanosheets as the dielectric layer in back-gated MoS2 FETs results in devices with significantly enhanced performance. This work provides a facile and controllable growth strategy for synthesizing ultrathin nonlayered CaNb2O6 nanosheets, offering valuable insights into the growth mechanism. Furthermore, it showcases the potential of 2D CaNb2O6 as high-κ single-crystalline dielectric for 2D electronic devices.

Methods

Growth of 2D CaNb2O6 nanosheets

The 2D CaNb2O6 nanosheets were synthesized in a single-temperature-zone tube furnace by van der Waals epitaxy. NaCl (99.9%, Aladdin) and Nb2O5 (99.9%, Macklin) powders were laid flat on an aluminum oxide plate containing 0.5 wt% CaO and placed in a ceramic crucible, then, the freshly cleaved substrates (fluorophlogopite) were placed directly above the powders. A small amount of selenium (99.9%, Aladdin, 20 mg to 100 mg) powder precursor and substrates were placed in a 25 mm diameter quartz tube in the upstream low temperature zone (24 cm away from the center of the tubular furnace) and the central constant temperature zone of the tube furnace, respectively. The mixture of argon and hydrogen gas was used as the carrier gas to transport the selenium precursors to the participate in the reaction for the CaNb2O6 nucleation and epitaxy growth. Before heating the system, 500 sccm argon was used to clean the air inside the quartz tube for 10 min. During the synthesis process, the furnace was heated up to 750–850 °C in 30 min with the argon of 20–80 sccm and hydrogen of 0–3 sccm. The growth time was maintained at 8–30 min, then the furnace was rapidly cooled down to room temperature.

Characterization

Optical microscope (Olympus, BX53M) and atomic force microscope (AFM, Oxford Instruments, MFP-3D) were used to characterize the surface morphology and thickness of CaNb2O6 nanosheets. X-ray diffraction measurement (XRD, Bruker, D8 Advance) and Raman spectrum (Raman, Thermo Fisher Scientific, DXR3, excitation wavelength of 532 nm) were carried out to make sure the structure, phase, and composition of CaNb2O6. The atomic-resolution cross-sectional STEM-HAADF, top-view STEM, SAED, and EDS were measured to characterize the atomic structure and elements composition. The potential distribution was measured by scanning Kelvin probe microcopy (SKPM). The microwave impedance microscopy measurement was carried out with a commercial AFM platform (AFM, Park, XE-70). Based on tuning-fork (TF) AFM, the dielectric constant can be fitted from microwave impedance microscopy signals of CaNb2O6 nanosheets at f = 1.0 GHz. The Ca K-edge and Nb L3-edge XANES spectra were collected at room temperature via the total electron yield (TEY) mode at the soft X-ray micro-characterization beamline (SXRMB) beamline in the Canadian Light Source (CLS).

Metal–insulator–metal device fabrication and electrical measurements

The device is fabricated via an Ultraviolet Maskless Lithography machine (TuoTuo Technology (Suzhou) Co., Ltd.). The metal-insulator-metal capacitor was fabricated as follows. A maskless UV lithograph (TTT-07-UV Litho) was used to expose the bottom electrode pattern on the surface of SiO2/Si substrate, and the Cr/Au electrodes (5/10 nm) were obtained by metal vapor deposition thermal evaporation system with a rate of 0.1 Å/s. Then, the CaNb2O6 nanosheets were transferred to the bottom electrodes, and the Cr/Au top electrodes (5/80 nm) also were prepared by lithography combined with vacuum thermal evaporation to obtain the metal-insulator-metal sandwich structure. The capacitance characteristics (C-f and C-V) were measured by Keysight LCR (E4980A). The breakdown voltage was obtained by measuring IV characteristics with a semiconductor analyzer (Fs-Pro).

FET device fabrication and measurements

For the back-gate MoS2 FETs, firstly, graphene was mechanically exfoliated onto SiO2/Si substrate as a bottom electrode, while CaNb2O6 nanosheets were transferred onto graphene with dry transfer technology. After that, exfoliated MoS2 also transferred onto CaNb2O6 dielectric with the help of two-dimensional transfer platform. Then, a standard electron beam lithography (EBL) process and thermal evaporation system were used to fabricate electrodes (Ni/Au, 10/60 nm). The electrical characteristics were measured by a semiconductor analyzer system (Agilent-B1500) with a probe station at room temperature in a lakeshore vacuum chamber of 10−4 Pa.

Details of computational methods

We employ DFT calculations with the meta-generalized-gradient approximation (meta-GGA)33 treatment of the exchange-correlation energy term using the Vienna ab initio simulation package (VASP)58,59. The projector augmented-wave (PAW) method and a plane wave basis set with a cutoff energy of 500 eV are applied to treat the core and valence electrons, respectively. The first Brillouin zone is represented by a Γ-point-centered Monkhorst–Pack k-mesh with a grid density of 2π × 0.02 Å−1 along each dimension. The convergence criteria of total energy and force component are set to be \(1\times {10}^{-6}\) eV and \(1\times {10}^{-3}\) eV/Å, respectively. All slabs possess the same number of each elemental atom with a vacuum thickness of 15 Å.