Introduction

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) are known for their tunable bandgaps and atomically thin thicknesses, which could meet the stringent requirements of advanced transistor technology to extend Moore’s law1,2,3,4,5. Nevertheless, some fundamental limitations are inevitable in monolayer TMDCs-based field-effect transistors (FETs) due to the layer-dependent electronic properties. For instance, low density of state and prominent phonon scattering limit carrier mobility of monolayer TMDCs comparing with multilayer counterparts6,7,8,9. The large bandgap induces large Schottky barrier height and contact resistance between monolayer TMDCs channels and electrodes, which ultimately restricts the current-drive capability10,11. Encouragingly, bilayer TMDCs are identified as the sweet spot to balance performance and power consumption for the sub-3-nm node12. Particularly, bilayer rhombohedral-stacked (3 R) TMDCs break symmetry, causing interlayer charge transfer through the hybridization between occupied state of one layer and unoccupied state of the other layer, and then lead to electric dipole moment and interfacial ferroelectricity generation13,14,15,16,17,18,19,20,21,22. From the point of device, the sliding ferroelectricity in bilayer 3R-TMDCs overcomes interlayer charge defect migration and aggregation, which can be used to construct high-performance ferroelectric semiconductor FETs (FeS-FETs)15,23. Even so, the synthesis of wafer-scale bilayer 3R-TMDCs single crystals is on the way due to the challenge of phase structure and grain orientation synergistic control.

To accurately control the grain orientations of 2D TMDCs, some methodologies have been proposed, such as step-edge-guided epitaxy24,25,26,27, buffer layer mediated growth28,29, and single type of atomic plane formation on sapphire surfaces30. However, the growth of wafer-scale bilayer TMDCs single crystals still faces a great challenge so far, due to the minimal formation energy difference between 3 R and hexagonal-stacked (2H) TMDCs. Interfacial epitaxy of bilayer/multilayer 3R-TMDCs single crystals has been developed on Ni(111) single crystals and a notable nonlinear optical enhancement based on thick 3R-TMDCs is observed8. The remote epitaxy method is designed to prepare centimeter-scale bilayer 3R-WS2 single crystal on a-plane sapphire31, yet, the single-crystal area should be further enlarged to meet the industrial demand. In addition, the homoepitaxy promoted by intrinsic defect is recently reported to grow bilayer/multilayer 3R-MoS2, nevertheless, the thickness uniformity and growth mechanism should be further improved and clarified32. In this regard, developing a new strategy to control the phase structure and grain orientation simultaneously during the growth process is crucial to synthesize bilayer TMDCs single crystals. Notably, the cation doping should modulate the interlayer coupling in bilayer TMDCs and the introduction of metal dopant contributes to the parallel steps formation on sapphire surfaces at low-temperature, which provide an alternative strategy for synthesizing wafer-scale bilayer TMDCs single crystals.

Here we design a hole-doping-assisted chemical vapor deposition (CVD) approach to synthesize a series of two-inch bilayer 3R-TMDCs single crystals on c-plane sapphire. The innovation of this methodology can be summarized as follows: 1) the introduction of hole dopants (e.g. Hf, V, Nb, Ta) increases the interlayer coupling to break the formation energy degeneracy of bilayer 3R- and 2H-stacked TMDCs, which contributes to the phase structure accurate control; 2) the uniform parallel steps are readily evolved on sapphire surfaces at low-temperature with the hole dopants assistance, which promotes the unidirectionally aligned bilayer TMDCs nucleation; 3) the FeS-FETs with high endurance and long retention time are fabricated in view that the unique sliding ferroelectricity restricts interlayer charge defect migration/aggregation. This work provides a choice for synthesizing wafer-scale bilayer 3R-TMDCs ferroelectric semiconductor single crystals and constructing multifunctional devices to extend Moore’s law.

Results

Synthesis of two-inch bilayer TMDCs single crystals

Phase structure and grain orientation controls are fundamentally important for growing wafer-scale bilayer TMDCs single crystals. Thereinto, interlayer coupling plays a significant role in determining the phase structure, and the strong interlayer interaction guarantees the 3R-TMDCs formation8,33. Particularly, the accurate doping is likely to modulate the interlayer spacing and coupling, which provides a pathway for controlling the phase structure of bilayer TMDCs. In addition, uniform parallel steps with appropriate height on the unique sapphire surfaces contribute to the unidirectionally aligned bilayer TMDCs grain nucleation6. The sapphire surface reconstruction is prerequisite for the evolution of parallel steps, and the introduction of metal dopants should trigger the emergence of such an interesting phenomenon. Bearing the above-mentioned point in mind, the hole-doping-assisted CVD strategy is designed to synthesize wafer-scale bilayer TMDCs single crystals on sapphire, with the growth recipe described in Fig. 1a. The hole dopants (e.g. Hf, V, Nb, Ta) increase the interlayer coupling and break the formation energy degeneracy of bilayer 3R- and 2H-TMDCs. Besides, parallel steps are readily evolved on sapphire surfaces, which contribute to the nucleation of unidirectionally aligned bilayer TMDCs grains. Such a synergistic effect of hole-doping promotes the preparation of wafer-scale bilayer TMDCs single crystals (Fig. 1b). Nevertheless, the electron dopants (e.g. Fe, Co) only control the grain orientation and are incapable for modulating the phase structure of bilayer TMDCs, because the interlayer coupling is not enhanced significantly. Furthermore, bilayer TMDCs with random orientations and mixed phase structures (3 R/2H) are achieved without introducing any dopants, as depicted in Fig. 1b and Supplementary Fig. 1.

Fig. 1: Hole-doping-assisted growth of two-inch bilayer transition-metal dichalcogenides (TMDCs) single crystals on sapphire.

a Schematic diagram of phase structure and grain orientation control, as well as the wafer-scale bilayer TMDCs single crystals synthesis on sapphire. b Radar map highlighting the growth results comparison for bilayer pristine, electron- and hole-doped TMDCs. c Interlayer spacing calculations of bilayer pristine, Fe-, Hf-, V-, Nb-, and Ta-MoS2 with 3 R and 2H phases, respectively. d Calculated formation energy difference between 3 R and 2H phases for bilayer pristine, Fe-, Hf-, V-, Nb-, and Ta-MoS2, respectively. e–h Optical microscopy (OM) images of as-grown bilayer Hf-, V-, Nb-, and Ta-MoS2 nanosheets, showing their unidirectional alignment features. i–l Large-area OM images of bilayer Hf-, V-, Nb-, and Ta-MoS2 single-crystal films on c-plane sapphire, revealing their high thickness uniformities. A scratch is made to enhance the contrast of the entire film against the substrate. m Photography of two-inch bilayer Hf-, V-, Nb-, and Ta-MoS2 single crystals on sapphire.

Atomic force microscopy (AFM) measurements are performed on c-plane sapphire substrates to uncover the role of Hf dopants, which are annealed (under Ar/O2 condition) with and without the presence of HfCl4, respectively (Supplementary Fig. 2). Continuous and parallel steps are readily and rapidly formed on sapphire surfaces after annealing at 900 °C for 10 min with HfCl4 assistance, and such steps determine the subsequent nucleation of unidirectionally aligned Hf-MoS2. Nevertheless, for the sapphire substrate after annealing at 1000 °C for 1 h without the presence of HfCl4, the parallel steps are not observed. These results indicate that the introduction of Hf dopants contributes to the sapphire surface reconstruction and the parallel step formation at low-temperature. Notably, prolonging the annealing time to more than 4 h, the parallel steps are also evolved on sapphire surfaces without the presence of HfCl46,24,30, however, the hole-doping-assisted method simplifies this process observably. Furthermore, the contact angle characterizations are also carried out on sapphire substrates, which are annealed (under Ar/O2 condition) with and without the presence of HfCl4. The strong hydrophilicity is observed on the sapphire substrate that is annealed at 900 °C for 10 min with HfCl4 assistance, indicating that the surface chemical property of sapphire is possibly changed by the Hf dopants and which should also influence the grain orientation of bilayer Hf-MoS2. Particularly, the parallel steps are also formed on sapphire surfaces after annealing at 900 °C for 10 min with V2O5, NbCl5, and TaCl4 assistances, which confirms the universality of such a strategy and the feasibility for low-temperature growth of bilayer TMDCs single crystals. Besides, during the CVD synthesis of bilayer TMDCs, the sapphire substrates are not pre-annealing treated, and the parallel steps are readily formed only under the growth temperature with the hole dopant assistances, highly indicative of the simplicity and convenience of this approach.

To uncover the influences of electron- and hole-doping on the interlayer coupling of bilayer TMDCs, density functional theory (DFT) calculations are performed, with the results shown in Fig. 1c,d and Supplementary Figs. 3,4. For bilayer pristine MoS2, the small interlayer spacing is observed in 3 R (2.980 Å) phase comparing with 2H (3.034 Å) counterpart. Intriguingly, such interlayer spacings are decreased for bilayer Fe- (3.022 and 2.979 Å), Hf- (2.978 and 2.868 Å), V- (2.965 and 2.896 Å), Nb (2.975 and 2.900 Å), and Ta-doped (2.986 and 2.913 Å) 2H- and 3R-MoS2, and the smaller values are obtained in bilayer hole-doped MoS2 (e.g. Hf, V, Nb, Ta), indicating that the hole-doping reduces the interlayer spacing and enhances the interlayer coupling in bilayer TMDCs (Fig. 1c). Although the layer spacing of bilayer MoS2 is reduced by the Fe-doping, it is not enough to increase the interlayer coupling significantly and then breaks the formation energy degeneracy of such two phases. In contrast, the hole-doping minimizes the layer spacing and maximizes the interlayer coupling of bilayer MoS2, which enables the bilayer 3R-MoS2 the most stable structure. Meanwhile, the interlayer charge density is increased with decreasing the interlayer spacing, which results in an enhanced interlayer coupling, as confirmed by the electron localization function calculated results in Supplementary Fig. 5. Atomic-resolution cross-sectional high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) measurements are performed in Supplementary Fig. 6. The smallest layer spacing is obtained in bilayer Hf-MoS2 (0.618 nm) comparing with those of bilayer pristine MoS2 (0.674 nm) and bilayer Fe-MoS2 (0.664 nm), reconfirming the strongest interlayer coupling in bilayer Hf-MoS2. Besides, the formation energy differences between 3 R and 2H phases are also obtained for bilayer pristine, Fe-, Hf-, V, Nb, and Ta-MoS2, and such values are changed from 0.4 meV/atom (pristine) to −0.3 meV/atom (Fe-doping), −4.4 meV/atom (Hf-doping), −3.7 meV/atom (V-doping), −3.4 meV/atom (Nb-doping), and −3.0 meV/atom (Ta-doping), as shown in Fig. 1d and Supplementary Table S1. Notably, the larger formation energy differences are observed in hole-doped samples, suggesting that the bilayer 3R-MoS2 becomes the most stable structure after introducing hole dopants (e.g. Hf, V, Nb, Ta), which clarifies the reason of phase structure controlling in bilayer TMDCs.

The hole-doping-assisted CVD method is thus developed to synthesize wafer-scale bilayer TMDCs single crystals on sapphire (Supplementary Fig. 7). Optical microscopy (OM) images in Fig. 1e−h and Supplementary Figs. 8–11 reveal that the CVD-synthesized bilayer Hf-, V-, Nb-, and Ta-MoS2 nanosheets possess nearly 100% unidirectionally aligned features, which contribute to the formation of single-crystal films. Notably, after introducing electron dopants (e.g. Fe, Co), bilayer MoS2 nanosheets with unidirectional orientations are also obtained, as presented in Supplementary Figs. 12,13. Such results manifest that the introduction of dopants promotes the parallel steps evolution and the unidirectionally aligned bilayer TMDCs nucleation (Supplementary Fig. 14). The doping type determination is implemented combining Raman, X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) results, as shown in Supplementary Figs. 15–20. By increasing the growth time, a series of two-inch bilayer MoS2 single-crystal films are synthesized (Fig. 1i−m and Supplementary Figs. 21–24). The digital photography of two-inch bilayer MoS2 single crystals in Fig. 1m reveals an identical color reflection as a symbol of macroscopic homogeneity. This ingenious hole-doping-assisted strategy can be extended to grow the other bilayer TMDCs single crystals, with the results shown in Supplementary Figs. 25–28. Notably, although the layer stacking and electronic band of bilayer/bulk MoS2 can be restructured by the Nb-doping34, the influence of hole-doping on the interlayer coupling should also been considered. Meanwhile, the epitaxy growth of wafer-scale bilayer TMDCs single crystals is crucial. The hole-doping-assisted CVD strategy has addressed such issues ideally.

Hole-doping modulating the phase structure of bilayer TMDCs

A series of electron- and hole-doping atoms, such as Hf, V, Nb, Ta, Fe, and Co, are accurately selected to further clarify their effects on the phase structure of bilayer TMDCs, as highlighted in the periodic table of elements in Fig. 2a. Notably, the symmetry of bilayer TMDCs can be flexibly modified by means of stacking two monolayers with a specific angle35. For example, stacking two monolayers in an antiparallel manner, the bilayer 2H-TMDCs are formed. On the contrary, the bilayer 3R-TMDCs are obtained when such two monolayers are parallelly stacked, which should break the symmetry and result in the formation of electric dipole moment and interfacial ferroelectricity, as shown in Supplementary Fig. 29. To experimentally corroborate the effects of electron- and hole-doping on the interlayer coupling in bilayer TMDCs, low-frequency Raman measurements are carried out on different samples (Fig. 2b and Supplementary Fig. 30). For the low-frequency Raman spectrum of bilayer TMDCs, the in-plane shear and out-of-plane breathing modes originate from the lateral and vertical layer displacements. The decreased Raman characteristic peak intensity indicate the limited shear vibration, and this can be used as the fingerprint for determining the enhanced interlayer coupling and 3R-stacking configuration in bilayer TMDCs, as has been demonstrated in the bilayer/multilayer TMDCs36,37 and multilayer graphene38. The shear/breathing mode intensity ratios are presented in Fig. 2b, the smaller values are observed in bilayer hole-doped 3R-MoS2 than those of bilayer pristine, electron-doped MoS2, and hole-doped 2H-MoS2, suggestive of the enhanced interlayer coupling39, which is beneficial to stabilizing the 3 R phase structure.

Fig. 2: Modulating the phase structure of bilayer TMDCs by hole-doping.

a Schematic diagram of the periodic table of elements, highlighting the electron- and hole-doping atoms. b Low-frequency Raman spectra of different samples. The characteristic peak (highlighted by the shaded areas) intensities of all the tested samples are calibrated by the characteristic peak of Si. The similar full width at half maximum implies the same crystal quality. The shear/breathing mode intensity ratios are presented. The smaller values in bilayer hole-doped 3R-MoS2 than those of bilayer pristine, electron-doped MoS2, and hole-doped 2H-MoS2 indicates the enhanced interlayer coupling. c Second-harmonic generation (SHG) spectra of bilayer 3R- and 2H-MoS2, as well as monolayer MoS2. The characteristic peaks are detected in bilayer 3R-MoS2 and monolayer MoS2. d Statistic distributions of phase structure for bilayer pristine, Hf-, V-, Nb-, Ta-, Fe-, and Co-doped MoS2 and WS2, after analyzing over 500 nanosheets for all the samples.

Second-harmonic generation (SHG) is a sensitive and nondestructive detection technology for identifying the stacking order of CVD-derived bilayer TMDCs, as depicted in Supplementary Fig. 31, which is then performed on bilayer pristine, Hf-, V-, Nb-, Ta-, Fe-, and Co-doped MoS2 and WS2, as well as monolayer MoS2, where the characteristic peaks at 532 nm are observed for bilayer 3R-TMDCs and monolayer MoS2, indicative of the broken inversion symmetry feature (Fig. 2c). 500 SHG spectra are captured from different nanosheets for all the samples, and most of them (more than 97.5%) present the 3 R phase structures for bilayer hole-doped MoS2 and WS2 (e.g. Hf, V, Nb, Ta). However, the coexisted 3 R/2H phases are obtained for bilayer electron-doped MoS2 and WS2 (e.g. Fe, Co), similar with the bilayer pristine counterparts (Fig. 2d). Such results indicate that the hole-doping is a high-efficiency strategy for modulating the phase structure of bilayer TMDCs, consistent with the theoretical results; nevertheless, the electron-doping is incapable. In addition, the bilayer pristine MoS2 has been synthesized on sapphire substrates, which possess the parallel steps (annealing at 1000 °C for 5 h), and the obtained bilayer MoS2 nanosheets own the unidirectionally aligned grain orientations. However, both the 3 R and 2H phases are observed (Supplementary Fig. 32), highly indicative of the crucial role of hole-doping in determining the phase structures of bilayer TMDCs.

Furthermore, a systematic exploration is carried out to uncover the influence of Hf dopant concentration on the phase purity and grain orientation of bilayer MoS2. By changing the usage amount of dopant precursor, the grain orientation and phase structure are altered accordingly, and the dopant dosage threshold is thus evaluated to be 3 mg (corresponding to the doping concentration of 1.0%) for synthesizing bilayer Hf-MoS2 single crystals (Supplementary Fig. 33). Notably, further increasing the dopant dosages, the 3 R phases are still preserved, as shown in Supplementary Fig. 34. Besides, bilayer 3R-stacked Hf-MoS2 nanosheets are obtained with the growth temperatures changing from 850 to 1000 °C (Supplementary Fig. 35), indicating that the temperature is not the dominant factor for determining the phase structure of bilayer MoS2. In short, the introduction of hole dopant reduces the formation energy of 3R-TMDCs and promotes the nucleation of unidirectionally aligned grains, which are crucial for synthesizing bilayer TMDCs single crystals.

Single-crystal determination of bilayer Hf-MoS2

CVD-synthesized bilayer Hf-MoS2 is chose as an example to determine the single-crystal property by performing the multiscale characterizations on as-grown and transferred samples. A low-magnification transmission electron microscopy (TEM) image captured from the merged area of two unidirectional bilayer Hf-MoS2 nanosheets is shown in Fig. 3a, the transparent feature indicates the atomically thin thickness. The selected-area electron diffraction (SAED) pattern presents only one set of hexagonally arranged diffraction spots, manifesting the unidirectional alignment and seamless stitching of these two bilayer Hf-MoS2 nanosheets (Fig. 3b). The 3 R phase structure is also confirmed because of the higher peak intensities of second-order diffractions than those of the first-order counterparts. In addition, a series of SAED patterns collected from different areas of bilayer Hf-MoS2 films shows nearly identical lattice orientations, highly suggestive of the single-crystal property (Fig. 3b and Supplementary Fig. 36).

Fig. 3: Multiscale characterizations of bilayer Hf-MoS2 single-crystal property.

a Low-magnification transmission electron microscopy (TEM) image taken from the merging area of two unidirectional bilayer Hf-MoS2 nanosheets. b Selected-area electron diffraction (SAED) patterns captured from different regions of bilayer Hf-MoS2 films. c–e Atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images obtained from the marked regions in (a). The doped atoms are highlighted by the red circles. f SHG mapping image of two merged unidirectional bilayer Hf-MoS2 nanosheets on sapphire. The edges are highlighted by the black dashed lines. g Large-area SHG mapping image of bilayer Hf-MoS2 single-crystal films. h OM images of as-grown bilayer Hf-MoS2 films on sapphire before and after Ar/O2 (with a flow rate ratio of 10:1) etching at 400 °C. i OM images of as-grown bilayer pristine MoS2 polycrystalline films on sapphire before and after Ar/O2 (with a flow rate ratio of 10:1) etching at 400 °C. The grain boundary is highlighted by the white arrow. j Raman intensity mapping image of E12 g mode for as-grown bilayer Hf-MoS2 films. k, l Color-coded images of Raman line scan mapping along the horizontal and longitudinal directions, respectively.

Atomic-resolution HAADF-STEM images in Fig. 3c−e clearly demonstrate the microscopic evidence of seamless stitching between adjacent bilayer Hf-MoS2 nanosheets without any grain boundaries. The electron energy loss spectroscopy (EELS), energy-dispersive spectroscopy (EDS), and simulative atomic-resolution HAADF-STEM results consistently confirm the presence of Hf dopant (Supplementary Fig. 37). In addition, the doped atoms (e.g. Hf, V, Nb, Ta, Fe, Co) are also identified by the STEM intensity analyses and EDS mapping images, as shown in Supplementary Figs. 38–41. The Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) results in Supplementary Fig. 42 show that some Hf dopants reside on the substrate surfaces; however, the atomic-resolution HAADF-STEM, EDS, EELS, and XPS results reconfirm that the Hf atoms are incorporated into the lattices of bilayer MoS2. Besides, the atomic-resolution HAADF-STEM and corresponding simulative images, as well as intensity curves of bilayer Hf-MoS2 in Supplementary Fig. 37c–f reveal that the doped Hf atoms are located at both upper and lower layers. This atomic structure is also adopted during the calculation processes. Atomic-resolution HAADF-STEM images are achieved from different locations of a large-area bilayer Hf-MoS2 film to determine the single-crystalline feature, the 3 R stacking configuration, and the uniform distribution of Hf doped atoms, as demonstrated in Supplementary Fig. 43. Notably, although the doping concentration is relatively low, the nucleation energy of hole-doped bilayer MoS2 is much lower than the pristine counterpart (Supplementary Table 1) and the amounts of dopant precursors are enough, which guarantee the phase structure control and the sapphire surface reconstruction.

Figure 3f displays the SHG mapping image captured from two merged bilayer Hf-MoS2 nanosheets; the uniform color contrast indicates their seamless stitching. Meanwhile, large-area SHG mapping are also performed on different regions of two-inch bilayer Hf-MoS2 films; no obvious intensity drop is observed, reconfirming the single-crystal property and consistent stacking sequence of 3 R phase over the wafer-scale (Fig. 3g and Supplementary Fig. 44). Low-energy electron diffraction (LEED) patterns measured at nine random locations across 1 cm2 bilayer Hf-MoS2 are displayed in Supplementary Fig. 45 the homogenous orientations indicate the single-crystal feature of bilayer Hf-MoS2. To further corroborate this result at macroscopic scale, Ar/O2 (with a flow rate ratio of 10:1) etching experiments were carried out on as-grown samples. Interestingly, almost no contrast variation is detected for the large-area bilayer Hf-MoS2 films, manifesting the single-crystal feature (Fig. 3h). However, the grain boundaries are observed for the bilayer pristine MoS2 polycrystalline films (indicated by the white arrow in Fig. 3i). Large-area Raman intensity mapping of E12 g mode is performed on different regions of wafer-scale bilayer Hf-MoS2 single-crystal films, the homogeneous distribution confirms the high thickness uniformity and outstanding crystalline quality (Fig. 3j and Supplementary Fig. 46). Additionally, using Raman line scanning, the thickness uniformity of CVD-derived bilayer Hf-MoS2 single-crystal films is also investigated, nearly the same peak positions suggest the high uniformity (Fig. 3k, l).

Device performances of bilayer Hf-MoS2 FeS-FETs

Bilayer 3R-TMDCs break the symmetry and induce the electric dipole moment and interfacial ferroelectricity formation through interlayer charge transfer22,23. Piezoresponse force microscopy (PFM) measurements are performed on transferred bilayer Hf-MoS2 onto Au/Si to determine its ferroelectricity (Supplementary Fig. 47). The well-defined phase contrast in PFM image and the switchable polarization characteristic collectively corroborate the intrinsic ferroelectric polarization in bilayer Hf-MoS2. In addition, the macroscopic ferroelectric hysteresis loop is also measured, and the remnant polarization value is calculated to be 3.87 μC/cm2 (Supplementary Fig. 48), higher than the other 2D ferroelectric materials, such as bilayer h-BN (0.68 μC/cm2)40, graphene/h-BN heterostructure (0.18 μC/cm2)41, and bilayer VS2 (0.202 μC/cm2)42. Furthermore, the positive up negative down (PUND) measurements are performed to obtain the remnant polarization of bilayer Hf-MoS2 accurately and the corresponding value is extracted to be 3.68 μC/cm2 at 1 V (Supplementary Fig. 49). To evaluate the device performances of bilayer Hf-MoS2, the back-gated FETs are fabricated in accordance with the benchmarking guidelines, as shown in Fig. 4a. The high electron mobility (53 cm2 V*−1 s−1) and remarkable on/off current ratio (107) are obtained in bilayer Hf-MoS2 (Fig. 4b), larger than those of bilayer pristine 2H-MoS2 (14 cm2 V−1 s−*1 and 105), as presented in Supplementary Fig. 50. The improved contact between channel and electrode, as well as the strong interlayer coupling are responsible for the improvement of device performances in bilayer Hf-MoS2, as revealed by the output characteristic curves in Fig. 4c and the contact resistances in Supplementary Fig. 51. Additionally, the controlled doping modulates the band structure of bilayer MoS2 and matches well with the work function of electrode, which reduces the Schottky barrier and contact resistance43. The low doping concentration of Hf and the work function difference between metal gate (Cr/Au) and semiconductor channel (bilayer Hf-MoS2) result in that the threshold voltage (Vth) shows insignificant shift comparing with the bilayer pristine MoS2. The electron mobility mapping of bilayer Hf-MoS2 is shown in Supplementary Fig. 52, the homogeneous color distribution suggests the high device performance uniformity.

Fig. 4: Device performances of chemical vapor deposition (CVD)-derived bilayer Hf-MoS2.

a Schematic diagram of the back-gated field-effect transistor (FET) of bilayer Hf-MoS2. b, c Transfer and output characteristic curves of bilayer Hf-MoS2 back-gated FETs. The length/width of channel is set to be 4 and 10 μm, respectively. VDS, LCH, W, and μ represent the source-drain voltage, length of channel, width of channel, and electron mobility, respectively. d Electronic band diagrams of transferred bilayer Hf-MoS2 with P-down and P-up states on SiO2/Si. LRS and HRS represent the low-resistance state and high-resistance state, respectively. e Hysteresis transfer characteristic loops of bilayer Hf-MoS2 under different VDS values. f, g Hysteresis transfer characteristic loops of bilayer Hf-MoS2 and bilayer pristine 2H-MoS2 under different sweeping rates. h Hysteresis loop windows plotted with different sweeping rates for bilayer Hf-MoS2 and bilayer pristine 2H-MoS2. i, j Endurance and retention time measurements of bilayer Hf-MoS2 by controlling the VGS values (+80 V write, 0 V read and –80 V erase, 0 V read). The fitting results are highlighted by the dashed lines. k Comparisons of endurance and retention time for bilayer Hf-MoS2 with the other 2D memory devices23,44,45,46,47,48,49,50,51,52,53,54,55.

In view of robust room-temperature ferroelectricity in bilayer Hf-MoS2, FeS-FETs are constructed using bilayer Hf-MoS2 as the channels. As shown in Fig. 4d, when the bilayer Hf-MoS2 is in a polarization-down (P-down) state, the electronic bands in channel are bended by the mobile carriers those are redistributed by the electric dipole moment, and which increases the carrier density. On the contrary, the carrier density in channel is reduced as the bilayer Hf-MoS2 ferroelectricity is in a polarization-up (P-up) state. Based on the above design principle, the gate voltage (VGS) should induce the polarization switch and produce the hysteresis effect and memory window in transfer characteristic curves. In this regard, by controlling the VGS, bilayer Hf-MoS2 FeS-FETs can be alternatively operated between on and off states. The transfer characteristic loops of bilayer Hf-MoS2 at different source-drain voltages (VDS) are shown in Fig. 4e, in which a large clockwise hysteresis window of 40 V is detected under −80 to +80 V sweeping, suggesting the application potential in logic-in-memory devices. The hysteresis window direction is related to the gate voltage and the effective oxide thickness (EOT) of dielectric layer. Due to the high EOT condition of SiO2 (with the thickness of 275 nm) dielectric layer in bilayer Hf-MoS2 FeS-FETs, the clockwise hysteresis loop is thus obtained. Similar phenomenon has also been demonstrated in bilayer pristine 3R-MoS244. To further confirm the ferroelectric polarization rather than the interface charge trapping results in such a memory effect, the transfer characteristic loops of bilayer Hf-MoS2 and bilayer pristine 2H-MoS2 are achieved under different scanning rates (Fig. 4f,g and Supplementary Fig. 53). Notably, the hysteresis window of bilayer Hf-MoS2 remains unchanged and the counterpart of bilayer pristine 2H-MoS2 is significantly reduced (Fig. 4h). To explore the influence of doping concentration on the electron mobility and hysteresis window of bilayer Hf-MoS2, the transfer characteristic loops and corresponding electron mobility/hysteresis window statistics are shown in Supplementary Fig. 54. With increasing the doping concentrations, the electron mobilities and hysteresis windows are increased accordingly, possibly due to the enhanced interlayer coupling and improved electrode contact as well as the increasement of ferroelectric polarization at high doping concentrations.

The non-volatile memory (NVM) performances of bilayer Hf-MoS2 FeS-FETs are evaluated through measuring the endurance and retention time by controlling the VGS. Figure 4i shows the endurance characteristic of a bilayer Hf-MoS2 FeS-FET that is switched up to 108,000 cycles between high-resistance states (HRS) and low-resistance states (LRS). Such two individual states possessing more than one order of magnitude differences (approximate 30) are stable and rewritable without significant degradation. Furthermore, the retention time of a bilayer Hf-MoS2 FeS-FET is obtained by programming and erasing with the VGS values of ±80 V for 1 s and reading at 0 V (Fig. 4j). The non-volatile program and erase states with the difference of more than two orders of magnitude (~1.7 × 102) are observed within the periods of up to 2 × 105 s. The projected retention time shows that the individual states should be able to be maintained for a timescale of the order of one year. Such results demonstrate that the high endurance and long data retention at room-temperature have been achieved in bilayer Hf-MoS2 FeS-FET, which are among the best results obtained for 2D ferroelectric semiconductors23,45,46,47,[48](#ref-CR48 “Han, W. et al. Phas