Epitaxy growth of centimeter-sized p-type monolayer molybdenum disulfide single crystals with high hole mobility

Abstract

Epitaxy growth and accurate doping of wafer-scale two-dimensional (2D) semiconductor single crystals are two crucial issues to break the scaling limitation of transistors. Despite remarkable progresses have been realized in preparing large-area 2D n-type semiconductor single crystals, the epitaxy growth of wafer-scale p-type semiconductor single crystals have yet to be realized. Here an in-situ hole doping strategy is proposed to control the domain orientation and modulate the electronic property of monolayer MoS₂, which enable the achievement of centimeter-sized p-type semiconductor single crystals. The introduction of hole dopants (e.g., V₂O₅, NH₄VO₃, and VCl₃) contributes to the parallel steps formation on sapphire surfaces to induce the unidirectional monolayer MoS₂ domains nucleation. Meanwhile, the electronic property of monolayer MoS₂ is also changed from n-type semiconducting to p-type. Benefiting from the different doping abilities of V₂O₅, NH₄VO₃, and VCl₃, the V doping concentrations can be regulated within a large range from 0.36 to 12.60 at%, which delivers an excellent hole mobility (17.6 cm²·V⁻¹·s⁻¹). This work provides a new avenue for synthesizing wafer-scale 2D p-type semiconductor single crystals, which will enrich the device functions and extend Moore’s law.

Graphical abstract

Keywords

epitaxy growth / centimeter-sized single crystal / p-type semiconductor / monolayer V-doped MoS ₂ / unidirectional domain orientation

1 Introduction

With the field-effect transistor (FET) size approaching to the physical limitation, developing high-performance integrated circuit faces great challenges from channel material selection to device structure design [1−4]. Encouragingly, two-dimensional (2D) semiconducting transition-metal dichalcogenides (TMDCs) possess an unprecedented potential to boost integrated circuit performances and extend Moore’s law beyond 1 nm technology nodes, because of their atomically thin thicknesses, tunable electronic structures, and silicon-compatible features [5−14]. To further improve the compatibility of 2D semiconducting TMDCs with the existing complementary metal oxide semiconductor (CMOS) technology, many fundamental challenges should be resolved for meeting the industry criteria of practical applications in electronics. For example, large-area growth of 2D semiconducting TMDCs single crystals to decrease grain boundaries [15, 16], the effective doping in 2D semiconducting TMDCs to modulate their electronic structures [17], and the optimization design of interfaces to reduce contact resistances [18−21]. Thereinto, epitaxy growth and controlled doping of wafer-scale 2D semiconducting TMDCs single crystals are two central tasks for extending Moore’s law beyond silicon. Notably, great breakthroughs have been made in growing wafer-scale 2D TMDCs single crystals. For instance, large-area 2H-MoTe₂ single-crystal films have been synthesized on Si/SiO₂ substrates using the transformation and recrystallization strategy [22, 23].

The domain orientation control and subsequently seamless splicing of unidirectionally aligned domains are crucial for synthesizing wafer-scale 2D single crystals, such as graphene [24] and h-BN [25−28]. To accurately modulate domain orientations of 2D semiconducting TMDCs, the step-edge-guided strategy is developed, and two-inch monolayer MoS₂ and WS₂ single crystals have been synthesized on custom-designed sapphire substrates and β-Ga₂O₃ single crystals, where the nucleation of domain is along with the step edge [29−32]. In addition, inducing the single type of atomic plane formation by reconstructing sapphire surface is considered as an alternative strategy for growing monolayer MoS₂ single crystals [33]. Besides, introducing a specific interfacial reconstructed layer is another effective method for synthesizing wafer-scale monolayer MoS₂ single crystals [34]. Notably, the bilayer [35] and multilayer [36] TMDCs single crystals have also been grown using the step-edge-guided strategy. However, the controlled synthesis of wafer-scale single crystals mainly focuses on 2D n-type semiconductors, and p-type semiconducting TMDCs single crystals have yet to be obtained due to the different growth thermodynamics and kinetics. In-situ substitution doping is an efficient and nondestructive methods to modulate the electronic properties of 2D semiconductors [37−40]. For example, after introducing V dopants and increasing discrete defect levels near the valence-band maximum, p-type monolayer WSe₂ FETs are thus fabricated [41]. The 4d electron orbital of Nb dopant contributes to increasing the density of states of monolayer WS₂ around the Fermi level, and resulting in the n- to p-type conversion [42, 43]. Nevertheless, the synergetic control of domain orientations and electronic properties of 2D semiconducting TMDCs, as well as the growth of wafer-scale p-type semiconductor single crystals are still facing many daunting challenges.

Here we develop a unique V-assisted chemical vapor deposition (CVD) approach to synthesize centimeter-sized p-type monolayer MoS₂ single crystals with tunable V doping concentrations on c-plane sapphire. The innovation of such a strategy can be summarized as follows: i) the introduction of V dopants facilitates the formation of parallel steps on c-plane sapphire surfaces without miscut angles and promotes the edge-nucleation of unidirectionally aligned monolayer MoS₂ domains; ii) the V doping concentration can be adjusted by varying the type of precursors, which contributes to changing the electronic property of monolayer MoS₂ from n-type semiconducting to p-type. This work provides a new paradigm for epitaxy growth of wafer-scale 2D p-type semiconductor single crystals, which enables the further device downscaling and extends Moore’s law.

2 Experimental section

2.1 CVD synthesis of monolayer V-MoS₂ single crystals

Before the CVD growth, c-plane sapphire substrates (purchased from Nanjing MKNANO Tech. Co., Ltd. (www.mukenano.com)) were ultrasonic cleaned in water and ethanol, respectively. The CVD growth of monolayer V-MoS₂ single crystals was conducted in a dual-heating-zone furnace. In detail, MoO₃ (10 mg, 99.99%, Alfa Aesar), VCl₃ (5 mg, 99.9%, Alfa Aesar), and S (excess, 99.99%, Alfa Aesar) powders were selected as the precursors and then placed in three separate open quartz boats, respectively. The c-plane sapphire with single-side polished feature had an orientation offset M-axis 0.2° ± 0.1° and the roughness of R_a < 0.5 nm. The temperatures of S and MoO₃ powders were raised to 120 and 900 °C, respectively, within 50 minutes and held for 8 minutes to obtain monolayer V-MoS₂ nanosheets with unidirectional orientations. To synthesize monolayer V-MoS₂ single-crystal films on c-plane sapphire substrates, the growth time was extended to 20 minutes. Before the CVD growth, the Ar carrier gas with a flow rate of 500 sccm was ventilated for 10 minutes to remove the water and oxygen. During the growth processes, the Ar carrier gas flow rate was maintained to be 50 sccm. VCl₃ powders were located at the middle of such two temperature zones, with the temperature of 700−800 °C. After completing the CVD growth process, the temperatures naturally dropped to 500−600 °C, and then the furnace cover was opened and cooled down to room-temperature. Monolayer V₂O₅- and NH₃VO₄-doped MoS₂ were synthesized in the same way, and the heat retention time was set to be 8 minutes.

2.2 Transfer and characterization of monolayer V-MoS₂

To perform various characterizations, CVD-synthesized monolayer V-MoS₂ single crystals were transferred onto different substrates using the polystyrene (PS)-assisted method. In detail, 13 g PS particles were dissolved into 100 mL toluene. The obtained PS solutions were spin coated onto V-MoS₂/sapphire and then baked for 20 minutes at 120 °C. The PS/monolayer V-MoS₂/sapphire edges were scraped for the fast separation. Then, the PS/monolayer V-MoS₂/sapphire was floated on the surface of water. When the PS/monolayer V-MoS₂ was transferred onto target substrates, the baking step (at 110 °C for 15 minutes) was carried out to remove the water residues. In the end, the PS/monolayer V-MoS₂ was soaked in toluene for 30 minutes to dissolve PS and then blown dry by the N₂ gas. The morphology, domain size, thickness, optical property, chemical component, and crystalline quality of monolayer V-MoS₂ were systematically characterized by OM (Olympus, BX53M), AFM (Dimension Icon, Bruker), XPS (ESCALAB 250Xi, Mg Kα as the excitation source), Raman spectroscopy (XploRA Plus, Horiba, with the excitation light of 532 nm), and TEM (JEOL JEM-F200 and JEM-NEOARM, with the acceleration voltage of 200 kV). The atomic-resolution HAADF-STEM and EDS results were achieved from a spherical-aberration-corrected STEM JEOL JEM-ARM200CF with an acceleration voltage of 80 kV. SHG characterizations were performed using the same system of Raman under the excitation light of 1064 nm with an average power of 800 μW. The ferromagnetic properties of monolayer V-MoS₂ were evaluated using a physical property measurement system (PPMS, Quantum Design, Dynacool).

2.3 Device fabrication and measurement

The UV lithography was used to pattern the source and drain. The electron beam evaporation system was employed for depositing Cr/Au electrodes with the thicknesses of 5/70 nm. The SiO₂ with the thickness of 275 nm was used as the dielectric layer and the length/width of channel were set to be 4 and 10 μm, respectively. The electrical transport measurements were performed under the vacuum (<1.3 mTorr) and dark conditions using a semiconductor characterization system (Keithley 4200-SCS).

2.4 Carrier mobility calculation

The carrier mobility of monolayer V-MoS₂ and pristine monolayer MoS₂ were calculated based on the transfer characteristic curves according to the following expression:

\mu ={\displaystyle \frac{L({\mathrm{d}I}_{\mathrm{D}\mathrm{S}}/{\mathrm{d}I}_{\mathrm{G}\mathrm{S}})}{W{C}_{\mathrm{O}\mathrm{X}}{V}_{\mathrm{D}\mathrm{S}}}},

where L (4 μm) and W (10 μm) were the length and width of channel, respectively, and C_OX was the gate capacitance of SiO₂, which could be calculated by C_OX = ϵ₀ϵ_r/d (ϵ₀ was the vacuum permittivity, and ϵ_r (3.9) and d (275 nm) were the dielectric constant and thickness of SiO₂, respectively).

3 Results and discussion

The periodic parallel steps can be formed on sapphire surfaces through the high-temperature annealing treatment or using the custom-designed sapphire with miscut angle [44]. Such steps play a pivotal role in controlling domain orientations and synthesizing wafer-scale 2D TMDCs single crystals [29, 30]. Particularly, the sapphire surface reconstruction is prerequisite for the parallel step evolution, and introducing metal dopants will induce the emergence of such a phenomenon at low temperature [45]. Meanwhile, the accurate metal doping will also modulate the electronic properties of 2D TMDCs, which can result in the conversion from n-type semiconducting to p-type. In this context, an ingenious V-assisted CVD approach is developed to synthesize centimeter-sized p-type monolayer MoS₂ single crystals on c-plane sapphire, with the growth recipe described in Fig.1(a) and Fig. S1, respectively. During the CVD growth process, the introduction of V dopants (e.g., V₂O₅, NH₄VO₃, and VCl₃) reduces the formation energy of parallel steps on sapphire surfaces and facilitates the edge-nucleation of unidirectionally aligned MoS₂ domains. Optical microscopy (OM) image in Fig.1(b) shows that the CVD-synthesized V-MoS₂ nanosheets exhibit nearly 100% unidirectional alignment feature, different from the pristine MoS₂ with two preferential orientations [Fig.1(c)]. The domain orientation distributions of V-MoS₂ and pristine MoS₂ in Fig.1(d) and (e) further corroborate this result. Atomic force microscopy (AFM) image and corresponding height profile analysis in Fig. S2 reveal that the thickness of V-MoS₂ is measured to be 0.74 nm, corresponding to the monolayer. These results indicate that the introduction of V dopants contributes to the evolution of parallel steps with uniform and optimal heights on sapphire surfaces and then results in the formation of unidirectionally aligned monolayer MoS₂ domains, as demonstrated in Fig. S3.

X-ray photoelectron spectroscopy (XPS) measurements were performed on transferred samples to determine the V doping in monolayer MoS₂, with the results shown in Fig. S4. The binding energies at 517.0 and 524.5 eV are attributed to V 2p_3/2 and 2p_1/2, respectively, consistent with the XPS result of monolayer V-doped WSe₂ [46], demonstrating that V atoms indeed exist in monolayer MoS₂. In addition, the Mo 3d and S 2p orbitals splitting of monolayer V-MoS₂ is observed comparing to the pristine monolayer MoS₂, suggesting that the bonding states of Mo and S atoms are modulated by the neighboring substituted V atoms. Besides, the Mo 3d and S 2p characteristic peaks in monolayer V-MoS₂ are shifted to low binding energies comparing to the pristine monolayer MoS₂, highly indicative of the p-type doping of V. After increasing the growth time to 20 minutes, centimeter-sized monolayer V-MoS₂ single-crystal films are synthesized on sapphire, as shown in Fig.1(f, g) and Fig. S5. The digital photography of one-centimeter-sized monolayer V-MoS₂ single crystals is manifested in Fig.1(f), and the same color reflection indicates the macroscopic thickness uniformity. Raman intensity mapping of A_1g mode for large-area monolayer V-MoS₂ films demonstrates the uniform color contrast, confirming the high thickness uniformity [Fig.1(h)]. Large-area Raman intensity mapping images are captured from different areas of centimeter-sized monolayer V-MoS₂ single-crystal films, and the uniform intensity distributions convince the excellent crystalline quality (Fig. S6). In addition, Raman line scanning was employed to comprehensively detect the thickness uniformity of monolayer V-MoS₂ single-crystal films, and the same characteristic peak positions along different directions suggest the high uniformity [Fig.1(i) and (j)]. In short, centimeter-sized monolayer V-MoS₂ single crystals have been synthesized on industry-compatible c-plane sapphire using a V-assisted CVD method, which offers a new pathway for epitaxy growth of wafer-scale 2D p-type semiconductor single crystals and a promising platform for constructing high-performance integrated devices.

Multiscale characterizations were performed on as-grown and transferred samples to confirm the single-crystal feature of CVD-derived monolayer V-MoS₂. The low magnification transmission electron microscopy (TEM) image captured from the merged area of two unidirectional monolayer V-MoS₂ nanosheets is shown in Fig.2(a), the well-defined domain edges and transparent feature suggest the high crystalline quality and atomically thin thickness. The selected-area electron diffraction (SAED) pattern reveals only one set of hexagonally arranged diffraction spots, proving the unidirectional alignment and seamless stitching of such two monolayer V-MoS₂ nanosheets [Fig.2(b)]. In addition, a series of SAED patterns collected from different areas of monolayer V-MoS₂ films display nearly identical lattice orientations, highly indicative of the single-crystal feature (Fig. S7). Atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images offer the microscopic evidence of seamless stitching between adjacent monolayer V-MoS₂ nanosheets without any grain boundaries, as demonstrated in Fig.2(c)−(f). Meanwhile, abundant V doped atoms are also observed, which should induce the conversion of monolayer MoS₂ from n-type semiconducting to p-type. Energy dispersive X-ray spectroscopy (EDS) measurements were performed to determine the chemical constitutions and their distributions (Fig. S8). The V doping concentration is calculated to be 12.60 at% according to HAADF-STEM and EDS results.

The polarized second harmonic generation (SHG) spectra captured from two monolayer V-MoS₂ nanosheets reveal nearly the identical patterns, indicating their same orientations, as shown in Fig.2(g) and (h). In contrast, the SHG spectra measured on two misoriented pristine monolayer MoS₂ nanosheets exhibit a twist angle, which should result in the formation of grain boundary (Fig. S9). SHG mapping images captured from two merged monolayer V-MoS₂ nanosheets and large-area monolayer films show no obvious intensity drop across the grain boundary, confirming the seamless stitching of such two V-MoS₂ nanosheets and the single-crystal feature [Fig.2(i) and (j)]. To further validate this conclusion at macroscopic scale, Ar/O₂ etching experiments were then performed on as-grown samples. The grain boundaries are observed in CVD-synthesized monolayer polycrystalline MoS₂ films (indicated by the white arrow). Nevertheless, for monolayer V-MoS₂ films, almost no contrast variation is presented, reconfirming the single-crystal characteristic [Fig.2(k) and (l)]. In short, centimeter-sized monolayer V-MoS₂ single crystals have been synthesized on c-plane sapphire through the seamless splicing of unidirectionally aligned domains, as have been verified by the multiscale characterization results.

The doping concentration plays a pivotal role in modulating electronic properties of monolayer MoS₂, and thus different dopants (e.g., V₂O₅, NH₄VO₃, and VCl₃) are selected as the precursors to synthesize monolayer V-MoS₂ with a large doping concentration range [Fig.3(a)−(d)]. In the atomic-resolution HAADF-STEM image, the contrast is related to the atomic number [47], which enables the direct observation of V doped atoms in monolayer MoS₂, as demonstrated in Fig.3(e) and (f). When V₂O₅ is used as the precursor, only a small number of V atoms are distinguished in monolayer MoS₂, and the doping concentration is calculated to be 0.36 at% [Fig.3(b)]. However, when NH₄VO₃ is chosen, many V doped atoms are identified, and the doping concentration is increased to 6.30 at% [Fig.3(c)]. Interestingly, with VCl₃ as the precursor, the doping concentration reaches to 12.60 at%, as shown in Fig.3(d). Such results confirm the effectiveness of selecting appropriate precursor to control the doping concentration in monolayer MoS₂ over a wide range. It is worth noting that unidirectionally aligned monolayer V-MoS₂ nanosheets are still observed on c-plane sapphire regardless the variation of dopants, as revealed in Fig.3(g) and (h).

The different doping ability of such three precursors results in the different V doping concentrations in monolayer MoS₂. As illustrated in Fig.3(i), V₂O₅ possesses the high melting point (690 °C), low reactivity, and large V−O binding energy (627 kJ/mol), which lead to the limited doping ability. In contrast, NH₄VO₃ and VCl₃ exhibit high doping capacity due to their low decomposition temperatures (200 and 350 °C) [48, 49]. Additionally, VCl₃ can be decomposed under the growth temperature (900−950 °C) to produce the active gas of VCl₄, which endows the highest doping ability among these three precursors [50]. These findings validate the feasibility of precursor-dependent strategy for growing monolayer V-MoS₂ with different doping concentrations. Raman characterizations were also carried out on transferred monolayer V₂O₅-, NH₄VO₃-, and VCl₃-doped MoS₂ and pristine monolayer MoS₂ onto SiO₂/Si substrates. The characteristic peaks (at 323 cm⁻¹) related to the doping concentration are observed in monolayer V₂O₅-, NH₄VO₃-, and VCl₃-doped MoS₂ [51]. Besides, both E_2g and A_1g peaks shift towards the low wavenumbers due to the charge transfer and strain in monolayer MoS₂ induced by V-doped atoms, as shown in Fig. S10.

The back-gated FETs were fabricated following the benchmarking guidelines [52] using monolayer V-MoS₂ as the channels, and the influence of V doping concentration on the carrier transport behavior was also analyzed. The schematic diagram and corresponding OM image of monolayer V-MoS₂ back-gated FET are demonstrated in Fig. S11. The transfer characteristic curves of pristine monolayer MoS₂ and monolayer V₂O₅-doped MoS₂ (with the V doping concentration of 0.36 at%) are shown in Fig.4(a) and (c), respectively. The n-type semiconducting features are observed in such back-gated FETs, which are also confirmed by the output characteristic curves in Fig.4(b) and (d). Particularly, for the monolayer NH₄VO₃- and VCl₃-doped MoS₂ FETs, the p-type output behaviors are clearly exhibited, and the corresponding V doping concentrations are determined to be 6.30 and 12.60 at%, respectively [Fig.4(e) and (g)]. The output characteristic curves reconfirm such an interesting phenomenon, as shown in Fig.4(f) and (h).

The corresponding electron mobilities of pristine monolayer MoS₂ and monolayer V₂O₅-doped MoS₂ are calculated to be 6.7 and 1.24 cm²·V⁻¹·s⁻¹, respectively. The hole mobilities of monolayer NH₄VO₃- and VCl₃-doped MoS₂ are obtained to be 0.85 and 17.6 cm²·V⁻¹·s⁻¹, respectively. Notably, this hole mobility value of monolayer V-MoS₂ is much higher than the other 2D p-type semiconductors [53−55]. The gate-source (V_GS) and drain-source (V_DS) voltages are set to be 50 and 1 V, respectively. The 4d electron orbitals of V doped atoms contribute to the density of states of monolayer V-MoS₂ around the Fermi level, which induces the conversion from n-type semiconducting to p-type. Such results indicate that the centimeter-sized p-type monolayer MoS₂ single crystals have been synthesized by tuning the V doping concentrations. Meanwhile, the long-range ferromagnetic order is also observed in CVD-derived monolayer V-MoS₂, which should broaden its application potential in spintronics (Fig. S12).

4 Conclusion

We have developed an ingenious V-assisted epitaxy strategy to synthesize centimeter-sized p-type monolayer MoS₂ single crystals on industry-compatible c-plane sapphire. The introduction of V dopants contributes to the formation of parallel steps on sapphire surfaces and the edge-nucleation of unidirectionally aligned monolayer MoS₂ domains, which is central to regulating domain orientations and growing wafer-scale 2D single crystals. Meanwhile, the electronic property conversion from n-type semiconducting to p-type is observed in monolayer V-MoS₂ by changing V doping concentrations. This work presents an advancement toward the controlled synthesis of wafer-scale 2D p-type semiconductor single crystals and provides a new approach for enriching the device functions.

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