Scalable high performance radio frequency electronics based on large domain bilayer MoS2

Atomically-thin layered molybdenum disulfide (MoS2) has attracted tremendous research attention for their potential applications in high performance DC and radio frequency electronics, especially for flexible electronics. Bilayer MoS2 is expected to have higher electron mobility and higher density of states with higher performance compared with single layer MoS2. Here, we systematically investigate the synthesis of high quality bilayer MoS2 by chemical vapor deposition on molten glass with increasing domain sizes up to 200 μm. High performance transistors with optimized high-κ dielectrics deliver ON-current of 427 μA μm−1 at 300 K and a record high ON-current of 1.52 mA μm−1 at 4.3 K. Moreover, radio frequency transistors are demonstrated with an extrinsic high cut-off frequency of 7.2 GHz and record high extrinsic maximum frequency of oscillation of 23 GHz, together with gigahertz MoS2 mixers on flexible polyimide substrate, showing the great potential for future high performance DC and high-frequency electronics.

T wo-dimensional (2D) semiconductors have received great research attention for applications in the emerging field of ubiquitous electronics, such as sensors, memory, and logic applications owing to their atomically thin body and excellent carrier transport properties [1][2][3][4][5][6][7][8][9] . Flexible electronics in wireless communication is one of the most promising field which has witnessed rapid development of flexible passive components and active components 10 . However, despite tremendous interest in graphene transistors for active radio frequency (RF) components 11,12 , it still remains a challenging issue that the gapless nature of graphene gives rise to poor current saturation and large output conductance in these transistors, which are detrimental for amplifying and mixing high frequency signals. Recently, great progress has been made on high frequency transistors and circuits based on 2D transition metal dichalcogenides, such as molybdenum disulfide (MoS 2 ), where the key disadvantage of graphene can be overcome [13][14][15][16][17] . Mechanically exfoliated MoS 2 on quartz substrates has shown high extrinsic radio frequency performances 13 . In order to provide a low-cost scalable solution, large-area synthesis of MoS 2 atomic films by chemical vapor deposition (CVD) was developed with progressive improvement by many research groups [18][19][20][21][22][23][24][25] . Recently, RF transistors on flexible polyimide substrates based on monolayer MoS 2 grown by CVD exhibited an extrinsic cut-off frequency f T of 2.7 GHz and maximum oscillation frequency f max of 2.1 GHz 16 and, furthermore, an extrinsic f T of 3.3 GHz and f max of 9.8 GHz were demonstrated using an embedded gate structure on SiO 2 /Si substrates 17 . However, these parameters are still well below the devices based on exfoliated MoS 2 , severely limiting their high frequency applications. It is well known that the carrier mobility of bilayer MoS 2 is higher than that of monolayer and, as a result, better performance can be obtained owing to the higher density of states and smaller bandgap which is more suitable for high frequency electronics 26,27 . However, bilayer MoS 2 growth by CVD suffers from small domain sizes and poor mobility, restricting its device performance [27][28][29][30] .
Here, high mobility large domain bilayer MoS 2 growth by CVD on molten glass is realized by adjusting the weight of MoO 3 precursor during growth. The largest domain size of 200 μm can be obtained and the resulting single-crystal triangular bilayer MoS 2 demonstrates a room temperature electron mobility of 36 cm 2 V −1 s −1 . A back-gated MoS 2 transistor with 40 nm channel length exhibits a record high ON-current (I on ) of 1.52 mA μm −1 at 4.3 K with optimized high-κ dielectrics. Stateof-the-art RF transistors based on bilayer MoS 2 are demonstrated with a record high extrinsic cut-off frequency f T of 7.2 GHz and maximum oscillation frequency f max of 23 GHz 15-17 . Moreover, MoS 2 RF transistors and frequency mixers on flexible substrates are demonstrated with a f T of 4 GHz and f max of 9 GHz where the mixer remains functional in gigahertz regime.

Results
Material synthesis and characterization. Bilayer MoS 2 was grown on molten glass by the vapor-phase reaction of sulfur and MoO 3 in a thermal CVD system. Schematic view of the CVD setup is shown in Fig. 1a. The CVD growth process was carried out at ambient pressure where the temperatures of sulfur powders and MoO 3 during growth were kept at 230 and 830°C, respectively. Optical microscopy images of the resulting MoS 2 domains on the molten glass with increasing weight of MoO 3 are shown in Fig. 1b-e, where well-defined triangular shapes and clear uniform color contrast of CVD bilayer MoS 2 are grown (for more details see Methods). As shown in Fig. 1b, monolayer triangular MoS 2 domains tend to grow when the weight of MoO 3 is less than 1 mg. As the weight of MoO 3 slowly increases from 1.5 to 6 mg, bilayer MoS 2 starts to grow with an increasing domain size for the same growth duration as shown in Fig. 1c-e. And, with the MoO 3 weight of 6 mg, the largest domain size up to 200 μm is obtained. The shrinking size of bilayer MoS 2 compared with the monolayer underneath may be due to the first layer has a faster growth rate than the second layer 31 , and the growth time decreases from the first to second layer 27 . This is the largest bilayer MoS 2 domain among reported results to the best of our knowledge (Supplementary Table 1) 27,29,30 . This result shows that the masstransport process controls the growth of bilayer MoS 2 and there is a positive linear relationship between the growth rate and the weight of the MoO 3 (detailed discussions in Supplementary Note 1) and the advantage of using molten glass as growth substrate (Supplementary Note 2). More optical images and scanning electron microscope (SEM) images are shown in Supplementary Note 3. As shown in Fig. 1f-h, the film thickness measured by atomic force microscope (AFM) of the monolayer and bilayer CVD MoS 2 is around 0.72 nm and 1.34 nm, respectively.
Raman spectroscopy is widely used to distinguish between monolayer and bilayer MoS 2 based on the spectral position of the characteristic E 2g 1 and A 1g peaks 32 . Figure 2a shows the typical Raman spectra of monolayer and bilayer MoS 2 after being transferred onto SiO 2 /Si substrates. The delta values between the E 2g 1 and A 1g peaks of monolayer and bilayer MoS 2 are 18.9 and 22.4 cm −1 , respectively, consistent with previous reports 30,32 . Figure 2b, c show Raman intensity mappings recorded at 385 cm −1 and 405 cm −1 , respectively. Bilayer MoS 2 region has a higher intensity of Raman signal than that of monolayer, and the uniform contrast indicates a good uniformity of the bilayer film. Figure 2d compares the typical photoluminescence (PL) spectra of the monolayer and bilayer MoS 2 , where the peaks corresponding to the A1 and B1 direct exciton transitions with the energy split from valence band spin-orbital coupling 33 . The PL intensity of the bilayer MoS 2 is about 60% lower than monolayer because of the transition from direct bandgap in monolayer to the indirect bandgap in bilayer. Figure 2e shows the PL intensity mapping of the bilayer domain recorded at 1.85 eV, further confirming the good uniformity of the bilayer MoS 2 . Transmission electron microscopy (TEM) and electron diffraction studies were performed to confirm the single crystalline nature and to determine the lattice structures of the bilayer MoS 2 domains. Figure 2f shows the low-resolution TEM image of a CVD MoS 2 domain transferred on copper grids. A magenta dotted line is used to indicate the boundary between monolayer and bilayer MoS 2 where the left side of the line is monolayer and the right side is bilayer. Selected area electron diffraction (SAED) was conducted at location 1 in Fig. 2f and the diffraction pattern is shown in Fig. 2g. These diffraction peaks yield (100) direction with a lattice plane spacing of 2.83 Å. SAED images on another four selected openings with same electron diffraction patterns are shown in Supplementary Note 4, confirming the uniform crystallinity of the bilayer MoS 2 domain. High-resolution TEM (HRTEM) analysis was also performed to evaluate the quality and crystallinity of the MoS 2 films on the atomic scale, as shown in Fig. 2h, revealing the AA stacking order of the bilayer MoS 2 domain. As depicted in Fig. 2i, a thickness of 1.26 nm can be determined from an HRTEM image recorded from a folded edge of the bilayer MoS 2 domain and consistent with the thickness of bilayer MoS 2 28 .
DC characterizations on back-gated devices. To characterize the electronic properties of bilayer MoS 2 , back-gated field-effect transistors (FETs) with channel lengths from 3 μm down to 40 nm were fabricated on HfLaO substrates with a Si back gate (details of MoS 2 transfer onto silicon substrate are discussed in Supplementary Note 5). High-κ dielectrics provide a better interface with the 2D semiconductors channel with smaller effective oxide thickness, which help to improve the output performance of the transistors 34,35 (details of the high-κ dielectrics are discussed in Supplementary Notes 6, 7). Optical microscope images and the corresponding SEM images of the back-gated MoS 2 transistors are shown in Fig. 3a, b. Transfer characteristics of the transistors in the linear region based on monolayer and bilayer MoS 2 with the same channel length of 3 μm are plotted in Fig. 3c. The current and transconductance are more than 50% higher in the bilayer MoS 2 devices compared with the monolayer devices. Note here that the two devices were made on the same substrate with the same oxide thickness and fabrication process to remove processing-induced differences between the two cases. Output characteristics of the same 3 μm channel bilayer MoS 2 device at 300 K and 4.3 K are shown in Fig. 3d, e, respectively. The output drain current increases from 35 μA μm −1 at 300 K to 65 μA μm −1 at 4.3 K, an improvement of over 80% compared to room temperature. Intrinsic fieldeffect mobility of bilayer FETs is calculated to be 36 and 127 cm 2 V −1 s −1 at 300 K and 4.3 K, respectively (details of the mobility calculations are discussed in Supplementary Note 8). The detailed temperature dependence of mobility is plotted in Fig. 3f, showing a steady increase of mobility with decreasing temperature, which can be mainly attributed to the reduced phonon scattering and the temperature dependence coefficient is consistent with previous work shown in Supplementary    As shown in Fig. 4c, the extrinsic cut-off frequency f T derived from the short-circuit current gain is 7.2 GHz, the highest extrinsic f T achieved for CVD MoS 2 17 and is consistent with the values extracted from Gummel's method (Supplementary Note 13). While the cut-off frequency f T defines the frequency at which short-circuit current gain becomes unity, the maximum oscillation frequency f max is defined as the frequency at which Mason's unilateral power gain equals unity. This figure of merit is more relevant in terms of power amplifying 44 . Figure 4d shows the unilateral power gain versus frequency with an extrinsic f max of 23 GHz, which is 2.3 times greater than the previously reported extrinsic f max for the CVD MoS 2 , and is also the highest value in all reported 2D semiconductors as shown in Supplementary  Table 3 17,45 . In this work, the improvement of f T and f max are attributed to the CVD bilayer MoS 2 with high carrier mobility and low contact resistance, short gate lengths designed, and the high output resistance. As shown in Supplementary Note 14, intrinsic de-embedded f T,int of 78 GHz and f max,int of 34 GHz are obtained, and saturation velocity of 4.4 × 10 6 cm s −1 can be obtained from the intrinsic f T . Voltage gain Av extracted as Z 21 / Z 11 is also an important parameter for MoS 2 RF transistors 15,17 and, as shown in Fig. 4e, the extrinsic voltage gain is positive up to 4.2 GHz. To demonstrate the overall performance of CVD bilayer MoS 2 RF transistors, cut-off frequencies and maximum oscillation frequencies with different gate lengths are plotted in Fig. 4f, g, respectively. Both f T and f max increase as the gate lengths decrease, and this positive scaling is benefited from the low contact resistance of bilayer MoS 2 devices especially in short channel devices (detailed RF characteristics of 190 nm and 300 nm devices are shown in Supplementary Note 15). It is well known that the high output conductance of graphene RF transistors due to the lack of bandgap typically results in an unsatisfactory f max /f T ratio. Figure 4h shows  Fig. 5a. Figure 5b shows the output spectrum of the I ds (µA GHz. The conversion gain versus the applied LO power is plotted in Fig. 5c, showing higher conversion gain can be achieved with increasing LO power from 3 to 9 dBm, consistent with previous work 47 . A conversion gain of −30.7 dB was obtained at the LO power of 9 dBm. It should be noted that conversion gain here is defined as the ratio between the IF output signal power and the RF input signal power 48 . The 2D semiconductors have received high expectations in flexible electronics due to their ultrathin body nature and RF devices are essential for analog signal transmitting, amplifying and processing in those applications. As a result, we fabricated high frequency bilayer MoS 2 transistors on flexible polyimide films from Dupont using the same fabrication and measurement techniques, where the DC characteristics of a representative flexible transistor can be found in Supplementary Note 16. An extrinsic cut-off frequency f T of 4 GHz and maximum oscillation frequency f max of 9 GHz are achieved in a 300 nm gate length device as shown in Fig. 5d, showing significant improvement over previous results based on monolayer CVD MoS 2 on flexible polyimide substrates 16 . The RF characteristics of the transistors after various bending conditions can be found in Supplementary Note 17. Moreover, we also constructed a gigahertz MoS 2 RF mixer on flexible substrates with the same test setup as on rigid substrates. The RF signal (f RF = 1.5 GHz, P RF = 9 dBm) and LO signal (f LO = 1.4 GHz, P LO = 9 dBm) are power combined and fed to the gate input of the mixer and the output spectra is measured with a signal analyzer, shown in Fig. 5e, where the intermediate frequency (100 MHz) along with all expected harmonics is clearly shown. This result represents the first demonstration of gigahertz MoS 2 mixer on flexible substrates showing great potential of bilayer MoS 2 for flexible RF communication. As shown in Fig. 5f, the conversion gain increases monotonically as the LO power increases from 3 to 9 dBm, similar to the rigid substrate case. The IF gains at various frequencies can be found in Supplementary Note 18. By further improving the DC performance and employing impedance matching techniques, the conversion gain can be further improved to match those on high resistivity rigid substrates 49,50 .

Discussion
Systematic study on the large area synthesis of single-crystal bilayer MoS 2 films on molten glass using chemical vapor deposition has been carried out. The largest domain size achieved is up to 200 μm with optimized growth condition. The transistors fabricated based on bilayer MoS 2 show a high field-effect mobility as well as high ON-current. Notably, the ON-current reaches a record high value at 4.3 K on a short channel 40 nm device, among the highest in 2D materials. Moreover, high performance radio frequency transistors based on these bilayer MoS 2 are successfully demonstrated with record high extrinsic f T and f max based on top-gated RF transistors. Furthermore, frequency mixers

Methods
Bilayer MoS 2 growth and characterization. The bilayer MoS 2 films were grown on molten soda-lime-silica glass substrates by atmospheric pressure CVD. Prior to growth, the substrates were cleaned in acetone, isopropyl alcohol, and deionized water, followed by 5 min of O 2 plasma treatment. Before the rise of temperature, the tube was pumped down to a base pressure, and followed by filling the tube with Ar to 1 atm pressure. Then, the temperatures of the zones Ι and ΙΙ were raised to 230°C and 830°C, respectively. In the growth stage, 40 sccm Ar was used as carrier gas. The sulfur precursor (1.4 g) was loaded in an alumina boat and placed in zone Ι. The sulfur weight is adequate, determined by the experiment results of different sulfur weight. The MoO 3 precursor was loaded in a SiO 2 /Si substrate and placed in zone ΙΙ. The molten glass was loaded in a piece of Mo foil, which was located on the surface of a quartz plate, placed in Zone ΙΙ and next to the MoO 3 precursor. The growth durations for all samples in this work were kept as 10 min The morphology and structure of the bilayer MoS 2 were characterized with optical microscopy, AFM (Shimadzu SPM-9700), Raman spectroscopy (LabRAM HR800, 532 nm laser wavelength) and HRTEM (Titan G2 60-300, at 300 kV).
Device fabrication. Back-gated devices are fabricated on HfLaO dielectrics on highly degenerated silicon substrates, where the high-κ dielectrics layer was deposited by atomic layer deposition (ALD). Bilayer MoS 2 was patterned with an electron beam lithography (EBL) step and etched using O 2 /Ar plasma. Source and drain electrodes were formed with 20 nm Ni/60 nm Au metal stack. Top-gated RF devices are fabricated on both silicon and polyimide substrates. Bilayer MoS 2 domains were transferred onto highly resistive HfLaO/Si or polyimide substrates and patterned with an EBL step, and etched using O 2 /Ar plasma. Source and drain electrodes were formed with 20 nm Ni/60 nm Au metals stack. A thin layer of naturally oxidized Al 2 O 3 and an additional layer of HfO 2 grown by ALD formed the top-gated dielectrics. The thickness of naturally oxidized Al 2 O 3 and ALD-grown HfO 2 layer are 6 nm and 11 nm, respectively. The overall gate capacitance is 0.36 μF cm −2 . Two-fingered top-gates (20 nm Ni/60 nm Au) were defined by a final EBL and lift-off process.
Device measurement. The DC transport measurements were carried out using a Lakeshore probe station and an Agilent B1500A semiconductor parameter analyzer with an Agilent vector network analyzer (N5225A) for high frequency measurement. The on-chip microwave measurements are carried out in the range of 10 MHz-30 GHz. Before the microwave measurements, Short-Open-Load-Thru calibrations are done with standard calibration substrates (GGB CS-5). The mixer measurements are carried out in Lakeshore probe station at room temperature using an Agilent 5182B (or Agilent N5224A) signal generator and Ceyear AV1464B signal generator as the RF and LO input source, and an Agilent DSA90804A digital (or Agilent N9030B signal analyzer) for the IF signal detection. Bias-Tee (Keysight 11612B) are used both at the input and the output to combine DC and RF signals, and provide isolation between them. The LO and RF inputs were combined using external power combiner (Keysight 11636C). Coaxial cable with SMA connectors (Rosenberger LA3-C138-100, Rosenberger LU8-C043-1500, SUCOFLEX 101PEA) were used for the signal transmission and the IF signal detection between output bias-tee and signal analyzer. All the instruments, cables, and connectors met the frequency requirements for the mixer measurement. It should be noted that none of the impedance matching techniques were used in this work. Our measurements were carried out in vacuum to avoid the effects of adsorbents from measurement environment.

Data availability
The data that support the findings within this study are available from the corresponding author upon reasonable request.