Scalable fabrication of a hybrid field-effect and acousto-electric device by direct growth of monolayer MoS2/LiNbO3.

Lithium niobate is the archetypical ferroelectric material and the substrate of choice for numerous applications including surface acoustic wave radio frequencies devices and integrated optics. It offers a unique combination of substantial piezoelectric and birefringent properties, yet its lack of optical activity and semiconducting transport hamper application in optoelectronics. Here we fabricate and characterize a hybrid MoS2/LiNbO3 acousto-electric device via a scalable route that uses millimetre-scale direct chemical vapour deposition of MoS2 followed by lithographic definition of a field-effect transistor structure on top. The prototypical device exhibits electrical characteristics competitive with MoS2 devices on silicon. Surface acoustic waves excited on the substrate can manipulate and probe the electrical transport in the monolayer device in a contact-free manner. We realize both a sound-driven battery and an acoustic photodetector. Our findings open directions to non-invasive investigation of electrical properties of monolayer films.

Through a combination of substantial piezoelectric and birefringent properties, LiNbO3 offers pronounced electro-optic, acousto-optic and acousto-electric activities1 that render it the material of choice for numerous applications in radio frequency (RF) signal processing23 and passive integrated optics4. LiNbO3, the ‘silicon of photonics'56, exhibits a large, ∼3.95 eV indirect bandgap7. For application as an optical sensor, hybridization with a lower bandgap material is of key relevance. Here we report millimetre-scale direct chemical vapour deposition of monolayer MoS2 onto 128°YX-cut LiNbO3. Field-effect transistors (FETs) fabricated on these films exhibit characteristics competitive with established transition metal dichalcogenide (TMD) devices on silicon891011. The acousto-electric activity of the LiNbO3 substrate permits concomitant control and measurement of the systems electronic and optical properties in a contact-free manner. In our hybrid device, surface acoustic waves (SAWs) excited directly on the LiNbO3 substrate induce a strong acousto-electric effect and sense remotely the photoconductance of an TMD monolayer. SAW photoconductance spectroscopy can be performed at any point along the propagation path of the wave that extends on the millimetre length scale of a chip. This is in strong contrast to contact-based transport measurements, for which only the sample area between the contacts can be probed.

TMD films have attracted considerable attention for opto-electronic applications because of their direct-bandgap-semiconducting property at the single-layer limit1213. Offering a tunable bandgap of 1.1–1.9 eV (MoTe2 to WS2), these two-dimensional (2D) semiconductors can complement the properties of graphene through their strong photoluminescence (PL), significant spin–orbit coupling1415, and ensuing valleytronics physics1617. Numerous reports have shown functional FET devices of MoS2 and other TMDs891011; prototypical TMD FETs have been applied for gas sensing18, extended to ferroelectric gating1920 and were used to construct a memory element2122. So far, most investigations have focused on conventional semiconductor platforms, typically utilizing TMD material exfoliated onto SiO2/Si substrates. A range of chemical vapour deposition-based growth methods has been developed for TMD films on different substrates, including SiO2, sapphire, and graphene2324252627282930. LiNbO3 is routinely used in numerous commercial SAW devices, ranging from RF filters to wirelessly interrogated and identity-tagged devices, as well as in microfluidics. In addition, LiNbO3 is the material of choice for integrated photonic devices that harness its native nonlinear optical properties for classical and quantum communication. Hence, the hybridization of TMDs with 128°YX-cut LiNbO3 is a key technological advancement.

Acousto-electric spectroscopy has a long-standing tradition in probing and controlling solid-state materials3132. SAWs in particular offer a versatile approach since these nanoscale sound waves can be excited and detected all-electrically on a chip. SAW spectroscopy has been performed on bulk semiconductors and their heterostructures even in the quantum regime333435. For more than 15 years, semiconductor-LiNbO3 hybrids have been realized by epitaxial lift-off and transfer onto the LiNbO3 substrate36. Recently, similar hybrids have been reported for graphene37383940. These applications have in common that the semiconducting layer is loosely attached to the substrate via weak van der Waals' interactions. This interaction preserves contact-free acousto-electric access but its ability to offer acousto-mechanical couplings has not been determined.

A particularly exciting aspect of transport measurements using SAWs is their inherently contact-free nature combined with their sensitivity in the limit of low conductivity. The transport properties reported for TMD materials, such as transconductance, carrier mobility and susceptibility to gating, vary widely even for measurements on the same TMD material. From the perspective of technological application, this is very undesirable. In particular, charge transfer at the interface between metal contacts and the 2D TMD films, resulting in band shifting/bending analogue to the formation of a Schottky barrier, has been reported to affect transport measurements414243. SAW-based devices will ultimately have the power to provide entirely contact-free measurements, thus opening a new avenue to shed light on this current issue.

Here we report on two technological advancements: the scalable fabrication of a hybrid MoS2/LiNbO3 FET-electro-acoustic device that combines FET functionality with response to SAWs and the cross-validation of the respective signals; we demonstrate the versatility and power of this approach by measurement of the photoconductivity of a single-layer MoS2 film.

Results

Device layout and characteristics

A schematic of our hybrid device is shown in Fig. 1a and the fabrication procedure is summarized in the Methods section. The device consists of two components: (i) a SAW delay line formed by a pair of interdigital transducers (IDTs) and (ii) a monolayer MoS2-FET centred in between the two IDTs. This configuration enables us to probe and manipulate the electrical characteristics of the FET by exciting and detecting SAWs interacting with carriers in the MoS2. IDTs are used for all-electrical excitation and detection of SAWs. On LiNbO3, SAWs propagate at a velocity vSAW=3,980 m s−1; our IDTs are designed for a frequency of fSAW=160 MHz corresponding to a design wavelength λSAW=25 μm. Their arrangement allows measurement of the scattering parameter, S21, that is, the SAW transmission from one IDT to the other. In Fig. 1b we plot S21 as a function of the RF signal applied to the sending IDT. In this trace, the delay line resonance frequency is resolved as a 40-dB high transmission maximum very close to the nominal design frequency of fSAW=160 MHz. This RF characterization demonstrates high efficiency generation, transmission and detection of SAWs on the LiNbO3 host substrate even after its exposure to the MoS2 growth conditions. FET fabrication was performed on a monolayer region of the as-grown MoS2 film; its single-layer thickness was validated by scanning PL spectroscopy. An overview map demonstrating millimetre-scale growth of monolayer MoS2 onto 128°YX-cut LiNbO3 is presented in Supplementary Fig. 1. After PL characterization, a 4-terminal MoS2 FET is monolithically defined on the LiNbO3 substrate in the region of maximum emission of monolayer MoS2. The FET is fabricated to be located in between a pair of IDTs. Figure 1c shows a 400 μm × 350 μm spatial map of the characteristic monolayer MoS2 PL emission in the channel region1213 of the FET. The PL intensity is encoded in colour scale with red/dark regions corresponding to high/low count rates, respectively. PL mapping is used to determine the extent of the monolayer film before device fabrication and it confirms the presence of a monolayer MoS2 channel in the completed FET. Intensity variations are likely due to variations in the film domain size as shown in ref. 25. In all other areas, the MoS2 film was selectively removed to avoid any signal contributions from these regions. The four vertical lines indicate reflection from the Au contact lines; the four diffuse spots close to the corners of the panel represent gold alignment marks used during fabrication of the FET channel. The upper and lower boundaries of the FET channel in Fig. 1c are aligned with the SAW propagation path. Thus, we ensure tight correspondence of the electrically and acoustically addressed film area. The LiNbO3 substrates serves as the dielectric for back gating to ensure full FET operation and we provide full electrical characterization of this device. We show the respective wiring diagrams for 4- and 2-point configuration as insets in Fig. 1a.

Figure 1

Sample.

(a) Schematic representation of our hybrid MoS2/LiNbO3 device. Four Ti/Au electrodes form the contacts of a FET fabricated on chemical vapour deposition-grown MoS2. Two opposing, non-impedance-matched IDTs are used to excite SAWs propagating across the MoS2 FET. The insets show the electrical wiring configurations for 4-point (left) and 2-point (right) measurements. The sample was excited optically using a × 50 microscope objective with a numerical aperture (NA) of 0.55. (b) SAW transmission between the IDTs across the FET device shows a pronounced 40 dB maximum at the design frequency fSAW=160 MHz of the 5.4-mm long delay line. (c) PL map of the active FET region (scale bar, 100 μm). Monolayer MoS2 PL intensity (colour coded: red high intensity, black low intensity) is detected only in the channel region. Reflection from the FET contacts and alignment marks is clearly visible. (d) Comparison of single-point PL spectra obtained on SiO2 (red) and our 128°YX-cut LiNbO3 substrate (black) reveals a blueshift attributed to compression of the MoS2 film. Norm., normalized.

In Fig. 1d, we compare normalized PL spectra from MoS2 films grown under nominally identical conditions on LiNbO3 (black) and on a SiO2/Si reference substrate (red). For both samples, a PL peak is clearly resolved, corroborating the monolayer nature of the MoS2 (Supplementary Fig. 2 also provides Raman spectroscopy). However, MoS2 grown on 128°YX-cut LiNbO3 yields PL signal at photon energies larger by 40–50 meV; we attribute this shift to the fivefold larger thermal expansion coefficient of LiNbO3 (ref. 44) of compared with that of Si (ref. 45) of near room temperature. These dissimilar values cause a net relative compressive strain of 0.35% for the MoS2 film on LiNbO3 during cool-down from growth temperatures. Compressive strain is expected to give rise to a blueshift of the PL emission. Extrapolating the data on uniaxial tensile stress reported in ref. 46 of ΔE=60–70 meV per 1% strain, we expect a blueshift of ∼23 meV, about ½ of the observed blueshift. Using the value of ΔE=300 meV per 1% strain reported by Hui et al.47 for compressive strain in trilayer MoS2, the expected blueshift amounts to 105 meV, which is larger than the value observed here. Thus, our observation of a blueshift is compatible in magnitude with recent work and corroborates a rigid connection of the MoS2 film to the LiNbO3 substrate, a precondition for maximum interaction with the SAW.

FET characterization

We expect our hybrid device to exhibit FET properties. Owing to the very large dielectric constant ɛr ∼50 for this cut of LiNbO3, moderate electric fields D=ɛrVGS/dsub=±40 kV cm−1 can be achieved by applying VGS=±40 V between the LiNbO3 backside (thickness dsub=500 μm) and the MoS2 layer. We test FET operation of our hybrid device by measurement of its transport characteristics as a function of a back gate voltage (VGS) across the LiNbO3 substrate. In Fig. 2a, we plot a set of output characteristics (ISD versus VSD), recorded in 4-point configuration, for different back gate voltages ranging between VGS=±40 V. As VGS is tuned from negative to positive polarity, we observe the expected reduction of the sheet resistance due to accumulation of electrons in the MoS2 monolayer. This observation clearly demonstrates FET operation in our hybrid device in the linear regime and the formation of an n-type transport channel, which is in agreement with prior work891011. Transfer characteristics (ISD versus VGS) recorded in 2-point configuration are plotted in Fig. 2b for different VSD. Again, the increase of ISD as VGS is tuned to positive bias at constant VSD is consistent with n-type character. We note that in the data, a small leakage current through the substrate was subtracted for clarity; Supplementary Fig. 3a contains the graphs before subtraction of gate leakage. The transfer characteristics exhibit a small hysteresis, which may arise from poling effects of LiNbO3 at the interface to the MoS2 layer19. We note that MoS2-based FETs on dielectric Si/SiO2 substrates have been found to be sensitive to the local environment and, in particular, to the surrounding gas atmosphere18. Adsorption and desorption of impinging gas atoms and molecules have been suggested as an origin of an hysteretic I–V characteristics10. From the turn-on behaviour, we are able to derive the threshold voltage Vth, marked in Fig. 2b. From this analysis, we can determine the field-effect mobility in our device given by , with L and W being the length and width of the channel, respectively and dsub denotes the thickness of the LiNbO3 substrate. An example analysis is presented in Supplementary Fig. 3b, and the VSD dependence of the values Vth and μFE are presented in Supplementary Fig. 3c and d, respectively. From the output characteristics set, we are able to extract the channel's electrical properties as a function of VGS. Figure 2c depicts the channel resistance and conductance extracted from 2-point output characteristics evaluated at VDS=0 and plotted versus VGS. Moreover, from a linear fit of the conductance for VGS>20 V, we can determine μFE and Vth from the slope and from the intersection at ISD=0, respectively. We performed an analogous analysis on the 4-point output characteristics that is presented in Supplementary Fig. 4. Table 1 summarizes the results for μFE and Vth; the error estimates originate from the accuracy of the linear fit and from the s.d. of 12 VGS up- and down-sweeps for output and transfer characteristics, respectively. The values for μFE and Vth derived from these independent sets of data are in good agreement. We note that the values obtained on our highly piezoelectric architecture are competitive with back-gated devices fabricated by exfoliation on the SiO2/Si platform48.

Figure 2

FET operation of hybrid MoS2/LiNbO3 device.

(a) Output characteristics (ISD versus VSD) for different gate voltages VGS recorded in 4-point configuration. For large negative VGS, the device is weakly conducting; an n-type channel is formed for positive VGS. (b) Transfer characteristics (ISD versus VGS) for different source-drain voltages VSD recorded in 2-point configuration shows pronounced increase of |ISD| at positive VGS due to formation of an n-type channel. (c) Channel resistance (red) and conductance (blue) as a function of VGS extracted from 2-point output characteristics at VSD=0. For positive VGS, a linear fit indicates a mobility μFE=33±5 cm2 V s−1 and a threshold voltage Vth=5.5±1.5 V. The latter agrees well with that derived from the data in b, as summarized in Table 1.

Table 1

Field-effect mobility and threshold voltage of our FET device determined by different techniques.

	2-Point output characteristics (VSD=0)	4-Point output characteristics (VSD=0)	2-Point transfer characteristics
μFE (cm2 V−1 s−1)	33±5	43±5	37±5
Vth (V)	5.5±1.5	5.2±1.5	5.8±2.5

FET, Field-effect transistor.

Acousto-electric effect

Having established the electrical functionality of our hybrid device, we validate its acousto-electric transport properties. Acousto-electric effects are expected within the transmission band of the SAW delay line. The frequency dependence of the corresponding scattering parameter S21 is plotted in Fig. 3a. First, we measure the short-circuit (VSD=0) acousto-electric current (AEC; Fig. 3b) in a two-probe configuration as a function of an RF signal of varying frequency and power PRF applied to each of the two IDTs. For a forward propagating SAW excited by constant PRF, we observe that the AEC exhibits a characteristic frequency dependence. This dependence faithfully reproduces the S21 data. As the propagation direction of the SAW is reversed, the polarity of the AEC reverses: this finding indicates that the propagation direction of the SAW determines the direction of the carrier flow between the two contacts (that is, momentum transfer between the SAW and the mobile carriers in the MoS2 film). The observed polarities provide an independent verification of n-type majority charge carriers in the MoS2 film. We note that the different AEC levels measured for the two IDTs at constant PRF arise from a combination of variations in their absolute conversion efficiencies, different distances from the location of our measurements and different SAW attenuation along the propagation path. The lower amplitudes of the AEC compared with reports on graphene on LiNbO3 (refs 37, 38) are expected due to the lower carrier concentrations in our MoS2 films compared with zero-bandgap graphene.

Figure 3

Acousto-electric spectroscopy.

(a) Frequency band of the SAW transmission between IDTs plotted as the scattering parameter S21. (b) AEC as a function of RF applied to the IDTs for different RF power levels PRF. Current measurements were performed in a 2-point short-circuit (VSD=0) configuration. The forward and reverse propagating SAWs were excited by either of the two opposing IDTs. They yield AECs of opposite sign. (c) AEV as a function of PRF measured in 4-point, open-circuit configuration (ISD=0). For both SAW propagation directions, the expected linear dependence is well reproduced. The signs of the AECs and voltages correspond to n-type conductivity of the film.

Second, we explore the PRF power dependence of the acousto-electric effect for both SAW directions in a complementary experiment by measuring the open-circuit voltage in a 4-point configuration (Fig. 1b) with open connection to the back contact. Here the total current between the two outer contacts is set to ISD=0 and the acousto-electric voltage (AEV) is picked up between the two inner contacts. In Fig. 3c, we plot the measured AEV as a function of PRF (in mW) applied for a forward (black) and reverse (red) propagating SAW. We observe the expected linear power dependence of the acousto-electric effect. We note that the AEC and voltage represent sound-driven constant current and voltage sources, respectively. Such an ‘acoustic battery' is remotely driven by the SAW.

Contact-free photoconductivity probe

Having established the functionality of our hybrid device, we proceed by applying it to the measurement of the photoconductivity of the MoS2 layer. MoS2 single-layer material exhibits a pronounced photoconductive response to optical (above bandgap) irradiation495051. SAWs provide an extremely sensitive and fast conductivity (σ) probe and are particular suitable to the characterization of poorly conductive films52. The SAW attenuation coefficient is given by:

In this expression, is the electromechanical coupling efficiency, kSAW is the SAW wave vector and is the characteristic sheet conductivity. This value corresponds to a characteristic channel conductance of ∼18.5 μS in our device. This value is larger than the measured channel conductance G<1.2 μS derived from FET the characteristics of Fig. 2.

To characterize the SAW transmission along the delay line, we measured the S21 scattering parameter, that is, the transmitted SAW intensity from one IDT to the other. In the upper panel of Fig. 4a, we plot the variation of S21 as a function of time. At t=6 s, a diffraction-limited spot in the centre of the hybrid device is irradiated for Δt=5 s by either a red (hν=1.87 eV) or an infrared laser (hν=1.46 eV) source at 1 mW power. The red laser is resonant with the fundamental optical transition of MoS2 on LiNbO3, while the photon energy of the infrared laser is less than the optical bandgap of MoS2. We resolve a pronounced photoresponse for the red laser, manifesting itself in a reduction of the transmitted SAW signal (ΔS21 <0). Such an increase of the attenuation is expected from equation (1) since photogeneration of electrons and holes leads to an increase of G, while still remaining in the G<MoS2. We confirm this interpretation by simultaneously measuring the electrical conductivity in 2-point configuration, which is plotted in the lower panel of Fig. 4a. The anticipated increase of the conductance (ΔG>0) is clearly resolved. The agreement of the global features of ΔS21 and ΔG is remarkable: both channels show quasi-instantaneous responses as the laser is switched on and off, which is attributed to the presence or absence of photogenerated carriers in the MoS2 layer. The associated processes occur on a timescale faster than the acquisition time of each data point of 250 ms. In addition to this fast contribution, a response on longer (seconds to minutes)50 timescales is resolved clearly.

Figure 4

Photoconductance spectroscopy.

(a) Comparison of the time-dependent photoresponse detected by the change of the transmitted SAW intensity (ΔS21) with the change of the 2-point conductance (ΔG) of the FET. Red and black traces were recorded for Plaser=1 mW excitation by a red and infrared laser, respectively. These lasers are switched on for Δt=5 s at t=6 s. Both the instantaneous and persistent features of the photoresponse are consistently resolved by both measurement techniques. For excitation with an infrared laser, no photoresponse is detected, proving that the signal detected for the red laser indeed stems from the MoS2 monolayer. (b) Comparison of ΔS21 with ISD under photoexcitation using a red laser. The laser is switched on every 1 min for Δt=5 s. Each successive minute Plaser is increased by 0.1 mW until Plaser=1 mW is reached. Subsequently, Plaser is decreased to 0 mW in steps of ΔPlaser=100 μW as shown in the lower panel. Upper and centre panel compare the SAW transmission (ΔS21, red) and photocurrent (ISD, black) for VSD=+100 mV and −100 mV, respectively. Direct correspondence between ΔS21 and ISD is confirmed: while ΔS21 reduces irrespective of voltages, the sign of ISD is determined by the polarity of VSD.

In Fig. 4b, we present a detailed optical pump power series. In this experiment, the laser source is repeatedly switched on for 5 s every minute. The optical pump power is initially increased in ΔPlaser=0.1 mW steps from Plaser=0.1–1 mW and then reduced to Plaser=0. The corresponding optical power pattern is plotted in the lower panel of Fig. 4b. The upper and centre panels compare the measured SAW (ΔS21) and current (ISD) responses for VSD=+100 mV and −100 mV, respectively. Clearly, both the SAW attenuation and the FET current scale with the laser power in a nonlinear manner similar to the observations of Yin et al.49 and Lopez-Sanchez et al.51 but different from the short-channel devices of ref. 50. While the sign of ISD depends on the polarity of VSD, ΔS21 decreases irrespective of the VSD polarity. Furthermore, the amplitude of ΔS21 only depends on Plaser and is independent of the applied VSD. These facts prove that electro-acoustic and FET operation do not interfere. The component of the photoresponse with the longer time constant50 leads to the accumulation of higher sheet conductivity over the full 20-min duration of the experiment. Such processes are frequently observed in 2D materials and are typically attributed to traps at the interface to the substrate or in the material itself. In ref. 50, we demonstrate the composition dependence of this phenomenon for MoS2(1-Se2 alloys.

Discussion

Direct growth of MoS2 onto the 128°YX-cut of LiNbO3 permits acousto-electric spectroscopy on the TMD overlayer as validated by the hybrid device assembled in this work. This finding opens many new avenues of research: while our hybrid device relied on metal contacts to the TMD film so as to validate congruence between electric transport and SAW-based conductivity measurements, subsequent experiments may dispense with the contacts, thereby allowing entirely contact-free transport measurements on TMD films. Moreover, the tight coupling of the TMD film to the underlying substrate as being indicated by the blueshift of the PL signal suggests that not only acousto-electric but also acousto-mechanic spectroscopy on TMD films may be possible. In such experiments, the SAW exerts tensile or compressive strain allowing measurement of the coupling of the dynamic deformation to the electronic degrees of freedom of the TMD material. Spin and charge excitations in recently discovered TMD-based quantum dots53 could also be controlled dynamically by SAW-driven deformation potential coupling and Stark effect54. The rigid connection of the TMD layer to the LiNbO3 is crucial for such acousto-mechanically driven approaches, since it ensures close coupling of the SAW to the film. We also highlight that our device fabrication used exclusively scalable techniques avoiding transfer or exfoliation steps. This paves the road towards the incorporation of TMD films as, for example, optically active elements, into conventional and inexpensive LiNbO3-based SAW devices of a type similar to those currently used, for example, as frequency filters in cell phones. As a consequence, we foresee that the fundamental device concept introduced in this article will attain widespread application both in the fundamental study of the properties of TMD films and in the technological realm where optically active thin, inorganic and durable films are desired: our SAW device remained functional for 9 months in air withstanding multiple intermittent thermal cycles of heating to temperatures as high as >450 K and cooling to as low as <10 K in vacuum in the meantime. Measurements on different TMD materials show promising initial results and will be reported on once completed.

Methods

Sample fabrication

TMD growth. Single-layer MoS2 synthesis follows the technique outlined in ref. 25: in a typical setup, 25 mg of molybdenum trioxide powder (99.99%, Sigma-Aldrich) was placed in an alumina boat and centred in the tube furnace. The LiNbO3 substrate (128°YX-cut, oxygen reduced and weakly conductive ‘black' LiNbO3, thickness dsub=500 μm) was mounted with a molybdenum mesh (Alfa Aesar) on the edge of the boat and 1 g of sulfur was placed upstream at a distance of 25 cm from the centre. The furnace was heated at a rate of 12.5 °C min−1, held at ∼650 °C for 20 min and then allowed to cool naturally to room temperature. N2 carrier gas aided the transfer of the sulfur vapour to the sample region for optimal growth.

Device layout and fabrication. After PL identification of the region with monolayer growth, we fabricated Ti/Au metal contacts and IDTs (split-2 design, 21 finger pairs, duty cycle 1:1, aperture 200 μm) for electric and acoustic interfacing to the MoS2 film, respectively. The IDT's design wavelength was chosen as λSAW=25 μm, corresponding to a design frequency of fSAW=160 MHz. Supplementary Fig. 5 shows a micrograph of an IDT. As shown in Fig. 1a, the two IDTs are located at opposite ends of the substrate in a delay line configuration (length 5.4 mm) to enable us to launch and record SAWs propagating at vSAW=3,980 m s−1 in opposite directions across the chip. The studied FET device features a channel length of L=35 μm and a total width of the conducting channel of W=360 μm. After fabrication of the electrical contacts, in a second lithographic step, an oxygen plasma treatment is used to remove the MoS2 single-layer film from the channel area except for a 180-μm-wide region across, which the SAW propagates. Thus, we ensure correspondence of the electrically and acoustically addressed film area. Figure 1c shows a PL map of the contact region after electrode deposition and removal of extraneous MoS2 film.

Lithographic patterning. All lithographic patterning of the IDTs and contact electrodes proceeded in a single exposure step using an electron beam writer and polymethyl methacrylate (PMMA) as resist. Contacts were fabricated by sequential deposition of 10 nm of Ti followed by 60 nm of Au in an e-beam evaporation tool. Subsequent lift-off defined the active structures. MoS2 was selectively removed in a second e-beam exposure step and subsequent oxygen plasma treatment at a plasma power of 200 W at a pressure of 500 mTorr for 13 s.

Measurement techniques

SAW excitation and SAW transmission experiments. For measurement of the AEC and AEV, the output of an RF signal generator was amplified and connected to one of the IDTs. The RF characteristics of the IDTs and SAW transmission lines were characterized using a vector network analyser measuring scattering parameters of the RF network, in particular the scattering parameters S11 (reflection) and S21 (transmission and insertion loss).

Electrical characterization. 2-Point characterization was performed using a Keithley K2400 source meter unit (SMU). For 4-point characterization, a K2400 SMU was used only as a constant current source (no measurement probes connected) and the voltage at the potential probes was recorded directly by a K2000 digital multimeter. The gate voltage was applied by a Keithley K2600 SMU, which measured the gate leakage current at the same time. Short-circuit AEC and open-circuit AEV were recorded using a K2400 SMU with VSD=0 and ISD=0, respectively.

Optical spectroscopy. The photoconductivity experiments relied on red (660 nm) and infrared (850 nm) pulsed semiconductor lasers with a 80-MHz repetition rate and a pulse duration ≤100 ps. PL and Raman spectroscopy, as well as mapping utilized a Horiba LabRAM HR spectroscopy system using a 532-nm excitation laser and a 1,800 lines per mm grating.

Additional information

How to cite this article: Preciado, E. et al. Scalable fabrication of a hybrid field-effect and acousto-electric device by direct growth of monolayer MoS2/LiNbO3. Nat. Commun. 6:8593 doi: 10.1038/ncomms9593 (2015).