Significant Suppression of Dark Current in a Surface Acoustic Wave Assisted MoS2 Photodetector

2D materials are considered as potential candidates for the next generation of optoelectronic materials. However, their optical absorption is typically weak due to thickness limitations, greatly restricting the photodetection capabilities of devices. To enhance the photoelectric gain of 2D materials or devices and improve detection sensitivity, various modulation methods such as strain, electric field, and magnetic field are commonly introduced. Among them, surface acoustic wave (SAW) represents a unique and effective modulation approach. In this study, photodetectors are fabricated based on few‐layer MoS2 on a SAW delay line on a LiTaO3 substrate. The interaction between SAW and MoS2 successfully manipulates the optoelectronic performance of the MoS2‐based devices. Under the influence of SAW, the dark current of the devices is significantly reduced by more than two orders of magnitude, while the photocurrent remains almost unchanged, resulting in excellent photoresponse performance. The devices provide a promising pathway for high‐performance optoelectronic applications and reveal a new possibility for acoustic devices in optoelectronics.


Introduction
2D materials, such as graphene, transition metal dichalcogenides, and black phosphorus, exhibit exceptional electronic, DOI: 10.1002/aelm.202300496optical, and mechanical properties at the atomic scale, garnering significant interest as potential next-generation optoelectronic materials. [1,2][5][6][7][8][9] These characteristics make them promising candidates for photodetector applications.Additionally, some 2D materials exhibit excellent mechanical flexibility, [3,10] making them suitable for the development of flexible devices.However, the limited thickness of 2D materials leads to inherently weak light absorption, [11] consequently resulting in diminished detection capabilities of the devices.This limitation poses a particular challenge for low signal-to-noise ratio scenarios, especially in the context of weak light detection, such as single-photon detection. [12]Therefore, enhancing the photoelectric gain of materials or devices to improve the detection sensitivity has been a prominent research topic.Various local field modulation approaches can be employed to enhance device performance, including but not limited to electric field modulation, [13][14][15] strain field modulation, [16][17][18] magnetic field modulation, [19,20] which primarily involve manipulating the electronic states of 2D materials.
Among the above approaches, the surface acoustic wave (SAW) is a unique effective mean of modulation.As a mechanical wave, SAW propagates along the surface of solids such as crystals, piezoelectric materials, and acoustic substrates, and the amplitude of SAW exponentially decays over a distance of one SAW wavelength. [21]In piezoelectric materials, SAWs can be generated by applying an alternating voltage at their resonant frequency to interdigital transducers (IDTs).When a SAW propagates on a piezoelectric material, the mechanical strain induced by the SAW generates an electric field.Consequently, a periodic potential is created that moves with the SAWs.This potential can influence the charge carriers near the surface (within approximately one SAW wavelength from the surface).24][25][26] The versatility and unique characteristics of SAWs enable them to be widespread utilization in diverse fields, including sensing, [27,28] communication, [29] microfluidics, [30] and nanotechnology. [31]2D materials, characterized by their atomic thinness and significant surface-to-volume ratio, exhibit remarkable flexibility as they readily deform in the direction perpendicular to their 2D planes.34][35][36][37][38] The integration of SAW with 2D materials has emerged as a compelling area of scientific research.[40][41][42] In this work, the photoconductive device based on multilayer MoS 2 is fabricated on a surface acoustic wave delay line on a LiTaO 3 substrate.The objective is to enhance the photoelectric detection performance of the devices by leveraging the unique properties of SAW and 2D materials.Through the interaction between SAW and MoS 2 , we can manipulate the optoelectronic properties of the MoS 2 -based device.When a voltage is applied to the electrodes of the piezoelectric crystal LiTaO 3 , mechanical strain is induced in the crystal lattice due to the piezoelectric effect.Furthermore, 2D materials are susceptible to deformation in the direction perpendicular to their 2D plane.As a result of local strain effects, strain of MoS 2 may introduce some trap states between the conduction band and valence band. [43,44]dditionally, surface acoustic waves induce a type II band edge modulation, [36,42] resulting in spatial separation of electrons and holes: electrons are pushed towards the conduction band minimum, while holes are pulled towards the valence band maximum.Under the combined action of these two mechanisms, the dark current of the device significantly decreases by more than two orders of magnitude, exhibiting a low dark current (I dark ) of 10 −12 A, while maintaining a photocurrent magnitude of 10 −7 A, demonstrating excellent photo-response performance.It can be expected that this prototype device could serve as a promising avenue for high-performance optoelectronic applications.

Results and Discussion
As depicted in the schematic diagrams of Figure 1a, our device is divided into two distinct components.The first component consists of a surface acoustic wave delay line, which is composed of a pair of interdigital transducers (IDTs) prepared on a piezoelectric substrate.The second component comprises a transistor based on multilayer MoS 2 , positioned on the surface acoustic wave delay line.Notably, the surface acoustic wave propagates along the direction of the transistor's source-drain current.Therefore, we can easily apply a radio frequency (RF) signal to the IDT, thereby exciting SAWs propagating along the delay line.The optical image of the device is shown in Figure 1b, where the IDTs and the MoS 2 -based device exhibit a well-defined morphology.The IDTs were patterned on a LiTaO 3 substrate using ultraviolet photolithography, followed by deposition of Ti/Au layers (15/45 nm), and each IDT consisting of 40 electrode pairs.Initially, a transfer method was employed to transfer several layers of MoS 2 onto the SAW delay line.To induce a pronounced modulation effect of SAW on MoS 2 , maintaining favorable interface conditions is crucial.Therefore, in order to mitigate the potential structural disruption to the ultrathin MoS 2 caused by conventional metal electrode deposition, [45] we opted for the technique of utilizing transferred metal electrodes to fabricate source and drain electrodes.Employing electron beam lithography for patterning, followed by the thermal evaporation deposition of a 100 nm-thick Au layer, the pre-patterned source-drain electrodes were transferred onto the MoS 2 layer.Fabricated on a YX-cut LiTaO 3 substrate, our IDTs support SAWs propagation along the substrate surface with a velocity of 3205 ms −1 .We designed the electrode width to be 7.8 μm, corresponding to a SAW wavelength of 31.2 μm.The device based on MoS 2 features a channel length of 7.8 μm, which corresponds to one-fourth of the wavelength of the SAW.This interdependency ensures that variations in the SAW wavelength do not significantly affect the device characteristics.The source and drain electrodes are connected with Cr/Au for applying sourcedrain voltage and collecting source-drain current.
Based on this configuration, we first characterize the transmission characteristics of the surface acoustic wave delay line used in the device, as it is generated by applying an RF signal to the IDTs at the resonance frequency.The fabricated device chip was encapsulated on a custom printed circuit board (PCB), with the IDTs' interconnections extended using conductive silver paste and Aluminum-silicon bonding wires and connected to a vector network analyzer and RF source via SMA connectors.The physical diagram of the device is shown in Figure 1c.This design allows for the measurement of the scattering parameter S 21 , which represents the transmission from one IDT to another IDT.Based on this, we measured the transmission characteristics (Sparameters) of the device using a vector network analyzer and plotted S 21 as a function of the RF signal applied to the transmitting IDT.Based on the formula f o = v SAW / SAW , where v SAW = 3205 m s −1 and  SAW = 31.2μm, the corresponding resonance frequency was calculated to be ≈f o = 102.7 MHz.This calculation aligns with our experimental measurements, as shown in Figure 1d.and A 1g , located at 382 cm −1 and 407 cm −1 , respectively.The E 1 2g mode corresponds to in-plane vibrations, while the A 1g mode corresponds to out-of-plane vibrations.Furthermore, the thickness information of MoS 2 is presented in Figure 2b, which was obtained by scanning few-layer MoS 2 flakes using atomic force microscopy (AFM).The measured thickness is ≈3.5 nm (corresponding to 5 layers).
Figure 2c,d depict the electrical characteristics of the fabricated device under SAW modulation in the absence of light (dark state).As a comparison, devices with 15 nm-thick MoS 2 were also tested under the same SAW excitation conditions, as shown in Figure S2 (Supporting Information).Two-point measurements were performed between the source and drain of the MoS 2 transistor in atmospheric environment.Figure 2c illustrates the current-voltage (I-V) characteristics under SAW-off and SAW-on conditions, revealing a substantial overall reduction in the dark current of the device by nearly two orders of magnitude upon SAW excitation.It is noteworthy that at lower bias voltage, the device dark current reaches the noise threshold of the measurement instrument (10 −12 A).Subsequently, we varied the RF power from 0 to 16 dBm with a step size of 1 dBm and measured the I-V characteristics of the device within a bias range of −1 to 1 V, as shown in Figure 2d.It can be clearly seen that the dark current of the device decreases rapidly with the increase of the SAW power.
Stability tests of the device's RF switch behavior were performed at a bias of −0.1 V, as shown in Figure 3a, where RF on represents the presence of SAW excitation, while RF off represents the inherent state without SAW excitation, with an RF power of 16 dBm.Under SAW-off excitation, the device exhibits a current level of 10 −10 A, whereas under SAW modulation, the dark current decreases by more than two orders of magnitude, reaching the instrument's limit of 10 −12 A. Due to the mechanical deformation induced by SAW propagation, the dark current of the device requires a certain relaxation time to return to a stable level.Notably, the long-duration vibration caused by SAW has limited impact on the device performance.As shown in Figure S1 (Supporting Information), the suppressed state of the dark current in the SAW-affected device remains nearly unchanged over a duration exceeding 70 s, and the current can recover to 10 −7 A after exposure to light.Moreover, even after continuous testing for over 60 min, the device continues to function without any observable issues.Subsequently, we demonstrated through experiments the significant enhancement of device photo-response aided by SAWs. Figure 3b presents the time-resolved I sd curve of repeated SAWs and optical pulses when the device is biased at -0.1 V, with an incident RF signal power of 16 dBm.The device exhibits an extremely low dark current (I dark ) < 1 × 10 −12 A under the influence of SAWs.Under laser irradiation with an incident wavelength of 520 nm and an incident optical power of 18.8 μW, the device's current sharply rises to 1 × 10 −7 A, showcasing a photocurrent gain exceeding 10 5 and exceptional optoelectronic switching ratio, showing a detection rate of 2.6 × 10 9 Jones.Calculation results indicate that in the presence of SAW, the device exhibits a responsivity of 3.8 mA W −1 and an EQE of 0.9%.This data is visually illustrated in Figure S3 (Supporting Information), where the light response for 520 nm illumination exhibits a rise time of roughly 13 s, coupled with a corresponding fall time of ≈4.4 s.
[45][46] These trapping states capture a large number of charge carriers, resulting in a significant reduction in dark current.The magnitude of RF power determines the amplitude of the SAW, [28,30] which influences the extent of mechanical deformation induced during SAW propagation and indirectly affects the number of introduced trapping states, ultimately impacting the degree of dark current reduction in the device.Furthermore, the SAW-induced modulation of the band edges (Type II modulation) restricts the movement of charge carriers in the channel near the minimum of the conduc-tion band and the maximum of the valence band, [36,42,47] which may partially affect the level of dark current.
As shown in the initial state (i) in Figure 4a, inherent defects exist in MoS 2 due to growth limitations, resulting in a small number of naturally occurring trap states between the conduction and valence bands, partially occupied by trapped carriers.Upon the application of an RF signal, as mentioned earlier regarding the two modulation mechanisms, surface acoustic wave introduce additional trap states in the energy band of MoS 2 and induce a type II band edge modulation, and the band structure is shown in state (ii).The introduction of trap states leads to a significant trapping of carriers in the device.Additionally, a fraction of carriers are confined near the bottom of the conduction band and the top of the valence band, respectively.Light is introduced on the basis of surface acoustic wave modulation, as shown in state (iii), incident photons(h > ∆E) are absorbed and used to excite electrons jump across energy gap into conduction band, leaving holes in valence band, and the trap states introduced by the surface acoustic waves are nearly fully occupied.Furthermore, the photo-generated electron-hole pairs are also captured within the conduction band minimum and valence band maximum.
The experimental results in Figure 4b,c also confirm the rationality of the mechanism.Figure 4b presents the relationship between the dark current at a bias of 1 V and the RF input power, showing a gradual decrease in dark current with increasing input  power.This result demonstrates that the decrease in dark current is attributed to the influence of acoustic surface waves, and the magnitude of the dark current can be modulated by the applied amplitude of the acoustic surface wave.Same as Figure 3b, using the same methodology, we characterized the device photoresponse at different laser powers, and the results are presented in Figure 4c, showcasing a certain degree of enhancement in the optoelectronic switching ratio of the device at varying laser powers.

Conclusion
In summary, we have observed the suppression of dark current (reduced by two orders of magnitude from 10 −10 to 10 −12 A) in the hybrid devices integrating SAW and MoS 2 -based photodetectors.Under the modulation of SAW, the introduction of additional trapping states in the band structure due to mechanical deformation caused by SAW propagation captures a significant number of charge carriers, resulting in a significant reduction in dark current.Furthermore, the SAW-induced edge modulation (type II modulation) restricts the movement of charge carriers near the minimum of the conduction band and the maximum of the valence band, partially affecting the level of dark current.Based on this, we have successfully demonstrated a substantial improvement in the optoelectronic switching ratio under SAW influence, reaching 10 5 , with a calculated device detectivity of 2.6 × 10 9 Jones.We firmly believe that this research on the device holds significant potential to offer an efficacious pathway for the investigation of high-performance optoelectronic detectors.

Figure 1 .
Figure 1.The structure of device and transmission characteristics of surface acoustic wave.a) Schematic diagram of the device structure showing IDTs, and the schematic diagram of MoS 2 photoconductive device, which is electrically connected by Cr/Au electrodes.c) Optical microscope image of device.The MoS 2 film was transferred onto a LiTaO 3 substrate.The IDTs are electrically connected by Ti/Au electrodes, each IDT consists of 40 pairs of electrodes.d) Schematic view of fabricated PCB with aluminium-silicon bonding wire and SMA connections for testing.e) SAW transmission between the IDTs at the design frequency ƒ SAW = 102.5 MHz of the delay line.Molecular vibrational modes of MoS 2 can be characterized by Raman spectroscopy.Similar to many layered materials, MoS 2 exhibits two vibrational modes: in-plane vibrational mode and out-of-plane vibrational mode.As shown in Figure 2a, our MoS 2 sample shows two characteristic Raman vibrational modes, E 1 2g

Figure 2 .
Figure 2. Characterization of materials and surface acoustic wave-mediated regulation of electrical properties in the dark state of devices.a) Raman spectra of a MoS 2 flake, showing two vibration peaks of MoS 2 , E 1 2g and A 1g .b) The AFM imaging of the multilayer MoS 2 flake, and the thickness of MoS 2 is measured as 3.5 nm (5 layers).c) Comparison of current-voltage (I-V) characteristics of the device in the dark before and after SAW excitation under an applied RF power of 16 dBm.d) I-V characteristics of the device in the dark under different power of RF, from 0 to 16 dBm with a step of 1 dBm.

Figure 3 .
Figure 3. Surface acoustic wave-mediated regulation of electrical properties in the dark and light state of devices.a) Stability test of RF-switching behavior of the device with an applied bias voltage of −0.1 V, under an applied RF power of 16 dBm.b) Photoelectric properties controlled by surface acoustic wave, a time -resolved RF -regulated light response.

Figure 4 .
Figure 4. a) Band structure of the carrier trapping states and the SAWs-induced band edge modulation of MoS 2 -based device.b) the current of the device in the dark under different input power of RF from 0 to 16 dBm with an applied bias voltage of 1 V. c) Ratio of Light-Dark current under illumination by a focused 520 nm laser beam at different laser power with V sd = −0.1 V and P SAW = 16 dBm.