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Molybdenum disulfide (MoS2) as a promising 2D material has attracted extensive attentions due to its unique physical, optical and electrical properties. In this work, we demonstrate an infrared (IR) light gated MoS2 transistor through a device composed of MoS2 monolayer and a ferroelectric single crystal Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT). With a monolayer MoS2 onto the top surface of (111) PMN-PT crystal, the drain current of MoS2 channel can be modulated with infrared illumination and this modulation process is reversible. Thus, the transistor can work as a new kind of IR photodetector with a high IR responsivity of 114%/Wcm−2. The IR response of MoS2 transistor is attributed to the polarization change of PMN-PT single crystal induced by the pyroelectric effect which results in a field effect. Our result promises the application of MoS2 2D material in infrared optoelectronic devices. Combining with the intrinsic photocurrent feature of MoS2 in the visible range, the MoS2 on ferroelectric single crystal may be sensitive to a broadband wavelength of light.
© 2015 Optical Society of America
1. Introduction
During the past decade, 2D nanosheet as a new class of material has attracted significant attention in a variety of applications [1–4 ]. Molybdenum disulfide (MoS2) is one of the most attractive members of transition metal dichalcogenide layered compounds. Its unique anisotropic structure, consisting of strong covalent bonding between Mo and S atoms but weak van der Waals attraction between lattice layers results in excellent electronic, optical and mechanical properties [5, 6 ]. By decreasing the thickness to a few atoms, the indirect bandgap of bulk MoS2 may change to a direct bandgap of 1.8 eV [7]. Therefore, MoS2 is becoming a very important semiconducting candidate in next-generation nanoelectronic devices such as field effect transistor (FET) [8], photodetectors [9,10 ], and heterojunction solar cells [11]. It is well known that the transport properties of MoS2 can be modulated by visible light due to the high absorption of 5~10% in the visible regime [12]. However, the intrinsic few-layer MoS2 is almost blind to infrared light [13,14 ]. This makes MoS2 unable to be used for infrared optoelectronic devices. Fortunately, the atomistic thin materials offer the opportunity of novel route for interfacial engineering of optical and electronic properties. Very recently, Wang et al. performed the first extensive investigation on the injection of hot electron into a bilayer MoS2 film, which provided additional bandwidth for MoS2 in near-infrared photodetection [15]. Moreover, the local dielectric environment has been reported to significantly affect carrier properties [16–18 ].
On the other hand, using ferroelectric materials as the dielectric and the charge storage medium to modulate the state of the FET is a common structure to take the advantage of multifunction of ferroelectric materials. For example, Lee et al. demonstrated a top-gate nonvolatile memory transistor with MoS2 nanosheets based on the bistable polarizations provided by a ferroelectric polymer [19]. Park et al. exploited the piezoelectric property of poly(vinylidene fluoride - trifluoroethylene) (P(VDF-TrFE)) in MoS2 field effect transistors to realize a pressure sensitive touch sensor [20]. Recently, we reported MoS2 transistors with a high ON/OFF ratio of 108 by combining the ultra high-k dielectric PZT thin film and CVD MoS2 flakes [21]. Besides, pyroelectric effect is also an important property of ferroelectric materials; the polarization of ferroelectric materials can change when temperature varies. However, there are no reports regarding MoS2 based FET utilizing the pyroelectric effect of ferroelectric materials.
In this letter, we demonstrate an infrared light gated MoS2 FET, of which the field effect is induced by the pyroelectric effect. The ferroelectric single crystal Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT) was chosen as the gate insulator due to its excellent pyroelectric effect [22]. Combining with the intrinsic photocurrent feature of MoS2 in the visible range, the MoS2 transistor integrated on a pyroelectric single crystal may show response to a broad bandwidth response to light ranging from visible to IR.
2. Experiments and results
PMN-0.26PT single crystal was cut into (111) specimen (5 × 5 × 0.5 mm3) and then thinned and polished to 60 μm thick using a mechanical method. Au/C (50/100 nm) electrode was deposited onto the bottom surface of the sample, and then mounted on an ITO (Indium Tin Oxide) glass as a holder with conductive glue. Au electrode (50 nm) was deposited onto the top surface of the sample by magnetron sputtering. Photolithography and Au etchant were used to define the source-drain contact pattern with a 20 μm channel length and 3 mm channel width. Then, the sample was poled under an applied electric field of 2 kV/mm in the direction perpendicular to the surface at 120 °C for 15 min. Highly crystalline monolayer MoS2 samples were prepared by chemical vapor deposition on silicon substrates. Then, the as-grown MoS2 was transferred to the top surface of PMN-PT single crystal via a standard transfer process. The process details can be found in our previous work [23]. Before transfer process, a soft oxygen plasma treatment at room temperature was employed to enhance the surface hydrophilicity of PMN-PT single crystal.
The MoS2 single layer was characterized by Raman and photoluminescence (PL) spectroscopy (Horiba HR800) with the excitation wavelength of 488 nm. X-ray diffractometer (XRD, Rigaku SmartLab) with Cu Kα radiation was used to analyze the crystallographic characteristic of the PMN-PT single crystal. The temperature dependent polarization of PMN-PT was measured with a TF2000 Analyzer (aixACCT Systems). The Keithley 2400 sourcemeter was used to assess the performance of the FET under the illumination of an infrared laser (1064 nm wavelength, focused within the outline of PMN-PT crystal) with different intensities. The photoresponse in the visible range was also measured by irradiating the MoS2 with different wavelengths. The monochromatic light was generated with a standard system equipped with a monochromator (Newport 66902) and a dual-channel power meter (Newport 2931-C). More detailed measuring process can be found in our previous work [24].
Figure 1(a) schematically illustrates the device structure of the ferroelectric gate FET. It should be pointed out that the gate electrode on the bottom of PMN-PT crystal was covered with 100 nm carbon electrode to improve the absorption of infrared radiation. Optical microscopy image of monolayer MoS2 film on PMN-PT single crystal is displayed in Fig. 1(b). As can be seen, highly crystalline monolayer MoS2 with an ultra large size (300μm × 300μm) has been successfully transferred on the PMN-PT single crystal.
Fig. 1 (a) A schematic view of the ferroelectric gate FET, (b)The photograph of monolayer MoS2 film transferred onto the PMN-PT single crystal, (c) Raman spectrum of the monolayer MoS2 film transferred on the transistor channel, (d) PL spectrum of transferred MoS2 on the transistor channel.
The MoS2 monolayer was characterized by Raman and PL spectroscopy with the excitation wavelength of 488 nm. The Raman spectrum of the MoS2 film on the transistor channel was shown in Fig. 1(c). Characteristic peaks can be found at 384 and 404 cm−1 which correspond to the in-plane vibrational (E2g 1) and the out-of-plane vibrational (A1g) modes of the MoS2 respectively. The difference of Raman shift between the two peaks is a convenient indicator to determine the layers of the MoS2 sample [25,26 ]. The peak difference is 20 cm−1 in this work, suggesting that the prepared MoS2 samples are monolayers. To further evaluate the optical properties of monolayer MoS2, PL measurement was performed. As shown in Fig. 1(d), the strong peak at 1.88 eV corresponds to the direct band transition between valence band maximum and conduction band minimum at the K point of the Brillouin zone [7]. The strong emission peak also indicates that the monolayer MoS2 is still of high quality after transferring onto the PMN-PT single crystal.
Composition and orientation dependent performances of PMN-xPT have been extensively studied since large-size single crystals of this system were successfully grown [27–29 ]. It is well-known that [111] oriented PMN-PT single crystals exhibit the excellent pyroelectric property [27], thus (111) plate of PMN-PT single crystal was used in this work. Figure 2(a) shows the X-ray diffraction pattern obtained from the polished PMN-PT single crystal. The sharp XRD peak of (111) illustrates that the polished crystal is along the [111] direction, which is desired for attaining an excellent pyroelectric property.
Fig. 2 (a) The XRD pattern of the [111]-oriented PMN-PT single crystal, (b) Temperature dependence of the polarization and the pyroelectric coefficient.
The temperature-dependent polarization of PMN-PT was measured using a TF Analyzer 2000 equipped with a FE-module (aixACCT Systems). Figure 2(b) shows the temperature dependence of polarization measured from 30 to 180 °C. As a typical ferroelectric material, PMN-PT crystal has a spontaneous polarization (Ps) in the absence of an applied electric field. Its pyroelectric behavior originates from the polarization intensity being dependent on the temperature. When the PMN-PT crystal is heated (dT/dt>0), the polarization intensity decreases monotonically as the average magnitude of dipole moment diminishes. The pyroelectric coefficient p is defined by [30]:
The three significant enhancements of the pyroelectric coefficients in the black curve can be ascribed to the structural transitions occurring with increased temperature [31].The Keithley 2400 sourcemeter was used to assess the performance of the FET under the illumination of an infrared laser (1064 nm wavelength). Figure 3(a) shows the I-V curves of PMN-PT/MoS2 transistor under the IR illumination with different laser power densities. The drain voltage is in the range of −0.5 to 0.5 V. As shown in Fig. 3(a), when the laser power density increases from 4 to 14 mW/mm2, the drain current increases gradually. Same as other ferroelectric FET [32], there are positive bound charges on the top surface of PMN-PT layer with an up polarization. As schematically shown in Fig. 3(b), when the n-type MoS2 is placed on the channel, the electron carriers in MoS2 will be attracted by the bound charge of PMN-PT, resulting in a small drain current. The IR illumination causes an increase in temperature, thus there is a decrease in spontaneous polarization, as schematically shown in Fig. 3(c). This fall in the polarization reduces the positive bound charges on the top surface of PMN-PT. As a result, the electron carrier concentration in the MoS2 increases, leading to the increase in drain current. The basic formula of classical device physics can be used to explain this phenomenon. In the linear region where drain voltage is small enough (0.5 V in this work), drain current depends on the channel conductance σ. The relation between the conductance and the carrier concentration is as follows [33]:
Where μ is carrier mobility, N is the total number of carriers per unit area in the MoS2, q is electron charge, Cox is capacitance of the PMN-PT gate insulator, Z is channel length, L is channel width, and Vth is an equivalent threshold voltage. In this way, the IR illumination on the PMN-PT works as a negative gate potential Vg to modulate the carrier concentration in MoS2 .Fig. 3 (a) Ids-Vds curve of the FET under different laser power densities (without any gate voltage). Inset is the change of Ids as a function of IR laser power density (at an external drain voltage of 0.5 V), (b) and (c) the working mechanism of the ferroelectric field effect transistor modulated with IR illumination, (d) The time-resolved photocurrent in response to IR on/off at an irradiance of 6 mW/mm2 with 1064 nm laser.
The IR response of the PMN-PT/MoS2 FET was measured under the 6 mW/mm2 laser illumination at a drain voltage of 0.5 V. By inserting light blockers periodically, the drain current is plotted as a function of time in Fig. 3(d). The drain current increases immediately (~5 s) when the IR laser is introduced but falls slowly in 2 min when the laser illumination is blocked. The relatively slow fall time is due to the slow cooling rate when the device reaches around the ambient temperature. From the data in Fig. 3(d), we can calculate the responsivity (R) of this PMN-PT/MoS2 transistor to IR by using the formula as follows [34]:
Where ION and IOFF is the drain current at the ON or OFF state, respectively. PIR is the IR intensity. Therefore, the responsivity of this device is found to be 114%/Wcm−2, which was 16 folds higher than the values reported in the reduced graphene oxide/P(VDF-TrFE) FET [34]. Moreover, the ON-OFF switching could be performed over multiple cycles, indicating the stability and reproducibility of this FET.Besides IR response, to demonstrate the broadband response of this PMN-PT/MoS2 FET, we have measured the change of drain current with different excitation wavelength in the visible range. It is worth mentioning that the visible light irradiated directly at the MoS2 rather than from the Au/C electrode side during measurement. As shown in Fig. 4 , the MoS2 shows an obvious response in the visible range between 450 to 700 nm, which is similar to the work reported previously [13,14,35 ]. As known, this intrinsic photocurrent feature originates from the excitation of electrons from the valence to the conduction band in MoS2.
Fig. 4 Wavelength-dependent photoresponse in the visible range, i.e., change of drain current (defined as ION-IOFF) under a bias of 0.5 V.
3. Conclusions
In summary, we have demonstrated a new ferroelectric transistor consisting of monolayer MoS2 and ferroelectric PMN-PT single crystal. This FET may be gated by IR illumination through the pyroelectric effect. The drain current responds strongly to illumination, even for modest drain voltage of 0.5 V. These results broaden the suitability of MoS2 in remote or wireless applications. Furthermore, combining with the intrinsic photocurrent feature of MoS2 in the visible range, the PMN-PT/MoS2 transistor shows a broad bandwidth response to light ranging from visible to IR, which would be beneficial to the development of MoS2 in optoelectronic devices.
Acknowledgment
This research was supported by the National Key Basic Research Program of China (973 Program) under Grant No. 2013CB632900, The Hong Kong Polytechnic University strategic project (No. 1-ZVCG) and the National Natural Science Foundation of China (NSFC) (No. 51173097 and 91333109). The Tsinghua University Initiative Scientific Research Program (No. 20131089202) and the Open Research Fund Program of the State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University (No. KF201516) are also acknowledged for partial financial support.
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