Pronounced Photovoltaic Response from Multi-layered MoTe2 Phototransistor with Asymmetric Contact Form

Nanoscale Research Letters, Nov 2017

In this study, we fabricate air-stable p-type multi-layered MoTe2 phototransistor using Au as electrodes, which shows pronounced photovoltaic response in off-state with asymmetric contact form. By analyzing the spatially resolved photoresponse using scanning photocurrent microscopy, we found that the potential steps are formed in the vicinity of the electrodes/MoTe2 interface due to the doping of the MoTe2 by the metal contacts. The potential step dominates the separation of photoexcited electron-hole pairs in short-circuit condition or with small V sd biased. Based on these findings, we infer that the asymmetric contact cross-section between MoTe2-source and MoTe2-drain electrodes is the reason to form non-zero net current and photovoltaic response. Furthermore, MoTe2 phototransistor shows a faster response in short-circuit condition than that with higher biased V sd within sub-millisecond, and its spectral range can be extended to the infrared end of 1550 nm.

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Pronounced Photovoltaic Response from Multi-layered MoTe2 Phototransistor with Asymmetric Contact Form

Liu et al. Nanoscale Research Letters Pronounced Photovoltaic Response from Multi-layered MoTe Phototransistor with 2 Asymmetric Contact Form Junku Liu 0 Nan Guo 0 Xiaoyang Xiao 2 Kenan Zhang 1 Yi Jia 0 Shuyun Zhou 1 Yang Wu 2 Qunqing Li 2 Lin Xiao 0 0 Nanophotonics and Optoelectronics Research Center, Qian Xuesen Laboratory of Space Technology, China Academy of Space Technology , Beijing 100094 , China 1 Department of Physics, Tsinghua University , Beijing 100084 , China 2 State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua University , Beijing 100084 , China In this study, we fabricate air-stable p-type multi-layered MoTe2 phototransistor using Au as electrodes, which shows pronounced photovoltaic response in off-state with asymmetric contact form. By analyzing the spatially resolved photoresponse using scanning photocurrent microscopy, we found that the potential steps are formed in the vicinity of the electrodes/MoTe2 interface due to the doping of the MoTe2 by the metal contacts. The potential step dominates the separation of photoexcited electron-hole pairs in short-circuit condition or with small Vsd biased. Based on these findings, we infer that the asymmetric contact cross-section between MoTe2source and MoTe2-drain electrodes is the reason to form non-zero net current and photovoltaic response. Furthermore, MoTe2 phototransistor shows a faster response in short-circuit condition than that with higher biased Vsd within sub-millisecond, and its spectral range can be extended to the infrared end of 1550 nm. MoTe2; Photovoltaic; Interface; Asymmetric Background Graphene and similar two-dimensional (2D) materials exist in bulk form as stacks of strongly bonded layers with weak interlayer attraction, allowing itself to be exfoliated into individual, atomically thin layers, which have opened up new possibilities for the exploration of 2D physics as well as that of new material applications [ 1–9 ]. Of them, semiconductor transition metal dichalcogenides (TMDs) with the common formula MX2, where M stands for a transition metal from group VI (M = Mo, W) and X for a chalcogen element (S, Se, Te), exhibit sizeable bandgaps [ 2, 3, 10, 11 ]. In addition, these 2D TMD flakes are flexible and free of dangling bonds between adjacent layers [ 12, 13 ]. These unique properties make TMDs promising candidates to construct electronic and optoelectronic devices [ 2–4, 14– 17 ], such as a next-generation field-effect transistor (FET) at sub-10 nm [ 18 ], on-chip light-emitting diode [ 19–21 ], and Van der Waals heterostructure devices [ 4, 5 ]. 2H-type molybdenum ditelluride (2H-MoTe2) is one of the typical 2D TMDs, which has an indirect bandgap of 0.83 eV in bulk form [ 22 ] and a direct bandgap of 1.1 eV when it is thinned to monolayer [ 23 ]. 2H-MoTe2 has been explored for applications in spintronics [ 24 ], FET [ 25–27 ], photodetector [ 28–32 ], and solar cell [ 33 ]. Like most 2D materials, electrical metal contacts with 2H-MoTe2 play an important role in realizing highperformance electronic and optoelectronic devices. It has been proven that p-type and n-type contact doping and ohm contact can be realized using suitable contact materials [ 34–40 ], and they can, in turn, be used to construct functional devices, such as photovoltaic photodetector [ 37, 38 ] and diode [37]. Up to now, the research focus has been concentrated on evaluating and studying metal-semiconductor contacts by comparing various electrode materials, but insufficient attention has been paid to comparing metal-semiconducting contact forms in-depth, for example, the same contact material with asymmetric contact cross-section. In this study, we fabricate air-stable p-type multi-layered MoTe2 phototransistor with asymmetric contact crosssection between MoTe2-source and MoTe2-drain electrodes and investigate its photoresponse using scanning photocurrent at different gate- and source-drain voltages. This study helps to reveal the spatial potential profiles and analyze the impact of contact in the device. Experimental data show that the device has non-zero net photocurrent in short-circuit condition and photovoltaic response. Scanning photocurrent map reveals that strong photocurrent is generated in the vicinity of contact interface in short-circuit condition or with small source-drain voltage (Vsd) biased, which indicates the potential steps are formed in the vicinity of the electrodes/MoTe2 interface due to the doping of the MoTe2 by the metal contacts. When biased voltage Vsd rises above the potential step, Vsd dominates the separation of photoexcited electron-hole pairs and photocurrent (IPC = Isd − Idark) peak appears in the center of the device channel. This indicates the asymmetric contact cross-section between MoTe2-source and MoTe2-drain electrodes is the reason to form non-zero net current and photovoltaic response. This finding is helpful to construct photovoltaic photodetector with low power consumption. Finally, we test the time-resolved and wavelength-dependent photocurrent of MoTe2 phototransistor, obtaining sub-millisecond response time and finding that its spectral range can be extended to the infrared end of 1550 nm. Results and Discussion We fabricate two back-gated multi-layered MoTe2 phototransistors (D1 and D2) and measure their photoresponse. The device is identified by an optical microscope, and the corresponding MoTe2 thickness and quality are characterized using atomic force microscopy (AFM) and Raman spectrum. All measurements are conducted in ambient condition. Figure 1a shows the optical image (left) and AFM image (right) of D1 (D2 is shown in Additional file 1: Figure S1. The following data are collected from D1 unless otherwise specified, and the data from D2 are shown in Additional file 1). The device consists of source electrode, drain electrode, and channel sample of multilayered MoTe2 on SiO2/p+-Si substrate. SiO2 film with the thickness of 300 nm is dielectric, and p+-Si works as a back-gate electrode. The details of D1 are characterized using AFM, which shows that multi-layered MoTe2 straddles source and drain electrodes. The channel length is 10 μm. MoTe2 sample in the channel is about 23 nm thick (height profile is shown in Additional file 1: Figure S2), and the widths of MoTe2-source and MoTe2-drain contact cross-section are 6.5 and 4.8 μm, respectively. Figure 1b shows the Raman spectrum of MoTe2 sample. The characteristics Raman-active modes of A1g (172 cm−1), E21g (233 cm−1), and B21g (289 cm−1) are clearly observed, confirming the good quality of MoTe2 in the channel. Electric measurement indicates that multi-layered MoTe2 phototransistor is p-type as shown in Fig. 1c, which is in on-state at negative gate voltage and in offstate at positive gate voltage. The current on-off ratio is 6.8 × 103 when source-drain voltage Vsd is 1 V. The field-effect mobility (μ) is 14.8 cm2/V s according to transfer characteristics. When biased voltage Vsd decreases from 1 V to 100 mV, on-current and offcurrent both decrease, and the on-off ratio is still above 6.0 × 103, as shown in Additional file 1: Figure S3(a) and (b). When the gate voltage is swept from − 20 to 20 V and then back to − 20 V, multi-layered MoTe2 phototransistor shows small hysteresis (see Additional file 1: Figure S3(c)) and air-stable p-type conductance, which benefits from the simple fabrication process and polymer-free MoTe2 sample. We also fabricate other multi-layered MoTe2 phototransistor with a thickness of 5, 10, 11, 12, 15.7, and 38 nm, respectively, as shown in Additional file 1: Figure S4. They all show air-stable p-type conductance. Figure 1d shows the output characteristics of multi-layered MoTe2 transistor as back-gate voltage (Vbg) varies from − 20 to 4 V. As seen, the response is essentially linear, especially at a low biased voltage of Vsd, which indicates that there is a low Schottky barrier between Au and MoTe2 in the air. Figure 2 shows the photoresponse of multi-layered MoTe2 phototransistor when it is illuminated by 637-nm continuous-wave laser in ambient condition, which is conducted by combining Agilent B1500A semiconductor analyzer with Lakeshore probe station. Laser spot size is larger than 200 μm in diameter, and the device is covered with uniform illumination intensity. Backgate-dependent and power-dependent photoresponse are shown in Additional file 1: Figure S5. As shown in Fig. 2a, when a back-gate voltage is 0 V, source-drain current (Isd) increases with laser power. Isd vs. Vsd curves at different illumination power levels all meet at Vsd = 0 V, which is clearly observed in a logarithmic plot of |Isd| shown in insert Figure of Fig. 2a. When Vbg = 5 V, the phototransistor is in off-state (see Fig. 1c), and the current of Isd increases with the illumination laser power, exhibiting clear nonlinear behavior, as shown in Fig. 2b. Furthermore, the phototransistor shows non-zero open-circuit voltage (VOC) and short-circuit current (ISC) with laser illumination, which is the evidence of photovoltaic response from multi-layered MoTe2 phototransistor. Figure 2c shows VOC and ISC as a function of illumination power. VOC remains unchanged at 50 mV (illumination power is higher than 500 μW), and |ISC| increases from 0 to 1.6 nA when laser power increases from 0 to 4175 μW. When we change the voltage direction, VOC and ISC remain unchanged as shown in Fig. 2d. Vsd represents the voltage loaded on source electrode and Vds is loaded on drain electrode, and the corresponding current is indicated by Isd and Ids, respectively. Insert image in Fig. 2d illustrates the voltage and current direction. Whether the voltage is loaded on the source or drain electrode, the VOC of 50 mV relative to source voltage and corresponding ISC of 680 pA flowing from drain electrode to source electrode both remain unchanged. This confirms the photovoltaic response of multi-layered MoTe2 phototransistor. In order to reveal the mechanism of photoresponse, especially the photovoltaic response, we perform a scanning photocurrent microscopy (SPCM) study, which helps to obtain the spatial potential profiles and to analyze the spatially resolved photoresponse. SPCM is performed using a home-made scanning photocurrent setup in ambient condition. Optical excitation is provided by SuperK EXTREME supercontinuum white light laser. Its wavelength ranges from 400 to 2400 nm. The beam, with adjustable wavelength using SuperK SELECT multi-line tunable filter, is focused on the device using a 20× objective lens. A galvanometer mirror positioning system is used to make the laser beam scan the device to obtain photocurrent maps. The reflected light and the photocurrent are recorded with a current preamplifier and a lock-in amplifier at chopper frequency of 1 KHz. Figure 3 shows the scanning photocurrent of D1 with an excitation wavelength of 1200 nm. Laser spot diameter is about 4.4 μm derived from the reflection image (see Additional file 1: Figure S7). Figure 3a shows the optical image, together with the electrical setup. IPC measurements are conducted in short-circuit condition, in which source electrode is grounded and IPC is collected from drain electrode. The current flowing from the source to drain electrode is positive. Figure 3b shows spatial-resolved photocurrent image collected at the gate voltage (Vbg) of − 5, 0, and 5 V, respectively. It can be seen that short-circuit IPC with opposite polarities is strong in the vicinity of the interfaces between MoTe2 and the electrodes. When Vbg is changed from − 5 to 0 V, IPC pattern remains unchanged but the intensity decreases. Vbg is further increased to 5 V; IPC not only switches polarity, the position of maximum IPC also moves away from contact interface and into the channel. Figure 3c shows the IPC profile taken from the black dashed line in Fig. 3b at Vbg = − 5, 0, and 5 V, respectively. It clearly demonstrates that IPC has a broad intensity peak near the interface between MoTe2 and electrodes at Vbg = − 5 and 0 V, while the peak moves into the channel, which is about 3 μm away from the contact interface and becomes narrower. The presence of IPC peaks indicates the existence of potential steps in short-circuit condition. According to the IPC distribution, we plot the corresponding potential profile along the device channel as shown in Fig. 3d. At Vbg = − 5 and 0 V, the potential steps are near the contact interface between MoTe2 and electrodes, and they move into the channel at Vbg = 5 V. According to the previous study [ 41 ], Au electrode contact introduces p-doping and pins the Fermi level of MoTe2 at contact part. Thus, the potential steps are formed in the vicinity of the electrode/MoTe2 interface as the Fermi level in the channel is modulated by the gate voltage. At Vbg = 0 V, a weak IPC is observed, which flows from the electrode to MoTe2 channel. It means photoexcited electrons drift to nearby electrode and holes to MoTe2 channel. At Vbg = − 5 V, the hole density in MoTe2 channel is enhanced and induces a larger potential step in the vicinity of the electrode/MoTe2 interface. Photoexcited electron-hole pairs can be separated effectively and IPC increases. When Vbg = 5 V, more electrons are injected into the MoTe2 channel, and potential well is formed in the channel. Because of electrostatics of electrode, the potential steps move away from the electrode and appear in the channel. The photoexcited electrons drift to the MoTe2 channel and holes toward the nearby electrode. IPC changes direction compared with that at Vbg = − 5 and 0 V. Figure 4 shows the spatial-resolved IPC at different Vsd as Vbg = 0 and 5 V, respectively. Figure 4a shows the optical image, together with the electrical setup. Vsd is loaded on the source electrode, and IPC is collected from the drain electrode. The current flowing from the source to drain electrode is positive. Figure 4b shows IPC as a function of Vsd at Vbg = 0 V. When Vsd = 0, − 0.01, and 0.01 V, strong IPC occurs in the vicinity of MoTe2/electrodes interface, then it moves toward the channel center as Vsd increases to 0.1 V. Similar trend is observed at Vbg = 5 V as Vsd increases as shown in Fig. 4c. Figure 4d shows a clear IPC peak in the center of the device channel as Vsd increases to 0.5 V. IPC profiles taken along the black dashed line in Fig. 4a are shown in Fig. 4e, f, which clearly show the IPC variation trend as Vsd increases. They both indicate the maximum IPC generated in the vicinity of contact interface in shortcircuit condition or with small Vsd biased. When the biased voltage is increased, photocurrent peak moves toward the center of the device channel. Based on these findings, we know that the potential step, formed in the vicinity of the electrodes/MoTe2 interface due to the doping of the MoTe2 by the metal contacts, dominates the separation of photoexcited electron-hole pairs in short-circuit condition or with small Vsd biased. Thus, IPC at MoTe2-source is larger than that at MoTe2-drain due to the larger contact interface at MoTe2-source, and the net current is not zero, while the non-zero net current is smaller than Isd at Vbg = − 5 and 0 V (in on-state), and larger than that at Vbg = 5 V (in offstate). Therefore, we observe clear ISC at Vbg = 5 V as shown in Fig. 2b and Additional file 1: Figure S6(b)–(f ). Therefore, both ISC and the corresponding VOC are the results of the potential step and asymmetric contact. Furthermore, we fabricate D2 sample with more asymmetric contact cross-section, as shown in Additional file 1: Figure S1, compared with D1. It shows a similar photovoltaic response, with VOC as high as 150 mV when Vbg = 5 V and illumination laser wavelength is 637 nm. When the illumination wavelength varies to 830, 940, 1064, and 1312 nm, D2 shows a similar photovoltaic response at Vbg = 5 V (see Additional file 1: Figure S6 ). We also fabricate other four devices as shown in Additional file 1: Figure S8, they demonstrate the similar behavior to that has been shown in D1 and D2. These data further confirm that photovoltaic response of multi-layered MoTe2 phototransistor is a result from the asymmetric contact cross-section between MoTe2-source and MoTe2-drain electrodes. Finally, we test the photoresponse time and spectral range of multi-layered MoTe2 phototransistor. Figure 5a shows the time-resolved photocurrent at Vbg = 5 V and Vsd = 0 and 1 V, respectively, which are recorded using a current preamplifier and oscilloscope. The excitation laser is a square wave with 2 ms width at 637 nm wave-length. The currents collected under Vsd = 0 and 1 V show opposite direction, which is consistent with the data given in Fig. 2b, and is a result from the difference between VOC and Vsd. The rise time and fall time of photoresponse are defined as the time between 10 and 90% of the total photocurrent. As seen, the rise time τrise is 20 μs and fall time τfall is 127 μs 0 0 at Vsd = 0 V, and the rise time τrise is 210 μs and fall time 1 τfall is 302 μs at Vsd = 1 V, which are both larger than that 1 at Vsd = 0 V. This is because of the different mechanism of photocurrent generation. At Vsd = 0 V, the potential stepdominated photocurrent is generated in the vicinity of electrode/MoTe2 interface. At Vsd = 1 V, the photocurrent is generated in the device channel, and the photoexcited carriers have to go through the channel to arrive at the electrode, which takes longer time than the generation near the electrode/MoTe2 interface. Thus, the device shows longer photoresponse time at Vsd = 1 V than that at Vsd = 0 V. In addition to working at the visible band, a multi-layered MoTe2 phototransistor has photoresponse at the nearinfrared band. Figure 5b shows that its photoresponse can be extended from 1200 to 1550 nm. Optical excitation, provided by SuperK EXTREME supercontinuum white light laser, is focused on the device channel center using a 20× objective lens with a spot diameter of 4.4 μm. The data indicate that multi-layered MoTe2 phototransistor can be used in the communication band. Conclusions In summary, we have fabricated air-stable p-type multilayered MoTe2 phototransistor with asymmetric contact form. Its photoresponse is investigated using scanning photocurrent at different gate and source-drain voltages, which helps to reveal the spatial potential profiles. The results indicate that potential step, formed in the vicinity of the electrodes/MoTe2 interface due to the doping of the MoTe2 by the metal contacts, plays an important role in separating photoexcited electron-hole pairs in short-circuit condition or with small Vsd biased. Net current is non-zero when potential step exists with asymmetric contact cross-section between MoTe2source and MoTe2-drain electrodes. When biased voltage Vsd rises above potential step, Vsd dominates the separation of photoexcited electron-hole pairs, and IPC peak appears in the center of the device channel. Moreover, MoTe2 phototransistor shows a faster response in shortcircuit condition than that with higher biased Vsd within sub-millisecond, and its spectral range can be extended to the infrared end of 1550 nm. Methods/Experimental Back-gated multi-layered MoTe2 phototransistors are fabricated in the following way. First, source, drain, and gate electrodes are patterned on 300-nm SiO2/p+-Si substrate using standard UV photolithography techniques, followed by selective etching of 300-nm SiO2 beneath the gate electrode and E-beam evaporation of a 5 nm/100 nm Cr/Au films. Second, the multi-layered MoTe2 sample is prepared on another 300-nm SiO2/p+-Si substrate by mechanical exfoliation of mm-size semiconducting 2H-MoTe2 single crystals, which is grown by chemical vapor transport using TeCl4 as the transport agent in a temperature gradient of 750 to 700 °C for 3 days. Finally, the prepared multi-layered MoTe2 sample is transferred onto patterned source-drain electrodes using polyvinyl alcohol (PVA) as a medium. PVA is dissolved in H2O and rinsed with isopropyl alcohol. Multi-layered MoTe2 samples are identified by an optical microscope, and the corresponding thickness is characterized using SPA-300HV atomic force microscopy (AFM). Raman signals are collected by a LabRAM HR Raman spectrometer with 514-nm wavelength laser excitation in the backscattering configuration using a 100 × objective. Electrical characterization and photoresponse for 637-nm laser excitation are performed by combining Agilent B1500A semiconductor analyzer with Lakeshore probe station. The laser is illuminated onto the device using fiber and, the spot size is larger than 200 μm. Timeresolved photocurrent is recorded using a DL1211 current preamplifier and Keysight MSOX3024T oscilloscope. Spatial-resolved photocurrent is conducted using a homemade setup. The excitation laser is provided by SuperK EXTREME supercontinuum white light laser with an accessory of SuperK SELECT multi-line tunable filter to adjust the wavelength. The light is focused onto the device using a 20× objective lens and is chopped with SR570. The reflected light and the photocurrent are recorded with DL1211 current preamplifier and SR830 lock-in amplifier. Additional file Additional file 1: Figure S1. D2 properties. Figure S2. AFM image and corresponding height profile D1. Figure S3. Electric properties of D1. Figure S4. Electric properties of MoTe2 phototransistor with different thickness. Figure S5. Backgate-dependent and power-dependent photoresponse of D1. Figure S6. Photoresponse of D2 with different excitation wavelength. Figure S7. Normalized reflection in the vicinity of electrode. Figure S8. The photoresponse of other four multi-layered MoTe2 phototransistors. (DOCX 1617 kb) Abbreviations 2D: Two-dimensional; 2H-MoTe2: 2H-type molybdenum ditelluride; AFM: Atomic force microscopy; FET: Field-effect transistor; IPC: Photocurrent; ISC: Short-circuit current; Isd: Source-drain current; PVA: Polyvinyl alcohol; TMDs: Transition metal dichalcogenides; Vbg: Back-gate voltage; VOC: Opencircuit voltage; Vsd: Source-drain voltage; τfall: Fall time; τrise: Rise time Acknowledgements This work was financially supported by National Natural Science Foundation of China (51502337, 11574171, and 51472019) and Open Research Fund Program of the State Key Laboratory of Low-Dimensional Quantum Physics. Availability of data and materials The datasets supporting the conclusions of this article are included within the article and its supporting information. Authors’ contributions JL and LX conceived the experiment. JL designed the experiment, performed the measurements, and drafted the manuscript. 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Junku Liu, Nan Guo, Xiaoyang Xiao, Kenan Zhang, Yi Jia, Shuyun Zhou, Yang Wu, Qunqing Li, Lin Xiao. Pronounced Photovoltaic Response from Multi-layered MoTe2 Phototransistor with Asymmetric Contact Form, Nanoscale Research Letters, 2017, 603, DOI: 10.1186/s11671-017-2373-5