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

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. Electronic supplementary material The online version of this article (10.1186/s11671-017-2373-5) contains supplementary material, which is available to authorized users.

2H-type molybdenum ditelluride (2H-MoTe 2 ) 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-MoTe 2 has been explored for applications in spintronics [24], FET [25][26][27], photodetector [28][29][30][31][32], and solar cell [33]. Like most 2D materials, electrical metal contacts with 2H-MoTe 2 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][35][36][37][38][39][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 MoTe 2 phototransistor with asymmetric contact crosssection between MoTe 2 -source and MoTe 2 -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 (V sd ) biased, which indicates the potential steps are formed in the vicinity of the electrodes/MoTe 2 interface due to the doping of the MoTe 2 by the metal contacts. When biased voltage V sd rises above the potential step, V sd dominates the separation of photoexcited electron-hole pairs and photocurrent (I PC = I sd − I dark ) peak appears in the center of the device channel. This indicates the asymmetric contact cross-section between MoTe 2 -source and MoTe 2 -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 MoTe 2 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 MoTe 2 phototransistors (D1 and D2) and measure their photoresponse. The device is identified by an optical microscope, and the corresponding MoTe 2 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 MoTe 2 on SiO 2 /p + -Si substrate. SiO 2 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 MoTe 2 straddles source and drain electrodes. The channel length is 10 μm. MoTe 2 sample in the channel is about 23 nm thick (height profile is shown in Additional file 1: Figure S2), and the widths of MoTe 2 -source and MoTe 2 -drain contact cross-section are 6.5 and 4.8 μm, respectively. Figure 1b shows the Raman spectrum of MoTe 2 sample. The characteristics Raman-active modes of A 1g (172 cm −1 ), E 1 2g (233 cm −1 ), and B 1 2g (289 cm −1 ) are clearly observed, confirming the good quality of MoTe 2 in the channel.
Electric measurement indicates that multi-layered MoTe 2 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 × 10 3 when source-drain voltage V sd is 1 V. The field-effect mobility (μ) is 14.8 cm 2 /V s according to transfer characteristics. When biased voltage V sd decreases from 1 V to 100 mV, on-current and offcurrent both decrease, and the on-off ratio is still above 6.0 × 10 3 , 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 MoTe 2 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 MoTe 2 sample. We also fabricate other multi-layered MoTe 2 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 MoTe 2 transistor as back-gate voltage (V bg ) varies from − 20 to 4 V. As seen, the response is essentially linear, especially at a low biased voltage of V sd , which indicates that there is a low Schottky barrier between Au and MoTe 2 in the air. Figure 2 shows the photoresponse of multi-layered MoTe 2 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 (I sd ) increases with laser power. I sd vs. V sd curves at different illumination power levels all meet at V sd = 0 V, which is clearly observed in a logarithmic plot of |I sd | shown in insert Figure of Fig. 2a. When V bg = 5 V, the phototransistor is in off-state (see Fig. 1c), and the current of I sd increases with the illumination laser power, exhibiting clear nonlinear behavior, as shown in Fig. 2b. Furthermore, the phototransistor shows non-zero open-circuit voltage (V OC ) and short-circuit current (I SC ) with laser illumination, which is the evidence of photovoltaic response from multi-layered MoTe 2 phototransistor. Figure 2c shows V OC and I SC as a function of illumination power. V OC remains unchanged at 50 mV (illumination power is higher than 500 μW), and |I SC | increases from 0 to 1.6 nA when laser power increases from 0 to 4175 μW. When we change the voltage direction, V OC and I SC remain unchanged as shown in Fig. 2d. V sd represents the voltage loaded on source electrode and V ds is loaded on drain electrode, and the corresponding current is indicated by I sd and I ds , respectively. Insert image in Fig. 2d illustrates the voltage and current direction. Whether the voltage is loaded on the source or drain electrode, the V OC of 50 mV relative to source voltage and corresponding I SC of 680 pA flowing from drain electrode to source electrode both remain unchanged. This confirms the photovoltaic response of multi-layered MoTe 2 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 Fig. 2 Photoresponse of multi-layered MoTe 2 phototransistor illuminated by 637-nm wavelength laser in ambient condition. a I sd vs. V sd curves at V bg = 0 V as illumination power increases. b I sd vs. V sd curves at V bg = 5 V as illumination power increases. c V OC and I SC as a function of illumination power. d Output current for biased voltage loaded on the source and drain electrode, respectively 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. I PC measurements are conducted in short-circuit condition, in which source electrode is grounded and I PC 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 (V bg ) of − 5, 0, and 5 V, respectively. It can be seen that short-circuit I PC with opposite polarities is strong in the vicinity of the interfaces between MoTe 2 and the electrodes. When V bg is changed from − 5 to 0 V, I PC pattern remains unchanged but the intensity decreases. V bg is further increased to 5 V; I PC not only switches polarity, the position of maximum I PC also moves away from contact interface and into the channel. Figure 3c shows the I PC profile taken from the black dashed line in Fig. 3b at V bg = − 5, 0, and 5 V, respectively. It clearly demonstrates that I PC has a broad intensity peak near the interface between MoTe 2 and electrodes at V bg = − 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 I PC peaks indicates the existence of potential steps in short-circuit condition. According to the I PC distribution, we plot the corresponding potential profile along the device channel as shown in Fig. 3d. At V bg = − 5 and 0 V, the potential steps are near the contact interface between MoTe 2 and electrodes, and they move into the channel at V bg = 5 V. According to the previous study [41], Au electrode contact introduces p-doping and pins the Fermi level of MoTe 2 at contact part. Thus, the potential steps are formed in the vicinity of the electrode/MoTe 2 interface as the Fermi level in the channel is modulated by the gate voltage. At V bg = 0 V, a weak I PC is observed, which flows from the electrode to MoTe 2 channel. It means photoexcited electrons drift to nearby electrode and holes to MoTe 2 channel. At V bg = − 5 V, the hole density in MoTe 2 channel is enhanced and induces a larger potential step in the vicinity of the electrode/MoTe 2 interface. Photoexcited electron-hole pairs can be separated effectively and I PC increases. When V bg = 5 V, more electrons are injected into the MoTe 2 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 MoTe 2 channel and holes toward the nearby electrode. I PC changes direction compared with that at V bg = − 5 and 0 V. Figure 4 shows the spatial-resolved I PC at different V sd as V bg = 0 and 5 V, respectively. Figure 4a shows the optical image, together with the electrical setup. V sd is loaded on the source electrode, and I PC is collected from the drain electrode. The current flowing from the source to drain electrode is positive. Figure 4b shows I PC as a function of V sd at V bg = 0 V. When V sd = 0, − 0.01, and 0.01 V, strong I PC occurs in the vicinity of MoTe 2 /electrodes interface, then it moves toward the channel center as V sd increases to 0.1 V. Similar trend is observed at V bg = 5 V as V sd increases as shown in Fig. 4c. Figure 4d shows a clear I PC peak in the center of the device channel as V sd increases to 0.5 V. I PC profiles taken along the black dashed line in Fig. 4a are shown in Fig. 4e, f, which clearly show the I PC variation trend as V sd increases. They both indicate the maximum I PC generated in the vicinity of contact interface in shortcircuit condition or with small V sd 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/MoTe 2 interface due to the doping of the MoTe 2 by the metal contacts, dominates the separation of photoexcited electron-hole pairs in short-circuit condition or with small V sd biased. Thus, I PC at MoTe 2 -source is larger than that at MoTe 2 -drain due to the larger contact interface at MoTe 2 -source, and the net current is not zero, while the non-zero net current is smaller than I sd at V bg = − 5 and 0 V (in on-state), and larger than that at V bg = 5 V (in offstate). Therefore, we observe clear I SC at V bg = 5 V as shown in Fig. 2b and Additional file 1: Figure S6(b)-(f). Therefore, both I SC and the corresponding V OC 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 V OC as high as 150 mV when V bg = 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 V bg = 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 MoTe 2 phototransistor is a result from the asymmetric contact cross-section between MoTe 2 -source and MoTe 2 -drain electrodes.
Finally, we test the photoresponse time and spectral range of multi-layered MoTe 2 phototransistor. Figure 5a shows the time-resolved photocurrent at V bg = 5 V and V sd = 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 V sd = 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 V OC and V sd . 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 τ 0 rise À Á is 20 μs and fall time τ 0 fall À Á is 127 μs at V sd = 0 V, and the rise time τ 1 rise À Á is 210 μs and fall time τ 1 fall À Á is 302 μs at V sd = 1 V, which are both larger than that at V sd = 0 V. This is because of the different mechanism of photocurrent generation. At V sd = 0 V, the potential stepdominated photocurrent is generated in the vicinity of electrode/MoTe 2 interface. At V sd = 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/MoTe 2 interface. Thus, the device shows longer photoresponse time at V sd = 1 V than that at V sd = 0 V. In addition to working at the visible band, a multi-layered MoTe 2 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 MoTe 2 phototransistor can be used in the communication band.

Conclusions
In summary, we have fabricated air-stable p-type multilayered MoTe 2 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/MoTe 2 interface due to the doping of the MoTe 2 by the metal contacts, plays an important role in separating photoexcited electron-hole pairs in short-circuit condition or with small V sd biased. Net current is non-zero when potential step exists with asymmetric contact cross-section between MoTe 2source and MoTe 2 -drain electrodes. When biased voltage V sd rises above potential step, V sd dominates the separation of photoexcited electron-hole pairs, and I PC peak appears in the center of the device channel. Moreover, MoTe 2 phototransistor shows a faster response in shortcircuit 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.

Methods/Experimental
Back-gated multi-layered MoTe 2 phototransistors are fabricated in the following way. First, source, drain, and gate electrodes are patterned on 300-nm SiO 2 /p + -Si substrate using standard UV photolithography techniques, followed by selective etching of 300-nm SiO 2 beneath the gate electrode and E-beam evaporation of a 5 nm/100 nm Cr/Au films. Second, the multi-layered MoTe 2 sample is prepared on another 300-nm SiO 2 /p + -Si substrate by mechanical exfoliation of mm-size semiconducting 2H-MoTe 2 single crystals, which is grown by chemical vapor transport using TeCl 4 as the transport agent in a temperature gradient of 750 to 700°C for 3 days. Finally, the prepared multi-layered MoTe 2 sample is transferred onto patterned source-drain electrodes using polyvinyl alcohol (PVA) as a medium. PVA is dissolved in H 2 O and rinsed with isopropyl alcohol. Multi-layered MoTe 2 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 MoTe 2 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