Defect-Affected Photocurrent in MoTe2 FETs - ACS Applied Materials

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Defect-affected Photocurrent in MoTe2 FETs Mohan Kumar Ghimire, Hyunjin Ji, Hamza Zad Gul, Hojoon Yi, Jinbao Jiang, and Seong Chu Lim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00050 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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ACS Applied Materials & Interfaces

Defect-affected Photocurrent in MoTe2 FETs Mohan Kumar Ghimire1,2,£, Hyunjin Ji1,£, Hamza Zad Gul1,2, Hojoon Yi1,2, Jinbao Jiang1,2, and Seong Chu Lim1,2,*

1Department

of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of

Korea 2Center

for Integrated Nanostructure Physics, Institute for Basic Science, Sungkyunkwan

University, Suwon 440-746, Republic of Korea

£These

authors contributed to this work equally.

*Corresponding author e-mail: [email protected]

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ABSTRACT Imperfections in the crystal lattice, such as defects, grain boundaries, or dislocations, can significantly affect the optical and electrical transport properties of materials. In this study, we report the effect of mid-gap trap states on photocurrent in 10 atomic layered 2H-MoTe2. Our study reveals that the photocurrent is very sensitive to the number of active traps, which can be controlled by Vgs. By fitting the measured transient drain current, our estimation shows that the trap state density is approximately 5 × 1011 cm-2. By analyzing the photocurrent data as a function of gate voltage, we realize how the ionized traps affect the photoexcited carriers. The model of hole traps, electron traps, and recombination centers inside the bandgap successfully describes our observed results.

Keywords: MoTe2, Defect, Photocurrent, Gate modulation, Trap and detrap, Metal-gate


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INTRODUCTION MoTe2, a 2D layered transition metal dichalcogenide (TMDC)1, has recently attracted great interest in the field of optoelectronics owing to its sizable bandgap2, strong photoluminescence of the monolayer3, high photoresponsivity4, and fast response5. These properties of semiconducting TMDs provide new platforms for electronic and optoelectronic devices such as next-generation field effect transistors (FETs)6, light emitting diodes7, and Van der Waals heterostructure devices8,9. In addition, 2H-MoTe2 has been explored for applications in spintronics10 and solar cells11. FETs fabricated with single-layer and a-fewlayer MoTe2 have shown both unipolar12 and ambipolar13 charge transport characteristics and the polarity can be controlled through the choice of the contact electrode14. However, harnessing of the various electrical and optoelectrical devices based on MoTe2 is hindered by the existence of defects and impurities. In order to obtain reliable performance and controllability of the device, understanding the role of these defects is of significance. In addition, the generation of defects in TMDs is relatively easy, therefore, the defect density is high in these systems15,16. There are various types of defects in TMDs, such as chalcogen atom vacancies, impurities, interstitials, antisite defects, and dislocations17. Some defects can induce mid-gap states, which impact various exotic electrical and optical properties. For instance, mid-gap states serve as electron and hole trap centers, as well as acting as electronhole recombination centers18. Exciton capture and release by mid-gap defects has been reported in monolayer MoSe2 by using femtosecond degenerate pump-probe spectroscopy19. The above example indicates that understanding the role of trap centers is important in electronic devices in order to optimize the device performance. Here, we report a photocurrent of MoTe2 that is affected by trap states. In order to control the energy level and the number of active trap sites, the channel of a MoTe2 FET is modulated 3 ACS Paragon Plus Environment


using the gate bias, which shifts the Fermi level and results in the occupation or inoccupation of the trap sites by mobile charges. The number of trap states at the Fermi level was calculated by fitting the time-dependent drain current measured in the dark. It is observed that the variation in the number of filled and empty trap centers significantly modulates the photocurrent of MoTe2. Our results are consistent with the photoexcited holes and electrons getting trapped in their respective trap centers and the electron and hole populations decaying through the recombination centers. RESULTS AND DISCUSSION In order to fabricate devices, bulk MoTe2 was purchased from 2D semiconductors. A few layers of the material were deposited onto a substrate by using the mechanical exfoliation method. Figure 1a illustrates the schematics of the fabrication process of a metal-back-gated MoTe2 device in which 2/18 nm of Cr/Au was used as the back-gate. The details of the fabrication process are given in the Methods section. Figure 1b is an optical microscope (OM) image of the multipurpose MoTe2 device fabricated. The inset of Figure 1c shows the atomic force microscopy (AFM) topographic image of MoTe2 and the height profile taken from the region corresponding to the blue square. From the height profile, the thickness of MoTe2 was estimated to be 7 nm, which corresponds to 10 atomic layers. In the study, the carrier fluctuation in the channel due to the capture/release and scattering by interface trap sites was investigated and it is found that the influence of interface trap density (NST) was minimal at the thickness of ~ 10 nm20. Since our MoTe2 is also prepared with mechanical exfoliation, for the above reason, we choose a flake with the thickness of 7 nm (10 layers), close to 10 nm, to minimize the effect by the interface traps on the photoresponse of MoTe2, and to maximize the effect of the midgap states in the experiment. The AFM image with the height profile for hexagonal boron nitride (h-BN) is shown in Supporting Information S1, from which the 4 ACS Paragon Plus Environment

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thickness of h-BN was calculated to be 25 nm. H-BN as a gate dielectric layer was selected for blocking the photogating effect from SiO2, because it is known to have less ionic impurities as compared to SiO2. In addition, a 25-nm-thick h-BN is believed to be durable under some extent of applied gate bias. A higher k-dielectric material could be a choice for the gate dielectric; however, it will have a larger number of impurities21 that will be ionized under illumination, thereby giving rise to the photogating effect. Figure 1d shows the results of Raman spectroscopy of MoTe2. The E12g peak at 234 cm-1 originates from the in-plane vibrations of the Te atoms. The FET device (shown in Figure 1b) was placed on a chip and mechanically wire bonded for electrical connection. It was then loaded onto a closed cycle refrigerator (CCR) chamber for electrical characterization. The temperature inside the chamber could be lowered to 20 K. We first measured the drain current (Ids) as a function of the gate-source voltage (Vgs) for different source-drain voltages (Vds). Ambipolarity was observed in the device, with electron current being slightly higher than hole current, as shown in Figure 2a. Since the band bending situations are similar for hole and electron injections, the height of the Schottky barrier (SB) for electron injection into the conduction band (CB) is supposed to be smaller than that for hole injection into the valence band (VB) in order to achieve asymmetric transconductance22. The ambipolarity indicates that the Fermi level (EF) can be tuned on both sides of the charge neutrality point (CNP) through Vgs. Further, on reversing the gate sweeping direction, hysteresis was observed, with a shift in CNP of about 0.5 V. The observation of hysteresis indicates the presence of charge trap sites that influence the channel current, because the device was annealed inside a vacuum chamber (Supporting Information S2). From the transfer curve shown in Figure 2a, we can estimate the field effect mobility, µFE23, and the subthreshold swing, SS. Using the device parameters L/W = 10/9.5 µm (representing the 5 ACS Paragon Plus Environment


length and width of MoTe2), thickness of h-BN = 25 nm, and dielectric constant of h-BN = 3.5, the maximum mobility of MoTe2 was calculated to be 3 cm2V-1s-1. From the subthreshold part of the curve, a SS of 2 V dec-1 was estimated based on SS = (dlog10Ids/dVgs)-1. The observation of hysteresis as well as the low value of the mobility and the high value of the SS are indicative of the presence of trap sites24. There is also a possibility of hysteresis due to the absorption of water molecules on the sample. If the hysteresis was due to the absorbed water molecules, then it should disappear after annealing the device in vacuum. To confirm the origin of the hysteresis, we measured the transfer curves of the device before and after the annealing step. The transfer curves measured at high vacuum (≈ 10-6 torr) before and after annealing (150 °C, 2 h) are shown in Supporting information S2. As seen from Figure S2, there is the shift in CNP, but the hysteresis remained intact after annealing. Hence, it is confirmed that the hysteresis originated from the trap sites present inside MoTe2, rather than from the admolecules on the surface of MoTe2. This motivated us to calculate the density of the mid-gap charge trap states. To calculate the trap density, we measured the transient drain current at different Vgs25, as shown in Figure 2b. The trap levels below EF are supposed to be occupied by electrons, which can be controlled through Vgs. A sudden change in Vgs shifts the position of EF inside the band gap, resulting in the occupation and emptiness of the trap sites. The time-stamped change in Ids represents the charging and discharging of the trap sites during the modulation of Vgs. All the measurements pertaining to the transient change in Ids were carried out at Vds = 0.1 V, unless otherwise specified. Figure 2b shows that at a given Vds, the change in Ids was monitored as a function of time by sweeping gate voltage from Vgs = -6 V to Vgs = 3 V, indicating a stepwise occupation of trap sites from the valence to the conduction band. When Vgs changes from -6 V to -5V, the time evolution of Ids is shown in Figure 2c. At each Vgs, we 6 ACS Paragon Plus Environment

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waited long enough to ensure that Ids was fully leveled out, as revealed in Figure 2c. The expression for the transient Ids can be written as 𝐼𝑑𝑠(𝑡) = 𝐼𝑑𝑠,

𝑠𝑎𝑡

―

𝑞𝑊𝜇𝐹𝐸𝑁𝑇𝑉𝑑𝑠 ―𝑡 𝐿

𝑒

𝜏 25

, where

Ids,sat is the saturated drain current, W and L are the width and length of the MoTe2 channel, q is the charge, µFE is the field effect mobility, NT is the number of traps, Vds is the bias voltage, and τ is the decay coefficient. Here, the time-independent component NT varies with Vgs, but it is obtained from Y intercept of the decay curve at different Vgs. In this equation, among the parameters constituting the pre-exponential factor, only NT is unknown. For the analysis, a single exponential curve expressed in the form 𝐼(𝑡) = 𝐼0 + 𝐴𝑒

―

𝑡 𝜏

was used for fitting. Here,

τ denotes the decay coefficient, which stems from the occupation/inoccupation of the traps. Comparing the fitted values of the pre-exponential factor A with that corresponding to the Ids equation, NT could be calculated. Similarly, the decay coefficient τ at each Vgs was obtained from the fitting. The number of trap states at each Vgs, NT (Vgs ) , was calculated and plotted as a function of Vgs in Figure 2d. It is seen that 0 V < Vgs < 2 V, the population of unfilled trap states sharply drops and levels out when Vgs changes from -6 V to -3 V. Then, NT shows a hump between Vgs = -2 V and Vgs = 0.5 V. When Vgs > 0.5 V, a rapid increase in NT is observed (See also Supporting Information S3 for NT at different thickness). In addition, we measured the temperature dependent transfer curves. As expected, the current level decreases on lowering the temperature as a characteristic of semiconducting nature of the sample. The measured data with temperature ranging from 300 to 210 K in a step of 15 K are shown in Fig. S4(a) of Supporting Information. We also measured the output curves at 300 K at different Vgs. The output curves were found linear for different Vgs indicating Ohmic contact nature between MoTe2 and Cr/Au electrode. The curves are shown in Fig. S4(a) of Supporting Information. 7 ACS Paragon Plus Environment


Figure 3a shows the transfer curves of MoTe2 in the dark and under laser illumination (wavelength = 638 nm). The laser power was controlled by using an optical density filter. From the curves, it is obvious that the behavior of the photoelectric current of MoTe2 depends on the gate bias. However, the photoinduced current of MoTe2 is not well distinguished from the channel current. A slight asymmetric behavior is observed in Figure 3a. The output curves of MoTe2 in dark and under laser illumination are shown in Supporting Information S5. The photoresponse of the MoTe2 FET will be further discussed in a following paragraph. We measured the photocurrent, shown in Figure 3b, by using the technique described in our previous work23, as well as that described in the Methods section. Since a thin Au layer is used as a metal-back-gate electrode, as shown in Figure 1c, current modulation by photogating is not a concern in our characterization. Figure 3b exhibits a modulation of the photocurrent as a function of the light intensity and Vgs. On increasing the laser power, the photocurrent increases, except CNP, Vgs ~ 0.8 V. This is common behavior, since the number of e-h pair increases with increasing laser power. As the channel exhibits electron conduction, corresponding to Vgs > 0.8 V, the photocurrent shows a monotonic increase with the backgate bias. Opposite to this, when the channel displays hole conduction, corresponding to Vgs < 0.8 V, the photocurrent fluctuates severely depending on the Vgs. In Figure 2d, we reveal the number of trap sites that influence the transport properties of our device. Therefore, in order to explain the behavior of the photocurrent observed in Figure 3b, we constructed the schematic diagrams shown in Figures 3c and 3d that revealed the presence of charge traps that capture and release optically excited carriers. Based on the figure, the imperfections can be classified into electron traps, hole traps, and recombination centers18. The traps of both the carriers are represented as open black circles when they are ionized and as solid circles when they are neutral. The optically excited electron and hole carriers are also shown as solid and


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open dark orange circles, respectively. Figure 3c illustrates the process of a hole and an electron being captured at the trap sites and their annihilation at a recombination center. Figure 3d illustrates the situation where the occupation and inoccupation of the gap states occurs depending on the position of EF (which in turn depends on Vgs) and the relaxation of the optically excited carriers. As shown in the schematic diagram, the trap states below EF are filled, while those above it are empty. The filled traps (solid black circles) are neutral and have no effect on the photoexcited carriers; on the other hand, the ionized traps state can capture the photoexcited carriers. After the laser illumination, holes and electrons populate the VB and the CB, respectively, as shown in Figure 3d (I). We swept Vgs from a negative to a positive back-gate bias; the schematics have also been drawn according to sweeping direction. The schematic for Vgs = -5.5 V is shown in Figure 3d (II). At this point, EF is close to the VB edge. The photoexcited holes start getting trapped in the hole trap states and the electrons in the electron trap states, as shown. Hence, the photocurrent, which is the sum of the photoexcited hole and electron currents, is expected to decrease. The trapping of both types of carriers occurs at the shallow trap states in this case. The number of active shallow traps is expected to be higher than that of the deep ones because of the lower ionization energy of the former. Therefore, the shallow traps prevail over their deep counterparts26, 27. Hence, the photocurrent will continuously decrease until EF remains in the shallow trap state region. When Vgs = -3 V, as shown in Figure 3d (III), EF is supposed to be close to the deep trap level. Since the number of ionized deep traps is lower, the trapping rate of holes starts decreasing. This causes the photocurrent to increase. The photocurrent increases continuously until EF remains within the deep trap region, corresponding to Vgs = -0.8 V, as shown in Figure 3b and 3d (IV). What needs to be taken into consideration here is that for both the shallow and deep trap sites, the trapping rates of electrons and holes may not be the same, 9 ACS Paragon Plus Environment


which can be inferred from the asymmetric curve shown in Figure 3a. The recombination centers existing inside the forbidden gap lie around the mid-gap18. When EF is close to the energy level of the recombination centers, as shown in Figure 3d (V), electrons and holes annihilate through trap-assisted recombination. This recombination is a symmetric process in that electrons and holes are lost in pairs. This is different from the case of an electron or hole getting trapped, which results in an asymmetric photocurrent. Owing to the annihilation of the carriers in pairs and the fact that this process is faster than carrier trapping19, the photocurrent is supposed to decrease quickly when EF crosses the energy level corresponding to the recombination centers. This behavior can be seen clearly in Figure 3b, resulting in a dome of the photocurrent corresponding to the hole conduction channel. Further sweeping of the Vgs towards the positive side results in EF crossing the CNP and reaching the CB edge, as shown in Figure 3d (VI) for Vgs = 3 V. What is important here is that all the hole trap sites are filled as EF crosses the CNP. Most of the holes that populate the VB will then contribute to the photocurrent. Hence, the photocurrent is only expected to increase during electron conduction, since the filled NT increases continuously. This can explain why such a dome shape is not observed in the electron conduction region, as was seen in the case of the hole conduction region. Reinspecting Figure 2d with regard to the calculated NT, our schematic model was found to follow the NT curve. Thus, our proposed model of carrier trapping in the forbidden gap successfully explains the dome of the photocurrent observed for the hole conduction channel of a metal-gated MoTe2 FET. Further, we have estimated the trap states from the time-resolved photoresponse technique (see Supporting Information S6 for detail). In addition, we intentionally introduce the surface trap states on MoTe2 by exposing the device to the air. The detailed of the engineering the photoresponse via introduction of surface trap states is provided on Supporting Information 10 ACS Paragon Plus Environment

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S7. The photo-responsivity calculated from our device is higher comparing to the previous reports. The reason for this may be that our sample is comparatively thicker so that there is a less chance for photoexcited carrier to be trapped at the dielectric interface comparing to the monolayer and few layer device. The detailed on the responsivity of the device is explained on Supporting Information S8. We also made effort to eliminate the factors from the device configurations such as, parasitic capacitance, current leakage or contact resistance that may attribute to the presence of trap states (See Supporting Information S9). CONCLUSIONS In summary, we have fabricated a metal back-gated 10 layer 2H-MoTe2 FET. Our device shows ambipolar transport behavior with a large asymmetry. In our estimation, the number of trap sites is approximately 1011 cm-2, and the time constant of the trapping and detrapping process varies over several tens of seconds. By using the position of the Fermi level and the recombination process involving electron and hole carriers, the photocurrent of ambipolar MoTe2 has been explained. Our charge trap model qualitatively explains our findings.

Methods Fabrication of metal back-gated MoTe2 FETs First, by using e-beam lithography, a metal-backgated electrode was patterned on a highly doped Si substrate covered with 300 nm of SiO2 and, then, Cr/Au (2/18 nm) was deposited with an electron beam evaporator. After lift-off, the metal back-gate was formed. h-BN was mechanically exfoliated from the commercially available bulk crystal onto a PVA/PMMAcoated Si/SiO2 substrate. Through the dry transfer technique, multilayered h-BN with a thickness of 25 nm was transferred onto the metal back-gate. Using the same transfer 11 ACS Paragon Plus Environment


technique, a few layers of 2H-MoTe2 were also transferred onto the h-BN and then, a plasma was used to define a MoTe2 channel with two wings for metal contact. To fabricate the FET, the electrodes were patterned on MoTe2 flakes by using e-beam lithography, and then, Cr/Au (10/90 nm) was deposited through electron beam evaporation.

Characterization of h-BN and MoTe2 AFM was performed to calculate the thicknesses of h-BN and MoTe2 (model – E-Sweep, manufacturer – Hitachi Hightech). Raman spectroscopy was also performed at the excitation wavelength of 532 nm to determine the number of layers of MoTe2. (Renishaw Raman, Witech system)

Electrical and optical measurements Electrical and photocurrent measurements were carried out inside a CCR chamber. The pressure inside the chamber was maintained at 1 × 10-6 Torr. The CCR chamber was equipped with a quartz window through which laser could be focused onto the sample. The electrical measurements were performed by using a double channel source-measure unit (SMU 2636). For ac photocurrent measurement, a low-noise current preamplifier (SR 570), SMU 2636, and an SR 830 lock-in amplifier were used. SMU 2636 was used to apply the gate voltage. The current preamplifier was used to apply the drain bias, as well as to convert the photocurrent into a voltage signal that was fed into the lock-in amplifier. In this preamplifier, the signal was amplified into a detectable voltage range by a lock-in amplifier. The lock-in amplifier reads the amplitude and phase of the signal that it receives from the current preamplifier with respect to the reference frequency provided externally by the optical 12 ACS Paragon Plus Environment

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chopper. A laser source of wavelength 638 nm was used to illuminate virtually the entire MoTe2 device. The laser had a maximum power of about 50 mW that was controlled through the use of an optical density filter. ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. AFM image and thickness of h-BN, Transfer curves before and after annealing, Density of trap sites from additional devices, Temperature dependent transfer and output characteristics, Output curves in the dark and under illumination, Estimation of trap states from timeresolved photoresponse technique, Engineering the photoresponse via introduction of surface trap states, Responsivity of the device, Effect of parasitic capacitance, current leakage and contact resistance.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ORCID Mohan Kumar Ghimire: 0000-0002-4076-975X Hyunjin Ji: 0000-0002-3967-6900 Seong Chu Lim: 0000-0002-0751-1458



Author Contributions M.K.G. conducted the experiment and acquired the data. H. J. analyzed the data. H.Z.G. and H.Y. assisted with the experimental setup. J.J. fabricated the MoTe2 FET and performed AFM measurement. S.C.L. organized the data and prepared the manuscript. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS The authors from SKKU are grateful for the financial support received from the civil-military technology cooperation program (15-CM-MA-14)


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REFERENCES (1) Wang, Q.H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotech. 2012, 7, 699-712. (2) Lezama, I. G.; Arora, A.; Ubaldini, A.; Barreteau, C.; Giannini, E.; Potemski, M.; Morpurgo, A. F. Indirect-to-Direct Band Gap Crossover in Few-Layer MoTe2. Nano Lett. 2015, 15, 2336-2342. (3) Ruppert, C.; Aslan, O. B.; Heinz, Tony F. Optical Properties and Band Gap of Singleand Few-layer MoTe2 Crystals. Nano Lett. 2014, 14, 6231-6236. (4) Yin, L.; Zhan, X.; Xu, K.; Wang, F.; Wang, Z.; Huang, Y.; Wang, Q.; Jiang, C.; He, J. Ultrahigh Sensitive MoTe2 Phototransistors Driven by Carrier Tunneling. App. Phys. Lett. 2016, 108, 043503. (5) Octon, T. J.; Nagareddy, V. K.; Russo, S.; Craciun, M. F.; Wright, C. D. Fast HighResponsivity Few-Layer MoTe2 Photodetectors. Adv. Opt. Mater. 2016, 4, 1750-1754. (6) Nourbakhsh, A.; Zubair, A.; Sajjad, R. N.; Tavakkoli, A.; Chen, W.; Fang, S.; Ling, X.; Kong, J.; Dresselhaus, M. S.; Kaxiras, E.; Berggren, K. K.; Antoniadis, D.; Palacious, T. MoS2 Field-Effect Transistor with Sub-10 nm Channel Length. Nano Lett. 2016, 16, 7798- 7806. (7) Withers, F.; Zamudio, O.D.; Mishchenko, A.; Rooney, A. P.; Gholinia, A.; Watanable, K.; Taniguchi, T.; Haigh, S. J.; Geim, A. K.; Tartakovskii, A. I.; Novoselov, K. S. Light-emitting Diodes by Band-structure Engineering in Van der Waals Heterostructures. Nat. Mater. 2014, 14, 301-306. (8) Geim, A. K.; Grigorieva, I. V. Van der Waals Heterostructures. Nature 2013, 499, 419-425. (9) Jeong, H.; Oh, H. M.; Bang, S.; Jeong, H. J.; An, S. J.; Han, G. H.; Kim, H.; Yun, S. J.; park, J. C.; Lee, Y. H.; Lerondel, G.; Jeong, M.S. Metal-Insulator-Semiconductor Diode Consisting of Two-Dimensional Nanomaterials. Nano Lett. 2017, 16, 18581862. (10) Xiao, D.; Liu, G. B.; Feng, W.; Xu, X.; Yao, W. Coupled Spin and Valley Physics in Monolayers of MoS2 and Other Group-VI Dichalcogenides. Phys. Rev. Lett. 2012, 108, 196802. (11) Pezeshki A.; Hossein, S.; Nazari, T.; Oh, K.; Im, S. Electric and Photovoltaic Behavior of a Few-Layer α-MoTe2/MoS2 Dichalcogenide Heterojunction. Adv. Mater. 2016, 28, 3216-3222. (12) Fathipour, S.; Ma, N.; Hwang, W. S.; Protasenko, V.; Vishwanath, S.; Xing, H. G.; Xu, H.; Jena, D.; Appenzeller, J.; Seabaugh, A. Exfoliated Multilayer MoTe2 Field-Effect Transistors. Appl. Phys. Lett. 2014, 105, 192101. (13) Lin, Y. F.; Xu, Y.; Wang, S. T.; Li, S. L.; Yamamoto, M.; Ferreira, A. A.; Li, W.; Sun, H.; Nakaharai, S.; Jian, W. B.; Ueno, K.; Tsukagoshi, K. Ambipolar MoTe2 Transistors and Their Applications in Logic Circuits. Adv. Mater. 2014, 26, 32633269. 15 ACS Paragon Plus Environment


(14) Nakaharai, S.; Yamamoto, M.; Ueno, K.; Tsukagoshi, K. Carrier Polarity Control in α-MoTe2 Schottky Junctions Based on Weak Fermi-level Pinning. ACS Appl. Mater. Interfaces 2016, 8, 14732-14739. (15) Rosenberger, M. R.; Chuang, H. J.; Mcreary, K. M.; Li, C. H.; Jonker, B. T. Electrical Characterization of Discrete Defects and Impact of Defect Density on Photoluminescence in Monolayer WS2. ACS Nano 2018, 12, 1793-1800. (16) Choi, K.; Raza, S. R. A.; Lee, H. S.; Jeon, P. J.; Pezeshki, A.; Min, S. W.; Kim, J. S.; Yoon, W.; Ju, S. Y.; Lee, K.; Im, S. Trap Density Probing on Top-gate MoS2 Nanosheet Field-effect Transistors by Photo-excited Charge Collection Spectroscopy. Nanoscale, 2015, 7, 5617. (17) Lin, Z.; Carvalho, B. R.; Kahn, E.; Lv, R.; Rao, R.; Terrones, H.; Pimenta, M. A.; Terrones, M. Defect Engineering of Two-Dimensional Transition Metal Dichalcogenides. 2D Mater. 2016, 3, 022002. (18) Stockmann, F. On the Classification of Traps and Recombination Centres. Phys. Stat. sol. (a) 1973, 20, 217 Subject classification: 13.4; 22. (19) Chen, K.; Ghosh, R.; Meng, X.; Roy, A.; Kim, J. S.; He, F.; Mason, S. C.; Xu, X.; Lin, J. F.; Akinwande, D.; Banerjee, S. K.; Wang, Y. Experimental Evidence of Exciton Capture by Mid-gap Defects in CVD Grown Monolayer MoSe2. npj 2D Mater. 2017, 1, 15. (20) Ji, H.; Lee, G.; Joo, M-K.; Yun, Y.; Yi, H.; Park, J-H.; Suh, D.; Lim, S. C. Thickness-Dependent Carrier Mobility of Ambipolar MoTe2: Interplay Between Interface Trap and Coulomb Scattering. Appl. Phys. Lett. 2017, 110, 183501. (21) Verma, A.; Mishra, A.; Jha, A.; Verma, K. Effect of High-K Oxide Layer on Carrier Mobility. IJAREEIE 2014, 3, 5. (22) Das, S.; Appenzeller, J. WSe2 Field Effect Transistors with Enhanced Ambipolar Characteristics. Appl. Phys. Lett. 2013, 103, 103501. (23) Ghimire, M. K.; Gul, H. Z.; Yi, Hojoon.; Dang. D. X.; Sakong, W. K.; Luan, N. V.; Ji, H. J.; Lim, S. C. Graphene-CdSe quantum Dot Hybrid as a Platform for the Control of Carrier Temperature. FlatChem 2017, 6, 77-82. (24) Ayari, A.; Cobas, E.; Ogundadegbe, O.; Fuhrer, M. S. Realization and Electrical Characterization of Ultrathin Crystals of Layered Transition-Metal Dichalcogenides. J. Appl. Phys. 2007, 101, 014507. (25) Amit, I.; Octon, T. J.; Townsend, N. J.; Reale, F.; Wright, C. D.; Mattevi, C.; Craciun, M. F.; Russo, S. Role of Charge Traps in the Performance of Atomically Thin Transistors. Adv. Mater. 2017, 29, 1605598. (26) Wang, Y.; Wu, D.; Fu, L. M.; Ai, X. C.; Xu, D.; Zhang, J. P. Density of State Determination of Two Types of Intra-gap Traps in Dye-sensitized Solar Cells and its Influence on Device Performance. Phys. Chem. Chem. Phys. 2014, 16, 11626. (27) Bartolomeo, A. D.; Grillo, A.; Urban, F.; Lemmo, L.; Giubileo, F.; Luongo, G.; Amato, G.; Croin, L.; Sun, L.; Liang, S. J.; Ang, L. K. Asymmetric Schottky Contacts in Bilayer MoS2 Field Effect Transistors. Adv. Funct. Mater. 2018, 28, 1800657.


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FIGURE CAPTIONS Figure 1. (a) Schematics of the processes involved in device fabrication, (b) optical microscope image of a metal-backgated MoTe2 FET, (c) height profile of the MoTe2 flake taken from the region corresponding to the blue square shown in the topographic image (inset), and (d) Raman spectrum of MoTe2.

Figure 2. (a) Semilog-transfer curve at different Vds under dual Vgs sweep, (b) timedependent Ids measured at different Vgs, (c) fit of the Ids curve of Figure (b) at Vgs = -5 V, and (d) calculated number of trap sites (NT) at different Vgs.

Figure 3 (a) Semilog-transfer curves in the dark and under various input optical powers of a 638 nm laser, (b) Vgs-sweep-dependent photocurrent as a function of laser power, (c) schematic diagram of the trapping of holes and electrons in the defect energy states inside the bandgap, and (d) schematic diagrams of charge trapping and recombination at different Vgs.


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Defect-Affected Photocurrent in MoTe2 FETs - ACS Applied Materials

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