Unique and Tunable Photodetecting Performance for Two

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Functional Inorganic Materials and Devices

Unique and Tunable Photo-Detecting Performance for TwoDimensional Layered MoSe2/WSe2 P-N Junction on 4H-SiC Substrate Wei Gao, Feng Zhang, Zhaoqiang Zheng, and Jingbo Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03709 • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 3, 2019

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ACS Applied Materials & Interfaces

Unique and Tunable Photo-Detecting Performance for TwoDimensional Layered MoSe2/WSe2 P-N Junction on 4H-SiC Substrate Wei Gao1, Feng Zhang2,3,4*, Zhaoqiang Zheng1, Jingbo Li1,3,4* 1

College of Materials and Energy, Guangdong University of Technology, Guangzhou

510006, P. R. China. 2

Department of Physics, Xiamen University, Xiamen, 361005, P. R. China.

3

Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P. R.

China. 4

College of Materials Science and Opto-Electronic Technology, University of Chinese

Academy of Sciences, Beijing 100049, China.

ACS Paragon Plus Environment

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ABSTRACT MoSe2/WSe2

two-dimensional

transition

metal

dichalcogenides

(TMDCs)

heterojunction photodetectors based on epitaxial growth n-doped 4H-Silicon Carbide (SiC) substrate are investigated and demonstrated firstly with low leakage, high stability and fast photo-response. The efficient separation of photo-generated carriers exist between TMDCs and 4H-SiC by the photoluminescence spectrum and the band alignment analysis under 532 nm. It shows an obvious rectification behavior and unique current-gate voltage (I-Vg) characteristics. The gate tunable photocurrent scanning maps display highest photocurrent in the MoSe2/WSe2 region including a certain intensive current region in individual TMDCs/4H-SiC junctions under a 532 nm laser. Besides, the maximum responsivity of the heterojunction photodetectors is 7.17 A·W-1 with the Vg of 10 V at positive bias. The corresponding maximum external quantum efficiency and detectivity also significantly increase to 1.67× 103 % and 5.51 × 1011 Jones with the largest Ilight/Idark ratio of ~103. Moreover, MoSe2/4H-SiC photodetector delievers a enhanced photo-response behavior with gate modulation, which is distinguished with previous paper. These results of our strudy demonstrate that MoSe2/WSe2 heterojunction photodetectors based on n doped 4H-SiC substrate will be a promising candidate for future optoelectronics applications in spectral responsivity.

KEYWORDS: two-dimensional transition metal dichalcogenides, heterostructure, silicon carbide, gate modulation, photodetector


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INTRODUCTION Recently, due to the quantum confinement effects and tunable band gap structures, two dimensional layered materials (2DLMs) such as graphene (Gr) family,1 transition metal dichalcogenides (TMDCs) family, IIIA-VIA group family and black phosphorene (BP) have been attracting much attentions and developing fast in optoelectronics and nanoelectronics field.2-4 Those multifarious 2DLMs can be stacked by van der Waals (vdWs) interactions along the vertical direction via a in-situ growth method or facile transfer technology.5 Photodetectors based on 2D Heterojunctions such as Gr/WS2,6 MoS2/WSe2,7 Gr/InSe/WSe2/Gr8 attract comprehensive attentions due to enhanced light-matter interaction, ultrahigh sensitivity and good flexibility for application in imaging, security checking and telecommunication and so on.9-12 However, high dark current and long response time can be induced by charge trapping effect of SiO2 in SiO2/Si system. Therefore, hexagonal-boron nitrile (h-BN) and Gr are usually used to prevent the formation of charge defects.13,14 Besides, a number of doped 3D substrates such as Si (band gap: 1.12 eV) and GaAs (band gap: 1.42 eV) are already used in the 2D material-based photodetector. However, lack of a dielectric layer such as SiO2, h-BN, Al2O3 between 2D and 3D substrate is existed in those structure. Therefore, a gate electric field cannot modulate and improve the (opto)electrical properties of the 2D/3D heterostructure, which is the limitation of the 2D/3D junction development. Moreover, the photo-response and the spectral range of commercial Si and its heterostructures are limited and it also needs to etch an area of SiO2 layer in array for further fabrication of electrodes and materials deposition, which restricts the



application fields of 2DLMs.15-17 As a wide band-gap 3D semiconductor, nowadays, the material quality and manufacturing process of silicon carbide (SiC) have been improved developed greatly.18 Utraviolet photodetectors based on SiC have been studied comprehensively and commercialized for detection applications of flame in mines, electric arc and weather forecast.19,20 Noticeably, high stability, high detectivity and ultralow leakage can be obtained in those SiC photodetector.21 Theoretically, n-type doped SiC with carrier concentration in the range of 1015 to 1019 cm-3 shows the optical absorption band from 1 to 3 eV.22 Therefore, high K constant (~9.7) of n-doped SiC can be used to integrate with 2DLMs to enhance the (opto)-electrical properties. Although the radiation stability, thermal and electrical characterizations of TMDCs/SiC heterostructures are studied in these years such as MoS2/SiC, WSe2/SiC etc.,23-25 the optoelectrical property, photoluminescence (PL), photon-generated carrier transition and transmission have not been studied in TMDCs/SiC system. Therefore, it is potential and indispensable to transfer the TMDCs heterojunction on 4H-SiC substrate for study of unique photoluminescence, rectification behavior and improved visible light detection by gate modulation. In this article, for the first time, an n-type 4H-SiC combined with individual n+MoSe2, p+-WSe2 and their van der Waals type-II heterojunction for visible light detection by mechanical exfoliation and wet transfer method. It is interesting and significant that the efficient photo-carriers transport can be achieved in TMDCs/4HSiC junctions based on the PL peak of 4H-SiC from 630 to 650 nm. The distinguishing


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and strong PL emission peaks are demonstrated in individual TMDCs on SiC compared to SiO2/Si substrate. The band alignment diagram between 4H-SiC and TMDCs is analyzed and consistent with the PL results. The correponding field effect transistor (FET) shows a rectification bahavior and mainly n-type semiconducting characteristics with a low dark current (~6 pA at Vg = +1.6 V). Furthermore, the gate tunable optoelectrical properties are impressively for WSe2, MoSe2 and MoSe2/WSe2 junction on the SiC substrate. Last but not least, unique photo-respose performance in this heterostructure is investigated via a photocurrent scanning map technology under 532 nm illumination with or without gate voltage. This novel 2D/3D TMDCs/4H-SiC devices pave the potential way to construct the high-speed, high sensitvity optoelectronics for optical imformation and communication application. EXPERIMENTAL SECTION Fabrication of MoSe2, WSe2 and the Corrsponding Heterostructure-Based Photodetector. Firstly, MoSe2 and WSe2 used for bottom and top layer were transferred onto the small SiO2 (300 nm)/Si substrate (0.3 cm × 0.3 cm) by mechanical cleavage method for convenient transferred operability. Firstly, to keep an insulatorsemiconducting behavior, the positive surface of 4H-SiC substrate should be rinsed in the buffered oxide etch (BOE) solution for 5 min to remove the native oxide layer at nanoscale. After that, the 4H-SiC is cleaned by deionized water. As a final substrate, n4H-SiC (The carrier density is 3×1015 cm-3, the thickness is about 11 μm)/n+ SiC (The carrier density is 3×1019 cm-3, the thickness is about 360 μm) substrate (1.5 cm × 1.5 cm) with back gate electrode (Ni/Ag) is fixed on a glass slide. To stack the MoSe2 on



the SiC substrate, 11 wt% poly (methyl methacrylate) (PMMA) anisole solution was spin-coated onto the small SiO2/Si substrate at 7000 rpm for 60 s and then baked at 150 °C on a hot plate for 20 min. Simultaneously, the substrate was soaked in 3 mol·L-1 KOH solution until the PMMA film separated from the substrate and floated on the liquid level. The PMMA film was rinsed with deionized water several times and the target materials were transferred onto the 4H-SiC substrate with the help of a micromanipulator in sequence. At last, the MoSe2-PMMA SiC sample was baked at 100°C on a hot plate for 30 min and rinsed with acetone to get rid of PMMA film. In a similar way, the WSe2 as a top layer was transferred onto the MoSe2 bottom layer by the above PMMA transferred method via a micromanipulator. For MoSe2/WSe2 based FETs and photodetectors. The devices were fabricated via an ultraviolet lithography technology (ULT). Firstly, a kind of photoresist (AR-P 5350 of ALLRESIST GmbH Co.,Ltd) was spin-coated onto the as-prepared SiC substrate at 3500 rpm for 60 s. After being baked on a hot plate for 4 min at 100 °C, the substrate was located and exposed to a Ultraviolet intensity of approximately 30 mW·cm-2 for 4 s via a lithography machine (ESCOPIA Co.,Ltd). Moreover, the electrode pattern will be developed in a developing solution and rinsed with deionized water. With the help of a vacuum thermal evaporation (OXFORD INSTRUMENT Co.,Ltd) at a rate of about 0.15 nm·s-1, a 100 nm gold layer was deposited onto the end of the MoSe2 and WSe2 to confirm the heterostructure locate between the electrode. Simultaneously, the FETs devices were achieved after being immersed in acetone to remove the remaining photoresist. Simultaneously, to eliminate the water and any solvent between the layers


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of heterostructure and form intimate contact between Au and TMDCs, the devices must be annealed in nitrogen atmosphere at 150 °C for 30 min before using the semiconductor characterization system. Characterizations. All optical images were captured by an optical microscope (Motic Moticcam Pro 205A). Raman measurement (NOST TECHNOLOGY Co.,Ltd, a laser excitation of 532 nm at 50 mW and a spot size of 120 μm) with a functional mode of Photoluminescence at room temperature were performed. The samples were characterized by Scanning Probe Microscope (SPM) with the functional modules of Atomic Force Microscopy (AFM) and Kelvin Probe Force Microscope (KPFM) (Dimension FastScan from BRUKER Co.,Ltd). Measurement. The (opto)-electrical properties of the devices were concluded by a three-probe station in air atmosphere at room temperature via a Keithley Agilent B2902A system. The laser devices with wavelengths of 405 nm and 532 nm are used to provide incident light. The response time was test via a shutter instrument. The spatially resolved photocurrent mapping measurement system includes a microscope objective (Olympus SLMPLN100×), a lock-in amplifier (SR830) with a light chopper and a micromechanical stage with a control system. RESULTS AND CONCLUSION Characterization of n-MoSe2/p-WSe2 heterostructure on 4H-SiC substrate. The 3D schematic image of the prepared heterostructure is demonstrated in Figure 1a. In particular, n-MoSe2/n-SiC and p-WSe2/n-SiC juntions should be also considered into the performace of the device. In Figure 1b, with the help of AFM, the thickness of the



top WSe2 layer and the bottom MoSe2 layer along the different lines are 70.52 nm and 10.06 nm, respectively. It is indicated that the lateral heterostructure is composed of few-layered MoSe2 and multilayered WSe2. The top n-doped 4H-SiC contact layer is epitaxial growth on the 360 μm n+-4H-SiC substrate by chemical vapour deposition method. The thickness of the top layer (~11 μm) is characterized by Fourier Transform Infrared Spectrophotometer (FTIR) (see in Figure S1) Besides, Raman spectroscopy is used to verify the crystal quality and phonon vibration modes of the TMDCs on the 4HSiC substrate. In Figure 1c, at spot 1 for 4H-SiC, the peaks at 202.9 cm-1, 778.4 cm-1 are E2 mode, corresponding to the planar acoustic (PA) mode and planar optical (PO) mode, respectively. Meanwhile, A1 modes at 611.5 cm-1, 967.9 cm-1 refer to axial acoustic (AA) and longitudinal optical (LO), the broaden of the A1(LO) phonon line shape can confirm the increasing carrier concentration in the 4H-SiC substrate. Thus, the sharp peak of A1(LO) is correlated to the low carrier concentration (3×1015 cm-3) of the top 4H-SiC layer (11 μm). Meanwhile, the peak at 797.7 cm-1 is E1 transverse optical (TO) mode.26 We note that all of the strong characterisitic peaks in 4H-SiC are consistent with our test. For thin MoSe2 on spot 2, the prominent A1g vibration mode (out-of-plane) at 238.5 cm-1 are observed and the weak characteristic peaks for 4H-SiC are also shown on spot 2. It is red-shifted compared to the few-layered MoSe2 on SiO2/Si substrate.27 The absence of E2g mode in the range of 285 to 290 cm-1 is due to the long-range Coulomb interactions between layers.28 For multilayered WSe2 on spot 3, the E12g and A11g prominent peaks at 245.4 cm-1, 253.9 cm-1 represent in-plane vibration and the out-of-plane direction, respectively.29 Last, the heterostructure on spot


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4 shows 240.3 cm-1, 245.2 cm-1 and 254.1 cm-1 vibration peaks, indicating that both WSe2 and MoSe2 layers appeared without significant shift on the 4H-SiC substrate.30 The E2 (PO) peak can also be shown in the junction. Moreover, as shown in Figure 1d, Raman intensity maps of the peaks at 238 cm-1 from MoSe2 and 252 cm-1 from WSe2 show a immediate signal in the junction area because of the quenching effect, which further confirms the van der Waals interaction at the interface.13 It is noteworthy to show the MoSe2 region from the WSe2 at 252 cm-1, attributing to the proximity of the peaks of MoSe2 and WSe2.31 Most importantly, the weak signal can also be seen in SiC/MoSe2 region. To investigate the interlayer coupling effect at the various interfaces and the optical emission bandgaps of 4H-SiC, thin MoSe2 and thick WSe2 on 4H-SiC, PL spectrum measurement is a suitable way as shown Figure 1e. Generally, 4H-SiC is a indirect wide band gap (~3.26 eV) semiconducting material.32 However, there are some in-gap impurity activation energy (~2 eV) locating in the band gap of the n-type doping 4H-SiC, where the majority electrons can be stimulated and shifted to several shallow levels by 532 nm laser (2.33 eV).33-35 For spot 1 (green line), a strong and broad peak at about 630 nm corresponding to about 1.97 eV under 532 nm, probably arising from c1 to c3 transistion in 4H-SiC band structure. Thus, the lower shallow conduction band of 4H-SiC contains a sufficient number of electrons contributes to the optical absorption.22,36 We also individually transfer the few-layered MoSe2 and multilayered WSe2 onto the SiO2/Si substrate. (the thickness can be seen in Figure S2). On the SiO2/Si, there is a broad overlapping peak including around 890 nm and 998 nm (belong to Si). (see in Figure S3) The former PL peak is probably corresponding to the K-K



direct band gap (1.39 eV deviated from 1.34 eV) of MoSe2 by the first pricinple calculation.37 For few-layered MoSe2 (violet line) on 4H-SiC, a strong PL peak was observed at the wavelength of 870 nm so that the K-K direct gap was changed from 1.39 eV to 1.43 eV on 4H-SiC. As for the multilayered WSe2 (pink line), two peaks can be obviously detected at 797 nm (1.55 eV, K-K direct emission) and 900 nm (1.38 eV, K- indirect emission).38 Ehanced peak intensities and a blueshift can be seen in fewlayered WSe2 on 4H-SiC. (see in Figure S4). Nevertheless, approximately 865 nm (indirect band gap of 1.43 eV) PL peak can only be exhibited for multilayered WSe2 on the SiO2/Si substrate.7 The above distinctive resultance is mainly due to the extra absorption band below 3 eV of the greenish transparent n type 4H-SiC and the strong interlayer coupling effect between n type 4H-SiC and TMDCs.22 In the measured heterostructure region (brown line), we found the enhancement (a factor of ~1.1) of the PL peaks (863 nm for MoSe2, 900 nm for WSe2 except for 801 nm,) with negligible shifts, which is reverse compared to the previous studies on MoSe2/WSe2.31,39,40 The PL peaks intensity of the overlapped region on SiO2/Si is generally suppressed due to the charge redistribution and the formation of spatial indirect, bond electron-hole pairs at the interface.39,40 Moreover, this result will be benefit for the improvement of rapid photo-response behavior because of the strong interlayer coupling at the junction.8,31 On the one hand, the reason is that the higher crystallinity and defect-free surface of epitaxial growth 4H-SiC can suppress the trapping of excitons by defects or radiative recombination at the interface.41-43 On the other hand, the in-gap defects levels of 4HSiC at about 2 eV are rectified according to the previous studies.22,33 Thus, under 532


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nm irradiation, some electrons on the deep levels of SiC can transfer to the CB of MoSe2 between SiC and MoSe2, leading to the improvement and blueshift of PL emission of MoSe2.44 The thick WSe2 may also have a prominent effect on the band alignment and PL intensities.45 Besides, the strong suppression of the loweast-energy PL peaks at TMDCs/4H-SiC part and those peaks are both redshifted to about 656 nm (1.89 eV) compared to 625-630 nm for 4H-SiC. It is confirmed that the efficient photo-carriers (mainly electrons) separation between n-type 4H-SiC/n-MoSe2 bottom layer, n-type 4H-SiC/p-WSe2 through the quenching effect. The photo-generated electrons on low conduction band from 4H-SiC by 532 nm can enhance the PL intensities of MoSe2 and WSe2. Furthermore, the band alignment between 4H-SiC and TMDCs is discussed based on the above PL result to better understand the transport and recombination of electron-hole carriers in Figure 1f. Based on the previous report, the minimum of conduction band (CBM) and the maximum of the valence band (VBM) of few-layered MoSe2 (multilayed WSe2) calculated by Perdew-Burke-Ernzerhof (PBE) method are 4.20 eV (3.85 eV) and 4.95 eV (4.75 eV).46 Thus, the offset of the corresponding conduction band and valence band are about 0.35 and 0.20 eV, respectively. Once in contact, a type-II heterojunction between MoSe2 and WSe2 can be obtained. Under 532 nm light, electron-hole pairs are generated. The photo-generated electrons from WSe2 move toward MoSe2 and holes move to WSe2 individually with a built-in electric field.47 Incorporating the semiducting role of n-type 4H-SiC, the CBM and VBM of 4H-SiC is about 2.7 eV and 6.0 eV.48 The experimental result (~3.17 eV) is consistant with the calculated band gap of nearly 3.26 eV. (see in Figure S5) Noticeabily, there is



a deep shallow energy level locating on the 4.0 eV (6.0 eV-1.97 eV) near the mid-gap of 4H-SiC. Under 532 nm laser, some photo-generated electrons on the deep levels motivated from VBM move toward MoSe2 because of the large offset while the minority electrons from WSe2 can transfer to the deep level of 4H-SiC, resulting in a positive Voc and a negative Isc (see in the Figure 3a).49 Consequently, the charge transfer from WSe2 to low-energy states in MoSe2 region on 4H-SiC is probably less than that of the region on SiO2/Si substrate. Thus, the heterojunction can produce its maximum photocurrent with the help of electrons-injection of n-4H-SiC top layer. In particular, some photocurrent at a certain intensity can be also obtained at the 4H-SiC/MoSe2, 4HSiC/WSe2 region due to the built-in electic field, which is distinguishing from the traditional TMDCs based on SiO2/Si substrate. Moreover, the photocurrent and the response-properties will be change prominently with the gate voltage at the back of the n+-4H-SiC substrate. Those forcast can be verified in electrical and optoelectrical measurement as follows. After the stuctural characterization, a high crystal quality MoSe2/WSe2 heterostructure on n-type 4H-SiC substrate has been prepared successfully. It is predicable to show the good optoelectrical properties of the device under 532 nm based on the PL characterization.


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Figure 1. MoSe2-WSe2 heterostructure on the n-4H-SiC substrate by mechanical exfoliation method. (a) Schematic 3D image MoSe2-WSe2 heterostructure of photodetector. (b) Atomic Force microscopy (AFM) topography of the semiconducting materials: 1-4H-SiC. 2-MoSe2. 3-WSe2. 4-MoSe2-WSe2. The corresponding thickness of the bottom MoSe2 and the top WSe2 along the different color lines. (c) Normalized Raman spectra of the test spot from (b) at room temperature. (d) Raman intensity mapping collected at E1 (TO) (1-SiC: 777 cm-1) and A1g (2-MoSe2: 238 cm-1, 3-WSe2: 252 cm-1), the scale bars are 5 μm. (e) Photoluminescence (PL) spectra of 1-4H-SiC, 2MoSe2, 3-WSe2 and 4-MoSe2/WSe2 heterostructure from 550 to 1050 nm under a 532 nm laser. Inset is the PL spectra of 4H-SiC from 550 to 850 nm. (f) The band alignment of the n-4H-SiC/n-MoSe2/p-WSe2/n-4H-SiC contact at the equilibrium condition under 532 nm laser. FET performance of 4H-SiC and MoSe2/WSe2 heterostructure on 4H-SiC. For comparison, the individual MoSe2 and WSe2 devices on n-4H-SiC and SiO2/Si substrate are also fabricated. The corresponding characterizations and properties can be seen in



Figure S6 to S11. The device is electrically measured at room temperature by applying drain (p-WSe2 end) to source (n-MoSe2 end) voltage (Vds). The top 4H-SiC (11 μm) layer acts as a light doping n-type insulator (dark)-semiconductor (light) and suppresses the leakage current.35 The back-gate voltage (Vg) below the low electrical resistance of the n+-type 4H-SiC (360 μm) should be chosen to positive value (in the range of 1 to 10 V) to make sure the stable operating condition at negative bias in the heterostructure configuration. As shown in Figure 2a, gate-modulated individual thin MoSe2 and thick WSe2 can be achieved on n-4H-SiC and mainly exhibits n-type (n-doping on/off ratio ~4.34×102) and p-type (on/off ratio ~2×102) channel behavior, respectively.50 The reason is that the different unintentional doping present in each crystal under gate control.7 Besides, the Ids-Vg transfer curve of the MoSe2-WSe2 junction shows a bipolar type. It is indicated that the dark current can be lower to ~6 pA under gate voltage of 1.6 V in the epitaxial 4H-SiC layer (11 μm). Besides, with the gate voltages increase from 1 to 10 V, we can visually find gate-voltage-tuned rectification of current as electrostatic doping by individual TMDCs modulates the concentration of electrons and holes in the heterostructure.7 The sudden downtrend of current for heterostructure at Vg from 1 to 1.6 V in Figure 2b can be attributed to the partly p-type doping of MoSe2 and the p-type of WSe2. When the Vg increases from 1.6 V to 10 V, the heavily electrons doping MoSe2 dominates the transfer channel of the n+-n heterostructure.39,47 As shown in Figure 2c, the gate tunable Ids-Vds curves is observed and show different rectification behaviors. The version in a log scale can be seen in Figure S12. Obviously, the Ids-Vds curve in Figure 2d for the device shows a rectifying behavior, indicating a depletion


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layer between TMDCs. The dark current at Vds = -1.5 V is only -7.78 × 10-11 A. Furthermore, to highlight the low leakage current of n type 4H-SiC substrate, the corresponding FETs was also electrically measured and the dark current is about -0.1 nA at Vds = -1.5 V in Figure 2e. It is indicated that the MoSe2/WSe2 heterostructure can suppress the dark current via an efficient p-n junction along reverse direction.

Figure 2. Electrical characteristics of MoSe2-WSe2 heterostructure on the 4H-SiC substrate. (a) Gate-dependent transport characteristics at Vds = 1.5 V for individual layers of thin MoSe2 and thick WSe2. (b) Transfer characteristic curve of the device correlated to Vg from 1 to 10 V. (c) 3D image of output characteristic curves of the device under Vg from 1 V to 10 V. (d) Ids-Vds curve of the device without Vg. Inset is the optical image of the FET device based on the heterostructure. The scale bar is 10 μm. (e) Ids-Vds curve of 4H-SiC substrate. Inset is the optical image of the FET device based on the 4H-SiC. The scale bar is 10 μm. Optoelectrical performance of the MoSe2/WSe2 heterostructure on 4H-SiC. A 532 nm laser is used to evaluate the photo-response properties of the heterostructure on



4H-SiC. As shown in Figure 3a, the current under 532 nm illumination are enhanced from 0.05 to 44.56 mW·cm-2. Meanwhile, apart from photoconductive effect, the photovoltaic effect is also exhibited (Isc = -4.07 × 10-10 A at Vds = 0 V, Isc-P curve see in Figure S13) and Voc is about 0.19 V at Ids = 0 V under 44.56 mW·cm-2 532 nm laser. When the Vg is applied at the back of the substrate, the Ids is enhanced significantly and also increases with increasing P under Vds = ± 1.5 V (see in Figure S14 and Figure 3b). The maximum Ids is about 3.96 × 10-9 A at the Ilight/Idark (3.96 × 10-9 A/1.67 × 10-10 A) ratio of ~40. In particular, the maximum Ilight/Idark (1.82 × 10-9 A/6.45 × 10-12 A) ratio of 2.82 × 102 can be achieved at Vg = 1.6 V. In Figure 3c, the photocurrent performs a sublinear behavior with the augment of P by a power law of 𝐼𝑝ℎ ∝ 𝑃𝛼, where α = 0.48, 0.49 and 1.34 at Vds = 1.5 V, 1.5 V at Vg = 10 V and Vds = -1.5 V, respectively. At forward bias (1.5 V), the high injection of majority carriers across the heterostructure result in the enhancement of the Auger recombination.51 While at backward bias (-1.5 V), a super-linear behavior is due to the decrement of Auger recombination sites and more photo-generated current transmits onto the surface of the junction without the trapping effect.7 Most importantly, the good stable photo-response curve and fast response time is the indispensable key of the photodetector. Figure 3d presents the timeresolved Ids under 1-second periods of alternated irradiance and darkness. The Ilight is such stable that the corresponding deviation is less than 5 %. Obviously, the Light to dark current ratio exceeds to 103, indicating the high optical response of our device with gate voltage of 10 V. The stable time trace of the photodetector at Vds = -1.5 V, Vg = 0 V is also added in the Figure S15. Furthermore, Figure 3e is the high-resolution time-


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resolved curve by using a faster measured point under Vg = 10 V. Response time including rise time (τr, photocurrent rises from 10 % to 90 % of its final value) and decay time (τd, photocurrent falls from 90 % to 10 % of its start value) is the ability of detecting the photo-signal at certain rate.52 The rise and decay time of our device at Vds = -1.5 V, Vg = 10 V are found to be 4.3 ms and 22.6 ms, respectively. Interestingly, the response time depends on the gate voltage as shown in Figure 3f. The rise time changes little (4.3 to 6.4 ms) when Vg is applied, while the decay time (26.1 to 22.6 ms) shows a decreasing trend. In this junction, the gate voltage can tune the Fermi level of the MoSe2, WSe2 and n-4H-SiC top layer, which will change the energy band barriers between them. Before applying a back-gate voltage, built-in electric fields can be obtained at MoSe2/WSe2, MoSe2/SiC and WSe2/SiC interfaces. The large photogenerated carriers can immediately separate to each side along the several electric fields, leading to the fast rise time speed for the heterostructure. However, the decay time is slower due to the increasing recombination rate of those large numbers of carriers. With increasing the Vg from 1 to 10 V, the energy band barriers between those interfaces decrease and the photo-generated carriers move easily, leading to a longer carrier lifetime. Thus, the recombination sites of the carriers along the vertical direction will decrease and the decay time will be faster.53,54



Figure 3. Optoelectrical characteristics of the MoSe2-WSe2 heterostructure on the 4HSiC substrate. (a) Ids-Vds curves dependent on the light power density under 532 nm illumination and in darkness. The unit is mW·cm-2 (b) Ids-Vg curves as a function of light power density under 532 nm illumination and in darkness. (c) Light power density dependence of the photocurrent. (d) Time trace of the photodetector under a 532 nm illumination at Vds = -1.5 V, Vg = 10 V. (e) Rising and decay time of the MoSe2-WSe2 heterostructure under a 532 nm illumination at Vds = -1.5 V, Vg = 10 V. (f) Gatedependent curves for response time including rise time and decay time. In Figure 4a, to exhibit the different material clearly of the heterojunction, the MoSe2 and WSe2 sheet region are highlighted in dashed line by pink and orange, respectively. The white lines are correlated to 100 nm Au electrode and the dark green region is ntype 4H-SiC top layer. To understand the photo-response characteristics of the heterostructure on 4H-SiC substrate, scanning photocurrent measurement is carried out with a confocal optical microscope at 6 μW 532 nm laser. Photocurrent mappings with or without gate voltage are shown in Figure 4b and c. Before applying a gate voltage,


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the photocurrent map shows the maximum photocurrent corresponding to the MoSe2/WSe2 junction, indicating a formation of p-n junction across the MoSe2/WSe2 area with strong interlayer coupling and fast charge carriers separation.7,49,51,55 It is necessary to note that the photocurrent correlated to the scoped area where WSe2/SiC, MoSe2/SiC are overlapped, which is different from other TMDCs photodetectors on SiO2/Si substrate.10,11,39,49,50 We believe that the photo-generated carriers in n-4H-SiC top layer/TMDCs separating efficiently can contribute to the MoSe2/WSe2 junction and enhance the measured Ilight value. This phenomenon is also consistent with the movement of the photo-generated electron-hole pairs between 4H-SiC and TMDCs in the PL spectrum and band alignment analysis. When the laser is far from the TMDCs and focused on the pure 4H-SiC substrate, the photo-current at a certain area can also reach to 2× 10-8 A. It is confirmed that a number of photo-generated carriers are existed in the substrate because of the in-gap defect levels.22,35 After supplying a gate voltage of 10 V, the region of the maximum photocurrent is increased and extended beyond the MoSe2/WSe2 heterostructure due to the decrement of the energy band barrier between MoSe2/WSe2 and SiC/TMDCs.50 Reasonably, the sharp photocurrent area of the n-type SiC top layer/TMDCs junction also slightly increases. Therefore, under 532 nm laser, n-type 4H-SiC top layer (11 μm) mainly acts as a photo-electrons donator toward the MoSe2/WSe2 junction dependent on gate voltage. To confirm the good photo-response of n-4H-SiC and MoSe2/WSe2 on n-4H-SiC, the optoelectrical parameters are calculated to evaluate the performance of the device later. Generally, Responsivity (Rλ), External quantum efficiency (EQE), Detectivity (D*)



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are calculated to evaluate the performance of the photodetector. First, the Rλ is defined as the photo-current generated per unit power of the incident light on the effective area of the photodetector.52 𝑅𝜆 =

𝐼𝑝ℎ

(1)

𝑃𝑆

Where Iph is the photocurrent (Iillumination - Idark), S is the photo-response region of the photodetector and P is the light power density. EQE is defined as the number of effective photo-generated carriers upon excitation of per incident photon. D* is a parameter evaluating the device’s capability to detect weak optical signal. The following equations can be shown.52 EQE = *

D =

ℎ𝑐𝑅𝜆

(2)

𝑒𝜆

𝑅𝜆S1 2

(3)

(2eIdark)1 2

Where h is the Planck constant, c is the light velocity, e is the electronic charge and λ is the wavelength of exciting light. For the photodetectors in our work, the overlapped and the adjacent area is calculated to be about 80 μm2 according to the photocurrent mapping images. In case of a wide band-gap at ~3.26 eV, 4H-SiC can be used as an insulating layer. In contrast, under a 532 nm illumination at P = 44.56 mW·cm-2, the drain current of the device can be enhanced to 1.24×10-9 A at Vds = 1.5 V. This unique response behavior compared to SiO2/Si substrate is owing to the extra optical absorption band below 3 eV of the greenish transparent 4H-SiC.22 Thus, a number of photo-charge can be generated beyond the in-gap impurity levels in 4H-SiC and then 11 μm 4H-SiC layer becomes a semiconducting layer. The corresponding Rλ of 89 mA·W-1, EQE of 21 % and D* of 8.16 × 1010 Jones (ie., cm·Hz1/2·W-1) at Vds = 1.5 V


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(see in Figure S8c, d). Its rise and decay time are both nearly 0.17 s with Ilight/Idark of about 9 (Figure S8f). After incorporating into the TMDCs heterostructure, our device shows enhanced photo-response and it can also operate at Vds = 0 V with maximum Rλ and EQE of up to 160 mA·W-1 and 37 %, which is due to the type II band alignment and built-in electric field at equilibrium condition.55 When a reverse bias (-1.5 V) is applied, both the Iph and Rλ increase because of the augment of the electric field in junction. Thus, the maximum of Rλ, EQE and D* can reach to 360 mA·W-1, 84 % and 4.05 × 1010 Jones at P = 0.05 mW·cm-2. Theoretically, the decrement of Rλ with increasing P is due to the Coulomb interaction between photo-generated carriers and radiative recombination.51 Most importantly, as shown in Figure 4e and f, the maximum of Rλ significantly increases from 3.77 to 7.17 A·W-1 with gate voltage of 0 to 10 V at Vds = 1.5 V. The corresponding EQE and D* is also enhanced to 1.67× 103 % and 5.51 × 1011 Jones. Generally, the photo-generated electron-hole pairs can cycle many times under the external electric field (Vds = 1.5 V). Many electrons in this n-type heterostructure can participate in the photocurrent. When the gate voltage is applied, the carrier concentration is enhanced to a certain extent. In this way, we can get an EQE much higher than 100%.56 Refer to the band alignment in Figure 1f, the barrier heights between type-II MoSe2/WSe2, n-type SiC top layer/TMDCs are decreased with the positive gate tunable effect. The tunable and good optoelectical parameters indicate that our n-MoSe2/p-WSe2 photodetector based on n type 4H-SiC substrate is much sensitive to weak optical input signals with or without gate voltage. In addition, we also used 405 nm laser to investigate the performance of the device. The results are inferior to the 532



nm and shown in Figure S16. By contrast, the responsivity, EQE and detectivity of n type MoSe2/4H-SiC photodetector can be significantly increased to 88 A·W-1, 2.05× 104 % and 2.02× 1012 Jones with the gate voltage of only 10 V. (see in Figure S10 and Table S1). The above results are better than other similar structure on the different substrates. (see in Table S2 and S3) While the response time is slightly prolonged and the Ilight/Idark ratio is poor due to the photoconductive effect. Nevertheless, the optoelectrical properties of the same thickness of MoSe2 on SiO2/Si substrate under gate modulation are very weak and not shown in Table S2 as other previous reported paper.

Figure 4. Photo-response performance of the MoSe2-WSe2 heterostructure on the 4HSiC substrate. (a) Optical microscope image of the heterostructure, component materials are outlined in different color including Au electrode. The scale bar is 5 μm. (b) and (c) Scanning photocurrent microscope images under a 6 μW 532 nm illumination laser with or without gate voltage. The laser beam is focused by microscope objective and the diameter of the spot size is about 1.5 μm (d) Responsivity


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and photocurrent by 532 nm laser as a function of light power density under Vds = -1.5 V (solid lines) and 0 V (dashed lines), Vg = 0 V. (e) Responsivity and photocurrent by 532 nm laser as a function of light power density under Vds = +1.5 V, Vg = 0 V (dash lines) and Vg = 10 V (solid lines). (f) EQE and D* as a function of light power density under Vds = 1.5 V, Vg = 10 V. In conclusion, we have fabricated a few-layered MoSe2/multilayered WSe2 heterostructure based on n-type 4H-SiC substrate. The electrical and 532 nm photoresponse properties are focused in this device. According to the unique PL spectrum and the band alignment analysis, there are not only a type II MoSe2/WSe2 heterostructure but also TMDCs/SiC junctions contributing to the enhancement and separation of the photo-generated charge carriers. Visually, the photocurrent scanning map is used to confirm the photo-response region in the device dependent on gate voltage. Except for MoSe2/WSe2 part, a certain intensity of the photocurrent can also be produced in the extended 4H-SiC/individual TMDCs junctions and 4H-SiC by 532 nm excitation. Furthermore, as a photovoltaic device, it shows the maximum Rλ and EQE of up to 160 mA·W-1 and 37 %. Besides, as a photoconductive device, the maximum Rλ increases from 3.77 to 7.17 A·W-1 with the Vg increasing from 0 to 10 V. The corresponding EQE and D* at Vg = 10 V also significantly increase to 1.67 × 103 % and 5.51 × 1011 Jones. The rise and decay time are 4.3 ms and 22.6 ms with a high Ilight/Idark ratio of ~103 at Vg = 10 V, Vds = -1.5 V. The above result is better than other similar structure on the different substrates. It is indicated that n-type 4H-SiC substrate mainly acts as a space-charge region and electrons donator to enhance the photocurrent



of the TMDCs junction by gate regulation. Hopefully, the MoSe2/WSe2 heterostructure on 4H-SiC can open a potential direction towards the development of the 2D/SiC heterostructures for novel nanoelectronics and optoelectronics. ASSOCIATED CONTENT Supporting Information FTIR spectrum of 4H-SiC substrate. AFM images of MoSe2 and WSe2 on the SiO2/Si substrate and 4H-SiC substrate. Photoluminescence spectrum of MoSe2 on the SiO2/Si substrate. Photoluminescence spectrum of 4H-SiC substrate. Thickness-dependent PL characterization of WSe2. Electrical and optoelectrical performance of MoSe2 and WSe2 on the SiO2/Si substrate and 4H-SiC substrate under 532 nm. Photo-response property of 4H-SiC under 532 nm illumination. Optoelectrical properties of MoSe2/WSe2 on the 4H-SiC under 405 nm. 3D image of output curves of the MoSe2/WSe2-based photodetector in a log scale. The short circuit current as a function of P. The Ids-Vds of the MoSe2/WSe2-based photodetector at Vg = 10 V. The time trace of the the MoSe2/WSe2-based photodetector at Vds = -1.5 V without Vg. Comparison of figures-of-merit for the photodetectors in this work or other reported photodetectors on the different substrates. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]. Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (11674310, 61805044 ， 61704034 ， 61474113 and 61574140), “Hundred Talents Program” of Guangdong University of Technology (GDUT), the National Basic Research Program of China (Grant No. 2015CB759600), the Science Challenge Project (TZ2017003), Beijing NOVA Program (2016071 and xxjc201801), Beijing Municipal Science and Technology Commission Project (Z161100002116018) and the Youth Innovation Promotion Association of CAS (2012098) REFERENCES 1. Kauling, A. P.; Seefeldt, A. T.; Pisoni, D. P.; Pradeep, R. C.; Bentini, R.; Oliveira, R. V. B.; Novoselov, K. S.; Neto, A. H. C.; The Worldwide Graphene Flake Production. Adv. Mater. 2018, 30, 1803784. 2. Xu, M.; Liang, T.; Shi, M.; Chen, H.; Graphene-Like Two-Dimensional Materials. Chem. Rev. 2013, 113, 3766-3798. 3. Wei, X.; Yan, F.; Shen, C.; Lv, Q.; Wang, K. Photodetectors Based on Junctions of Two-Dimensional Transition Metal Dichalcogenides. Chinese Phys. B. 2017, 6, 038504. 4. Yan, F.; Wei, Z.; Wei, X.; Lv, Q.; Zhu, W.; Wang, K. Toward High-Performance Photodetectors Based on 2D Materials: Strategy on Methods. Small Methods. 2018, 2, 1700349. 5. Huo, N.; Yang, Y.; Li, J. Optoelectronics Based on 2D TMDs and Heterostructures.



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Unique and Tunable Photodetecting Performance for Two

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