Enhanced Electrical and Optoelectronic Characteristics of Few-Layer

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Enhanced Electrical and Optoelectronic Characteristics of Few-Layer Type-II SnSe/MoS2 van der Waals Heterojunctions Shengxue Yang, Minghui Wu, Bin Wang, Li-Dong Zhao, Li Huang, Chengbao Jiang, and Su-Huai Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15288 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 15, 2017

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ACS Applied Materials & Interfaces

Enhanced Electrical and Optoelectronic Characteristics of Few-Layer Type-II SnSe/MoS2 van der Waals Heterojunctions Shengxue Yang1,+, Minghui Wu2,+, Bin Wang1, Li-Dong Zhao1, Li Huang2, Chengbao Jiang1,*, Su-Huai Wei 3,*

1

School of Materials Science and Engineering, Beihang University, Beijing 100191, P.

R. China Email: [email protected] 2

Department of Physics, South University of Science and Technology of China,

Shenzhen 518005, P. R. China 3

Beijing Computational Science Research Center, Beijing 100094, P. R. China

Email: [email protected] KEYWORDS: Van der Waals heterostructure, type-II band alignment, dissimilar material systems, rectification, self-power photocurrent

ABSTRACT Van der Waals heterojunctions formed by stacking various two-dimensional (2D) materials have a series of attractive physical properties, thus offering an ideal platform for versatile electronic and optoelectronic applications. Here, we report few-layer SnSe/MoS2 van der Waals heterojunctions and study their electrical and optoelectronic characteristics. The new heterojunctions present excellent electrical transport characteristics with a distinct rectification effect and a high current on/off

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ratio (~ 1 × 105). Such type-II heterostructures also generate a self-powered photocurrent with a fast response time (< 10 ms) and exhibit high photoresponsivity of 100 A/W, together with high external quantum efficiency (EQE) of 23.3 × 103 % under illumination by 532-nm light. Photoswitching characteristics of the heterojunctions can be modulated by bias voltage, light wavelength and power density. The designed novel type-II van der Waals heterojunctions are formed from a combination of a transition metal dichalcogenide (TMD) and a group IV-VI layered 2D material, thereby, expanding the library of ultrathin flexible 2D semiconducting devices.

INTRODUCTION Van der Waals heterojunctions built by assembling different two-dimensional (2D) atomic crystals, such as graphene (Gr), hexagonal boron nitride (h-BN), transition metal

dichalcogenides

(TMDs),

phosphorene

and

group

III-VI

layered

semiconductors, have revealed many fascinating new phenomena that have stimulated a new understanding of 2D-system physics and have expanded the applications of 2D heterostructures in electronics, optoelectronics and valleytronics.

1-8

Such van der

Waals heterostructures present a series of attractive physical properties, including high electrical conductivity and mobility,1 Klein tunnelling,9 ultrafast charge transfer,4 efficient photocurrent generation,10 strong light-matter interactions,11 novel quantum Hall effect12 and spin/valley polarization13. The vertical stacking of graphene on h-BN was the most successful example. The combined van der Waals heterostructures


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exhibited enhanced mobility and reduced carrier inhomogeneity compared to the graphene devices on SiO2.1 The photoluminescence mapping and femtosecond pump-probe spectroscopy experiments indicated that ultrafast charge transfer (50 fs) was taking place in photoexcited MoS2/WS2 heterostructures, due to the formation of type-II band alignment between MoS2 and WS2.4 In vertically stacked WSe2/MoS2 heterojunctions, the photocurrent could be efficiently generated with an external quantum efficiency (EQE) up to 12%. Moreover, strong electroluminescence with rich spectral features was also observed.10 Photoresponse measurements under illumination by

various-wavelength

lasers

showed

strong

light-matter

interactions

in

h-BN/Gr/WS2/Gr/h-BN heterojunction, allowing for the development of extremely efficient photovoltaic devices with an EQE of above 30%.11 Various 2D materials have differences in their band structures, work functions, and spin-orbit coupling, which can be utilized to design novel 2D van der Waals heterostructure devices for a variety of useful applications.6 These include vertical field-effect transistors (FETs),2,14 resonant tunneling diodes,15 photodetectors,6 light-emitting diodes,5 photovoltaic cells,11 lasers,16 and spin-valleytronic devices.13 The theoretical calculations predict that artificial 2D semiconducting heterostructures can form type-II band alignment, where the conduction band minimum (CBM) and the valence band maximum (VBM) reside on opposite sides of the heterostructure. In type-II band alignment, photoexcited electrons and holes can be efficiently separated, which leads to ultrafast charge transfer, long photoexcited carrier lifetimes, tuning interlayer coupling, and reduced interfacial electron-hole recombination.6,17,18 Such



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alignment offers good opportunities for optoelectronic and light-harvesting devices. Group IV-VI metal dichalcogenides, such as GeS, GeSe, SnS2, SnSe, PbS, form another family of 2D semiconducting materials. The combination of this type of semiconductor with other 2D layered materials has already been used to construct some heterostructures.19-22 By decorating monolayer WS2 with SnSe nanocrystals (NCs), phototransistors with a hugely enhanced photoresponse was obtained.21 Later, an infrared photodetector was successfully developed by integrating non-layered PbS nanoplates and multilayer MoS2 nanostructures, which exhibited a fast response, high photoresponsivity and high detectivity.

22

Such high performance was attributed to a

strong chemical hybridization between highly-crystalline PbS and MoS2, together with a small van der Waals interaction distance in this heterojunction. However, in the above studies, group IV-VI semiconductors are in the form of non-layered structures. Considering the great variety of layered semiconducting structures, further exploration of van der Waals heterojunctions based on layered group IV-VI semiconductors and TMDs with type-II band alignment is potentially of great significance. In this work, we report on the electrical and optoelectronic characteristics of few-layer type-II van der Waals heterojunctions consisting of hexagonal-structure MoS2 and orthorhombic-structure SnSe thin layers (Figure 1). Few-layer MoS2 is an indirect gap semiconductor with a high crystal symmetry from the TMDs family, where the covalently bonded S−Mo−S atomic planes are held together by weak van der Waals forces.6 While few-layer SnSe is an indirect gap semiconductor with a low


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degree of symmetry from the family of group IV-VI metal dichalcogenides, where tightly bound double layers of Sn and Se atoms are stacked along the c-axis.23 The SnSe/MoS2 van der Waals heterojunctions present excellent electrical transport characteristics with a high current on/off ratio and a distinct rectification behavior. In addition, such type-II heterostructures generate self-powered photocurrents with a fast response time, as well as a tunable photoresponse with high photoresponsivity and high EQE. Photoswitching characteristics of SnSe/MoS2 heterojunctions depend on bias voltage, light wavelength and power density. Our studies demonstrate that novel devices formed by new type of material systems –– TMDs and group IV-VI layered 2D materials have superior performance, which expands the library of ultrathin flexible van der Waals heterojunctions.

Figure 1. (a, b) HRTEM images of MoS2 and SnSe flakes. The insets are corresponding SAED patterns. The scale bar is 2 nm. (c) Heterojunction based on orthorhombic structure of SnSe and hexagonal structure of MoS2 flakes. The yellow,



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green, purple and gray balls show sulfur, selenium, molybdenum and tin atoms, respectively. (d) Optical microscopy (OM) image of a SnSe/MoS2 van der Waals heterojunction on a SiO2/Si substrate. (Black and red dashed lines indicate MoS2 and SnSe flakes, respectively). The scale bar is 3 µm. (e) The selected region (black square frame in (d)) of AFM image of a SnSe/MoS2 type-II heterostructure. The scale bar is 2 µm. (f) AFM height profiles along the red and black lines in (e) show the thicknesses of few-layer SnSe (28 nm) and MoS2 (7 nm) flakes.

RESULTS AND DICUSSION MoS2 single crystals were grown by the vapor transport technique, while the SnSe single crystals were obtained by using the Bridgman crystal growth method.24 To confirm the atomic structure and crystal quality of MoS2 and SnSe crystals, we measured the transmission electron microscopy (TEM) images and corresponding selected area electron diffraction (SAED) patterns. Few-layer MoS2 and SnSe flakes were first exfoliated from bulk single crystals using Scotch tape, and then they were deposited onto the micro grids using the transfer method for the TEM measurements. From the high-resolution TEM (HRTEM) image shown in Figure 1a, the lattice spacing of MoS2 is 0.27 nm, corresponding to the structure of 2H-MoS2.6 The HRTEM image of SnSe flake shows orthogonal lattice fringes with the lattice spacing of 0.3 nm, and the intersection angle between the (011) and (0-11) planes is approximately 92° (Figure 1b).23 The clear lattice fringes prove that both the MoS2 and SnSe crystals have high crystallinity. The SAED patterns shown in the insets of


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Figures 1a and b indicate that hexagonal MoS2 orients along the [0 0 1] zone axis (JCPDS 37−1492) and the orientation of SnSe is along the [1 0 0] zone axis, which is consistent with the PDF card (JCPDS 48−1224). Then, the van der Waals heterostructures were created by vertical stacking of few-layer SnSe flakes on top of the exfoliated MoS2 thin layers using dry stamping with the aid of a polymethyl methacrylate (PMMA) transfer mediator (Figure 1c). The detailed method is shown in the Supporting Information Figure S1. The heterojunction devices were fabricated by conventional electron beam lithography (EBL), and two Ni (20 nm)/Au (50nm) electrodes were deposited by the electron beam evaporation (EBE), as shown in Figure 1d. The thicknesses of the few-layer MoS2 and SnSe flakes were obtained by atomic force microscopy (AFM), and the AFM height profiles indicated that the values were 7 nm and 28 nm, respectively (Figure 1e, and 1f).

Figure 2. (a-c) Room-temperature Raman spectra of different positions on a heterojunction: MoS2 flake (red), SnSe flake (blue), and the SnSe/MoS2 heterostructure (black). The excitation wavelength was 532 nm. The insets show the schematic illustration of phonon modes. (d, e) Calculated band structures of few-layer



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MoS2 and SnSe flakes, respectively. (f) Calculated band alignment of a SnSe/MoS2 heterojunction, showing the formation of a type-II structure.

We then measured the Raman spectra to characterize the SnSe/MoS2 heterojunctions. In the Raman spectrum of few-layer MoS2 flake shown in Figure 2a, two typical peaks are observed at 383 and 407 cm-1, corresponding to the in-plane E2g mode and out-of-plane A1g mode.6 For the few-layer SnSe flake, there are four typical peaks at 69.5, 109.0, 130.6, and 149.5 cm-1 in the Raman spectrum, which represent the Ag(1), B3g, Ag(2) and Ag(3) modes, respectively (Figure 2b).23 The phonon vibrations of typical Raman modes for few-layer MoS2 and SnSe flakes are illustrated in the insets of Figure 2a and 2b. Figure 2c is the Raman spectrum of a SnSe/MoS2 heterojunction, it can be seen that all the modes are consistent with the typical Raman peaks of the individual MoS2 and SnSe flakes, but the intensities of the peaks are quenched in the heterojunction, due to the ultrafast charge transfer process within the SnSe/MoS2 heterojunction. Moreover, first-principles calculations were performed to estimate the band structures of few-layer MoS2 and SnSe flakes, as well as the band alignment and charge transfer of the SnSe/MoS2 heterojunction. The details of the calculation methods are given in the Supporting Information. Figure 3d shows the calculated band structure of few-layer MoS2 flake, where the CBM locates between K and Γ points, and VBM is at Γ point, thus it forms an indirect bandgap of 1.44 eV. For few-layer SnSe flake, the CBM lies between Γ and Y points, and the location of VBM is between Z and Γ points, forming an indirect bandgap of 0.86 eV (Figure 2e). The


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predicted band alignment of the SnSe/MoS2 van der Waals heterojunction is shown in Figure 2f. From Figure 2f, it can be seen that the VBM of few-layer MoS2 locates between the CBM and VBM of few-layer SnSe, so a type-II band alignment is formed between MoS2 and SnSe. Then, the electrons tend to accumulate in MoS2, while the holes tend to be located in SnSe. The electrons and holes are separated efficiently, which suppresses the recombination of electron-hole pairs.

Figure 3. (a) Schematic of a SnSe/MoS2 FET on a 300 nm SiO2/Si substrate. The FET devices of few-layer SnSe/MoS2 heterojunctions were fabricated using standard EBL followed by EBE of 20 nm Ni and 50 nm Au. Two Ni/Au electrodes were used as source and drain electrodes, where a doped Si substrate served as a back-gate electrode. (b) Current-voltage (Ids – Vds) curves of a heterojunction in dark (black line) and under illumination (red line; 8 mW cm−2 white light). The source-drain bias



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voltage Vds = −1 ~ 1 V, and the back-gate voltage Vbg = 20 V. (c) Room-temperature transfer (Ids − Vbg) characteristics of a SnSe/MoS2 heterojunction. The black and blue lines represent a linear scale (left y-axis) and a log scale (right y-axis), respectively. The Vds was 1 V, and the Vbg were from −30 to 30 V. (d) Output characteristics recorded for different back-gate voltages Vbg (10 to 60 V), where Vds were from −0.2 to 1 V.

We studied the electrical characteristics of few-layer SnSe/MoS2 heterojunctions by the electrical transport measurements in vacuum at room temperature. A schematic of a few-layer SnSe/MoS2 field-effect transistor (FET) is shown in Figure 3a, where the transport behavior was measured through using a semiconductor parameter analyzer and a shielded probe station. It is noted that good Ohmic contacts between the material and electrodes are necessary. Thus, the electrical contacts of few-layer SnSe and MoS2 flakes with the Ni (20 nm)/Au (50 nm) electrodes were first measured. As shown in the Supporting Information Figure S2, the current-voltage (Ids – Vds) curves of both few-layer SnSe and MoS2 FETs are linear, proving the low contact resistances of Au/Ni/SnSe junctions and Au/Ni/MoS2 junctions. The transfer and output characteristics of few-layer SnSe and MoS2 FETs indicate that the conductance of these two devices increases with higher positive back-gate voltage (Vbg), revealing their typical n-type behavior (see Supporting Information Figure S3). For a few-layer SnSe/MoS2 heterojunction, the Ids – Vds curve in the dark exhibits distinct rectification with forward-to-reverse current ratio of ~105 (Figure 3b).6,20 In the SnSe/MoS2


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heterojunction, electrons were initially transferred from SnSe to MoS2 to achieve the balance of electronic potential, due to the formation of type-II band alignment, which leads to a built-in potential at the interface of heterojunction. Consequently, under positive source-drain bias voltages (Vds), the SnSe/MoS2 heterojunction shows an on state because of the decrease of the built-in potential. The heterojunction is off under negative Vds, which is because increased barrier height restrains the electrons from crossing the barrier. The strong interlayer coupling in SnSe/MoS2 heterostructure enables efficient photocurrent generation under light illumination. Comparing the Ids – Vds curves measured in the dark and under white light illumination, an evident photoresponse is observed (Figure 3b). The transfer characteristics of a SnSe/MoS2 heterojunction are shown in Figure 3c, where the source-drain current (Ids) is strongly modulated by the back-gate voltage (Vbg). As Vbg varies from −30 to 30 V, Ids changes from the off-state current of 4.0 × 10−12 A to the on-state current of 4.2 × 10−7 A, indicating an n-type conducting behavior with an on/off current ratio of ~ 1 × 105. The field-effect mobility can be calculated by the following equation

µ = Lgm/WCiVds

(1)

where L (13 µm) and W (5 µm) are the length and width of channel, gm is the transconductance (namely, the slope of dIds/dVbg), and Ci = 1.15 × 10−4 F m−2 is the gate capacitance of 300 nm SiO2 insulating layer.25 The typical electron mobility of this heterojunction based on the data shown in Figure 3c is determined to be ~8 cm2 V−1 s−1, which is higher than the values of few-layer SnSe (~ 1.5 cm2 V−1 s−1)26 and



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few-layer MoS2 (~ 0.11 cm2 V−1 s−1)27 back-gate devices at room temperature. The enhanced mobility is attributed to reduced scattering in the heterostructure with type-II band alignment.20 Figure 3d shows the output characteristics of this heterojunction by applying different Vbg, where the variation of Ids with Vds is dependent on the Vbg. It is observed that the rectification behavior is more notable when increasing positive Vbg.

Figure 4. (a) Schematic of a high-efficiency separation process of the photo-excited electrons and holes. (b) Self-powered photoswitching behavior of a SnSe/MoS2 type-II heterojunction under illumination of 1 mW cm−2 white light (Vds = 0 V). (c) The on/off photoswitching response as a function of positive bias voltages (Vds = 1, 3, and 5 V). The power density of white light is 1 mW cm−2. (d) The photoresponse curves for selected wavelengths of incident light (~1 mW cm−2 488 nm, ~1 mW cm−2 532 nm, and ~1 mW cm−2 white light) under 1 V bias voltage. (e) Photocurrent (defined as Iph = Ilight − Idark) as a function of light power density with Vds = 0 V. Black dots are experimental data, and red line is the fitted curve. (f) Dynamic temporal


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response of a SnSe/MoS2 FET measured at Vds = 0 V under 1 mW cm−2 white light. The optoelectronic properties of few-layer SnSe/MoS2 heterojunctions were investigated by exploring their photoswitching characteristics under varied bias voltage, light wavelength and power density. Figure 4b shows the time-dependent photocurrent output characteristic of a SnSe/MoS2 type-II van der Waals heterojunction with no applied bias voltage. The self-powered photocurrent initially generates in this type-II heterostructure, and rapidly responds to the light switching on and off. The photocurrent on/off ratio of this heterojunction reaches to ~100 without any bias voltage, which is higher than that of few-layer MoS2 devices at Vds = 1 V (Supporting Information Figure S4), while there is no photocurrent generation in few-layer SnSe devices under bias voltage because of the strong recombination capability of electron-hole pairs. Due to the formation of type-II band alignment between few-layer SnSe and MoS2, the photoexcited electrons and holes separately accumulate in MoS2 and SnSe (Figure 4a), resulting in an open-circuit voltage, which causes a short-circuit current and induces the self-powered photoswitching characteristics of heterojunction. As shown in Figure 4c, the output photocurrent increases with enhancement of bias voltages (1 – 5 V), and it also exhibits stably switching response and long-term repeatability. This is because that the photoexcited electrons and holes can quickly separate and reach the electrodes under a high bias voltage. To confirm the capability of multiwavelength photodetection, we measured the photoresponse of few-layer SnSe/MoS2 heterojunctions under illumination of 488 nm,



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532 nm, and white light at 1 V bias voltage. From Figure 4d, it can be seen that the photocurrent exhibits distinct on/off states for all the measured wavelengths, but the photocurrent on/off ratio varies with the change of wavelengths. We then calculated the photoresponsivity (Rλ) and external quantum efficiency (EQE) of heterostructure devices through the following two equations Rλ = Iph/(PS)

(2)

EQE = hcRλ/(eλ)

(3)

where Iph represents the photocurrent defined as Ilight − Idark, P is the light power density, S is the effective illuminated area, h is Planck’s constant, c is the light velocity, e is the electronic charge, and λ is the light wavelength.6 Based on the experimental data in Figure 4d, the Rλ and EQE are calculated to be 30 A/W and 7.6 × 103 % for ~1 mW cm−2 488 nm light, and they are 100 A/W and 23.3 × 103 % for ~1 mW cm−2 532 nm light. The photoresponse performance of our few-layer SnSe/MoS2 type-II van der Waals heterojunctions is better than that of GaTe/MoS26 and SnS2/MoS220 type-II heterojunctions. Table 1 compares the photoresponse performance of our few-layer SnSe/MoS2 FETs with other vertical heterojunction devices. The output photocurrent can also be modulated by the incident light power density. The photoresponse characteristics of a few-layer SnSe/MoS2 FET were then measured under different light power densities (0.1 to 8 mW cm−2 white light) at Vds = 0 V. The plot of Iph as a function of P can be fitted by the equation Iph ~ Pα

(4)


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where α is an exponent. As the fitted curve in Figure 4e shows, the change of Iph with P appears linear with a power dependence of ~ 0.97. The linear relationship indicates that the transformation of absorbed photons to photo-induced carriers during light illumination is highly efficient. Figure 4f shows the dynamic temporal response of a SnSe/MoS2 FET, and the photocurrent changes quickly between on state and off state, revealing a fast response time. The photoresponse speed for the rise and decay processes can be deduced from these two equations I(t) = I0 [1 − exp(−t/τr)]

(5)

I(t) = I0 exp(−t/τd)

(6)

where τr and τd are the time constants of rise process (the photocurrent increases from 10% to 90%) and decay process (the photocurrent decreases from 90% to 10%). According to the data in Figure 4f, τr and τd can be estimated to be 4 ms and 6 ms, respectively. These two values are both smaller than the results of few-layer MoS2 FETs, where τr and τd are 15 ms and 11 ms, indicating that the photoresponse is slow in few-layer MoS2 devices. The photoresponse speed of few-layer SnSe/MoS2 heterojunctions in this study is much faster than that reported for multilayer MoTe2/MoS2 heterojunctions (~ 68 ms)28 and PbS quantum dots/few-layer MoS2 photodetectors (300 − 400 ms)29 (see Table 1). All the above results confirm that our few-layer

SnSe/MoS2

van

der

heterojunctions

exhibit

huge

potential

in

high-performance FETs, ultrafast photoswitches, and highly efficient photodetectors.



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CONCLUSION We have fabricated few-layer type-II van der Waals heterojunctions consisting of a group IV-VI layered semiconductor –– SnSe and a TMD –– MoS2 by the dry transfer technique. The new SnSe/MoS2 van der Waals heterojunctions showed excellent electrical transport characteristics and stable photoswitching responses. The transfer curves of few-layer SnSe/MoS2 FETs presented an n-type conducting behaviors with a high current on/off ratio of ~ 1 × 105. Due to the formation of type-II band alignment, few-layer SnSe/MoS2 heterostructures exhibited a self-powered photocurrent with an ultrafast photoresponse (< 10 ms). Photoswitching performance of few-layer SnSe/MoS2 heterojunctions was sensitive to the bias voltage, light wavelength and power density. The photoresponsivity of few-layer SnSe/MoS2 heterojunctions reached to 100 A/W, and their EQE were high to 23.3 × 103 % under 532-nm light. All these enhanced electrical and optoelectronic characteristics suggest that type-II SnSe/MoS2 van der Waals heterojunctions show great promise for ultrathin flexible 2D semiconducting devices.

Table 1. The photoresponse performance comparison of our few-layer SnSe/MoS2 FETs to other vertical heterojunction devices. Response -1

Rλ (AW )

EQE (%) time

Our few-layer SnSe/MoS2 FETs

100

23.3×103

< 10 ms

Few-layer GaTe/MoS2 photodetectors6

1.365

266

< 10 ms


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Bilayer SnS2/MoS2 heterojunctions20

1.36

264

Multilayer MoTe2/ MoS2 heterojunctions28

0.15

39.4

68 ms

Hybrid 2D–0D MoS2–PbS quantum dot 106

0.3 − 0.4 s

photodetectors29

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Detailed experimental process, theoretical calculation methods, and Figures S1−S4 (PDF).

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT SXY and MHW contributed equally to this work. CBJ is supported by the National Natural Science Foundations of China (NSFC) under Grant No. 51331001. SXY is



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supported by the National Natural Science Foundation of China (NSFC) under Grant No. 51602014 and the Fundamental Research Funds for the Central Universities (50100002017101022). SHW is supported by the National Natural Science Foundation of China (NSFC) under Grant Nos. 51672023 and U1530401. We thank Prof. J. M. D Coey for discussing the manuscript.

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Enhanced Electrical and Optoelectronic Characteristics of Few-Layer

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