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Tunable Polarity Behavior and High-Performance Photosensitive Characteristics in Schottky-Barrier Field-Effect Transistors Based on Multilayer WS2 Yibin Yang, Le Huang, Ye Xiao, Yongtao Li, Yu Zhao, Dongxiang Luo, Lili Tao, Menglong Zhang, Tiantian Feng, Zhaoqiang Zheng, Xing Feng, Zhongfei Mu, and Jingbo Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18370 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 23, 2017

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ACS Applied Materials & Interfaces

Tunable Polarity Behavior and High-Performance Photosensitive Characteristics in Schottky-Barrier Field-Effect Transistors Based on Multilayer WS2 Yibin Yang, Le Huang, Ye Xiao, Yongtao Li, Yu Zhao, Dongxiang Luo, Lili Tao, Menglong Zhang, Tiantian Feng, Zhaoqiang Zheng, Xing Feng, Zhongfei Mu, and Jingbo Li*

School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China

KEYWORDS: Tunable polarity behavior, photosensitive characteristics, Schottky barrier, WS2 field-effect transistors, 2D materials

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Abstract Schottky-barrier field-effect transistors (SBFETs) based on multilayer WS2 with Au as drain/source contacts are fabricated in this paper. Interestingly, the novel polarity behavior of the WS2 SBFET can be modulated by drain bias, ranging from p-type to ambipolar and finally to n-type conductivity, due to the transition of band structures and Schottky barrier heights under different drain and gate biases. The electron mobility and the on/off ratio of electron current can reach as high as 23.4 cm2/Vs and 8.5×107, respectively. Moreover, the WS2 SBFET possesses high-performance photosensitive characteristics with response time of 40 ms, photo responsivity of 12.4 A/W, external quantum efficiency of 2420%, and photo detectivity as high as 9.28×1011 cmHz1/2/W. In conclusion, the excellent performance of the WS2 SBFETs may pave the way for next-generation electronic and photoelectronic devices.

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1. INTRODUCTION Recently, layered two-dimensional (2D) materials have drawn tremendous attentions because of their unique electrical, optical, mechanical, and chemical properties, and also the access to novel physical phenomena.1 Graphene, as the forefather of layered 2D materials, has demonstrated a series of new physics and potential applications in quantum physics, condensed matter, and electronic devices.2-4 However, the zero bandgap of graphene limits its possible applications in electronics, even though it demonstrates the highest reported carrier mobility in excess of 106 cm2/Vs.5-7 On the other hand, 2D transition metal dichalcogenides (TMDs) such as MoS2 and WS2 possessing reasonable band gaps around 1~2 eV and reasonable carrier mobilities,8-10 have shown significant promise recently as semiconductors with tunable electronic, optical, spin, valley, and sensing properties.11-16 In order to apply 2D TMDs to low-power and high-performance complementary logic applications, both n- and p-type FETs need to be fabricated. The polarity of FETs is determined by the type of charge carriers which can be injected from the source/drain contact into the semiconductor channel.17 In conventional metal-oxide semiconductor FETs (MOSFETs), a common approach to deal with this issue is to heavily dope the semiconductor, so that charge carriers can easily tunnel from the metal to the semiconductor.18 However, as for TMDs, due to the lack of the controlled heavy doping technology and the existence of Fermi level pinning,19-21 the effect of Schottky barrier at the metal-TMD interfaces must be considered. In addition, Schottky barrier 3



heights, in principle, can be modulated by the work function of metals. Therefore, low work-function metals such as Ti,22-24 Sc,1 Au,11 and Mo25 are used to make MoS2 n-type conductivity. While Pd26 and MoOx17 with high work function are applied in MoS2 p-type FETs. As for WS2, another promising TMD, the Fermi-level tends to be pinned near the charge neutrality level, resulting in a larger Schottky barrier than MoS2.2,8,27 Thus, WS2 FETs often show ambipolar behaviors.28-32 Recently, Huo et al. have revealed that the polarity behavior of the WSe2/WS2 heterojunction can be modulated by drain and gate biases, which ranges from p- or n-type to ambipolar or anti-bipolar behaviors.33 To the best of our knowledge, such novel tunable polarity behaviors based on WS2 or other isolated TMD FETs have not been reported in other literatures. Here, multilayer WS2 SBFETs with Au as drain/source electrodes are fabricated without annealing, in order to form Schottky contacts at Au-WS2 interfaces. The novel tunable polarity transition along a route of p-ambipolar-n is observed in the WS2 SBFET when increasing the drain bias from 0.1 to 15 V. The band structure of Au-WS2 is calculated by the first principle calculation. The transition of band structures and Schottky barrier heights under different drain and gate biases can well explain this novel tunable polarity behavior. In addition, the on/off ratio of electron current and the electron mobility can reach as high as 8.5×107 and 23.4 cm2/Vs, respectively. Moreover, the WS2 SBFET possesses high-performance photosensitive properties with response time of 40 ms, photo responsivity of 12.4 A/W, external quantum efficiency (EQE) of 2420%, and photo detectivity as high as 9.28×1011 cmHz1/2/W. 4


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2. EXPERIMENTAL SECTION WS2 flakes were mechanically exfoliated from bulk WS2 crystals onto a 300 nm SiO2/Si substrate by using a standard scotch tape technique. Photoresist (AR-P 5350) was coated under 4000 r/min and baked at 100 °C for 5 min. The samples were exposed to UV light for 6 s under photomask aligner for drain/source pattern by using a lithography machine (ECOPIA，M150). The samples were subsequently patterned with 1% TMAH developer solution for 40 s. Then, a 30 nm Au layer was deposited by electron beam evaporation (ECOPIA，EB400S). For the liftoff process to finally define drain/source electrodes, acetone, isopropanol and DI water were used, respectively. In order the form Schottky contacts at Au-WS2 interfaces, annealing was not needed. The thickness and room-temperature Raman spectrum of WS2 flakes were measured by atomic force microscope (AFM) (Bruker Dimension Edge SPM) and Raman spectroscopy (LabRAM HR 800), respectively. All output and transfer characteristics and photocurrent were characterized by using a semiconductor parameter analyzer (KEITHLEY，2614B) at room temperature.

3. RESULTS AND DISCUSSION Figure 1(a) displays the Schematic diagram of the WS2 SBFET. WS2 flakes were mechanically exfoliated from bulk WS2 crystals onto a 300 nm SiO2/Si substrate through a standard scotch tape technique. Au electrodes were deposited on the WS2 flakes to form 5



source/drain contacts via photolithography, electron beam evaporation, and lift-off processes, respectively. In order to form Schottky contacts at Au-WS2 interfaces, annealing was not needed. Considering the reproducibility, we fabricated three WS2 SBFETs, and the optical micrographs of these three devices are shown in Figure S1. The thickness of the WS2 flakes is about 80 nm, confirmed by atomic force microscope (AFM), as exhibited in Figure 1(b). Room-temperature Raman spectrum of the multilayer WS2 is shown in Figure 1(c), with the excitation laser wavelength of 633 nm. The values of the distinct typical Raman modes of E12g and A1g are 351.25 and 422.5 cm-1, respectively, with the distance of 71.25 cm-1, which agrees well with the published articles.34-38 The nonlinear output characteristics in Figure 1(d) suggest the existence of a considerable level of Schottky barrier in the WS2 SBFET with Au electrodes.

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Figure 1. (a) Schematic diagram of the WS2 SBFET. (b) The AFM image and the thickness profile near the edge of WS2 flakes, indicating the thickness of about 80 nm. (c) Room-temperature Raman spectrum of the multilayer WS2, with the excitation laser wavelength of 633 nm. (d) Output characteristics of the WS2 SBFET.

Figure 2(a-d) shows the drain current (IDS) in linear scale for the WS2 SBFET with gradually varying drain bias (VDS) at 0.5, 3, 5 and 10 V, respectively, as a function of gate bias (VG). For all the measurements, VG sweeps from −50 to 50 V. Complete transfer characteristics with VDS increasing from 0.1 to 15 V are displayed in Figure S2. It is interesting to observe that the polarity behavior of the WS2 SBFET develops from p-type 7



to ambipolar and finally to n-type conductivity, with the increase of VDS. Although transfer characteristics in logarithmic scale demonstrate ambipolar behavior of the device under different VDS, as shown in Figure 2(e-h), the asymmetric transfer curves at low and high VDS reveal that the dominant carriers are holes (at low VDS) and electrons (at high VDS), respectively.

Figure 2. Transfer characteristics for the WS2 SBFET (a-d) in linear and (e-h) in logarithmic scale, respectively, with gradually varying VDS at 0.5, 3, 5 and 10 V, respectively.

To consider the reproducibility and to further research the transition of the polarity behavior, electrical properties of three devices were measured. Statistic data of hole current (defined as Ih at VG = -50V), electron current (defined as Ie at VG = 50 V), hole mobility (µh) and electron mobility (µe) as functions of VDS are listed in Tables S1 and S2, 8


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and the plots are exhibited in Figure 3(a). The mobility (µ) is obtained from the equation

µ=

∂I DS  L    , where L and W are the channel length and width, respectively. Ci is ∂VG  WCiVDS 

the gate capacitance between the channel and the silicon back gate per unit area, which can be given by the equation Ci = ε 0ε r / d , where ε 0 (8.85×10-12 F/m) is vacuum dielectric constant, and ε r (3.9) and d (300 nm) are the dielectric constant and the thickness of SiO2, respectively. In Figure 3(a), it is clearly seen that there are two stages for the transition of Ih, Ie, µh and µe. At stage I (VDS < 5 V), µh and µe increase very slowly whereas Ih and Ie rise relatively quickly with the increase of VDS. At this stage, both Ih and µh are larger than Ie and µe, indicating mainly p-type conductivity of the WS2 SBFET. However, at stage II (VDS > 5 V), both Ih and µh keeps constant, while Ie and µe increase extremely rapidly and µe can reach 23.4 cm2/Vs. Both Ie and µe are much higher than Ih and µh at this stage, revealing dominantly n-type conductivity of the device. Thus, the WS2 SBFET demonstrates symmetrically ambipolar behavior between stages I and II (around VDS of 5 V). Figure 3(b) shows the on/off ratios of Ih and Ie dependent on VDS, and the statistic data are listed in Table S3. The plots are also divided into two stages, in agreement with Figure 3(a). The on/off ratio of Ie is lower than that of Ih at stage I, while it exceeds the latter and reaches as high as 8.5×107 at stage II, due to the rapid increase of µe and Ie.

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Figure 3. (a) Ih, Ie, µh and µe, and (b) on/off ratios of Ih and Ie as functions of VDS, respectively. The plots are divided into stages I and II.

To explain this novel phenomenon of tunable polarity behavior, the transition of band structures and Schottky barrier heights modulated by different VG and VDS is investigated. First principle calculations are performed to calculate the band structure of WS2 and Au before and after contact, as depicted in Figure 4(a, b). It is clearly seen that the Fermi level of Au is nearer the valence band than the conduction band of WS2, which results in a higher Schottky barrier height of the conduction band and a relatively lower Schottky barrier height of the valence band. At stage I when increasing VDS, although the Schottky barrier height of the conduction band at drain side (ΦCD) is lowered, both Schottky barrier heights of the conduction band at source side (ΦCS) and ΦCD are still larger than Schottky barrier heights of the valence band at source side (ΦVS) and drain side (ΦVD), as shown in Figure 4(c, d). Therefore, it is more difficult for the electron transport (at high positive VG) than the hole transport (at high negative VG), which leads to Ih>Ie and mainly p-type conductivity of the WS2 SBFET. At stage II when further 10


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increasing VDS to higher values, ΦCD keeps reducing and finally diminishes, leaving only ΦCS, which promotes the transport of electrons,18 as illustrated in Figure 4(e). On the other hand, ΦVS and ΦVD almost remain constant, since they are fixed by VG. As a result, Ie and µe increase sharply, while Ih and µh keep constant, which results in Ie>Ih and dominantly n-type conductivity of the device. Therefore, the tunable polarity transition along a route of p-ambipolar-n is well explained by this model of band structure transition.

Figure 4. (a) The band alignment and (b) the band structure of WS2 and Au before and after contact, calculated by first principle calculations. The Fermi level (EF), the conduction band minimum (CBM) and the valence band maximum (VBM) are marked with dashed lines. Schematic band alignments of the WS2 SBFET under (c) positive and (d) negative VG at stage I, and under (e) positive and (f) negative VG at stage II, respectively. 11



Figure 5(a) shows time-dependent photocurrent responses during the light switching on/off, with a 635-nm laser of 80 mW/cm2, at VG of 0 V and VDS of 1, 5 and 10 V, respectively. It is seen that the drain current (IDS) can change instantly between on and off state, by switching the light on/off quickly and repetitively, indicating a high reversibility and stability of the device. The device also exhibits very fast dynamic response for both rise and fall process, with a quick response and recovery time of 40 ms (Figure S3). The IDS will take relatively long time to be saturated at high VDS, due to the thermo effect caused by high injection current. Furthermore, the photo responsivity (R) under different VDS are calculated and shown in Figure 5(b), and the statistic data of photo responsivity and EQE are listed in Table S4, which can be expressed as R = I ph / PS and

EQE = hcR / eλ , where I ph ( I ph = I light − I dark ) is the photocurrent; P is the light power intensity; S is the effective illumination area; h is the Planck’s constant; c is the light velocity; e is the electron charge; and λ is the incident laser wavelength.37 It is clearly observed that the photo responsivity is very low when VDS is below 5 V. While it quickly reaches as high as 12.4 A/W when VDS rises to 10 V, because of the rapidly increase of electron mobility under high VDS at stage II, as discussed above. Furthermore, the photo detectivity of this device is as high as 9.28×1011 cmHz1/2/W, calculated by the equation39,40 D* =

SR . In addition, the upward (at source side) and downward (at drain side) 2eI dark

bending of conduction and valence band edges at high VDS leads to the high efficient 12


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separation of photo-generated electrons and holes, which also results in the high-performance photosensitive characteristics, as illustrated in Figure 5(c). Meanwhile, Table 1 summarizes the corresponding figures-of-merit of recently reported photodetectors based on 2D materials. Importantly, our device stands out in the comprehensive consideration of these optical properties, which indicates that the WS2 SBFET can be applied as a high-performance photodetector.

Figure 5. (a) Time-dependent photocurrent responses during the light switching on/off at VDS of 1, 5 and 10 V, respectively. (b) The photo responsivity as a function of VDS. (c) The band alignment at high VDS and under the incident laser of 635 nm.

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Table 1. Summary of the figures-of-merit of previously reported photodetectors based on related 2D materials. ND: No data; Gr: Graphene. Device

Responsivity (A/W)

EQE (%)

Detectivity (cmHz1/2/W)

Response time (ms)

Wavelength range (nm)

Ref.

Au-WS2-Au

5.7

1118

ND

20

633

(37)

Au-WS2-Au

0.51

137

2.7×109

4100

370-1064

(38)

Gr-WS2-Gr

3.5

933

1.6×1010

ND

532

(41)

Gr-WS2-Gr

0.1

30

ND

ND

48-633

(42)

Au-MoWS2-Au

5.8

1135

ND

150

370-1064

(43)

Ti-WS2/SnSe-Ti

0.099

26

1.2×108

8.2

473-1064

(44)

Bi2Te3-SnSe-Bi2Te3

5.5

1833

6×1010

ND

370-808

(45)

Au-WS2-Au

12.4

2420

9.28×1011

40

635

This work

4. CONCLUSION In summary, the WS2 SBFET with Au as drain/source electrodes has been configured and 14


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investigated. The Schottky barrier at the Au-WS2 interface plays a crucial role in determining the electronic and optoelectronic transport properties of this structure. Constantly, the novel tunable polarity behavior can be modulated by VDS, ranging from p-type to ambipolar and finally to n-type properties, which is attributed to the transition of band structures and Schottky barrier heights under different drain and gate biases. High-performance photosensitive characteristics such as quick photo response, high EQE, high photo responsivity and detectivity have also been observed in the device. In a word, all these novel emerging properties and device functionalities would suggest SBFETs of 2D TMDs possess great application potentials in next-generation of electronics, optoelectronics, and photodetections.

ASSOCIATED CONTENT Supporting Information Additional experimental details and statistic data in Figures S1−S3 and Tables S1-S4.

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

Notes The authors declare no competing financial interest. 15



ACKNOWLEDGMENTS This work was supported by the Youth Fund of Guangdong University of Technology (Grant No. 16QNZD004), the Guangdong Science and Technology Plan of China (Grant No. 2016A010101026), the Pearl River S&T Nova Program of Guangzhou (Grant No. 201710010143), the Key Platforms and Research Projects of Department of Education of Guangdong Province (Grant Nos. 2016KTSCX034 and 2016KTSCX031), the Science and Technology Program of Guangzhou, China (Grant No. 201707010324), the National Key Research and Development Program of China (Grant No. 2016YFF0203604), and the National Natural Science Foundation of China (Grant Nos. 51602065, 61704034, 61705044 and 11674310).

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