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Ultraviolet Wavelength-Dependent Optoelectronic Properties in Two-Dimensional NbSe-WSe van der Waals Heterojunction-Based Field-Effect Transistors 2

2

Seung Bae Son, Yonghun Kim, Ah Ra Kim, Byungjin Cho, and Woong-Ki Hong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11983 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

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ACS Applied Materials & Interfaces

Ultraviolet

Wavelength-Dependent

Optoelectronic

Properties in Two-Dimensional NbSe2-WSe2 van der Waals Heterojunction-Based Field-Effect Transistors

Seung Bae Son1,†, Yonghun Kim2,†, Ah Ra Kim2, Byungjin Cho3,*, Woong-Ki Hong1,*

1

Jeonju Center, Korea Basic Science Institute, Jeonju, Jeollabuk-do 54907, Republic of Korea

2

Department of Advanced Functional Thin Films, Surface Technology Division, Korea

Institute of Materials Science (KIMS), 797 Changwondaero, Sungsan-gu, Changwon, Gyeongnam 51508, Republic of Korea 3

Department of Advanced Material Engineering, Chungbuk National University, Seowon-gu,

Cheongju, Chungbuk 28644, Republic of Korea.

†These authors contributed equally to this work. *Correspondence to: [email protected], [email protected]

Keywords: van der Waals junction, tungsten diselenide, niobium diselenide, optoelectronic property, ultraviolet irradiation, field-effect transistor

ACS Paragon Plus Environment


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Abstract Atomically thin two-dimensional (2D) van der Waals (vdW) heterostructures are one of the very important research issues for stacked optoelectronic device applications. In this study, using the transferred and stacked NbSe2-WSe2 films as electrodes and a channel, we fabricated the field-effect transistor (FET) devices based on 2D-2D vdW metalsemiconductor heterojunctions (HJs) and systematically studied their ultraviolet (UV) wavelength-dependent electrical and photoresponse properties. Upon the exposure to UV light with a wavelength of 365 nm, the NbSe2-WSe2 vdW HJFET devices exhibited threshold voltage shift toward positive gate bias direction, a current increase, and a nonlinear photocurrent increase upon applying a gate bias due to the contribution of the photogenerated hole current. In contrast, for the 254 nm-UV irradiated FET devices, the drain current was decreased dramatically, and the threshold voltage was negatively shifted. The time-resolved photoresponse properties showed that the device current after turning off the 254 nm-UV light was completely and much more rapidly recovered compared to the case with the persistent photocurrent after turning off the 365 nm-UV light. Interestingly, we found that the wettability of the WSe2 surface was changed with increasing irradiation time only after 254 nm-UV irradiation. The measured wetting behavior on the WSe2 surface provided direct evidence that the experimentally observed UV wavelength-dependent phenomena was attributed to the UV-induced dissociative adsorption of oxygen and water molecules, leading to the modulation of charge trap states on the photogenerated and intrinsic carriers in the ptype WSe2 channel. This study will help provide understanding the influence of environmental and electrical measurement conditions on the electrical and optical properties of 2D-2D vdW HJ devices for a variety of device applications through the stacking of 2D heterostructures.


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Introduction 2D nanomaterials including transition metal dichalcogenides (TMDs)-layered materials have been attracting intense research efforts for possible applications in electronic and optoelectronic devices such as field-effect transistors (FETs),1–4 gas sensors,5–8 photodetectors,9–15 and solar cells.16–18 Among many TMDs, MoS2 and WSe2 are currently attracting increasing interest due to their layer-dependent indirect-to-direct band gap transition and surface sensitivity to adsorbents,19,20 which can be useful for photodetection and gas sensing applications. Both multilayer MoS2 (n-type semiconductor) and WSe2 (ptype semiconductor) have an indirect band gap of approximately 1.2 eV, whereas their monolayers exhibit a direct band gap of approximately 1.8 eV for MoS2 and 1.65 eV for WSe2.21,22 The photoresponse properties as the intensive research subject of two-dimensional (2D) nanomaterials have been extensively studied in lateral metal-TMD-metal structure devices. In this regard, the effect of the metal contact on the electrical and optical properties of atomically thin TMDs plays an important role for the photodetection applications. In addition, stacked van der Waals (vdW) p-n heterostructures based on 2D nanomaterials, such as WS2-MoS2,23 WSe2-MoS2,24 SnSe2-WSe2,25 WS2-WSe2,26 ReSe2-WS2,27 GaTe-MoS2,28 and ReS2-ReSe229 have recently attracted much attention for ultrathin photodetection and photovoltaic device applications. Until now, most studies on the detectable wavelength for 2D TMDs-based photodetectors have been mainly focused on the visible wavelength range. Recently, the photodetection properties from visible to near infrared (IR) have been demonstrated

for

a

vertically

integrated

phototransistor

based

on

single-layer

MoS2/multilayer WSe2 van der Waals heterojunction and multilayer MoS2 FETs.24,30 However, the research on ultraviolet (UV) photodetection of 2D TMDs-based devices is



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crucial for exploiting broadband responsive photodetectors from the UV to IR region. Accordingly, Eom and co-workers reported the influence of deep UV light on the layerdependent photoresponse properties of MoS2 FETs.31 These researchers also demonstrated the reversible modulation of the charge carrier (doping) in different layers of MoS2 nanosheets under N2 and O2 gases using deep UV light. Cho et al.32 showed the effect of UV light on the photoconductive characteristics of MoS2 FETs that were fabricated with mechanically exfoliated multilayer MoS2 flakes. Such previous studies have demonstrated the UV photoresponse of lateral metal-TMD-metal structure devices, especially MoS2 as an ntype semiconductor. In these studies, regardless of the UV wavelength, the photocurrent of the lateral metal-TMD-metal structure devices became larger than that under dark condition. Meanwhile, most recently, Kim et al.33,34 reported a novel device architecture of realizing a 2D-2D metal-semiconductor system with a relatively lower contact resistance for use in all 2D TMD-based electronic devices. The authors demonstrated that the synergistic combination of Schottky barrier lowering and enhanced tunneling efficiency at the 2D-2D metal–semiconductor vdW heterojunctions (HJ) contributes to improved charge transport behavior. To the best of our knowledge, there is no study of the UV wavelength-dependent optoelectronic properties in 2D-2D vdW HJ, which could provide important progress for the applications of various electronic and optoelectronic devices through the stacking of 2D heterostructures. In this work, we fabricated the FET devices based on 2D-2D metal-semiconductor vdW HJ using the transferred and stacked chemical vapor deposition (CVD)-grown NbSe2WSe2 films as electrodes and a channel, respectively. Then, we systematically explored the UV-wavelength dependence of the electrical and photoresponse properties of the NbSe2WSe2 vdW HJFET devices using UV light with wavelengths of 254 and 365 nm. Additionally, we studied the transition of the water contact angle after UV irradiation for the


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SiO2/Si and WSe2/SiO2/Si substrate. Based on these results, we suggested the experimentally observed UV wavelength-dependent optoelectronic properties are attributed to the UV irradiation-induced modulation of the charge trap states in the WSe2 channel, which was explained using a vdW HJ-based energy band diagrams.

Experimental Section Semiconducting WSe2 and metallic NbSe2 were synthesized with thermal chemical vapor deposition (CVD) technique.33 First of all, using a thermal evaporator, WO3 and Nb2O5 thin films were deposited onto SiO2/Si wafers for the formation of WSe2 and NbSe2 films, respectively. The metal oxide deposition samples were placed on the center of a furnace and evacuated with rotary pump. Then, the furnace was heated to the desired temperature (900 ~ 1000 °C) with flowing Ar (95 %) + H2 (5 %) gas and at the same time, the selenium source on inlet of the furnace was heated to 500 °C for 1 hour to selenize the WO3 and Nb2O5 thin films. After that, the furnace was cooled down to room temperature. The thicknesses of the as-synthesized films were characterized by AFM. Height and phase AFM images of the NbSe2 and WSe2 films were obtained in tapping mode using a NanoScope IV (Bruker). The Raman spectra of the WSe2 and NbSe2 films were obtained at a laser excitation wavelength of 512 nm. The structural analyses of as-synthesized WSe2 and NbSe2 were performed using an HR-TEM (JEM-ARM200F, JEOL). The electrical and photoresponse properties were examined at room temperature in air under dark and irradiation of UV light with wavelengths of 365 and 254 nm using a Keithley 4200-SCS. The water contact angle measurement was carried out using a contact angle analyzer attached to a



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CCD camera (Phoenix 300 SEO), by placing a water droplet on the surface of the SiO2/Si and WSe2/SiO2/Si substrates, at room temperature.

Results and Discussion Figure 1a shows the schematic drawing of the 2D NbSe2-WSe2 vdW HJs and optical images of their back gated-HJFET devices with the channel lengths of 10 and 30 µm. In the 2D NbSe2-WSe2 vdW HJs, the NbSe2 and WSe2 function as metallic electrodes and a semiconducting channel, respectively. The detailed synthesis process of the 2D NbSe2 and WSe2 films and their HJFET device fabrication process has been described elsewhere33 (Also see Supporting Information (SI) Figure S1). In the Raman spectra of the WSe2 and NbSe2 layers (Figure 1b), the corresponding phonon frequencies of E12g and A1g for the WSe2 and NbSe2 indicate in-plane vibrational modes of W-Se for the WSe2 (Nb-Se for the NbSe2) and out-of-plane vibrational modes of Se-Se, respectively. Figure 1c shows the cross-sectional high-resolution transmission electron microscopy (HR-TEM) images of the WSe2 and NbSe2 layers. Electron diffraction patterns of the planar NbSe2 and WSe2 films were also explored, revealing hexagonal structures. (SI Figure S2). The measured thickness of the as-grown semiconducting WSe2 and metallic NbSe2 films on the SiO2 substrate was approximately 2.3 and 2.8 nm, respectively, consistent with the thicknesses of the 2D WSe2 and NbSe2 films measured by the atomic force microscopy (AFM) after the fabrication of FET devices (SI Figure S3). To investigate the electrical properties of the 2D NbSe2-WSe2 vdW HJFET devices shown in Figure 1a, we examined the output characteristics (drain-source current versus drain-source voltage, IDS-VDS) and transfer characteristics (drain-source current versus gatesource voltage, IDS-VGS) of the fabricated NbSe2-WSe2 vdW HJFET devices under dark


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condition, as shown in Figure 1d,e (see also SI Tables S1-S2 and Figures S4-S5). Figure 1d shows the IDS-VDS curves of the NbSe2-WSe2 vdW HJFET with a 10 µm channel length at various gate voltages under dark condition. The IDS-VGS curves of the same device were measured at different drain biases (VDS = –1 and -5 V), with the back-gate voltage sweeping from 40 to –60 V ((Figure 1e). Additionally, the IDS-VDS and IDS-VGS curves of the NbSe2WSe2 vdW HJFET with a 30 µm channel length are shown in Figure S6a,b. All these I-V curves show that the NbSe2-WSe2 vdW HJFETs exhibit typical p-type semiconductor characteristics with almost linear IDS-VDS curves at low drain biases. Additionally, Figure S7 shows the IDS-VDS curves for different gate voltages between –60 and +20 V, which exhibit ohmic-like characteristics for all the gate voltages. These results indicate the absence of blocking contacts and the exceptionally narrow Schottky barrier due to the presence of a vdW gap in the atomically thin 2D-2D HJs, which enable low resistance tunneling across the 2D2D HJ.35,36 Accordingly, the influence of photoinduced Schottky barrier lowering on the photoconductivity can be disregarded in the NbSe2-WSe2 vdW HJFETs. Figure S8 shows evidently that direct tunneling dominates carrier transport in the NbSe2-WSe2 contacts. Next, we measured the UV wavelength-dependent optoelectronic properties of the NbSe2-WSe2 vdW HJFETs, as shown in Figure 2. Interestingly, from the IDS-VDS and IDS-VGS characteristics of the NbSe2-WSe2 vdW HJFET device under UV irradiation (Figure 2a,b), It was clearly observed that the NbSe2-WSe2 vdW HJFET devices were noticeably influenced by UV wavelength. The UV irradiation with a wavelength of 365 nm leads to an increase in the device current, whereas the device current under UV irradiation of 254 nm wavelength decreased dramatically. This effect was similarly observed in the IDS-VGS curves (Figure 2b). In particular, the device current increased dramatically at VGS < –42 V under 365 nm-UV irradiation, whereas under 254 nm-UV irradiation, the current increased gradually at the



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whole region of the applied gate voltages. The NbSe2-WSe2 vdW HJFET device with 30 µm channel length also exhibited almost similar photoresponse behaviors in the IDS-VGS curves (SI Figure S6c). The photocurrent changed nonlinearly with a negatively increased gate bias, indicating that the channel resistance with an increasing the applied electric field contributes significantly to the photocurrent change under UV irradiation of different wavelengths (Also see SI Figure S9a,b). The photocurrent values also increased linearly with the bias voltage VDS (SI Figure S9c,d), which is likely due to the change in carrier drift velocity and the related carrier transit time.9 This effect was more pronounced under larger negative gate and drain bias conditions. Figure 2c and Figure S6d show the UV wavelength-dependent photocurrent (Iph = Iphoto – Idark, where Iphoto and Idark are the drain currents with and without incident light, respectively) as a function of the gate voltage VGS at VDS = –1 V for the NbSe2WSe2 vdW HJFET devices with channel lengths of 10 and 30 µm, respectively. In the contour plots of the conductance (Figure 2d-f and SI Figures S10-S11), it is clearly seen that the conductance increased (decreased) with negatively applied VDS and VGS under UV irradiation of 365 (254) nm wavelength. According to previous reports,37–42 oxygen and/or water molecules largely affect the electrical properties of 2D nanomaterials, which are likely due to the trapping of charge carriers in the host materials. In addition, 2D WSe2 show a ptype semiconducting feature the adsorption of oxygen and water molecules in air ambient.43 More importantly, irradiation with 254 nm-UV light wavelength can induce the dissociative adsorption of oxygen and/or water molecules in air ambient, which can generate the trap states of carriers.44,45 Additionally, the trap states like defects or impurities and the adsorption of oxygen and/or water molecules in air ambient can be induced on either the channel surface or interfaces during the fabrication process of the vdW HJs. From all these results, the UV wavelength-dependent photoresponse properties reflects the influence of the filling and modulation of trap states existing either in WSe2 or at the interfaces of WSe2/the underlying


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SiO2 layer and WSe2/air on the intrinsic and photogenerated carriers, which will be described later (Figure 5). Figure 3a,b shows the photoresponsivity and photogain as a function of the gate voltage at a fixed VDS = –1 V of the NbSe2-WSe2 vdW HJFET device with a 10 µm channel length (Also see SI Figure S6e,f). The photoresponsivity R is defined as Iph/Popt, where Popt is the absorbed power of approximately 4 nW for 254 nm-UV light and approximately 6.1 nW for 365 nm-UV light. The photogain G is defined as Rhν/ηq, assuming that the incident light was completely absorbed (η = 100%), where h is the Planck constant, ν is the frequency of the UV light, η is the external quantum efficiency, and q is the electronic charge.46 It is clearly seen that the photoresponsivity and photogain for the case of 254 nm-UV irradiation are much higher than those for 365 nm-UV irradiation. These results are supported by the specific detectivity (SI Figure S12) and the photoresponsivity (R) as a function of the drainsource voltage at various gate voltages under 254 and 365 nm-UV irradiation (Figure 3c,d). In Figure 3c,d, R increases linearly with an increasing VDS and the photoresponsivity is increased by applying an increased VGS. R as a function of VDS under 254 and 365 nm-UV irradiation is reasonably fitted to a power law (R ∝ VDSα) (marked by dashed lines), where α represents the effect of the defects on the photocurrent under UV irradiation. Interestingly, the extracted value of α exhibit sublinearity for the case of 254 nm-UV irradiation, whereas for 365 nm-UV irradiation, α values are divided into a sublinear regime (α < 1) at a high gate bias and superlinear regime (1< α < 2) at a low gate bias. The superlinearity with a α > 1 at relatively lower gate voltages (VG > -45 V) is likely ascribed to the existence of the intrinsic defects or disorders in the WSe2 channel region, which can trap the charge carriers induced by the applied gate voltage. As shown in Figure 2b, the current values of the vdW HJFET device dramatically increase at VG < -45 V under 365 nm-UV irradiation. Accordingly, in our



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devices, the transition from superlinearity to sublinearity under 365 nm-UV irradiation can be accompanied by the trap state filling with negatively increasing the gate voltage, reflecting the increased influence of photogenerated carrier concentration rather than that of the intrinsic carrier concentration. In addition, these results imply an entirely different photodetection mechanisms of the NbSe2-WSe2 vdW HJFET device under 254 and 365 nmUV irradiation, which can be associated with the modulation of trap states on intrinsic charge carriers and photogenerated carriers in the WSe2 channel layer.47 To obtain further insight into the UV-induced modulation of the trap states and the influence of the trap states on the dynamics of the NbSe2-WSe2 vdW HJFET device, we explored the transient photoresponse characteristics of the vdW HJFET device with a 10 µm channel length in which the photoresponse were recorded at VDS = –1 V and VGS = 0 V under different UV wavelengths of 254 and 365 nm in ambient air condition. Figure 4 shows the photocurrent variations upon turning on and off the UV light for the different wavelength conditions. After the UV irradiation of 254 nm wavelength, the drain current of the NbSe2WSe2 vdW HJFET device decreased approximately from –2.0 to –1.66 nA and subsequently the device current was fully recovered back to its initial current state after the UV light was turned off. In contrast, after the 365 nm-UV irradiation, the device current increased approximately from –2.3 to –2.79 nA and then was partially recovered after turning off the UV light. The influence of the trap states on the photoconductivity of 2D TMDs can be understood through the dynamics of the photocurrent variations in which the time constant is obtained by fitting the experiment data with a stretched-exponential function:32,48,49 ‫ܫ‬௣௛ ሺ‫ݐ‬ሻ = ‫ܫ‬௣௛,௢௙௙ + ‫݌ݔ݁ܣ‬ሾ−ሺ‫ݐ‬/߬௥ଵ ሻሿ + ‫݌ݔ݁ܤ‬ሾ−ሺ‫ݐ‬/߬௥ଶ ሻሿ where Iph,off is the photocurrent after turning off the UV light, τr1 and τr2 are two relaxation time constants. As shown in Figure 4, the photoresponse is characterized by τr1 = 15 and τr2 =


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143.9 s for 254 nm-UV irradiation and τr1 = 40.8 and τr2 = 307.2 s for 365 nm-UV irradiation. Intriguingly, the device current was recovered much more rapidly for the case of 254 nm-UV irradiation, while after turning off the 365 nm-UV light, the current was not recovered completely, showing a photocurrent persisting for a long time. According to previous reports,50,51 the photoinduced molecular desorption from graphene, carbon nanotubes, and reduced graphene oxides could modify the electrical transport properties, resulting in a significant change in resistance. Similarly, the surroundings of the 2D TMDs play an important role in the electrical and optical properties.37,43,52,53 In particular, the oxygen/water molecule could induce the p-type doping in low-dimensional nanomaterials such as graphene,54 carbon nanotube,55 and WSe2.43 In our device, the UV-irradiated current behavior under vacuum was similar to that under ambient air, indicating that the water molecule or oxygen species could be strongly adsorbed on 2D TMD surfaces (see SI Figure S13).56,57 The adsorbates and defect states significantly affect the photoresponse behavior of 2D TMDs, especially decay dynamics.58,59 Accordingly, these differences in the photoresponse dynamics for our vdW HJFET devices can be originated from either intrinsic defects or disorders in the TMD materials or extrinsic sources such as gas adsorbates and/or chemical impurities on the SiO2 interface.44,45,52 The results of photoresponse dynamics indicate that carrier-capturing states can be different between 254 nm- and 365 nm-UV irradiation in ambient air, considering the possible surface reaction processes associated with water and oxygen through UV irradiation. The observed phenomena of UV irradiation on the electrical and photoresponse properties of the NbSe2-WSe2 vdW HJFET device can be explained, as shown in Figure 5. Figure 5 shows schematic illustrations of the surface processes and corresponding energy band diagrams under an equilibrium state and UV irradiation with different wavelengths. With no irradiation and without applying a gate or drain bias, the energy level alignment



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between the electrodes (NbSe2) and the channel (WSe2) reaches an equilibrium state with the vdW gap between the NbSe2 contact and WSe2 channel (Figure 5a, left). Upon applying a gate and drain bias to the HJFET device, the majority holes of the WSe2 channel can undergo tunneling process passing across the HJ due to the presence of a vdW gap in the 2D-2D HJ (Figure 5a, right). The hole transport can be modulated by the gate bias and at the same time, the charge carriers can be trapped in the intrinsic defects or disorders in the WSe2 channel, resulting in trap state filling as the gate voltage is increased. The defects or disorders in 2D TMDs can be originated from the synthesis of 2D TMDs and the device fabrication.60-62 Additionally, the p-type semiconducting property of the WSe2 channel is attributed to the adsorption of oxygen and water molecules in ambient air in which water plays an important role in achieving significant p-doping.43 UV irradiation to the HJFET device in the ON state (VGS < Vth) results in light absorption and excitation of electron–hole pairs in the WSe2 channel (Figure 5b,c). For the case of 254 nm-UV irradiation, the current may decrease due to UV-induced dissociative adsorption/desorption of oxygen or water molecules on the WSe2 surface (Figure 2 and SI Figure S6), resulting in the creation of charge trap states and in turn, trapping of photogenerated carriers and/or intrinsic holes (Figure 5b). Recent studies have suggested that there is a wettability transition from hydrophobic to hydrophilic surfaces through the UV-induced dissociative adsorption of oxygen or water molecules in graphene and carbon nanotube.44,45,63 Interestingly, we found that the wettability of the WSe2 surface was increasingly changed approximately from 75.4° to 62.6° with increasing irradiation time after 254 nm-UV irradiation (Figure 5d), whereas there is no change of the surface wettability of WSe2 was observed after 365 nm-UV irradiation (Figure 5e). The surface wettability of a SiO2 layer without the WSe2 channel was not changed at all under both 254 nm- and 365 nmUV irradiation (SI Figure S14 and Table S3). This is direct evidence to supporting the


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different charge transport mechanisms in our vdW HJFET devices depending on the UV wavelength. Accordingly, compared to the case of 254 nm-UV irradiation, 365 nm-UV irradiation can lead to a considerable contribution of the photogenerated holes to the device current (Figure 5c). As shown in Figure 5c, the trap states in the band gap of WSe2 can become filled with the photogenerated holes, resulting in significant increase of the photocurrent as the photogenerated and intrinsic carrier concentration exceed the concentration of trapping states (Figure 2 and SI Figure S6).

Conclusion In summary, we fabricated the FET devices based on a 2D-2D metal-semiconductor vdW HJ where the transferred and stacked CVD-grown NbSe2 and WSe2 films functioned as electrodes and a channel, respectively. We also studied systematically the influence of UV irradiation on the optoelectronic properties of the 2D-2D NbSe2-WSe2 vdW HJFET devices. The electrical and time-resolved photoresponse properties of the NbSe2-WSe2 vdW HJFET devices strongly depended on UV wavelength of 254 and 365 nm. The contact angle measurements provide direct evidence that the experimentally observed UV wavelengthdependent optoelectronic properties were attributed to the UV irradiation-induced modulation of the charge trap states. The wettability change of the WSe2 surface occurred only after 254 nm-UV irradiation (approximately from 75.4° to 62.6° with increasing irradiation time), while there was no significant change in the measured water contact angle was observed after 365 nm UV irradiation. This indicates that 254 nm UV irradiation resulted in dissociative adsorption of the oxygen or water molecules on the WSe2 surface, significantly affecting hole conductivity in the p-type WSe2 channel. Finally, the UV wavelength-dependent transport mechanism was suggested using a vdW HJ-based energy band diagrams. Our study will help provide understanding of the influence of environmental and electrical measurement



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conditions on the electrical and optical properties of 2D-2D vdW HJ devices, which can be an important progress for the applications of a variety of electronic and optoelectronic devices through the implementation of all 2D heterostructures.

Conflict of Interest: The authors declare no competing financial interest. Acknowledgements. W.-K.H acknowledges the financial support from the KBSI grant (T37417). B.C. is grateful for the support from the Basic Science Research Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2017R1C1B1005076). Supporting Information Available: Device fabrication process of 2D NbSe2-WSe2 vdW HJFETs, thickness measurement of NbSe2-WSe2, transistor and photoresponse characteristics of the 30 µm channel vdW HJFETs with, photocurrent characteristics of 10 µm channel vdW HJFET device, and water contact angle on the SiO2 surface after UV irradiation.


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Figures

Figure 1. (a) Schematic drawing of the 2D NbSe2-WSe2 vdW HJ (top) and optical images of the back gated-HJ FET devices with channel lengths of 10 and 30 µm (bottom). (b) Raman spectra of the WSe2 (left) and NbSe2 (right) films. (c) Cross-sectional HR-TEM images of the WSe2 (left) and NbSe2 (right) layers. Thicknesses of the semiconducting WSe2 and metallic NbSe2 films on the SiO2 substrate were measured to be approximately 2.3 and 2.8 nm, indicating three and four layers, respectively. (d) Output characteristics showing a representative IDS-VDS curves of the NbSe2-WSe2 vdW HJFETs (10 µm channel) under dark condition. (e) Transfer characteristics showing the linear IDS-VGS curves of the same device at different drain biases at –1 and –5 V. The inset shows the semi-logarithmic scale plots of the IDS-VGS curves.



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Figure 2. (a) IDS-VDS and (b) IDS-VGS characteristics of the NbSe2-WSe2 vdW HJFET device (10 µm channel) under UV irradiation (254 and 365 nm wavelength). The inset in (b) shows the semi-logarithmic scale plot of the IDS-VGS curves. (c) UV wavelength-dependent photocurrent (Iph = Iphoto – Idark) of the vdW HJFET device as a function of gate voltage VGS at VDS = –1 V. (d-f) Contour plots of the conductance of the vdW HJFET device at different VGS and VDS under (d) dark condition, (e) 254 nm-, and (f) 365 nm-UV irradiation.


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Figure 3. (a,b) Photoresponsivity (R) (a) and photogain (G) (b) as a function of the gate voltage of the NbSe2-WSe2 vdW HJFET device (10 µm channel) at fixed VDS = –1 V under different UV irradiation. (c,d) Photoresponsivity as a function of the drain-source voltage at various gate voltages under (c) 254 nm- and (d) 365 nm-UV irradiation. The R-VDS log-log plots are reasonably fitted to a power law (R ∝ VDSα).



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Figure 4. (a,b) Transient photoresponse of the NbSe2-WSe2 vdW HJFET device (10 µm channel) at VDS = -1 V and VGS = 0 V before and after (a) 254 nm- and (b) 365 nm-UV irradiation. Two relaxation time constants (τr1 and τr2) are obtained by fitting the experiment data

with

a

stretched-exponential

function

( ‫ܫ‬௣௛ ሺ‫ݐ‬ሻ = ‫ܫ‬௣௛,௢௙௙ + ‫݌ݔ݁ܣ‬ሾ−ሺ‫ݐ‬/߬௥ଵ ሻሿ +

‫݌ݔ݁ܤ‬ሾ−ሺ‫ݐ‬/߬௥ଶ ሻሿ).


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Figure 5. (a) Energy level alignment between the electrodes (NbSe2) and the channel (WSe2) under an equilibrium state with the vdW gap and trap states (left) and energy band diagram under bias condition. Schematic illustrations of the surface reaction processes involving water and oxygen molecules and corresponding energy band diagrams for the NbSe2-WSe2 vdW HJFET device under (b) 254 nm- and (c) 365 nm-UV irradiation. Note that the applied gate voltage is smaller than threshold gate voltage (VGS < Vth), indicating the accumulation (ON) state of the p-type WSe2 channel, and UV light induces the generation of photogenerated electron–hole pairs in the WSe2 channel. The gray/blue/magenta arrows indicate current flow. (d,e) Water contact angles on the WSe2 surface as a function of the irradiation time of (d) 254 nm- and (e) 365 nm-UV light. The decrease of contact angle with increasing UV irradiation indicates the surface wettability of relatively more hydrophilic property. The inset shows images of a water droplet on the WSe2/SiO2 substrate.



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