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Enhanced photoresponse and surface charge transfer mechanism of graphene-tungsten disulfide heterojunction
Optical Materials 98 (2019) 109426
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Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Enhanced photoresponse and surface charge transfer mechanism of graphene-tungsten disulfide heterojunction Muhammad Zahir Iqbal a, *, Sana Khan a, Muhammad Arshad Kamran b, Salma Siddique c, Muhammad Javaid Iqbal d, Thamer Alharbi b, Saman Siddique a, Mian Muhammad Faisal a, Syed Shabhi Haider a a
Nanotechnology Research Laboratory, Faculty of Engineering Sciences, GIK Institute of Engineering Sciences and Technology, Topi, 23640, Khyber Pakhtunkhwa, Pakistan Department of Physics, College of Science, Majmaah University, Al-Majmaah, 11952, Saudi Arabia c Institute of Industrial Biotechnology, GC University Lahore, 54000, Pakistan d Centre of Excellence in Solid State Physics, University of Punjab, Quaid-e-Azam Campus Lahore, 54590, Pakistan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene Tungsten disulfide Deep ultraviolet light (DUV) Heterostructure p-n junction Photoresponse
Two dimensional (2D) materials based heterostructures have gained profound interest in optoelectronics and electronic technology due to additional functionalities over the individual structures. This study demonstrates the fabrication and characterization of van der Waal heterostructure by selective coverage of graphene (Gr) with tungsten disulfide (WS2). The electrical transport measurements divulge the tweaking of charge carriers in graphene after WS2 coverage. Such architecture provides route towards the formation of heterojunction within graphene FET based on surface charge transfer between Gr/WS2 heterointerface. Furthermore, the exposure of device towards deep ultraviolet light (DUV) enhances the charge transfer mechanism and as a result more pronounced junction is observed. The photoelectrical characterization of heterostructure is also investigated by calculating detectivity (D*), external quantum efficiency (EQE) photoresponsivity (Rλ). Our results suggest that 2D heterostructures in combination with DUV irradiations are more efficient and suitable choice to selectively tune the properties of 2D material-based optoelectronic devices.
1. Introduction Two-dimensional (2D) materials like graphene (Gr) and transition metal di-chalcogenides (TMDCs) have gained significant attention because of their unique physical properties [1]. Gr has attracted considerable interest in photo-sensing materials and transparent elec trodes because of its high thermal conductivity, carrier mobility, flexi bility, mechanical strength and tunable absorption of light [2–7]. But, due to one atomic thickness, its light absorption is reasonably weak (only 2.7% in the visible spectrum) [2]. Additionally, small values of photo responsivity (Rλ � 10 3 A/W) and no band gap nature of graphene constrained its utilization for fabrication of highly efficient graphene-based optoelectronic devices [7–12]. For this different TMDCs generally WS2 and MoS2 are emerging as strong candidates to be used as photodetectors along with Gr [8]. Optoelectronic characteristics of MoS2 and WS2 have been explored because of their direct band gaps and
are thought to balance the zero band gap nature of Gr [13–17]. Addi tionally, fabricating heterostructure is an important concept to contrive the optical and electronic properties of semiconductors and are elementary building block for advanced technology like transistors and lasers [18]. Different reports on tunneling FETs, photodetectors, photovoltaic cells and biosensors have proved the distinctive electronic properties and improved light-matter interactions offered by Gr/TMDC heterojunctions [19–22]. Weak van der Waals interactions (vdW), interlayer interaction also proposes that strict lattice matching and mismatching is not desired to form heterostructures [23,24]. Therefore, 2D heterostructures can be formed without involving any sort of chemical bonding at heterointerface [19,23]. Different drawbacks of conventional heterostructures are reduced in these vdW hetero structures like interlayer atomic diffusion, strain, interfacial defects and disorders. Moreover, these heterostructures are chemically stable and flexible [19]. Therefore, the controllable Fermi level and high
* Corresponding author. E-mail address: [email protected] (M.Z. Iqbal). https://doi.org/10.1016/j.optmat.2019.109426 Received 6 August 2019; Received in revised form 23 September 2019; Accepted 25 September 2019 Available online 3 October 2019 0925-3467/© 2019 Published by Elsevier B.V.
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Fig. 1. The Gr/WS2 based van der Waal heterostructure: (a) Schematic illustration of the Gr based field effect transistor, (b–e) Device fabrication steps including transfer of layer, lithography etc.
Fig. 2. (a) Raman spectrum of CVD grown graphene, (b) Raman analysis of WS2 classifying E2g and A1g modes. (Inset: Lorentz fitting of 2LA and E2g modes of WS2).
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Fig. 3. Transport characteristics of Gr without DUV irradiation and under different DUV irradiation time. (a) Current as function of gate voltage (Vg), (b) Shift in dirac position with respect to DUV treatment time, indicating positive shift (p-doping) in Gr, (c) Change in carrier concentration with increasing DUV treatment time, (d) Trend of mobility for different treatment time.
Fig. 4. Transport characteristics of WS2 without DUV irradiation and under different DUV irradiation time. (a) The behavior of current vs gate voltage (Vg), (b) Shift in threshold voltage for different treatment time, (c) Trend of mobility of WS2 at different treatment times. 3
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transparency of graphene in combination with remarkable optical properties of TMDCs opens the innovative thoroughfare for highly effi cient optical devices [5]. Here, we reported the fabrication and characterization of chemical vapor deposition (CVD) grown Gr and WS2 heterostructure based on surface charge transfer mechanism between 2D materials. By applying the gate voltage, the majority carriers from WS2 are injected to Gr layer, resulting into the alteration of carrier’s concentration in Gr and forma tion of p-n junction. The doping introduced in Gr by WS2 was further modulated by DUV treatment. More prominent p-n junction peaks were observed under DUV exposure, which shows enhancement in properties of field effect transistor (FET). Moreover, photoelectrical response of heterostructure is also investigated by measurements of optical param eters such as detectivity (D*), external quantum efficiency (EQE) pho toresponsivity (Rλ) and relaxation time for the device. These results offer an approach to selectively tune the properties 2D-material’s under DUV exposure and to be employed in photosensing applications.
room temperature and then shifted to SiO2/Si by adopting the wet transfer technique as reported in our previous report [25]. For fabri cating the heterostructure, Gr layer is partially covered by depositing WS2 flakes, exfoliated by using scotch tape method. Then the photoli thography and reactive ion etching is performed simultaneously to remove the undesirable area of Gr and the Hall bar formed. Patterning is done by electron beam lithography and finally by process of thermal evaporation, metallic contacts are made on device. Renishaw Micro-Spectrometer is used to measure the Raman spectra of WS2 and Gr, with the laser wavelength of 514.5 nm. The laser spot size of ~1 μm and power of ~1.0 mW is used to avoid heating of the substrate. 3. Results and discussion The schematic representation of Gr/WS2 heterostructure is illus trated in Fig. 1(a), where the half channel of Gr is covered with WS2. The light green layer with a brown bottom represents the SiO2/Si substrate. WS2 (black Tungsten and yellow Sulphur atoms) is deposited over desired area of Gr layer (black). The metal contacts are represented by yellow color. Fig. 1(b–e) demonstrates step wise device fabrication process. At first step CVD grown graphene is transferred on the SiO2/Si substrate followed by WS2 flakes transfer. Photolithography and metal deposition is performed in second step. E-beam lithography is carried out for O2 plasma etching to form Hall bar. Finally inner electrodes are patterned with e-beam lithography and gold is deposited. Raman spectroscopy is used to examine the quality of Gr and WS2, illustrated in Fig. 2. Raman shift of single layer Gr is shown in Fig. 2(a).
2. Experimental A thin Gr layer is prepared by CVD technique on Copper foil (LS Mtron LSU-STN). For this purpose, Cu was placed inside the CVD furnace under Argon environment (rate of 100 sccm for 30 min and 1000 � C) whereas to avoid the oxidation of Cu, hydrogen gas was injected inside the furnace (10 sccm for 20 min). Argon and methane’s mixture was injected into the furnace (at 10 sccm flow rate and 230 mTorr pressure) for 15 min to grow Gr film. Gr along with Cu was then cooled down to
Fig. 5. Electrical transport measurements of Gr and Gr/WS2 heterostructure. (a) Resistance as a function gate voltage (Vg) for Gr, (b) Resistance as a function Vg for Gr/WS2 heterostructure. (c) Schematics illustration of charge transfer process at Gr/WS2 heterojunction, (d) Zoom in view of charge transfer mechanism from WS2 to grahene. 4
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The characteristic G and 2D-peaks lies at 1592.5 cm 1 and 2665.2 cm 1, respectively and I2D/IG is 4.85 identifying the single layer of graphene (SLG) [25,26]. In all sp2 hybridized carbons the G-peak refers to the phonon (E2g) at the Brillouin zone center [27,28]. The D-peak corre sponds to the carbon atom’s ring breathing modes and always require some defects for activation [27]. The absence of D-peak represents the defect free SLG. The second ordered D peak (2D peak) is always a single peak in SLG while in BLG it branches into four peaks [23,25]. The appearance of single 2D peak again confirms the single layer of graphene in our work. The G peak and 2D peak always present in Raman spectrum of graphene because they did not need any defect for activation [29]. Fig. 2(b) represents the Raman spectrum of WS2, which includes second order Raman peak (2LA(M)) present at M-point of Brillouin zone center and first order in plane (E12g) and out of plane (A1g) optical peaks at Brillouin zone [29]. Here, the letter M represents the particular magnitude and direction of phonons’ momentum [30]. The first order optical modes (E12g and A1g) are located at 354 cm 1 and 420 cm 1 respectively, and are used to study 2D materials properties. The differ ence between two optical modes is ~64 cm 1 which confirm the multilayer WS2 film [29–31]. The 2LA (M)s peak intensity is dominant
for WS2 films and overlapped with the E12g peak. The individual contribution of each mode is identified by using the multipeak Lor entzian fitting as shown in inset of Fig. 2(b). Electrical transport measurements of Gr and WS2 were carried out at different DUV treatment time, to investigate the effect of DUV irradia tion on individual structures. Fig. 3(a) displays the current as a function of Vg for untreated and DUV treated Gr. The VDirac of pristine Gr was observed at Vg ¼ 4.5 V, which is the identification of p-doping in Gr and a more positive shift in VDirac is observed as DUV treatment time in creases (Fig. 3(b)). After 15 min treatment, VDirac reached at 27 V (highly p-doped) which is attributed to photoexcitation of sample under DUV light treatment [32]. Considering the theoretical discussion, one can consider that oxygen atoms dissociate and become more reactive as they gain enough energy by DUV light and react chemically with graphene [7,33]. In the chemical mechanism, oxygen atoms bind with certain – O and also electron from pi-electrons of graphene and formed C–O of C– graphene transferred to oxygen molecules (O2) or ozone (O3) which, suggests p-doping of graphene [33]. Change in carrier density (Δn) of Gr obtained calculated by the relation Δn ¼ Cg (jVDi V DPjÞ=e, for different DUV treatment time is
Fig. 6. Transport properties of Gr and Gr/WS2 heterostructure with DUV exposure. (a) Resistance corresponding to different gate voltage (Vg) for Gr, (b) Resistance corresponding to different Vg for Gr/WS2 heterostructure. (c) Schematic illustration of charge carriers (excitons) formation at Gr/WS2 heterojunction, (d) Zoom in view of charge transfer mechanism from WS2 to grahene layer. 5
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illustrated in Fig. 3(c) [34]. Cg represents the value of gate capacitance (115 aF/μm 2) obtained for silicon substrate with oxide of thickness 300 nm, whereas VDP and VDi symbolize the Dirac point of pristine and DUV treated Gr, respectively [34,35]. As affirmed in Fig. 3(c), the car rier’s density upsurges with treatment time and is related to the regu lation of Gr Fermi level [34]. The mobility (μ) is extracted from the slope 1 ∂σ of transfer curve, and is given by the relation; μ ¼ Cg ð∂Vg) where, σ rep
voltage (Vg), the appearance of second Dirac point indicates the transfer of electron at heterointerface from WS2 to Gr. A lateral separation with altered Fermi levels of Gr is formed, resulting in reduction of doping (ptype) inside the heterostructure domain. The schematic illustration of carrier transfer at Gr/WS2 hetero interface is presented in Fig. 5(c and d). The injection of electrons from WS2 to Gr layer is illustrated in Fig. 5 (c), which might be credited to the electric field induces at heterointerface due to mismatch in work func tion of both materials. The energy band exemplification of WS2-Gr is illustrated in Fig. 5(d) which, demonstrates the electron transfer process and modulation of Gr Fermi level covered by WS2. For clear under standing of charge transfer mechanism, the energy diagram of Gr/WS2 before and after junction formation is elucidated in Fig. 7(a and b). WS2 having an electron affinity (χw) of 4.1 eV and work function (фw) of 4.9 eV shows a semiconducting nature [36,37]. The semi-metallic behavior of Gr is confirmed by its conic structure with the work func tion (фw) of 4.6 eV [29,38]. Modulation of Fermi level at Vg sweep is observed when WS2 makes heterojunction with Gr as illustrated in Fig. 7 (b) [29,39]. The open region shows p-doping while a shift from p-type to n-type doping is observed in covered region because of electrons transfer from WS2 to Gr layer. The transport characteristics of heterostructure upon DUV light treatment was also investigated and presented in Fig. 6. Under DUV il luminations, the Gr Dirac point shifts to 27 V, showing highly p-type doping in uncovered Gr channel as illustrated in Fig. 6(a). The appear ance of sharp Dirac peak at more positive Vg and the peak at less positive Vg under DUV treatment depicts highly p-doping in Gr and relatively low
resents the conductivity of device, and is illustrated in Fig. 3(d) [34]. Similarly, the transport characteristics of WS2 was also characterized under dark and DUV treatment. As clear from Fig. 4(a), the threshold voltage (Vth) of WS2 shifts to less negative values as treatment time in creases which might be attributed to the decrease in n-doping (electron concentration) in WS2 layer. This change in Vth for WS2 with respect to DUV treatment time is plotted in Fig. 4(b). This positive shift in Vth is attributed to the adsorption of oxygen atoms or molecules in defect sites on surface of WS2 or in sulphur which, in turns behave as electron trap centers. DUV light dissociates oxygen molecules into oxygen atoms which intermingles with WS2 flakes. Besides of dissociation, oxygen molecules can directly adsorb on WS2 surface. The electron mobility of WS2 as a function of irradiation time is presented in Fig. 4(c). Fig. 5 illustrates the transport properties of pristine CVD grown Gr and shift in Dirac point after doping induced by WS2 coverage. For the pristine Gr, single Dirac point appears at Vg ¼ 4.6 V, indicating the pdoped sample and is shown in Fig. 5 (a). After the selective coverage of Gr channel with WS2, second peak is appeared at 54 V, affirming the formation of p-n junction as illustrated in Fig. 5(b). By changing the
Fig. 7. Energy level diagram of WS2 and Gr, (a) Before in contact, фW and χ represent work function and electron affinity of WS2 (b) Before formation of p-n junction, (c) Under DUV irradiation. DUV treatment rises the Fermi level of Gr, resulting reduction in junction’s height. 6
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p-doping in Gr/WS2 domains. The result divulges that upon photo excitation more electrons are transferred from WS2 to Gr, along with the carries injection by simple contact between both materials [33]. Thus, highly p-doped Gr channel and weakly p-doped Gr/WS2 region give rise to formation of p-p type junction. This photo-assisted doping is accredited to the splitting of photo-generated excitons. The electrons from WS2 enters into Gr region through hetero-interface and holes re mains confined to the WS2 lattice (Fig. 7(c)). Fig. 6(c) shows the sche matics illustration of charge carrier’s creation in Gr/WS2 heterostructure domain. The charge transfer mechanism at Gr/WS2 heterointerface is presented in Fig. 6(d). The I–V results for Gr/WS2 heterojunction are plotted in Fig. 6(3), which demonstrates the linear response because the graphene act as channel. The optical parameters of Gr/WS2 are characterized by photo electrical characterization of under DUV exposure of intensity 11 W/cm2 with a wavelength of 220 nm. Gr/WS2 divulges the consistent photo response for different cycles as shown in Fig. 8(a). The photocurrent at different Vg is illustrated in Fig. 8(b), demonstrating linear dependence on voltage. The relaxation time (τdecay) for the sensor is extracted by experimental fitting of the photoresponse according to equation; Iph ¼ t Idark þ Aexp ð τdecay Þ, where A is a scaling constant and t symbolizes the
WS2 layer whereas electrons transfer to graphene channel, and this mechanism would be responsible for enhanced photoresponsivity of heterostructure (illustrated in Fig. 6(c and d)) [4,40]. Fig. 9(b) shows the D* of the device for different Vg inferring that D* increases from 4.9 � 109 to 9.2 � 1010 Jones as Vg increased. Similarly, the calculated EQE significantly varied from 4.6 � 105 to 2.7 � 106 for Vg ranges from 0 to 60 V and is illustrated in Fig. 9(c). The result divulges that the selective coverage of Gr with WS2 can leads to controllable tweaking of its properties by where WS2 act as electron injector. Whereas photo-irradiation increased the carriers transfer along the carries injection by simple contact between WS2 and Gr. The optical response divulges the significant enhancement in the performance of device as function of Vg. Rλ, D* and EQE increased to by the magnitude of about 10 times as Vg increased from 0 to 60 V. 4. Conclusion We have demonstrated the fabrication and characterization of 2D material’s based van der Waal heterostructure FET by ingenious com bination of controllable Fermi level and high mobility of graphene with remarkable optical properties of WS2. The carriers injection occurs be tween Gr/WS2 interfaces based on surface charge transfer mechanism between 2D materials whereas the photoresponse is credited to sepa ration of photogenerated excitons. Electrical transport characteristics of heterostructure confirmed the n-doping in Gr after WS2 coverage resulting in the formation of a p-n junction between p-doped Gr and ndoped Gr/WS2. However, the junction become more prominent after DUV exposure, which reveals that charge transfer between Gr and WS2 enhanced after light exposure because of photogenerated charge car riers. Moreover, the optical response of the device is also studies by evaluating the parameters such as Rλ, D* and EQE to understand the sensing properties of heterostructure. The values obtained of optical
UV light switching time. Other important optical figure of merits such as 1 pffiffiffiffiffiffiffiffiffiffiffiffiffi detectivity (D* ¼ Rλ A2 = 2eIdark ), photoresponsivity (Rλ ¼ Iph = PA) and external quantum efficiency (EQE ¼ hcRλ =eλ) have also been extracted by the gate dependent photoresponse of the sensor. Rλ varies from 818 to 4800 A/W as Vg increased from 0 to 60 V and is illustrated in Fig. 9(a) with the error bars depicting the consistent response for different cycles. Upon light exposure, transfer of photogenerated charge carriers take place at Gr/WS2 interface which might be attributed to the electric field induces at hetero-interface due to mismatch in work function of both materials. Photoexcited holes remain trapped into the
Fig. 8. (a) Photocurrent response of DUV light at different gate voltages (b) The value of photocurrent at different gate voltages (c) Decay time as a function of different gate voltages. 7
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Fig. 9. (a) Photoresponsivity as a function of different backgate voltages (b) Detectivity at different backgate voltages (c) External quantum efficiency (EQE) at different backgate voltages.
parameters for heterostructure are far above than that reported for in dividual structures and a significant enhancement is observed by changing Vg. So, we can conclude that DUV treatment along with gate tunable properties of graphene can be opted as a good approach for fabrication of future electronic and optoelectronic devices involving surface charge transfer between the materials.
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Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Enhanced photoresponse and surface charge transfer mechanism of graphene-tungsten disulfide heterojunction Muhammad Zahir Iqbal a, *, Sana Khan a, Muhammad Arshad Kamran b, Salma Siddique c, Muhammad Javaid Iqbal d, Thamer Alharbi b, Saman Siddique a, Mian Muhammad Faisal a, Syed Shabhi Haider a a
Nanotechnology Research Laboratory, Faculty of Engineering Sciences, GIK Institute of Engineering Sciences and Technology, Topi, 23640, Khyber Pakhtunkhwa, Pakistan Department of Physics, College of Science, Majmaah University, Al-Majmaah, 11952, Saudi Arabia c Institute of Industrial Biotechnology, GC University Lahore, 54000, Pakistan d Centre of Excellence in Solid State Physics, University of Punjab, Quaid-e-Azam Campus Lahore, 54590, Pakistan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene Tungsten disulfide Deep ultraviolet light (DUV) Heterostructure p-n junction Photoresponse
Two dimensional (2D) materials based heterostructures have gained profound interest in optoelectronics and electronic technology due to additional functionalities over the individual structures. This study demonstrates the fabrication and characterization of van der Waal heterostructure by selective coverage of graphene (Gr) with tungsten disulfide (WS2). The electrical transport measurements divulge the tweaking of charge carriers in graphene after WS2 coverage. Such architecture provides route towards the formation of heterojunction within graphene FET based on surface charge transfer between Gr/WS2 heterointerface. Furthermore, the exposure of device towards deep ultraviolet light (DUV) enhances the charge transfer mechanism and as a result more pronounced junction is observed. The photoelectrical characterization of heterostructure is also investigated by calculating detectivity (D*), external quantum efficiency (EQE) photoresponsivity (Rλ). Our results suggest that 2D heterostructures in combination with DUV irradiations are more efficient and suitable choice to selectively tune the properties of 2D material-based optoelectronic devices.
1. Introduction Two-dimensional (2D) materials like graphene (Gr) and transition metal di-chalcogenides (TMDCs) have gained significant attention because of their unique physical properties [1]. Gr has attracted considerable interest in photo-sensing materials and transparent elec trodes because of its high thermal conductivity, carrier mobility, flexi bility, mechanical strength and tunable absorption of light [2–7]. But, due to one atomic thickness, its light absorption is reasonably weak (only 2.7% in the visible spectrum) [2]. Additionally, small values of photo responsivity (Rλ � 10 3 A/W) and no band gap nature of graphene constrained its utilization for fabrication of highly efficient graphene-based optoelectronic devices [7–12]. For this different TMDCs generally WS2 and MoS2 are emerging as strong candidates to be used as photodetectors along with Gr [8]. Optoelectronic characteristics of MoS2 and WS2 have been explored because of their direct band gaps and
are thought to balance the zero band gap nature of Gr [13–17]. Addi tionally, fabricating heterostructure is an important concept to contrive the optical and electronic properties of semiconductors and are elementary building block for advanced technology like transistors and lasers [18]. Different reports on tunneling FETs, photodetectors, photovoltaic cells and biosensors have proved the distinctive electronic properties and improved light-matter interactions offered by Gr/TMDC heterojunctions [19–22]. Weak van der Waals interactions (vdW), interlayer interaction also proposes that strict lattice matching and mismatching is not desired to form heterostructures [23,24]. Therefore, 2D heterostructures can be formed without involving any sort of chemical bonding at heterointerface [19,23]. Different drawbacks of conventional heterostructures are reduced in these vdW hetero structures like interlayer atomic diffusion, strain, interfacial defects and disorders. Moreover, these heterostructures are chemically stable and flexible [19]. Therefore, the controllable Fermi level and high
* Corresponding author. E-mail address: [email protected] (M.Z. Iqbal). https://doi.org/10.1016/j.optmat.2019.109426 Received 6 August 2019; Received in revised form 23 September 2019; Accepted 25 September 2019 Available online 3 October 2019 0925-3467/© 2019 Published by Elsevier B.V.
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Fig. 1. The Gr/WS2 based van der Waal heterostructure: (a) Schematic illustration of the Gr based field effect transistor, (b–e) Device fabrication steps including transfer of layer, lithography etc.
Fig. 2. (a) Raman spectrum of CVD grown graphene, (b) Raman analysis of WS2 classifying E2g and A1g modes. (Inset: Lorentz fitting of 2LA and E2g modes of WS2).
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Fig. 3. Transport characteristics of Gr without DUV irradiation and under different DUV irradiation time. (a) Current as function of gate voltage (Vg), (b) Shift in dirac position with respect to DUV treatment time, indicating positive shift (p-doping) in Gr, (c) Change in carrier concentration with increasing DUV treatment time, (d) Trend of mobility for different treatment time.
Fig. 4. Transport characteristics of WS2 without DUV irradiation and under different DUV irradiation time. (a) The behavior of current vs gate voltage (Vg), (b) Shift in threshold voltage for different treatment time, (c) Trend of mobility of WS2 at different treatment times. 3
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transparency of graphene in combination with remarkable optical properties of TMDCs opens the innovative thoroughfare for highly effi cient optical devices [5]. Here, we reported the fabrication and characterization of chemical vapor deposition (CVD) grown Gr and WS2 heterostructure based on surface charge transfer mechanism between 2D materials. By applying the gate voltage, the majority carriers from WS2 are injected to Gr layer, resulting into the alteration of carrier’s concentration in Gr and forma tion of p-n junction. The doping introduced in Gr by WS2 was further modulated by DUV treatment. More prominent p-n junction peaks were observed under DUV exposure, which shows enhancement in properties of field effect transistor (FET). Moreover, photoelectrical response of heterostructure is also investigated by measurements of optical param eters such as detectivity (D*), external quantum efficiency (EQE) pho toresponsivity (Rλ) and relaxation time for the device. These results offer an approach to selectively tune the properties 2D-material’s under DUV exposure and to be employed in photosensing applications.
room temperature and then shifted to SiO2/Si by adopting the wet transfer technique as reported in our previous report [25]. For fabri cating the heterostructure, Gr layer is partially covered by depositing WS2 flakes, exfoliated by using scotch tape method. Then the photoli thography and reactive ion etching is performed simultaneously to remove the undesirable area of Gr and the Hall bar formed. Patterning is done by electron beam lithography and finally by process of thermal evaporation, metallic contacts are made on device. Renishaw Micro-Spectrometer is used to measure the Raman spectra of WS2 and Gr, with the laser wavelength of 514.5 nm. The laser spot size of ~1 μm and power of ~1.0 mW is used to avoid heating of the substrate. 3. Results and discussion The schematic representation of Gr/WS2 heterostructure is illus trated in Fig. 1(a), where the half channel of Gr is covered with WS2. The light green layer with a brown bottom represents the SiO2/Si substrate. WS2 (black Tungsten and yellow Sulphur atoms) is deposited over desired area of Gr layer (black). The metal contacts are represented by yellow color. Fig. 1(b–e) demonstrates step wise device fabrication process. At first step CVD grown graphene is transferred on the SiO2/Si substrate followed by WS2 flakes transfer. Photolithography and metal deposition is performed in second step. E-beam lithography is carried out for O2 plasma etching to form Hall bar. Finally inner electrodes are patterned with e-beam lithography and gold is deposited. Raman spectroscopy is used to examine the quality of Gr and WS2, illustrated in Fig. 2. Raman shift of single layer Gr is shown in Fig. 2(a).
2. Experimental A thin Gr layer is prepared by CVD technique on Copper foil (LS Mtron LSU-STN). For this purpose, Cu was placed inside the CVD furnace under Argon environment (rate of 100 sccm for 30 min and 1000 � C) whereas to avoid the oxidation of Cu, hydrogen gas was injected inside the furnace (10 sccm for 20 min). Argon and methane’s mixture was injected into the furnace (at 10 sccm flow rate and 230 mTorr pressure) for 15 min to grow Gr film. Gr along with Cu was then cooled down to
Fig. 5. Electrical transport measurements of Gr and Gr/WS2 heterostructure. (a) Resistance as a function gate voltage (Vg) for Gr, (b) Resistance as a function Vg for Gr/WS2 heterostructure. (c) Schematics illustration of charge transfer process at Gr/WS2 heterojunction, (d) Zoom in view of charge transfer mechanism from WS2 to grahene. 4
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The characteristic G and 2D-peaks lies at 1592.5 cm 1 and 2665.2 cm 1, respectively and I2D/IG is 4.85 identifying the single layer of graphene (SLG) [25,26]. In all sp2 hybridized carbons the G-peak refers to the phonon (E2g) at the Brillouin zone center [27,28]. The D-peak corre sponds to the carbon atom’s ring breathing modes and always require some defects for activation [27]. The absence of D-peak represents the defect free SLG. The second ordered D peak (2D peak) is always a single peak in SLG while in BLG it branches into four peaks [23,25]. The appearance of single 2D peak again confirms the single layer of graphene in our work. The G peak and 2D peak always present in Raman spectrum of graphene because they did not need any defect for activation [29]. Fig. 2(b) represents the Raman spectrum of WS2, which includes second order Raman peak (2LA(M)) present at M-point of Brillouin zone center and first order in plane (E12g) and out of plane (A1g) optical peaks at Brillouin zone [29]. Here, the letter M represents the particular magnitude and direction of phonons’ momentum [30]. The first order optical modes (E12g and A1g) are located at 354 cm 1 and 420 cm 1 respectively, and are used to study 2D materials properties. The differ ence between two optical modes is ~64 cm 1 which confirm the multilayer WS2 film [29–31]. The 2LA (M)s peak intensity is dominant
for WS2 films and overlapped with the E12g peak. The individual contribution of each mode is identified by using the multipeak Lor entzian fitting as shown in inset of Fig. 2(b). Electrical transport measurements of Gr and WS2 were carried out at different DUV treatment time, to investigate the effect of DUV irradia tion on individual structures. Fig. 3(a) displays the current as a function of Vg for untreated and DUV treated Gr. The VDirac of pristine Gr was observed at Vg ¼ 4.5 V, which is the identification of p-doping in Gr and a more positive shift in VDirac is observed as DUV treatment time in creases (Fig. 3(b)). After 15 min treatment, VDirac reached at 27 V (highly p-doped) which is attributed to photoexcitation of sample under DUV light treatment [32]. Considering the theoretical discussion, one can consider that oxygen atoms dissociate and become more reactive as they gain enough energy by DUV light and react chemically with graphene [7,33]. In the chemical mechanism, oxygen atoms bind with certain – O and also electron from pi-electrons of graphene and formed C–O of C– graphene transferred to oxygen molecules (O2) or ozone (O3) which, suggests p-doping of graphene [33]. Change in carrier density (Δn) of Gr obtained calculated by the relation Δn ¼ Cg (jVDi V DPjÞ=e, for different DUV treatment time is
Fig. 6. Transport properties of Gr and Gr/WS2 heterostructure with DUV exposure. (a) Resistance corresponding to different gate voltage (Vg) for Gr, (b) Resistance corresponding to different Vg for Gr/WS2 heterostructure. (c) Schematic illustration of charge carriers (excitons) formation at Gr/WS2 heterojunction, (d) Zoom in view of charge transfer mechanism from WS2 to grahene layer. 5
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illustrated in Fig. 3(c) [34]. Cg represents the value of gate capacitance (115 aF/μm 2) obtained for silicon substrate with oxide of thickness 300 nm, whereas VDP and VDi symbolize the Dirac point of pristine and DUV treated Gr, respectively [34,35]. As affirmed in Fig. 3(c), the car rier’s density upsurges with treatment time and is related to the regu lation of Gr Fermi level [34]. The mobility (μ) is extracted from the slope 1 ∂σ of transfer curve, and is given by the relation; μ ¼ Cg ð∂Vg) where, σ rep
voltage (Vg), the appearance of second Dirac point indicates the transfer of electron at heterointerface from WS2 to Gr. A lateral separation with altered Fermi levels of Gr is formed, resulting in reduction of doping (ptype) inside the heterostructure domain. The schematic illustration of carrier transfer at Gr/WS2 hetero interface is presented in Fig. 5(c and d). The injection of electrons from WS2 to Gr layer is illustrated in Fig. 5 (c), which might be credited to the electric field induces at heterointerface due to mismatch in work func tion of both materials. The energy band exemplification of WS2-Gr is illustrated in Fig. 5(d) which, demonstrates the electron transfer process and modulation of Gr Fermi level covered by WS2. For clear under standing of charge transfer mechanism, the energy diagram of Gr/WS2 before and after junction formation is elucidated in Fig. 7(a and b). WS2 having an electron affinity (χw) of 4.1 eV and work function (фw) of 4.9 eV shows a semiconducting nature [36,37]. The semi-metallic behavior of Gr is confirmed by its conic structure with the work func tion (фw) of 4.6 eV [29,38]. Modulation of Fermi level at Vg sweep is observed when WS2 makes heterojunction with Gr as illustrated in Fig. 7 (b) [29,39]. The open region shows p-doping while a shift from p-type to n-type doping is observed in covered region because of electrons transfer from WS2 to Gr layer. The transport characteristics of heterostructure upon DUV light treatment was also investigated and presented in Fig. 6. Under DUV il luminations, the Gr Dirac point shifts to 27 V, showing highly p-type doping in uncovered Gr channel as illustrated in Fig. 6(a). The appear ance of sharp Dirac peak at more positive Vg and the peak at less positive Vg under DUV treatment depicts highly p-doping in Gr and relatively low
resents the conductivity of device, and is illustrated in Fig. 3(d) [34]. Similarly, the transport characteristics of WS2 was also characterized under dark and DUV treatment. As clear from Fig. 4(a), the threshold voltage (Vth) of WS2 shifts to less negative values as treatment time in creases which might be attributed to the decrease in n-doping (electron concentration) in WS2 layer. This change in Vth for WS2 with respect to DUV treatment time is plotted in Fig. 4(b). This positive shift in Vth is attributed to the adsorption of oxygen atoms or molecules in defect sites on surface of WS2 or in sulphur which, in turns behave as electron trap centers. DUV light dissociates oxygen molecules into oxygen atoms which intermingles with WS2 flakes. Besides of dissociation, oxygen molecules can directly adsorb on WS2 surface. The electron mobility of WS2 as a function of irradiation time is presented in Fig. 4(c). Fig. 5 illustrates the transport properties of pristine CVD grown Gr and shift in Dirac point after doping induced by WS2 coverage. For the pristine Gr, single Dirac point appears at Vg ¼ 4.6 V, indicating the pdoped sample and is shown in Fig. 5 (a). After the selective coverage of Gr channel with WS2, second peak is appeared at 54 V, affirming the formation of p-n junction as illustrated in Fig. 5(b). By changing the
Fig. 7. Energy level diagram of WS2 and Gr, (a) Before in contact, фW and χ represent work function and electron affinity of WS2 (b) Before formation of p-n junction, (c) Under DUV irradiation. DUV treatment rises the Fermi level of Gr, resulting reduction in junction’s height. 6
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p-doping in Gr/WS2 domains. The result divulges that upon photo excitation more electrons are transferred from WS2 to Gr, along with the carries injection by simple contact between both materials [33]. Thus, highly p-doped Gr channel and weakly p-doped Gr/WS2 region give rise to formation of p-p type junction. This photo-assisted doping is accredited to the splitting of photo-generated excitons. The electrons from WS2 enters into Gr region through hetero-interface and holes re mains confined to the WS2 lattice (Fig. 7(c)). Fig. 6(c) shows the sche matics illustration of charge carrier’s creation in Gr/WS2 heterostructure domain. The charge transfer mechanism at Gr/WS2 heterointerface is presented in Fig. 6(d). The I–V results for Gr/WS2 heterojunction are plotted in Fig. 6(3), which demonstrates the linear response because the graphene act as channel. The optical parameters of Gr/WS2 are characterized by photo electrical characterization of under DUV exposure of intensity 11 W/cm2 with a wavelength of 220 nm. Gr/WS2 divulges the consistent photo response for different cycles as shown in Fig. 8(a). The photocurrent at different Vg is illustrated in Fig. 8(b), demonstrating linear dependence on voltage. The relaxation time (τdecay) for the sensor is extracted by experimental fitting of the photoresponse according to equation; Iph ¼ t Idark þ Aexp ð τdecay Þ, where A is a scaling constant and t symbolizes the
WS2 layer whereas electrons transfer to graphene channel, and this mechanism would be responsible for enhanced photoresponsivity of heterostructure (illustrated in Fig. 6(c and d)) [4,40]. Fig. 9(b) shows the D* of the device for different Vg inferring that D* increases from 4.9 � 109 to 9.2 � 1010 Jones as Vg increased. Similarly, the calculated EQE significantly varied from 4.6 � 105 to 2.7 � 106 for Vg ranges from 0 to 60 V and is illustrated in Fig. 9(c). The result divulges that the selective coverage of Gr with WS2 can leads to controllable tweaking of its properties by where WS2 act as electron injector. Whereas photo-irradiation increased the carriers transfer along the carries injection by simple contact between WS2 and Gr. The optical response divulges the significant enhancement in the performance of device as function of Vg. Rλ, D* and EQE increased to by the magnitude of about 10 times as Vg increased from 0 to 60 V. 4. Conclusion We have demonstrated the fabrication and characterization of 2D material’s based van der Waal heterostructure FET by ingenious com bination of controllable Fermi level and high mobility of graphene with remarkable optical properties of WS2. The carriers injection occurs be tween Gr/WS2 interfaces based on surface charge transfer mechanism between 2D materials whereas the photoresponse is credited to sepa ration of photogenerated excitons. Electrical transport characteristics of heterostructure confirmed the n-doping in Gr after WS2 coverage resulting in the formation of a p-n junction between p-doped Gr and ndoped Gr/WS2. However, the junction become more prominent after DUV exposure, which reveals that charge transfer between Gr and WS2 enhanced after light exposure because of photogenerated charge car riers. Moreover, the optical response of the device is also studies by evaluating the parameters such as Rλ, D* and EQE to understand the sensing properties of heterostructure. The values obtained of optical
UV light switching time. Other important optical figure of merits such as 1 pffiffiffiffiffiffiffiffiffiffiffiffiffi detectivity (D* ¼ Rλ A2 = 2eIdark ), photoresponsivity (Rλ ¼ Iph = PA) and external quantum efficiency (EQE ¼ hcRλ =eλ) have also been extracted by the gate dependent photoresponse of the sensor. Rλ varies from 818 to 4800 A/W as Vg increased from 0 to 60 V and is illustrated in Fig. 9(a) with the error bars depicting the consistent response for different cycles. Upon light exposure, transfer of photogenerated charge carriers take place at Gr/WS2 interface which might be attributed to the electric field induces at hetero-interface due to mismatch in work function of both materials. Photoexcited holes remain trapped into the
Fig. 8. (a) Photocurrent response of DUV light at different gate voltages (b) The value of photocurrent at different gate voltages (c) Decay time as a function of different gate voltages. 7
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Fig. 9. (a) Photoresponsivity as a function of different backgate voltages (b) Detectivity at different backgate voltages (c) External quantum efficiency (EQE) at different backgate voltages.
parameters for heterostructure are far above than that reported for in dividual structures and a significant enhancement is observed by changing Vg. So, we can conclude that DUV treatment along with gate tunable properties of graphene can be opted as a good approach for fabrication of future electronic and optoelectronic devices involving surface charge transfer between the materials.
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