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Dual characteristics of molybdenum disulfide based PN heterojunction photodetector prepared via drop-cast technique
Optik - International Journal for Light and Electron Optics 188 (2019) 8–11
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Dual characteristics of molybdenum disulfide based PN heterojunction photodetector prepared via drop-cast technique
T
⁎
H. Ahmada,b, , T.M.K. Thandavana, K. Thambiratnama a b
Photonics Research Centre, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia Faculty of Science and Technology, Airlangga University, Surabaya 60115, Indonesia
A R T IC LE I N F O
ABS TRA CT
Keywords: Molybdenum disulfide Dual characteristics Responsivity Photodetector Ultraviolet
A p-silicon/molybdenum disulfide (p-Si/MoS2) p-n heterojunction photodetector (PD) is proposed and fabricated using the drop-casting technique. The composition of elements in the localized surface morphology enables for excellent photoconduction under 380 nm illumination at various ultraviolet (UV) powers. The uneven and even distribution of the current-voltage I-V curve in the negative and positive bias regions indicate substantial dual characteristics in the fabricated device. A high responsivity of about 9.6 and 0.388 AW−1 is measured at the negative and positive bias regions respectively, allowing the p-Si/MoS2 p-n heterojunction PD to operate at UV powers lower than 830 μW.
1. Introduction Transition metal dichalcogenides (TMDs) such as MoS2, WS2, MoSe2, WSe2 and MoTe2 as well as other materials that form twodimensional (2D) semiconducting monolayers are being investigated for its emerging characteristics in the field of electronics and optoelectronics as emitters and detectors [1–3]. These semiconductors are utilized in various forms such as nanotubes or other nanoparticles which can help to detect or emit light. This is due to the intrinsic properties of the charge carriers in these 2D semiconductors which offers numerous advantages such as high mobility, high sensitivity and fast response with chemical stability [4,5]. These properties, as reported, have been able to enhance the optoelectronic characteristics of semiconducting devices such as photodiodes (PDs) by trapping the effective photons [6]. Lin et al. [7] reported metamaterials and metasurfaces decorated with nanoantenna shaped semiconducting materials offer facile integration with electronics in order to control the flow of light at the nanoscale level. This structure also enables the manipulation of the polarization states, creating structural colors and acting as anti-reflection coatings to further enhance the absorption of effective photons [8,9]. Of late, the application of MoS2 based PDs as a viable replacement for GaAs and Si based PDs have become the focus of significant research attention due to the broad absorption spectrum of MoS2 ranging from 350 to 950 nm [10–12]. Radisavljevic et al. [13] demonstrated an electron mobility of at least 200 cm2V-s−1 in a single layer of MoS2 that is similar to that of graphene nanoribbons together with high turn ON/OFF ratio of 1 × 108· This, combined with ultralow standby power dissipation has made MoS2 an interesting alternative to graphene. Many attempts have thus been made to improve the potential of MoS2 by overcoming its limitations such as its low scalability in the fabrication processes as well as the need for mechanical or chemical exfoliation that leads to poor thickness control [14]. This is of particular importance as the bandgap of MoS2 can be varied from 1.2 to 1.8 eV as the thickness reduces from its bulk form to a monolayer form. The increased bandgap in the monolayer structure results in singularities near the
⁎
Corresponding author at: Photonics Research Centre, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia. E-mail address: [email protected] (H. Ahmad).
https://doi.org/10.1016/j.ijleo.2019.05.033 Received 9 March 2019; Accepted 11 May 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 188 (2019) 8–11
H. Ahmad, et al.
conduction and valence band edges that significantly increases the absorption probability of photons that have energies close to the bandgap [15]. Furthermore, the availability of unsaturated d-orbitals in MoS2 allows the bandgap to be easily tuned. This work investigates the dual characteristics of a MoS2 based p-n heterojunction PD fabricated using the drop-casting technique that favors significant changes in the positive and negative bias regions. The repetition of results shows substantial consistency and reproducibility of the current-voltage (I-V) characteristics under UV illumination. Thus the produced p-n heterojunction PD may lead the way for dual functionality of the device in the electronic and optoelectronic devices. 2. Experimental A boron doped p-Si substrate with a thickness of 1.0 mm is utilized as the p- source while a MoS2 thin film is used as the n- source in order to obtain the p-Si/MoS2 p-n heterojunction PD. The p-Si substrate is selectively cut into 2 cm × 2 cm squares and then cleaned ultrasonically in a 10% hydrofluoric acid bath solution for 30 min. The substrate is then rinsed thoroughly in deionised water as to remove remaining traces of hydrofluoric acid and then cleaned again ultrasonically in an acetone bath solution for 30 min followed by an ethanol solution for another 30 min. The substrate is rinsed a final time with DI water and blow-dried with dried nitrogen gas as to completely remove any remaining chemical and deionized water traces on the substrate. The 2D MoS2 material is prepared by mixing the pristine MoS2 solution obtained from the Graphene Market with ethanol in a volume ratio of 1:1 and ultrasonicated for an hour. This allows for the MoS2 nanoflakes to break and be homogeneously dispersed in the solution. This makes it easy for the solution to dry fast once drop-casted onto the p-Si substrate as the ethanol evaporates. An Eppendorf pipette is used to drop 1.5 μL of the MoS2 solution onto the p-Si substrate which has been heated to a constant 80 °C. The MoS2 solution is allowed to dry on the p-Si substrate and to cool to room temperatures before being transferred into a desiccator filled with a drying agent. This is done to avoid any contamination of the p- and n- layers. Silver contacts are then incorporated onto the pSi surface and MoS2 thin film to serve as the drain and source electrodes respectively for photoconduction measurement. An insulation tape is also used to cover the edge of the p-Si substrate so as to avoid direct illumination by UV light that would interrupt the I-V measurement. The photoconduction characteristics of the MoS2 thin film is determined under UV illumination. The UV source is fixed at a distance of 1 cm from the fabricated PD, and the I-V characteristics are taken for a direct current (DC) bias with a voltage ranging from −10 to +10 V using a Keithley 2410 Sourcemeter. The UV powers are made to vary between 1.0–2277 μW. The UV signal is aligned perpendicularly to the surface of the PD and focused on the active area formed by the MoS2 thin film which is determined using a S130VC slim photodiode power sensor. 3. Results and discussion Fig. 1a shows the 2D surface morphology scan image that is obtained from JEOL JSM-7600 F field emission scanning electron microscope (FESEM). From the image, it is clearly observable that MoS2 nanostructures are formed on the p-Si substrate in sizes up to 100 nm. The detailed element composition is taken from the energy-dispersive X-ray spectroscope (EDX) spectrum as in Fig. 1b, which shows a high count rate of C and O elements as well as lower counts of Mo, S and Si elements. This is due to the mixture of the ethanol solution during the preparation of MoS2, which forms active functional groups in the lattice structure of the MoS2 thin film. The inset of Fig. 1b shows the detailed element composition and EDX image of scanned area. The elements such as C, O, Si and S are traces due to the electrons transfer from the K line series whereas for the trace for the Mo element arises from the L line series. Fig. 1c shows the I-V characteristics of the p-Si/MoS2 p-n heterojunction PD for a DC bias voltage ranging from −10 to 10 V under various UV power illumination. The n-type MoS2 thin film is set as the positive terminal while the p-Si layer is connected to the negative terminal of the Keithley 2410 sourcemeter as shown in Fig. 1c. As the DC bias voltage is swept from −10 to 10 V, significant changes are noticed in the positive and negative bias regions. The negative bias region shows a large flow of dark current and photocurrent as compared to the positive region, with the dark current measured to be about −94.7 and 3.5 μA in the negative and positive regions respectively. When not illuminated, the applied potential difference across the p-n junction would narrow the depletion region thus permitting the flow of pre-existing charge carriers in the device. Under UV illumination, the generation of electron-hole (e-h) pairs in the MoS2 thin film further enhances the generation of photocurrent. Thus, the photocurrent in the positive bias region is found to increase as the UV power increases. Unexpectedly, an uneven distribution of photocurrent is observed as the UV power is increased in the negative bias region. UV powers from 1 to 829.9 μW shows a higher photocurrent than the dark current whereas at UV powers of 1049–2277 μW a lower photocurrent than the dark current is observed. Thus, the p-Si/MoS2 p-n heterojunction PD is observed to have dual characteristics under increasing UV powers and can be operated at powers less than 830 μW. Fig. 1d shows the distribution of PD’s dual characteristics at selected DC bias voltages of −10 and +10 V. As the UV power increases, the photocurrent is found to increase as well with a higher photocurrent increment in the positive and negative bias region occurring at a UV power of 112.5 μW. As the power is further increased, a steady increment of the photocurrent is observed for UV powers ranging from 576.4 to 2277 μW. In contrast however, the photocurrent begins to unexpectedly drop in the negative bias region for the same UV power range. In the negative bias region, at UV power ranging from 112.5 to 695.8 μW a decrement in the photocurrent is observed along with a large reduction in the photocurrent at a UV power of 915 μW. Photocurrents lower than the dark current occur at UV powers from 1049 to 2277 μW. Thus, the fabricated p-Si/MoS2 p-n heterojunction PD can be selectively operated at UV powers ranging from 112.5 to 575.4 μW and 112.5 to 2277 μW at the negative and positive bias region respectively. These dual characteristics of the p-Si/MoS2 p-n heterojunction PD has not been reported elsewhere, and is therefore a new challenge for the PDs. 9
Optik - International Journal for Light and Electron Optics 188 (2019) 8–11
H. Ahmad, et al.
Fig. 1. (a) FESEM image, (b) EDX spectrum with respective scanned area and composition of elements (inset), (c) I-V characteristics with experimental setup, (d) distribution of current at bias −10 V, (e) responsivity and (f) EQE of the p-Si/MoS2 PN heterojunction PD.
Two important parameters to quantify the fabricated PD are its responsivity, Rλ = (Iph-Idk)/Pi and external quantum efficiency (EQE), where EQE = hcRλ/eλ [16,17]. Iph, Idk, Pi, h, c, e and λ are the photocurrent, dark current, incident UV power, Plank constant, speed of light, charge of an electron and the UV wavelength respectively. As shown in Fig. 1e, the responsivity in the negative bias region is found to have a rising trend for all UV powers except at powers of 1049, 1798 and 2277 μW. A high responsivity of about 9.6 AW-1−1 at a UV power of 1.0 μW and DC bias of −10 V is observed. In the positive bias region, all UV powers show rising trends in the responsivity except at powers of 1.0 and 40.0 μW. The responsivity is found to have a normal distribution with a peak responsivity of about 0.388 and 0.034 AW−1 at 2.5 V for UV powers of 1.0 and 40.0 μW respectively. This shows that the p-Si/MoS2 p-n heterojunction PD is strongly dependent on the bias voltage and can be operated selectively at preferred UV powers. Fig. 1f shows the distribution of the EQE in the p-Si/MoS2 p-n heterojunction PD. The EQE expresses the number of electrons detected per incident photon [18], and in the positive bias region of 2.5–5.0 V the EQE is greater than 1, indicating the generation of large numbers of e-h pairs from a single photon in the active MoS2 photoconduction area. The use of the drop-casting method to obtain the MoS2 n-layer of the PD is an important development, as it shows that a simple approach can be employed to obtain a highly responsive and effective opto-electronic device. This is compared to more complicated practices in the industry, such as chemical, modified chemical and plasma enhanced vapor deposition which are complicated and henceforth costly approaches. While it may be argued that these approaches provide a uniform and repeatable outcome in terms of the fabricated layer, current advances have also allowed the drop-casting method to generate highly uniform layers repeatedly. This would allow for increased research to be undertaken in PDs, as researchers are not bound to the availability of expensive and complicated to operate equipment. Additionally, the dual-characteristics observed in this work are unique, and to the best-knowledge of the authors have not been observed elsewhere. This would hopefully encourage the development of new PD technologies and 10
Optik - International Journal for Light and Electron Optics 188 (2019) 8–11
H. Ahmad, et al.
techniques. 4. Conclusion The drop casting technique is successfully used to form a p-Si/MoS2 p-n heterojunction PD. EDX analysis confirms the unintentional functionalization on the surface of MoS2 which leads to excellent photoconduction under UV illumination. Furthermore, the distribution of the uneven surface morphology with nanostructured particles provides excellent photoconduction due to the availability of effective photons, giving a high responsivity about 9.6 AW−1. At increased UV powers, dual characteristics are observed in the device, an interesting development which would spur more research into this phenomenon. Conflict of interest None. Acknowledgements Funding for this work was supported by Ministry of Higher Education (MoHE), Malaysia under grants LRGS (2015) NGOD/UM/ KPT and GA 010-2014 (ULUNG) as well as the University of Malaya under the grants RU 013-2018. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C.-Y. Chim, G. Galli, F. Wang, Nano Lett. 10 (4) (2010) 1271–1275. B. Radisavljevic, A. Radenovic, J. Brivio, i.V. Giacometti, A. Kis, Nat. Nanotechnol. 6 (3) (2011) 147. R. Sundaram, M. Engel, A. Lombardo, R. Krupke, A. Ferrari, P. Avouris, M. Steiner, Nano Lett. 13 (4) (2013) 1416–1421. G.R. Bhimanapati, Z. Lin, V. Meunier, Y. Jung, J. Cha, S. Das, D. Xiao, Y. Son, M.S. Strano, V.R. Cooper, ACS Nano 9 (12) (2015) 11509–11539. S. Riazimehr, A. Bablich, D. Schneider, S. Kataria, V. Passi, C. Yim, G.S. Duesberg, M.C. Lemme, Solid. Electron. 115 (2016) 207–212. M.L. Brongersma, Y. Cui, S. Fan, Nat. Mater. 13 (5) (2014) 451. D. Lin, P. Fan, E. Hasman, M.L. Brongersma, Science 345 (6194) (2014) 298–302. P. Spinelli, M. Verschuuren, A. Polman, Nat. Commun. 3 (2012) 692. Y. Yang, W. Wang, P. Moitra, I.I. Kravchenko, D.P. Briggs, J. Valentine, Nano Lett. 14 (3) (2014) 1394–1399. G. Wu, X. Wang, Y. Chen, Z. Wang, H. Shen, T. Lin, W. Hu, J. Wang, S. Zhang, X. Meng, Nanotechnology 29 (48) (2018) 485204. R. Kumar, N. Goel, R. Raliya, P. Biswas, M. Kumar, Nanotechnology 29 (40) (2018) 404001. N. Goel, R. Kumar, B. Roul, M. Kumar, S. Krupanidhi, J. Phys. D Appl. Phys. 51 (37) (2018) 374003. A. Molina-Sanchez, L. Wirtz, Phys. Rev. B 84 (15) (2011) 155413. H. Li, J. Wu, Z. Yin, H. Zhang, Acc. Chem. Res. 47 (4) (2014) 1067–1075. L. Britnell, R. Ribeiro, A. Eckmann, R. Jalil, B. Belle, A. Mishchenko, Y.-J. Kim, R. Gorbachev, T. Georgiou, S. Morozov, Science 340 (6138) (2013) 1311–1314. J. Cheng, Y. Zhang, R. Guo, J. Cryst. Growth 310 (1) (2008) 57–61. L. Zeng, L. Tao, C. Tang, B. Zhou, H. Long, Y. Chai, S.P. Lau, Y.H. Tsang, Sci. Rep. 6 (2016). B. Chitara, S. Krupanidhi, C. Rao, Appl. Phys. Lett. 99 (11) (2011) 113114.
11
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Dual characteristics of molybdenum disulfide based PN heterojunction photodetector prepared via drop-cast technique
T
⁎
H. Ahmada,b, , T.M.K. Thandavana, K. Thambiratnama a b
Photonics Research Centre, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia Faculty of Science and Technology, Airlangga University, Surabaya 60115, Indonesia
A R T IC LE I N F O
ABS TRA CT
Keywords: Molybdenum disulfide Dual characteristics Responsivity Photodetector Ultraviolet
A p-silicon/molybdenum disulfide (p-Si/MoS2) p-n heterojunction photodetector (PD) is proposed and fabricated using the drop-casting technique. The composition of elements in the localized surface morphology enables for excellent photoconduction under 380 nm illumination at various ultraviolet (UV) powers. The uneven and even distribution of the current-voltage I-V curve in the negative and positive bias regions indicate substantial dual characteristics in the fabricated device. A high responsivity of about 9.6 and 0.388 AW−1 is measured at the negative and positive bias regions respectively, allowing the p-Si/MoS2 p-n heterojunction PD to operate at UV powers lower than 830 μW.
1. Introduction Transition metal dichalcogenides (TMDs) such as MoS2, WS2, MoSe2, WSe2 and MoTe2 as well as other materials that form twodimensional (2D) semiconducting monolayers are being investigated for its emerging characteristics in the field of electronics and optoelectronics as emitters and detectors [1–3]. These semiconductors are utilized in various forms such as nanotubes or other nanoparticles which can help to detect or emit light. This is due to the intrinsic properties of the charge carriers in these 2D semiconductors which offers numerous advantages such as high mobility, high sensitivity and fast response with chemical stability [4,5]. These properties, as reported, have been able to enhance the optoelectronic characteristics of semiconducting devices such as photodiodes (PDs) by trapping the effective photons [6]. Lin et al. [7] reported metamaterials and metasurfaces decorated with nanoantenna shaped semiconducting materials offer facile integration with electronics in order to control the flow of light at the nanoscale level. This structure also enables the manipulation of the polarization states, creating structural colors and acting as anti-reflection coatings to further enhance the absorption of effective photons [8,9]. Of late, the application of MoS2 based PDs as a viable replacement for GaAs and Si based PDs have become the focus of significant research attention due to the broad absorption spectrum of MoS2 ranging from 350 to 950 nm [10–12]. Radisavljevic et al. [13] demonstrated an electron mobility of at least 200 cm2V-s−1 in a single layer of MoS2 that is similar to that of graphene nanoribbons together with high turn ON/OFF ratio of 1 × 108· This, combined with ultralow standby power dissipation has made MoS2 an interesting alternative to graphene. Many attempts have thus been made to improve the potential of MoS2 by overcoming its limitations such as its low scalability in the fabrication processes as well as the need for mechanical or chemical exfoliation that leads to poor thickness control [14]. This is of particular importance as the bandgap of MoS2 can be varied from 1.2 to 1.8 eV as the thickness reduces from its bulk form to a monolayer form. The increased bandgap in the monolayer structure results in singularities near the
⁎
Corresponding author at: Photonics Research Centre, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia. E-mail address: [email protected] (H. Ahmad).
https://doi.org/10.1016/j.ijleo.2019.05.033 Received 9 March 2019; Accepted 11 May 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 188 (2019) 8–11
H. Ahmad, et al.
conduction and valence band edges that significantly increases the absorption probability of photons that have energies close to the bandgap [15]. Furthermore, the availability of unsaturated d-orbitals in MoS2 allows the bandgap to be easily tuned. This work investigates the dual characteristics of a MoS2 based p-n heterojunction PD fabricated using the drop-casting technique that favors significant changes in the positive and negative bias regions. The repetition of results shows substantial consistency and reproducibility of the current-voltage (I-V) characteristics under UV illumination. Thus the produced p-n heterojunction PD may lead the way for dual functionality of the device in the electronic and optoelectronic devices. 2. Experimental A boron doped p-Si substrate with a thickness of 1.0 mm is utilized as the p- source while a MoS2 thin film is used as the n- source in order to obtain the p-Si/MoS2 p-n heterojunction PD. The p-Si substrate is selectively cut into 2 cm × 2 cm squares and then cleaned ultrasonically in a 10% hydrofluoric acid bath solution for 30 min. The substrate is then rinsed thoroughly in deionised water as to remove remaining traces of hydrofluoric acid and then cleaned again ultrasonically in an acetone bath solution for 30 min followed by an ethanol solution for another 30 min. The substrate is rinsed a final time with DI water and blow-dried with dried nitrogen gas as to completely remove any remaining chemical and deionized water traces on the substrate. The 2D MoS2 material is prepared by mixing the pristine MoS2 solution obtained from the Graphene Market with ethanol in a volume ratio of 1:1 and ultrasonicated for an hour. This allows for the MoS2 nanoflakes to break and be homogeneously dispersed in the solution. This makes it easy for the solution to dry fast once drop-casted onto the p-Si substrate as the ethanol evaporates. An Eppendorf pipette is used to drop 1.5 μL of the MoS2 solution onto the p-Si substrate which has been heated to a constant 80 °C. The MoS2 solution is allowed to dry on the p-Si substrate and to cool to room temperatures before being transferred into a desiccator filled with a drying agent. This is done to avoid any contamination of the p- and n- layers. Silver contacts are then incorporated onto the pSi surface and MoS2 thin film to serve as the drain and source electrodes respectively for photoconduction measurement. An insulation tape is also used to cover the edge of the p-Si substrate so as to avoid direct illumination by UV light that would interrupt the I-V measurement. The photoconduction characteristics of the MoS2 thin film is determined under UV illumination. The UV source is fixed at a distance of 1 cm from the fabricated PD, and the I-V characteristics are taken for a direct current (DC) bias with a voltage ranging from −10 to +10 V using a Keithley 2410 Sourcemeter. The UV powers are made to vary between 1.0–2277 μW. The UV signal is aligned perpendicularly to the surface of the PD and focused on the active area formed by the MoS2 thin film which is determined using a S130VC slim photodiode power sensor. 3. Results and discussion Fig. 1a shows the 2D surface morphology scan image that is obtained from JEOL JSM-7600 F field emission scanning electron microscope (FESEM). From the image, it is clearly observable that MoS2 nanostructures are formed on the p-Si substrate in sizes up to 100 nm. The detailed element composition is taken from the energy-dispersive X-ray spectroscope (EDX) spectrum as in Fig. 1b, which shows a high count rate of C and O elements as well as lower counts of Mo, S and Si elements. This is due to the mixture of the ethanol solution during the preparation of MoS2, which forms active functional groups in the lattice structure of the MoS2 thin film. The inset of Fig. 1b shows the detailed element composition and EDX image of scanned area. The elements such as C, O, Si and S are traces due to the electrons transfer from the K line series whereas for the trace for the Mo element arises from the L line series. Fig. 1c shows the I-V characteristics of the p-Si/MoS2 p-n heterojunction PD for a DC bias voltage ranging from −10 to 10 V under various UV power illumination. The n-type MoS2 thin film is set as the positive terminal while the p-Si layer is connected to the negative terminal of the Keithley 2410 sourcemeter as shown in Fig. 1c. As the DC bias voltage is swept from −10 to 10 V, significant changes are noticed in the positive and negative bias regions. The negative bias region shows a large flow of dark current and photocurrent as compared to the positive region, with the dark current measured to be about −94.7 and 3.5 μA in the negative and positive regions respectively. When not illuminated, the applied potential difference across the p-n junction would narrow the depletion region thus permitting the flow of pre-existing charge carriers in the device. Under UV illumination, the generation of electron-hole (e-h) pairs in the MoS2 thin film further enhances the generation of photocurrent. Thus, the photocurrent in the positive bias region is found to increase as the UV power increases. Unexpectedly, an uneven distribution of photocurrent is observed as the UV power is increased in the negative bias region. UV powers from 1 to 829.9 μW shows a higher photocurrent than the dark current whereas at UV powers of 1049–2277 μW a lower photocurrent than the dark current is observed. Thus, the p-Si/MoS2 p-n heterojunction PD is observed to have dual characteristics under increasing UV powers and can be operated at powers less than 830 μW. Fig. 1d shows the distribution of PD’s dual characteristics at selected DC bias voltages of −10 and +10 V. As the UV power increases, the photocurrent is found to increase as well with a higher photocurrent increment in the positive and negative bias region occurring at a UV power of 112.5 μW. As the power is further increased, a steady increment of the photocurrent is observed for UV powers ranging from 576.4 to 2277 μW. In contrast however, the photocurrent begins to unexpectedly drop in the negative bias region for the same UV power range. In the negative bias region, at UV power ranging from 112.5 to 695.8 μW a decrement in the photocurrent is observed along with a large reduction in the photocurrent at a UV power of 915 μW. Photocurrents lower than the dark current occur at UV powers from 1049 to 2277 μW. Thus, the fabricated p-Si/MoS2 p-n heterojunction PD can be selectively operated at UV powers ranging from 112.5 to 575.4 μW and 112.5 to 2277 μW at the negative and positive bias region respectively. These dual characteristics of the p-Si/MoS2 p-n heterojunction PD has not been reported elsewhere, and is therefore a new challenge for the PDs. 9
Optik - International Journal for Light and Electron Optics 188 (2019) 8–11
H. Ahmad, et al.
Fig. 1. (a) FESEM image, (b) EDX spectrum with respective scanned area and composition of elements (inset), (c) I-V characteristics with experimental setup, (d) distribution of current at bias −10 V, (e) responsivity and (f) EQE of the p-Si/MoS2 PN heterojunction PD.
Two important parameters to quantify the fabricated PD are its responsivity, Rλ = (Iph-Idk)/Pi and external quantum efficiency (EQE), where EQE = hcRλ/eλ [16,17]. Iph, Idk, Pi, h, c, e and λ are the photocurrent, dark current, incident UV power, Plank constant, speed of light, charge of an electron and the UV wavelength respectively. As shown in Fig. 1e, the responsivity in the negative bias region is found to have a rising trend for all UV powers except at powers of 1049, 1798 and 2277 μW. A high responsivity of about 9.6 AW-1−1 at a UV power of 1.0 μW and DC bias of −10 V is observed. In the positive bias region, all UV powers show rising trends in the responsivity except at powers of 1.0 and 40.0 μW. The responsivity is found to have a normal distribution with a peak responsivity of about 0.388 and 0.034 AW−1 at 2.5 V for UV powers of 1.0 and 40.0 μW respectively. This shows that the p-Si/MoS2 p-n heterojunction PD is strongly dependent on the bias voltage and can be operated selectively at preferred UV powers. Fig. 1f shows the distribution of the EQE in the p-Si/MoS2 p-n heterojunction PD. The EQE expresses the number of electrons detected per incident photon [18], and in the positive bias region of 2.5–5.0 V the EQE is greater than 1, indicating the generation of large numbers of e-h pairs from a single photon in the active MoS2 photoconduction area. The use of the drop-casting method to obtain the MoS2 n-layer of the PD is an important development, as it shows that a simple approach can be employed to obtain a highly responsive and effective opto-electronic device. This is compared to more complicated practices in the industry, such as chemical, modified chemical and plasma enhanced vapor deposition which are complicated and henceforth costly approaches. While it may be argued that these approaches provide a uniform and repeatable outcome in terms of the fabricated layer, current advances have also allowed the drop-casting method to generate highly uniform layers repeatedly. This would allow for increased research to be undertaken in PDs, as researchers are not bound to the availability of expensive and complicated to operate equipment. Additionally, the dual-characteristics observed in this work are unique, and to the best-knowledge of the authors have not been observed elsewhere. This would hopefully encourage the development of new PD technologies and 10
Optik - International Journal for Light and Electron Optics 188 (2019) 8–11
H. Ahmad, et al.
techniques. 4. Conclusion The drop casting technique is successfully used to form a p-Si/MoS2 p-n heterojunction PD. EDX analysis confirms the unintentional functionalization on the surface of MoS2 which leads to excellent photoconduction under UV illumination. Furthermore, the distribution of the uneven surface morphology with nanostructured particles provides excellent photoconduction due to the availability of effective photons, giving a high responsivity about 9.6 AW−1. At increased UV powers, dual characteristics are observed in the device, an interesting development which would spur more research into this phenomenon. Conflict of interest None. Acknowledgements Funding for this work was supported by Ministry of Higher Education (MoHE), Malaysia under grants LRGS (2015) NGOD/UM/ KPT and GA 010-2014 (ULUNG) as well as the University of Malaya under the grants RU 013-2018. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C.-Y. Chim, G. Galli, F. Wang, Nano Lett. 10 (4) (2010) 1271–1275. B. Radisavljevic, A. Radenovic, J. Brivio, i.V. Giacometti, A. Kis, Nat. Nanotechnol. 6 (3) (2011) 147. R. Sundaram, M. Engel, A. Lombardo, R. Krupke, A. Ferrari, P. Avouris, M. Steiner, Nano Lett. 13 (4) (2013) 1416–1421. G.R. Bhimanapati, Z. Lin, V. Meunier, Y. Jung, J. Cha, S. Das, D. Xiao, Y. Son, M.S. Strano, V.R. Cooper, ACS Nano 9 (12) (2015) 11509–11539. S. Riazimehr, A. Bablich, D. Schneider, S. Kataria, V. Passi, C. Yim, G.S. Duesberg, M.C. Lemme, Solid. Electron. 115 (2016) 207–212. M.L. Brongersma, Y. Cui, S. Fan, Nat. Mater. 13 (5) (2014) 451. D. Lin, P. Fan, E. Hasman, M.L. Brongersma, Science 345 (6194) (2014) 298–302. P. Spinelli, M. Verschuuren, A. Polman, Nat. Commun. 3 (2012) 692. Y. Yang, W. Wang, P. Moitra, I.I. Kravchenko, D.P. Briggs, J. Valentine, Nano Lett. 14 (3) (2014) 1394–1399. G. Wu, X. Wang, Y. Chen, Z. Wang, H. Shen, T. Lin, W. Hu, J. Wang, S. Zhang, X. Meng, Nanotechnology 29 (48) (2018) 485204. R. Kumar, N. Goel, R. Raliya, P. Biswas, M. Kumar, Nanotechnology 29 (40) (2018) 404001. N. Goel, R. Kumar, B. Roul, M. Kumar, S. Krupanidhi, J. Phys. D Appl. Phys. 51 (37) (2018) 374003. A. Molina-Sanchez, L. Wirtz, Phys. Rev. B 84 (15) (2011) 155413. H. Li, J. Wu, Z. Yin, H. Zhang, Acc. Chem. Res. 47 (4) (2014) 1067–1075. L. Britnell, R. Ribeiro, A. Eckmann, R. Jalil, B. Belle, A. Mishchenko, Y.-J. Kim, R. Gorbachev, T. Georgiou, S. Morozov, Science 340 (6138) (2013) 1311–1314. J. Cheng, Y. Zhang, R. Guo, J. Cryst. Growth 310 (1) (2008) 57–61. L. Zeng, L. Tao, C. Tang, B. Zhou, H. Long, Y. Chai, S.P. Lau, Y.H. Tsang, Sci. Rep. 6 (2016). B. Chitara, S. Krupanidhi, C. Rao, Appl. Phys. Lett. 99 (11) (2011) 113114.
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