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Comprehensive photoresponse study on high performance and flexible π-SnS photodetector with near-infrared response
Materials Science in Semiconductor Processing 100 (2019) 270–274
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Comprehensive photoresponse study on high performance and flexible πSnS photodetector with near-infrared response
T
Mohamed S. Mahdia,b,∗, Naser M. Ahmedb, A. Hmoodc,∗∗, K. Ibrahimb, M. Bououdinad a
Ministry of Science and Technology, Baghdad, Iraq Universiti Sains Malaysia, School of Physics, Penang, Malaysia c Microelectronics and Nanotechnology Research Laboratory (M. N. R. Lab.), University of Basrah, College of Science, Physics Department, Basrah, Iraq d University of Bahrain, College of Science, Bahrain b
ARTICLE INFO
ABSTRACT
Keywords: π-SnS Flexible Photoresponse Chemical bath deposition
Tin sulfide (SnS) has attracted a great interest recently due to its high absorption coefficient (∼104 cm−1), facile deposition and low-cost. This research work consists on a comprehensive photoresponse investigation of flexible π-SnS photodetector analyzed under illumination of near-infrared (NIR). The π-SnS film has been deposited onto a flexible substrate polyethylene terephthalate (PET) via facile and relatively low-cost chemical bath deposition. The photoresponse characteristics are studied extensively at different bias voltages, various illumination power densities, and different bending angles. The as-fabricated photodetector shows an excellent stability and reproducibility characteristics as well as good photoresponse properties under light illumination of NIR (750 nm); i.e. sensitivity (1635), response time (0.55 s) and recovery time (0.53 s) at bias voltage 5 V. Based on the obtained exceptional characteristics, besides its flexibility, non-toxic nature and low-cost, the as-developed π-SnS photodetector can be considered as a promising optoelectronic device in the range of NIR.
1. Introduction Recently, flexible optoelectronic devices have attracted much consideration owing to their unique characteristics, such as light weight, biocompatibility, shock resistance, and softness. These features have encouraged their use in many applications such as wearable devices, energy-storage, and future paper displays [1]. Moreover, the low-toxicity of Sn compared with other metals such as Pb and Cd, presents an advantage for SnS over other sulfide metal based compounds. Furthermore, SnS received a particular interest due to its specific characteristics, including natural abundance and stable properties under ambient environmental conditions [2]. Since the first report about the new cubic crystal structure of π-SnS [3], it has been extensively investigated [4–14]. Numerous methods have been utilized to synthesize π-SnS films, including spray pyrolysis [7], chemical vapor deposition [5], thermal evaporation [6] and chemical bath deposition (CBD) [8–14]. Based on literature review, CBD technique has been widely used for the deposition of π-SnS thin films onto glass substrates [8–14]. Prior studies focused mostly on structure, surface morphology, and optical properties. However, up to the knowledge of the authors, there is only one report about the use of deposited π-SnS film onto glass
∗
substrate to fabricate near-infrared (NIR) photodetector [14]. Therefore, the current research work offers a new successful approach to deposit π-SnS film onto PET substrate via CBD technique, as well as fabricating a high performance flexible NIR photodetector which makes it more applicable for applications requiring flexibility such as wearable devices. Moreover, comprehensive photoresponse measurements under illumination of NIR were performed. The present flexible photodetector showed sensitivity more than 2 times higher compared to the photodetector based on cubic structure onto glass substrate [14] and more than 16 times higher compared to that flexible photodetector based on the orthorhombic structure [15]. Moreover, the photodetector exhibited excellent photoresponse characteristics (fast response time and recovery time, high sensitivity) and reproducibility. 2. Experimental details SnS film was deposited onto the PET substrate by CBD technique. The bath reaction solution was prepared as follows: 0.15 M of thioacetamide (C2H5NS), 0.1 M of stannous chloride (SnCl2·2H2O) as a source of S2− and Sn2+ ions, respectively and 0.2 M of trisodium citrate (Na3C6H5O7) as a complex agent. The pH value was adjusted to 7.0 by
Corresponding author. Ministry of Science and Technology, Baghdad, Iraq. Corresponding author. University of Basrah, College of Science, Department of Physics, Iraq E-mail addresses: [email protected] (M.S. Mahdi), [email protected] (A. Hmood).
∗∗
https://doi.org/10.1016/j.mssp.2019.05.019 Received 12 March 2019; Received in revised form 1 May 2019; Accepted 16 May 2019 Available online 21 May 2019 1369-8001/ © 2019 Elsevier Ltd. All rights reserved.
Materials Science in Semiconductor Processing 100 (2019) 270–274
M.S. Mahdi, et al.
84, 172, 187 and 304 cm−1. Based on earlier studies [6,9], the vibrational modes at 84, ∼177, and 187 cm−1 correspond to the π-SnS phase. The vibrational mode at 304 cm−1 denotes the presence of Sn2S3 [22,23].
adding aqueous ammonia. Prior to deposition, proponal-2 solution and distilled water were used for cleaning PET substrate in that order for 10 min. The deposition process was performed at 80 °C for 4 h. The structure of SnS film was checked by X-ray diffraction (XRD) using CuKα radiation source in the 2θ range of 10° to 70°. Raman spectrum was recorded at wavelength of 514 nm as the excitation source emitted from argon ion (Ar+) laser by using Jobin Yvon HR 800 UV operating at 20 mW. Morphological observations of SnS film surface were analyzed by field-emission scanning electron microscopy (FESEM) using FEI Nova NanoSEM 450 equipped with energy dispersive X-ray (EDX) spectrometer for elemental chemical analysis. Diffuse reflectance spectrum was recorded in the wavelength range of (300–1400 nm) by using UV–Vis–NIR Cary model spectrophotometer. The photoresponse measurements were performed using source meter Keithley 2400 and light emitting diode (LED) with wavelength peak of 750 nm.
3.2. Optical properties The energy gap (Eg ) of SnS film was estimated from diffuse reflectance spectroscopy. As can be seen from Fig. 2(a), there is a significant change in the spectral dependence of diffuse reflectance near 760 nm, where there is an abrupt decrease in the reflectance, which shows an onset of a fundamental absorption edge. Furthermore, a large slope in the graph can be observed when absorption begins from SnS film. To determine the direct energy gap of SnS film from the diffuse reflectance spectrum, the Kubelka–Munk function F (R) = (1 R )2 /2 R has been applied, where R represents the diffuse reflectance [14]. Fig. 2 (b) shows a plot of (F(R) hν)2 versus hν, where hν represents the photon energy. The value of Eg can be obtained by extrapolating the intercept of the linear segment from (F(R)hυ)2 to the hυ axis. The value of Eg was determined as 1.6 eV.
3. Results and discussion 3.1. Structural analysis X-ray diffraction and Raman spectroscopy were used to analyze the structure of SnS thin film.
3.3. Surface morphology
3.1.1. X-ray diffraction The XRD pattern of the deposited SnS film onto PET substrate is shown in Fig. 1 (a). It can be noted that a high intense peak at 25.93° with four less intense peaks at 17.98°, 22.88°, 46.28° and 53.43° were observed. These peaks are indexed to PET substrate [16–18]. Moreover, one peak at 39.78° is ascribed to (510) plane of π-SnS cubic structure, which is consistent with earlier reports [7,8]. In earlier study [14], the XRD pattern of the deposited π-SnS (cubic structure) film onto glass substrate showed high intense peaks at 26.58°, 30.77° and 31.68°. Therefore, the presence of one peak belonging for the as-deposited SnS film onto PET substrate may be ascribed to a high intensity and broad peak of the PET substrate which is located between 10° and 35°.
Fig. 3 illustrates FESEM image of SnS thin film deposited onto PET substrate. It can be observed that the surface of the substrate is uniformly covered with spherical grains having sizes around 300–500 nm, as well few voids and pinholes. Furthermore, some flakes start to form as shown in the inset of Fig. 3. The presence of flakes in film surface enhances the light absorption efficiency [24] due to the increase in the surface-to-volume ratio for these flakes [25,26]. This increase in light absorption enhances electron-hole pairs generation, which leads to an increase in the resultant photocurrent and consequently improves the performance of the photodetector [14,27]. The observed surface morphology characteristics is expected to have a strong effect on the optical properties of the film such as light absorption, which provide a large surface area for light absorption. Therefore, the sharp decrease in the reflection value (Fig. 2(a)) near the wavelength, which corresponds to the energy gap of the film, can be attributed to the strong absorption at this specific wavelength. The elemental chemical composition as estimated by EDX, indicates the presence of Sn and S elements, with an atomic ratio of 57.85 and 42.15. The film thickness was calculated by a gravimetric method, and the film thickness is approximately 300 nm.
3.1.2. Raman spectroscopy Raman spectroscopy is effectively utilized as an appropriate technique to distinguish the possible existing phases (polymorph), which may be formed during the synthesis of SnS film such as SnS2 and Sn2S3 because of its efficacy in examining the structural changes and vibration properties of materials [19–21]. Fig. 1 (b) exhibits the Raman spectrum of the deposited film onto PET substrate. It can be seen that the peaks of vibrational modes vary in intensity and are located around
Fig. 1. (a) XRD pattern of deposited π- SnS film onto PET substrate; the magnified view demonstrates the intensity of the plane (510) of π-SnS cubic structure. (b) The Raman spectrum of the deposited π- SnS film. 271
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Fig. 2. (a) The diffuse reflectance spectrum of the π-SnS film deposited onto PET substrate (b) Plot of (F(R) hν)2 versus hν for π-SnS film grown onto PET substrate.
maximum current value is steady at all bias voltages, which demonstrates a high stability of the photodetector. The dependence of photocurrent on the bias voltage is depicted in Fig. 4(d). It can be seen that the photocurrent value increases with increasing bias voltage because of the increase of carriers drift velocity v, that is directly proportional with the bias voltage V (defined as v= µ. V / l , where l is the distance between the electrodes, µ represents the carrier's mobility) and reduction of the carrier's transit time Tt (defined as Tt = l2/µ. V ) [15,30]. The photodetector sensitivity (S) has been calculated using the following relationship S (%) = (Iph/ Idark ) x100 , where photocurrent Iph = Ilight – Idark , Ilight and Idark denote the currents under illumination and dark, respectively [14]. The sensitivity values of the as-fabricated photodetector as calculated from Fig. 4(c) are found to be approximately 1635, 1480 and 1196 at 5, 7, and 10 V, respectively. It can be noted that the S value is found to decrease with increasing bias voltage. This result may be ascribed to the increase in dark current which is higher than photocurrent associated with the increase in bias voltage; hence, the S value decreases with increasing bias voltage. The sensitivity value at bias voltage of 5 V is highest compared with the sensitivity values of SnS photodetectors onto glass substrates based on cubic [14] and orthorhombic structures [10,31,32]. It also exceeds the value of SnS photodetector based on orthorhombic structure onto PET [15] and SiO2/Si substrates [33,34]. The high sensitivity value of the device could be ascribed to the improved crystanallity of SnS film and low dark current (∼120 pA at 5 V). The low value of dark current is an important prerequisite to attain a high performance photodetector [35]. Furthermore, the low dark current allows to detect low power density light [36] and as well enhances the detector signal-to-noise ratio due to the shot noise in proportion with the dark current [37]. Table 1 summarizes the value of sensitivity (S), as obtained in this study besides the values reported in previous studies of SnS photodetectors [10,14,15,31–34]. The photoresponse time is a significant parameter to evaluate the photodetector performance. Relatively faster photoresponse time (response and recovery time) of photodetector will expand the range of its applications. From Fig. 4(c), it can be seen that the vertical increase and decrease in the current value at ON and OFF illumination, respectively, is a consequence of electrons transition band-to-band without contribution of sub-band states, while, the exponential increase and decrease is due to the sub-band states electrons transition [14,38]. Hence, the current dramatically increases to steady value, and thereafter drastically decreases to its initial value when the illumination is switched off, revealing the fast photoresponse time characteristic of the photodetector. The response times at 5, 7, and 10 V bias voltages are determined to be 0.55, 0.56, and 0.53 s respectively, whereas the recovery times are found to be 0.53, 0.53, and 0.51 s at 5, 7 and 10 V bias voltages, respectively.
Fig. 3. FESEM image of π-SnS film.
3.4. Photoconductivity measurements SnS thin film is considered as a suitable absorber material in optoelectronic applications, and hence it could manifest better photocurrent response characteristics. In the case of a metal-semiconductor junction with an ohmic contact, the carriers are free to flow between the metal and semiconductor. Hence, there is a minimum resistance across the contact. To achieve that in the case of SnS (p-type semiconductor), it requires that the metal work function should be close to or larger than the sum of the energy band gap (1.6 eV) and the electron affinity (4.20 eV) [28]. In this study, platinum (Pt) (work function is 5.6 eV) [29] has been chosen as contact metal, since it satisfies the above condition. Fig. 4 (a and b) shows the schematic and currentvoltage measurements in dark and under the illumination of NIR (750 nm) of the as-fabricated photodetector. The linear properties and symmetry of the (I-V) curves indicate that an ohmic contact has been successfully achieved between SnS film and the Pt electrodes. Furthermore, the current remarkably increases under the light illumination compared with the dark condition. The photoresponse stability is a key parameter to determine the capability of photodetector in applications. Fig. 4(c) exhibits currenttime curves measured at 5, 7, and 10 V bias voltages upon NIR illumination (alternating dark and illumination) condition with OFF/ON cycles of 20 s. It can be obviously noted that after several cycles, the 272
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Fig. 4. (a) Schematic diagram of the photocurrent measuring process in the photodetector. (b) The (I-V) properties for SnS photodetector in the dark and under NIR illumination. (c) Photoresponse properties of the photodetector under NIR illumination at various bias voltages. (d) The dependence of photocurrent on the bias voltage.
To further investigate the photoresponse properties of the device, the current was measured at 5 V bias voltage under various incident light power density (Pinc ), as shown in Fig. 5(a). This figure exhibits a significant increase in the device current with the increase of light power density. The corresponding photocurrent - light power density curve is plotted as Fig. 5(b). The curve can be fitted by a power law Iph = KPinc , where K is the proportionality constant [39,40]. Fitting of the curve gives = 0.63. In the semiconductor materials, the complex processes such as electron–hole generation and recombination resulted in the non-unity exponent [39,40]. To investigate the durability and flexibility of the device, the current–time (I-T) curves are measured under light illumination and different bending angles, as shown in Fig. 5 (c). The reduced values of dark and light currents after bending may be attributed to the cracks occurring within metal electrodes, which reduce collection efficiency of photogeneration carriers [41,42]. The reduction of dark and light currents values is consistent with that for SnS2 flexible photodetector [42]. The values of dark current and sensitivity of photodetector without and with various bending angles as shown in Fig. 5 (d). It can be noted that the sensitivity value increases with increasing the bending angle. The enhancement of the device sensitivity with bending angle may be ascribed to the reduction in the dark current value more than
photocurrent value; where the sensitivity is proportional inversely with dark current [14,15]. In addition, the sensitivity tends to be higher for photodetector which is low in dark current [10,43,44]. The determined response times are found to be 0.55, 0.51 and 0.58 s, whereas the recovery times are 0.53, 0.50 and 0.53 s in the cases of without bending, with bending at 10° and 15°, respectively. Thus, the time-resolved curves of the device reveal an excellent stability before and after bending the device. 4. Conclusion In this study, flexible NIR photodetector based on π-SnS film with cubic phase was fabricated onto PET substrate by low-cost and simple chemical bath deposition technique. The photodetector showed high sensitivity, fast responce and excellent stability. The excellent photoresponse characteristics of the photodetector suggest that π-SnS film offers a prominent future in the applications of flexible optoelectronic devices. Acknowledgments The researchers are very thankful to the Ministry of Science and
Table 1 Comparison the results of the present photodetector with previous reports of SnS photodetectors. Substrate
Crystal Structure
Bias Voltage (V)
Illumination source
Pinc(mW/cm2)
Sensitivity (%)
Reference
glass glass glass glass SiO2/Si SiO2/Si PET PET
orthorhombic orthorhombic orthorhombic cubic orthorhombic orthorhombic orthorhombic cubic
10 5 5 5 5 2 5 5
Tungsten halogen lamp LED (750 nm) Visible light LED (750 nm) Visible light 650 nm LED (850 nm) LED (750 nm)
100 38 100 38 100 50 55 38
∼250 260 ∼80 698 ∼1300 ∼70 102 1635
[31] [10] [32] [14] [33] [34] [15] This work
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Fig. 5. (a) Time-dependent photoresponse properties of the SnS photodetector upon NIR illumination with various power densities. (b) The dependence of photocurrent on light power density. (c) Photoresponse characteristics of the photodetector without and various bending angles. (d) The values of dark current and sensitivity of photodetector without and with various bending angles.
Technology-Iraq and Nano-Optoelectronics Research LaboratoryUniversiti Sains Malaysia-Malaysia for the financial supports to achieve this research work.
[20] M. Vasudeva Reddy, G. Sreedevi, C. Park, R.W. Miles, K.T. Ramakrishna Reddy, Curr. Appl. Phys. 15 (2015) 588–598. [21] M.S. Mahdia, A. Hmood, K. Ibrahim, N.M. Ahmed, M. Bououdina, Superlattice. Microst. 128 (2019) 170–176. [22] Y. Jayasree, U. Chalapathi, V. Sundara Raja, Thin Solid Films 537 (2013) 149–155. [23] A. Supee, Y. Tanaka, M. Ichimura, Mater. Sci. Semicond. Process. 38 (2015) 290. [24] B.D. Boruah, A. Misra, ACS Appl. Mater. Interfaces 8 (2016) 18182–18188. [25] G.M. Kumar, X. Fu, P. Ilanchezhiyan, S.U. Yuldashev, D.J. Lee, H.D. Cho, T.W. Kang, ACS Appl. Mater. Interfaces 9 (2017) 32142–32150. [26] Lou Zheng, Ludong Li, Guozhen Shen, Nanoscale 8 (2016) 5219–5225. [27] M.S. Mahdi, K. Ibrahim, Naser M. Ahmed, A. Hmood, F.I. Mustafa, S.A. Azzez, M. Bououdina, J. Alloy. Comp. 735 (2018) 2256–2262. [28] M. Devika, N. Koteeswra Reddy, F. Potolsky, K.R. Gunasekhar, J. Appl. Phys. 104 (2008) 124503. [29] X.F. Wang, L. He, S. Halas, T. Pie, J.G. Lin, T. Durakiewicz, Appl. Phys. Lett. 102 (2013) 223504. [30] O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic, A. Kis, Nat. Nanotechnol. 8 (2013) 497–501. [31] Chao Gao, Honglie Shen, Lei Sun, Appl. Surf. Sci. 257 (2011) 6750–6755. [32] T. Srinivasa, M. Kumar, RSC Adv. 6 (2016) 95680. [33] Renfu Zhuo, Shiyong Zuo, Weiwei Quan, De Yan, Baisong Geng, Jun Wang, Xuehu Men, RSC Adv. 8 (2018) 761. [34] J. Wang, G. Lian, Z. Xu, C. Fu, Z. Lin, L. Li, Q. Wang, D. Cui, C.-P. Wong, Appl. Mater. Interfaces 8 (2016) 9545–9551. [35] Z. Wang, X. Zhan, Y. Wang, S. Muhammad, Y. Huang, J. He, Nanoscale 4 (2012) 2678. [36] Y. Fang, J. Huang, Adv. Mater. 27 (2015) 2804–2810. [37] O.F. Farhat, M.M. Halim, M.J. Abdullah, M.K.M. Ali, N.M. Ahmed, N.K. Allam, Appl. Phys. Mater. Sci. Process 119 (2015) 1197–1201. [38] Yashika Gupta, P. Arun, Phys. Status Solidi B 253 (2016) 509–514. [39] Zhi Zheng, Lin Gan, Huiqiao Li, Ying Ma, Yoshio Bando, Dmitri Golberg, Tianyou Zhai, Adv. Funct. Mater. 25 (2015) 5885–5894. [40] H. Kind, H.Q. Yan, B. Messer, M. Law, P.D. Yang, Adv. Mater. 14 (2002) 158–160. [41] S.R. Tamalampudi, Y.Y. Lu, U.R. Kumar, R. Sankar, C.D. Liao, B.K. Moorthy, C.H. Cheng, F.C. Chou, Y.T. Chen, Nano Lett. 14 (2014) 2800. [42] X. Zhou, Q. Zhang, L. Gan, H. Li, T. Zhai, Adv. Funct. Mater. (2016), https://doi.org/10. 1002/adfm. 201600318. [43] T.H. Sajeesh, A.S. Cherian, C.S. Kartha, Energy Procedia 15 (2012) 325–332. [44] T.H. Sajeesh, C.S. Kartha, C. Sanjeeviraja, T. Abe, Y. Kashiwaba, K.P. Vijayakumar, J. Phys. D Appl. Phys. 43 (2010) 445102.
References [1] X. Wang, W. Song, B. Liu, G. Chen, D. Chen, C. Zhou, G. Shen, Adv. Funct. Mater. 23 (2013) 1202–1209. [2] X. Zhou, L. Gan, Q. Zhang, X. Xiong, H. Li, Z. Zhong, J. Han, T. Zhai, J. Mater. Chem. C 4 (2016) 2111. [3] A. Rabkin, S. Samuha, R.E. Abutbul, V. Ezersky, L. Meshi, Y. Golan, Nano Lett. 15 (2015) 2174–2179. [4] Jonathan M. Skelton, Lee A. Burton, Fumiyasu Oba, Aron Walsh, Apl. Mater. 5 (2017) 036101. [5] Devika Mudusu, Koteeswara Reddy Nandanapalli, Sreekantha Reddy Dugasani, Jang Won Kang, Sung Ha Park, Charles W. Tu, RSC Adv. 7 (2017) 41452. [6] Kosuke O. Hara, Shintaro Suzuki, Noritaka Usami, Thin Solid Films 639 (2017) 7–11. [7] S. Polivtseva, A. Katerski, E. Kärber, I. Oja Acik, A. Merea, V. Mikli, M. Krunks, Thin Solid Films 633 (2017) 179–184. [8] R.E. Abutbul, A.R. Garcia-Angelmo, Z. Burshtein, M.T.S. Nair, P.K. Nair, Y. Golan, CrystEngComm 18 (2016) 5188–5194. [9] R.E. Abutbul, E. Segev, L. Zeiri, V. Ezersky, G. Makov, Y. Golan, RSC Adv. 6 (2016) 5848–5855. [10] M.S. Mahdi, K. Ibrahim, N.M. Ahmed, A. Hmood, S.A. Azzez, F.I. Mustafa, M. Bououdina, Mater. Lett. 210 (2018) 279–282. [11] P.K. Nair, A.R. Garcia-Angelmo, M.T.S. Nair, Phys. Status Solidi (A) 213 (2016) 170–177. [12] U. Chalapathi, B. Poornaprakash, Si-Hyun Park, Superlattice. Microst. 103 (2017) 221–229. [13] A. Javed, Qurat-ul- Ain, M. Bashir, J. Alloy. Comp. 759 (2018) 14–21. [14] M.S. Mahdi, K. Ibrahim, A. Hmood, N.M. Ahmed, F. Mustafa, S.A. Azzez, Mater. Lett. 200 (2017) 10–13. [15] Mohamed S. Mahdi, K. Ibrahim, Naser M. Ahmed, A. Kadhim, Shrook A. Azzez, M. Bououdina, Mater. Res. Express 4 (2017) 105033. [16] B. Demirel, A.H. Yaras, B.F. Elcicek, BAÜ Fen Bil. Enst. Dergisi Cilt 13 (2011) 26–35. [17] M.G. Faraj, K. Ibrahim, A. Salhin, Mater. Sci. Semicond. Process. 15 (2012) 165–173. [18] J.E. JOHNSO, J. Appl. Polym. Sci. 11 (1959) 205–209. [19] J. Chao, Z. Wang, X. Xu, Q. Xiang, W. Song, G. Chen, J. Hu, D. Chen, RSC Adv. 3 (2013) 2746–2753.
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Comprehensive photoresponse study on high performance and flexible πSnS photodetector with near-infrared response
T
Mohamed S. Mahdia,b,∗, Naser M. Ahmedb, A. Hmoodc,∗∗, K. Ibrahimb, M. Bououdinad a
Ministry of Science and Technology, Baghdad, Iraq Universiti Sains Malaysia, School of Physics, Penang, Malaysia c Microelectronics and Nanotechnology Research Laboratory (M. N. R. Lab.), University of Basrah, College of Science, Physics Department, Basrah, Iraq d University of Bahrain, College of Science, Bahrain b
ARTICLE INFO
ABSTRACT
Keywords: π-SnS Flexible Photoresponse Chemical bath deposition
Tin sulfide (SnS) has attracted a great interest recently due to its high absorption coefficient (∼104 cm−1), facile deposition and low-cost. This research work consists on a comprehensive photoresponse investigation of flexible π-SnS photodetector analyzed under illumination of near-infrared (NIR). The π-SnS film has been deposited onto a flexible substrate polyethylene terephthalate (PET) via facile and relatively low-cost chemical bath deposition. The photoresponse characteristics are studied extensively at different bias voltages, various illumination power densities, and different bending angles. The as-fabricated photodetector shows an excellent stability and reproducibility characteristics as well as good photoresponse properties under light illumination of NIR (750 nm); i.e. sensitivity (1635), response time (0.55 s) and recovery time (0.53 s) at bias voltage 5 V. Based on the obtained exceptional characteristics, besides its flexibility, non-toxic nature and low-cost, the as-developed π-SnS photodetector can be considered as a promising optoelectronic device in the range of NIR.
1. Introduction Recently, flexible optoelectronic devices have attracted much consideration owing to their unique characteristics, such as light weight, biocompatibility, shock resistance, and softness. These features have encouraged their use in many applications such as wearable devices, energy-storage, and future paper displays [1]. Moreover, the low-toxicity of Sn compared with other metals such as Pb and Cd, presents an advantage for SnS over other sulfide metal based compounds. Furthermore, SnS received a particular interest due to its specific characteristics, including natural abundance and stable properties under ambient environmental conditions [2]. Since the first report about the new cubic crystal structure of π-SnS [3], it has been extensively investigated [4–14]. Numerous methods have been utilized to synthesize π-SnS films, including spray pyrolysis [7], chemical vapor deposition [5], thermal evaporation [6] and chemical bath deposition (CBD) [8–14]. Based on literature review, CBD technique has been widely used for the deposition of π-SnS thin films onto glass substrates [8–14]. Prior studies focused mostly on structure, surface morphology, and optical properties. However, up to the knowledge of the authors, there is only one report about the use of deposited π-SnS film onto glass
∗
substrate to fabricate near-infrared (NIR) photodetector [14]. Therefore, the current research work offers a new successful approach to deposit π-SnS film onto PET substrate via CBD technique, as well as fabricating a high performance flexible NIR photodetector which makes it more applicable for applications requiring flexibility such as wearable devices. Moreover, comprehensive photoresponse measurements under illumination of NIR were performed. The present flexible photodetector showed sensitivity more than 2 times higher compared to the photodetector based on cubic structure onto glass substrate [14] and more than 16 times higher compared to that flexible photodetector based on the orthorhombic structure [15]. Moreover, the photodetector exhibited excellent photoresponse characteristics (fast response time and recovery time, high sensitivity) and reproducibility. 2. Experimental details SnS film was deposited onto the PET substrate by CBD technique. The bath reaction solution was prepared as follows: 0.15 M of thioacetamide (C2H5NS), 0.1 M of stannous chloride (SnCl2·2H2O) as a source of S2− and Sn2+ ions, respectively and 0.2 M of trisodium citrate (Na3C6H5O7) as a complex agent. The pH value was adjusted to 7.0 by
Corresponding author. Ministry of Science and Technology, Baghdad, Iraq. Corresponding author. University of Basrah, College of Science, Department of Physics, Iraq E-mail addresses: [email protected] (M.S. Mahdi), [email protected] (A. Hmood).
∗∗
https://doi.org/10.1016/j.mssp.2019.05.019 Received 12 March 2019; Received in revised form 1 May 2019; Accepted 16 May 2019 Available online 21 May 2019 1369-8001/ © 2019 Elsevier Ltd. All rights reserved.
Materials Science in Semiconductor Processing 100 (2019) 270–274
M.S. Mahdi, et al.
84, 172, 187 and 304 cm−1. Based on earlier studies [6,9], the vibrational modes at 84, ∼177, and 187 cm−1 correspond to the π-SnS phase. The vibrational mode at 304 cm−1 denotes the presence of Sn2S3 [22,23].
adding aqueous ammonia. Prior to deposition, proponal-2 solution and distilled water were used for cleaning PET substrate in that order for 10 min. The deposition process was performed at 80 °C for 4 h. The structure of SnS film was checked by X-ray diffraction (XRD) using CuKα radiation source in the 2θ range of 10° to 70°. Raman spectrum was recorded at wavelength of 514 nm as the excitation source emitted from argon ion (Ar+) laser by using Jobin Yvon HR 800 UV operating at 20 mW. Morphological observations of SnS film surface were analyzed by field-emission scanning electron microscopy (FESEM) using FEI Nova NanoSEM 450 equipped with energy dispersive X-ray (EDX) spectrometer for elemental chemical analysis. Diffuse reflectance spectrum was recorded in the wavelength range of (300–1400 nm) by using UV–Vis–NIR Cary model spectrophotometer. The photoresponse measurements were performed using source meter Keithley 2400 and light emitting diode (LED) with wavelength peak of 750 nm.
3.2. Optical properties The energy gap (Eg ) of SnS film was estimated from diffuse reflectance spectroscopy. As can be seen from Fig. 2(a), there is a significant change in the spectral dependence of diffuse reflectance near 760 nm, where there is an abrupt decrease in the reflectance, which shows an onset of a fundamental absorption edge. Furthermore, a large slope in the graph can be observed when absorption begins from SnS film. To determine the direct energy gap of SnS film from the diffuse reflectance spectrum, the Kubelka–Munk function F (R) = (1 R )2 /2 R has been applied, where R represents the diffuse reflectance [14]. Fig. 2 (b) shows a plot of (F(R) hν)2 versus hν, where hν represents the photon energy. The value of Eg can be obtained by extrapolating the intercept of the linear segment from (F(R)hυ)2 to the hυ axis. The value of Eg was determined as 1.6 eV.
3. Results and discussion 3.1. Structural analysis X-ray diffraction and Raman spectroscopy were used to analyze the structure of SnS thin film.
3.3. Surface morphology
3.1.1. X-ray diffraction The XRD pattern of the deposited SnS film onto PET substrate is shown in Fig. 1 (a). It can be noted that a high intense peak at 25.93° with four less intense peaks at 17.98°, 22.88°, 46.28° and 53.43° were observed. These peaks are indexed to PET substrate [16–18]. Moreover, one peak at 39.78° is ascribed to (510) plane of π-SnS cubic structure, which is consistent with earlier reports [7,8]. In earlier study [14], the XRD pattern of the deposited π-SnS (cubic structure) film onto glass substrate showed high intense peaks at 26.58°, 30.77° and 31.68°. Therefore, the presence of one peak belonging for the as-deposited SnS film onto PET substrate may be ascribed to a high intensity and broad peak of the PET substrate which is located between 10° and 35°.
Fig. 3 illustrates FESEM image of SnS thin film deposited onto PET substrate. It can be observed that the surface of the substrate is uniformly covered with spherical grains having sizes around 300–500 nm, as well few voids and pinholes. Furthermore, some flakes start to form as shown in the inset of Fig. 3. The presence of flakes in film surface enhances the light absorption efficiency [24] due to the increase in the surface-to-volume ratio for these flakes [25,26]. This increase in light absorption enhances electron-hole pairs generation, which leads to an increase in the resultant photocurrent and consequently improves the performance of the photodetector [14,27]. The observed surface morphology characteristics is expected to have a strong effect on the optical properties of the film such as light absorption, which provide a large surface area for light absorption. Therefore, the sharp decrease in the reflection value (Fig. 2(a)) near the wavelength, which corresponds to the energy gap of the film, can be attributed to the strong absorption at this specific wavelength. The elemental chemical composition as estimated by EDX, indicates the presence of Sn and S elements, with an atomic ratio of 57.85 and 42.15. The film thickness was calculated by a gravimetric method, and the film thickness is approximately 300 nm.
3.1.2. Raman spectroscopy Raman spectroscopy is effectively utilized as an appropriate technique to distinguish the possible existing phases (polymorph), which may be formed during the synthesis of SnS film such as SnS2 and Sn2S3 because of its efficacy in examining the structural changes and vibration properties of materials [19–21]. Fig. 1 (b) exhibits the Raman spectrum of the deposited film onto PET substrate. It can be seen that the peaks of vibrational modes vary in intensity and are located around
Fig. 1. (a) XRD pattern of deposited π- SnS film onto PET substrate; the magnified view demonstrates the intensity of the plane (510) of π-SnS cubic structure. (b) The Raman spectrum of the deposited π- SnS film. 271
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Fig. 2. (a) The diffuse reflectance spectrum of the π-SnS film deposited onto PET substrate (b) Plot of (F(R) hν)2 versus hν for π-SnS film grown onto PET substrate.
maximum current value is steady at all bias voltages, which demonstrates a high stability of the photodetector. The dependence of photocurrent on the bias voltage is depicted in Fig. 4(d). It can be seen that the photocurrent value increases with increasing bias voltage because of the increase of carriers drift velocity v, that is directly proportional with the bias voltage V (defined as v= µ. V / l , where l is the distance between the electrodes, µ represents the carrier's mobility) and reduction of the carrier's transit time Tt (defined as Tt = l2/µ. V ) [15,30]. The photodetector sensitivity (S) has been calculated using the following relationship S (%) = (Iph/ Idark ) x100 , where photocurrent Iph = Ilight – Idark , Ilight and Idark denote the currents under illumination and dark, respectively [14]. The sensitivity values of the as-fabricated photodetector as calculated from Fig. 4(c) are found to be approximately 1635, 1480 and 1196 at 5, 7, and 10 V, respectively. It can be noted that the S value is found to decrease with increasing bias voltage. This result may be ascribed to the increase in dark current which is higher than photocurrent associated with the increase in bias voltage; hence, the S value decreases with increasing bias voltage. The sensitivity value at bias voltage of 5 V is highest compared with the sensitivity values of SnS photodetectors onto glass substrates based on cubic [14] and orthorhombic structures [10,31,32]. It also exceeds the value of SnS photodetector based on orthorhombic structure onto PET [15] and SiO2/Si substrates [33,34]. The high sensitivity value of the device could be ascribed to the improved crystanallity of SnS film and low dark current (∼120 pA at 5 V). The low value of dark current is an important prerequisite to attain a high performance photodetector [35]. Furthermore, the low dark current allows to detect low power density light [36] and as well enhances the detector signal-to-noise ratio due to the shot noise in proportion with the dark current [37]. Table 1 summarizes the value of sensitivity (S), as obtained in this study besides the values reported in previous studies of SnS photodetectors [10,14,15,31–34]. The photoresponse time is a significant parameter to evaluate the photodetector performance. Relatively faster photoresponse time (response and recovery time) of photodetector will expand the range of its applications. From Fig. 4(c), it can be seen that the vertical increase and decrease in the current value at ON and OFF illumination, respectively, is a consequence of electrons transition band-to-band without contribution of sub-band states, while, the exponential increase and decrease is due to the sub-band states electrons transition [14,38]. Hence, the current dramatically increases to steady value, and thereafter drastically decreases to its initial value when the illumination is switched off, revealing the fast photoresponse time characteristic of the photodetector. The response times at 5, 7, and 10 V bias voltages are determined to be 0.55, 0.56, and 0.53 s respectively, whereas the recovery times are found to be 0.53, 0.53, and 0.51 s at 5, 7 and 10 V bias voltages, respectively.
Fig. 3. FESEM image of π-SnS film.
3.4. Photoconductivity measurements SnS thin film is considered as a suitable absorber material in optoelectronic applications, and hence it could manifest better photocurrent response characteristics. In the case of a metal-semiconductor junction with an ohmic contact, the carriers are free to flow between the metal and semiconductor. Hence, there is a minimum resistance across the contact. To achieve that in the case of SnS (p-type semiconductor), it requires that the metal work function should be close to or larger than the sum of the energy band gap (1.6 eV) and the electron affinity (4.20 eV) [28]. In this study, platinum (Pt) (work function is 5.6 eV) [29] has been chosen as contact metal, since it satisfies the above condition. Fig. 4 (a and b) shows the schematic and currentvoltage measurements in dark and under the illumination of NIR (750 nm) of the as-fabricated photodetector. The linear properties and symmetry of the (I-V) curves indicate that an ohmic contact has been successfully achieved between SnS film and the Pt electrodes. Furthermore, the current remarkably increases under the light illumination compared with the dark condition. The photoresponse stability is a key parameter to determine the capability of photodetector in applications. Fig. 4(c) exhibits currenttime curves measured at 5, 7, and 10 V bias voltages upon NIR illumination (alternating dark and illumination) condition with OFF/ON cycles of 20 s. It can be obviously noted that after several cycles, the 272
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Fig. 4. (a) Schematic diagram of the photocurrent measuring process in the photodetector. (b) The (I-V) properties for SnS photodetector in the dark and under NIR illumination. (c) Photoresponse properties of the photodetector under NIR illumination at various bias voltages. (d) The dependence of photocurrent on the bias voltage.
To further investigate the photoresponse properties of the device, the current was measured at 5 V bias voltage under various incident light power density (Pinc ), as shown in Fig. 5(a). This figure exhibits a significant increase in the device current with the increase of light power density. The corresponding photocurrent - light power density curve is plotted as Fig. 5(b). The curve can be fitted by a power law Iph = KPinc , where K is the proportionality constant [39,40]. Fitting of the curve gives = 0.63. In the semiconductor materials, the complex processes such as electron–hole generation and recombination resulted in the non-unity exponent [39,40]. To investigate the durability and flexibility of the device, the current–time (I-T) curves are measured under light illumination and different bending angles, as shown in Fig. 5 (c). The reduced values of dark and light currents after bending may be attributed to the cracks occurring within metal electrodes, which reduce collection efficiency of photogeneration carriers [41,42]. The reduction of dark and light currents values is consistent with that for SnS2 flexible photodetector [42]. The values of dark current and sensitivity of photodetector without and with various bending angles as shown in Fig. 5 (d). It can be noted that the sensitivity value increases with increasing the bending angle. The enhancement of the device sensitivity with bending angle may be ascribed to the reduction in the dark current value more than
photocurrent value; where the sensitivity is proportional inversely with dark current [14,15]. In addition, the sensitivity tends to be higher for photodetector which is low in dark current [10,43,44]. The determined response times are found to be 0.55, 0.51 and 0.58 s, whereas the recovery times are 0.53, 0.50 and 0.53 s in the cases of without bending, with bending at 10° and 15°, respectively. Thus, the time-resolved curves of the device reveal an excellent stability before and after bending the device. 4. Conclusion In this study, flexible NIR photodetector based on π-SnS film with cubic phase was fabricated onto PET substrate by low-cost and simple chemical bath deposition technique. The photodetector showed high sensitivity, fast responce and excellent stability. The excellent photoresponse characteristics of the photodetector suggest that π-SnS film offers a prominent future in the applications of flexible optoelectronic devices. Acknowledgments The researchers are very thankful to the Ministry of Science and
Table 1 Comparison the results of the present photodetector with previous reports of SnS photodetectors. Substrate
Crystal Structure
Bias Voltage (V)
Illumination source
Pinc(mW/cm2)
Sensitivity (%)
Reference
glass glass glass glass SiO2/Si SiO2/Si PET PET
orthorhombic orthorhombic orthorhombic cubic orthorhombic orthorhombic orthorhombic cubic
10 5 5 5 5 2 5 5
Tungsten halogen lamp LED (750 nm) Visible light LED (750 nm) Visible light 650 nm LED (850 nm) LED (750 nm)
100 38 100 38 100 50 55 38
∼250 260 ∼80 698 ∼1300 ∼70 102 1635
[31] [10] [32] [14] [33] [34] [15] This work
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Fig. 5. (a) Time-dependent photoresponse properties of the SnS photodetector upon NIR illumination with various power densities. (b) The dependence of photocurrent on light power density. (c) Photoresponse characteristics of the photodetector without and various bending angles. (d) The values of dark current and sensitivity of photodetector without and with various bending angles.
Technology-Iraq and Nano-Optoelectronics Research LaboratoryUniversiti Sains Malaysia-Malaysia for the financial supports to achieve this research work.
[20] M. Vasudeva Reddy, G. Sreedevi, C. Park, R.W. Miles, K.T. Ramakrishna Reddy, Curr. Appl. Phys. 15 (2015) 588–598. [21] M.S. Mahdia, A. Hmood, K. Ibrahim, N.M. Ahmed, M. Bououdina, Superlattice. Microst. 128 (2019) 170–176. [22] Y. Jayasree, U. Chalapathi, V. Sundara Raja, Thin Solid Films 537 (2013) 149–155. [23] A. Supee, Y. Tanaka, M. Ichimura, Mater. Sci. Semicond. Process. 38 (2015) 290. [24] B.D. Boruah, A. Misra, ACS Appl. Mater. Interfaces 8 (2016) 18182–18188. [25] G.M. Kumar, X. Fu, P. Ilanchezhiyan, S.U. Yuldashev, D.J. Lee, H.D. Cho, T.W. Kang, ACS Appl. Mater. Interfaces 9 (2017) 32142–32150. [26] Lou Zheng, Ludong Li, Guozhen Shen, Nanoscale 8 (2016) 5219–5225. [27] M.S. Mahdi, K. Ibrahim, Naser M. Ahmed, A. Hmood, F.I. Mustafa, S.A. Azzez, M. Bououdina, J. Alloy. Comp. 735 (2018) 2256–2262. [28] M. Devika, N. Koteeswra Reddy, F. Potolsky, K.R. Gunasekhar, J. Appl. Phys. 104 (2008) 124503. [29] X.F. Wang, L. He, S. Halas, T. Pie, J.G. Lin, T. Durakiewicz, Appl. Phys. Lett. 102 (2013) 223504. [30] O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic, A. Kis, Nat. Nanotechnol. 8 (2013) 497–501. [31] Chao Gao, Honglie Shen, Lei Sun, Appl. Surf. Sci. 257 (2011) 6750–6755. [32] T. Srinivasa, M. Kumar, RSC Adv. 6 (2016) 95680. [33] Renfu Zhuo, Shiyong Zuo, Weiwei Quan, De Yan, Baisong Geng, Jun Wang, Xuehu Men, RSC Adv. 8 (2018) 761. [34] J. Wang, G. Lian, Z. Xu, C. Fu, Z. Lin, L. Li, Q. Wang, D. Cui, C.-P. Wong, Appl. Mater. Interfaces 8 (2016) 9545–9551. [35] Z. Wang, X. Zhan, Y. Wang, S. Muhammad, Y. Huang, J. He, Nanoscale 4 (2012) 2678. [36] Y. Fang, J. Huang, Adv. Mater. 27 (2015) 2804–2810. [37] O.F. Farhat, M.M. Halim, M.J. Abdullah, M.K.M. Ali, N.M. Ahmed, N.K. Allam, Appl. Phys. Mater. Sci. Process 119 (2015) 1197–1201. [38] Yashika Gupta, P. Arun, Phys. Status Solidi B 253 (2016) 509–514. [39] Zhi Zheng, Lin Gan, Huiqiao Li, Ying Ma, Yoshio Bando, Dmitri Golberg, Tianyou Zhai, Adv. Funct. Mater. 25 (2015) 5885–5894. [40] H. Kind, H.Q. Yan, B. Messer, M. Law, P.D. Yang, Adv. Mater. 14 (2002) 158–160. [41] S.R. Tamalampudi, Y.Y. Lu, U.R. Kumar, R. Sankar, C.D. Liao, B.K. Moorthy, C.H. Cheng, F.C. Chou, Y.T. Chen, Nano Lett. 14 (2014) 2800. [42] X. Zhou, Q. Zhang, L. Gan, H. Li, T. Zhai, Adv. Funct. Mater. (2016), https://doi.org/10. 1002/adfm. 201600318. [43] T.H. Sajeesh, A.S. Cherian, C.S. Kartha, Energy Procedia 15 (2012) 325–332. [44] T.H. Sajeesh, C.S. Kartha, C. Sanjeeviraja, T. Abe, Y. Kashiwaba, K.P. Vijayakumar, J. Phys. D Appl. Phys. 43 (2010) 445102.
References [1] X. Wang, W. Song, B. Liu, G. Chen, D. Chen, C. Zhou, G. Shen, Adv. Funct. Mater. 23 (2013) 1202–1209. [2] X. Zhou, L. Gan, Q. Zhang, X. Xiong, H. Li, Z. Zhong, J. Han, T. Zhai, J. Mater. Chem. C 4 (2016) 2111. [3] A. Rabkin, S. Samuha, R.E. Abutbul, V. Ezersky, L. Meshi, Y. Golan, Nano Lett. 15 (2015) 2174–2179. [4] Jonathan M. Skelton, Lee A. Burton, Fumiyasu Oba, Aron Walsh, Apl. Mater. 5 (2017) 036101. [5] Devika Mudusu, Koteeswara Reddy Nandanapalli, Sreekantha Reddy Dugasani, Jang Won Kang, Sung Ha Park, Charles W. Tu, RSC Adv. 7 (2017) 41452. [6] Kosuke O. Hara, Shintaro Suzuki, Noritaka Usami, Thin Solid Films 639 (2017) 7–11. [7] S. Polivtseva, A. Katerski, E. Kärber, I. Oja Acik, A. Merea, V. Mikli, M. Krunks, Thin Solid Films 633 (2017) 179–184. [8] R.E. Abutbul, A.R. Garcia-Angelmo, Z. Burshtein, M.T.S. Nair, P.K. Nair, Y. Golan, CrystEngComm 18 (2016) 5188–5194. [9] R.E. Abutbul, E. Segev, L. Zeiri, V. Ezersky, G. Makov, Y. Golan, RSC Adv. 6 (2016) 5848–5855. [10] M.S. Mahdi, K. Ibrahim, N.M. Ahmed, A. Hmood, S.A. Azzez, F.I. Mustafa, M. Bououdina, Mater. Lett. 210 (2018) 279–282. [11] P.K. Nair, A.R. Garcia-Angelmo, M.T.S. Nair, Phys. Status Solidi (A) 213 (2016) 170–177. [12] U. Chalapathi, B. Poornaprakash, Si-Hyun Park, Superlattice. Microst. 103 (2017) 221–229. [13] A. Javed, Qurat-ul- Ain, M. Bashir, J. Alloy. Comp. 759 (2018) 14–21. [14] M.S. Mahdi, K. Ibrahim, A. Hmood, N.M. Ahmed, F. Mustafa, S.A. Azzez, Mater. Lett. 200 (2017) 10–13. [15] Mohamed S. Mahdi, K. Ibrahim, Naser M. Ahmed, A. Kadhim, Shrook A. Azzez, M. Bououdina, Mater. Res. Express 4 (2017) 105033. [16] B. Demirel, A.H. Yaras, B.F. Elcicek, BAÜ Fen Bil. Enst. Dergisi Cilt 13 (2011) 26–35. [17] M.G. Faraj, K. Ibrahim, A. Salhin, Mater. Sci. Semicond. Process. 15 (2012) 165–173. [18] J.E. JOHNSO, J. Appl. Polym. Sci. 11 (1959) 205–209. [19] J. Chao, Z. Wang, X. Xu, Q. Xiang, W. Song, G. Chen, J. Hu, D. Chen, RSC Adv. 3 (2013) 2746–2753.
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