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Systematic investigations on the effect of prolong UV illumination on optoelectronic properties of ZnO honeycomb nanostructures
Scripta Materialia 163 (2019) 1–4
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Systematic investigations on the effect of prolong UV illumination on optoelectronic properties of ZnO honeycomb nanostructures Jitesh Agrawal a, Tejendra Dixit b, I.A. Palani c, Vipul Singh a,⁎ a b c
Molecular and Nanoelectronics Research Group (MNRG), Department of Electrical Engineering, IIT Indore, Indore 453552, Madhya Pradesh, India Department of Physics and Nano Functional Materials Technology Centre, IIT Madras, Chennai 600036, India Mechatronics and Instrumentation Lab, Department of Mechanical Engineering, IIT Indore, Indore 453552, Madhya Pradesh, India
a r t i c l e
i n f o
Article history: Received 13 November 2018 Accepted 25 December 2018 Available online xxxx Keywords: Zinc oxide Honeycomb structure XRD Deep UV photodetector Lattice defects
a b s t r a c t Herein, the effect of prolong UV illumination over ZnO optoelectronic characteristics has been investigated. The photoluminescence analysis has shown significant enhancement in deep level emission (DLE) after sample being exposed to UV radiations. The formation of photo-induced oxygen vacancies (VO) over the ZnO surface was found to be responsible for such noteworthy enhancement in DLE. The observed phenomenon was further utilized for controlled incorporation of VO in ZnO via UV illumination, towards obtaining optimal device performance. The UV treated photo-detector has shown significantly high photo-responsivity and photo-sensitivity in the deep UV region. © 2018 Published by Elsevier Ltd on behalf of Acta Materialia Inc.
ZnO has attracted a lot of interest of researchers for past two decades because of its wide bandgap of 3.37 eV and various intriguing optical and electronic applications, i.e. LEDs, photo-detectors [1–4], gas sensors [5], piezoelectric transducers [6], transparent conducting electrodes [7] and many more [8,9]. ZnO based deep UV photo-detectors find application in flame detection, UV radiation detection, missile launching systems, etc. [10]. Therefore, various efforts have been made towards the development of high sensitivity UV photo-detectors viz. growth of complex nanostructures to obtain high surface to volume ratio and doping of certain impurities i.e. Ge, Cd, Be etc. [11–14]. However, doping is a complex process, which increases the complexity of the fabrication process and also the cost involved in device development. Notably, recent studies have suggested that the intrinsic lattice defects viz. VO, Zni etc. could significantly improve the device performance. Additionally, high VO concentration over the surface would encourage more atmospheric Oxygen to get adsorbed over the ZnO nanostructures which in turn leads to reduction in the dark current [15]. Therefore, a lot of efforts have been made towards the development of VO rich ZnO nanostructures by controlling growth ambient in PLD and sputtering systems and by postgrowth annealing [16–18]. However, limited success has been achieved and the development of high responsivity and sensitivity deep UV ZnO based photodetector is still a challenging task. Furthermore, in various optoelectronic applications like solar cells, photodetectors etc., the devices were subjected to high energy UV radiations which could significantly modulate the device characterstics. ⁎ Corresponding author. E-mail address: [email protected] (V. Singh).
https://doi.org/10.1016/j.scriptamat.2018.12.029 1359-6462/© 2018 Published by Elsevier Ltd on behalf of Acta Materialia Inc.
Therefore, it is desirable to investigate the effect of UV irradiation over the ZnO properties. Notably, the formation energy of VO at the surface is comparatively lower than the bulk [19]. Therefore, there are high possibilities of the formation of VO defect states over the surface, as the device exposed to UV radiation. Hence, intentional prolong UV illumination could also pave the way for tuning of defect states which in turn significantly affect optoelectronic characteristics of ZnO nanostructures. However, to the best of our knowledge, no such investigation has been reported till now. Moreover, along with high VO concentration, incorporation of Zni could not only increase the n-type behavior but also improve deep UV absorption which will extend the applicability of ZnO in the deep UV region [15]. In this work, we have performed hydrothermal growth of Zni rich and thinner ZnO honeycomb nanostructures (NSs), which could provide a large surface to volume ratio for enhanced deep UV sensitivity. Further, the effect of UV illumination has been methodically analyzed using photoluminescence (PL), electron paramagnetic resonance (EPR) and diffuse reflectance analysis (DRS) analysis. The photodetector has shown significant enhancement in photo-sensitivity and photo-responsivity in deep UV region. Fig. 1(a) shows FESEM images of honeycomb NSs obtained via citrate assisted hydrothermal growth of ZnO (see supplementary material for detailed experimental procedure). The growth mechanism of the honeycomb nanostructure can be explained as follows: The citrate ions are negatively charged which act as a chelating agent, therefore get adsorbed over the polar plane of ZnO lattice which consists of positively charged Zn ions. As a consequence, the growth along polar c-axis inhibited and the same is along nonpolar a-axis [11,15]. The XRD plot in
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J. Agrawal et al. / Scripta Materialia 163 (2019) 1–4
Fig. 1. (a) FESEM image of ZnO honeycomb nanostructures, inset shows high magnification FESEM image; (b) XRD plot of ZnO honeycomb nanostructures.
Fig. 1(b) shows the diffraction peaks at 31.72°, 34.38° and 36.20° which is in good agreement with the JCPDS file of ZnO (JCPDS 01-089-7102) and were accordingly assigned to (100), (002) and (101) planes of ZnO Wurtzite structure. XRD plot also shows intense (101) and (100) peaks, whereas (002) peak intensity was almost suppressed, which has further confirmed that the growth along the polar c-axis has been inhibited and promoted along nonpolar a-axis. In order to investigate the types of defects present in the sample, PL analysis has been performed. The PL spectra in Fig. 2(a) show a sharp emission peak centered around 385 nm, which is the characteristic peak of ZnO. However, the Gaussian peak extended towards the higher wavelength region. The deconvolution of the peak has shown two separate peaks centered at 380 and 420 nm. The peak centered at 420 nm can be assigned to the Zni defects present in the crystal lattice [15]. Further, a low intensity deep level emission (DLE) peak in the visible region has been observed, which indicates towards the lower Oxygen vacancy related defects concentration in the sample. Further, towards analyzing the effect of UV illumination over the sample, PL characterization of the UV illuminated sample has been performed. Interestingly, a shoulder peak in the near band emission (NBE)
region starts dominating as the UV illumination time has been increased. The shoulder peak corresponds to the increasing Zni defect states. Notably, significant enhancement in the DLE has been observed which was attributed the formation of new defect states in the sample. The DLE peak was centered nearly at 580 nm that has been assigned to the doubly ionized oxygen vacancies (VO++). Henceforth, increasing DLE peak intensity with UV illumination can be assigned to the formation of the doubly ionized oxygen vacancies [20–22]. Moreover, DLE was observed to increase for continuous illumination of the sample by 254 nm light for duration of 1 h. Therefore, it indicated that the intrinsic defects viz. Zni and VO, concentration can be efficiently tuned by controlling the UV illumination time. The formation of the doubly ionized oxygen vacancies on UV illumination can be explained via following equations [23–25]: þ
hv→h þ e− þ
ðGeneration of excitonsÞ þ
O2− þ h ↔O− þ h ↔1
2
O2 þ V O
ðFormation of VO Þ
ð1Þ ð2Þ
Fig. 2. (a) PL spectra of UV illumination sample for different durations; (b) Normalized PL spectra (inset shows magnified image of NBE peak) of sample for different durations of UV illumination; (c) Diffuse reflectance analysis of pristine and UV illuminated samples; (d) EPR analysis of pristine and UV illuminated sample.
J. Agrawal et al. / Scripta Materialia 163 (2019) 1–4
3
Fig. 3. (a) Device schematic; (b) I–V photo response in dark and after 30, 90, 150 and 210 s. of UV exposer; (c) UV switching response of ZnO honeycomb nanostructures.
− V O →V 2þ O þ 2e
ðPhoto−excitation of Oxygen vacanciesÞ
ð3Þ
As the ZnO sample surface comes in contact with atmospheric Oxygen, the Oxygen molecules get chemically adsorbed over the nanostructure surface and trap the free electron from the surface of ZnO which leads to the formation of space charge region near the surface [15]. When the sample was exposed to UV light, excitons were generated over the surface (Eq. (1)). The built-in electric field at the surface space charge region was high enough to surmount the coulombic attraction between the excitons, therefore increasing the dissociation rate to photo-excited charge carriers. Further, some of the holes reach to lattice oxygen at the surface and release the lattice Oxygen which results in the formation of VO (Eq. (2)). Some of the VO defect states were photoexcited to metastable defect states in conduction band i.e. VO2+ (Eq. (3)), resulting in the enhanced DLE emission [19]. Furthermore, confocal measurement of the samples by sub bandgap excitation i.e. 559 nm and 635 nm has been recorded (see supplementary material S1 for confocal images). A significant enhancement in the luminescence of photo-excited sample has been observed, which was assigned to the formation of VO defects states in the bandgap. Moreover, 2-photon analysis has been performed for the confirmation of the formation of new energy states in the mid-bandgap due to the photo-induced VO defects states. For the 2-photon analysis samples were photo-excited with different wavelengths ranging from 720 to 820 nm (see supplementary material S1 for 2-photon images). The pristine samples have shown feeble emission which clearly suggests the absence of defect states in 1.5–1.72 eV energy region above the vacancy band. However, the prolong UV irradiated sample showed significant enhancement in the luminescence intensity for the same excitation wavelength which confirmed the incorporation of new defects i.e., VO in the samples. In order to probe in more detail about the formation of defect states after UV illumination DRA analysis has also been performed. As shown in Fig. 2(c), UV illuminated samples showed significant blue shift in the near UV absorption peak from 372 to 360 nm which was assigned to the high electron concentration in the conduction band that resulted in bandgap widening. Therefore, in order to estimate the variation in the bandgap, Tauc plot has been used (see supplementary material S2 for Tauc plot image). A significant enhancement in the bandgap from 3.21 to 3.32 eV has been observed which could be assigned to the BursteinMoss effect [26]. Furthermore, UV illuminated samples have shown notable enhancement in the absorption in visible region, indicating the formation of new defect states after UV exposure. The obtained results were completely correlated with PL analysis. To further analyze the effect of UV illumination, EPR analysis has been performed (Fig. 2(d)). A sharp EPR peak at 1.96 g-factor has been observed. However, the origin of the peak is still unclear; several research groups have assigned this peak to the hole trapped at neutral VO i.e. VO+ [27–29]. The peak intensity of the pristine ZnO sample was quite low which has been significantly increased after 1 h of UV illumination. Therefore, the EPR results show a correlation with the PL
analysis and confirm the formation of oxygen vacancies after UV illumination. Furthermore, another peak close to free electron value (g ≈ 2.005) has also been observed which was attributed to the electron trapped at oxygen vacancies [30–32]. A slight enhancement in the peak intensity has been assigned to the formation of oxygen vacancies. In order to analyze the deep UV sensing performance, I–V characterization has been performed (shown in Fig. 3(b)). The device active area was 0.001 cm2. Under the dark conditions, a significantly small current of 5 nA at 20 V bias has been observed which can be explained as follows: when the sample left in the air, electrons were trapped by adsorbed oxygen molecule resulting in the formation of the depletion region. As the thickness of the nanostructures was comparable to the width of the depletion region (~20 nm), NSs got wholly depleted from free electrons. Therefore low free charge carrier concentration resulting in the low dark current. Additionally, the Schottky junction at Ag/ZnO interface has further reduced the dark current [33]. As the sample was illuminated with 254 nm, 2.11 mW·cm−2 light source, excitons were generated. Large electric field in the surface space charge region would surmount the attractive Coulomb potential between the excitons and dissociate electron-hole pairs. As a result, electrons move towards the inner zone of the nanostructures whereas photo-excited holes move towards the surface, releasing the adsorbed oxygen molecules. As a consequence, NSs were again populated with free electrons, resulting in significant enhancement in the current from 5 nA to 2.45 mA. The measured device photo-sensitivity and responsivity was around 106 and 1150 A/W, respectively. Further, as shown in the Fig. 3 (c), enhancement in the photo-current with UV illumination time has been observed that can be attributed to increasing free electron concentration due to the formation of photo-induced n-type defects i.e. Zni and VO, in the sample. Iph reach to its maximum (Iph(max)) in 210 s, thereafter rapid decrease in the Iph has been observed. The abrupt enhancement in the defects concentration due to prolong UV illumination were assigned to the substantially degraded device performance. In the next switching cycle, noteworthy suppression in the Iph(max) has been observed, which indicates the permanent damage to the sample caused by prolong UV illumination. In conclusion, the effect of photo-induced defects viz. Zinc interstitials and Oxygen vacancies have been investigated. Photoluminescence has shown a shoulder peak and increasing DLE with increasing UV illumination time. A systematic analysis of the photo-induced n-type intrinsic defect states has been performed. The device has shown a significantly low dark current of 5 nA, whereas photocurrent increased to 2.45 mA. The resulting photo-current to dark current ratio was around of 6 orders. The prepared device was a potential candidate for the future optoelectronic application. Acknowledgments This work was supported by UGC, India (3526(NET-DEC. 2014)). J. A. acknowledges the Sophisticated Instrument Centre, IIT Indore for
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providing required facilities. Authors would further like to acknowledge Dr. Pankaj R Sagdeo for allowing the usage of UV Visible DRS facility. Appendix A. Supplementary data The supplementary material contains the experimental section (explaining the device fabrication steps) and Figs. S1 and S2. Supplementary data to this article can be found online at https://doi.org/10. 1016/j.scriptamat.2018.12.029. References [1] P. Ivanoff Reyes, C.-J. Ku, Z. Duan, Y. Xu, E. Garfunkel, Y. Lu, Appl. Phys. Lett. 101 (3) (2012), 031118. . [2] S. Hullavarad, N. Hullavarad, D. Look, B. Claflin, Nanoscale Res. Lett. 4 (12) (2009) 1421–1427. [3] A. Bilgaiyan, T. Dixit, I.A. Palani, V. Singh, Phys. E. 86 (2017) 136–141. [4] A. Kumar, T. Dixit, I.A. Palani, D. Nakamura, M. Higashihata, V. Singh, Phys. E. 93 (2017) 97–104. [5] A.P. Chatterjee, P. Mitra, A.K. Mukhopadhyay, J. Mater. Sci. 34 (17) (1999) 4225–4231. [6] T. Yamamoto, T. Shiosaki, A. Kawabata, J. Appl. Phys. 51 (6) (1980) 3113–3120. [7] H. Nanto, T. Minami, S. Shooji, S. Takata, J. Appl. Phys. 55 (4) (1984) 1029–1034. [8] T. Dixit, A. Bilgaiyan, I.A. Palani, D. Nakamura, T. Okada, V. Singh, J. Sol-Gel Sci. Technol. 75 (3) (2015) 693–702. [9] A. Janotti, C.G. Van de Walle, Rep. Prog. Phys. 72 (12) (2009), 126501. . [10] M. Razeghi, A. Rogalski, J. Appl. Phys. 79 (10) (1996) 7433–7473. [11] J. Agrawal, T. Dixit, I.A. Palani, M.S.R. Rao, V. Singh, J. Phys. D. Appl. Phys. 51 (18) (2018), 185106. . [12] C. Li, Y. Bando, M. Liao, Y. Koide, D. Golberg, Appl. Phys. Lett. 97 (16) (2010) 161102. [13] J. Zúñiga-Pérez, V. Muñoz-Sanjosé, M. Lorenz, G. Benndorf, S. Heitsch, D. Spemann, M. Grundmann, J. Appl. Phys. 99 (2) (2006), 023514. .
[14] Y.R. Ryu, T.S. Lee, J.A. Lubguban, A.B. Corman, H.W. White, J.H. Leem, M.S. Han, Y.S. Park, C.J. Youn, W.J. Kim, Appl. Phys. Lett. 88 (5) (2006), 052103. . [15] J. Agrawal, T. Dixit, A.I. Palani, M.S.R. Rao, V. Singh, Phys. Status Solidi (RRL) 12 (10) (2018), 1800241. . [16] B.J. Jin, S.H. Bae, S.Y. Lee, S. Im, Mater. Sci. Eng. B 71 (1–3) (2000) 301–305. [17] H.S. Kang, J.S. Kang, J.W. Kim, S.Y. Lee, J. Appl. Phys. 95 (3) (2004) 1246–1250. [18] S.-H. Jeong, B.-S. Kim, B.-T. Lee, Appl. Phys. Lett. 82 (16) (2003) 2625–2627. [19] M.-P. Lu, C.-W. Chen, M.-Y. Lu, Phys. Rev. Appl. 6 (5) (2016). [20] K. Bandopadhyay, J. Mitra, RSC Adv. 5 (30) (2015) 23540–23547. [21] A.K. Srivastava, M. Arora Praveen, S.K. Gupta, B.R. Chakraborty, S. Chandra, S. Toyoda, H. Bahadur, J. Mater. Sci. Technol. 26 (11) (2010) 986–990. [22] T. Dixit, I.A. Palani, V. Singh, J. Phys. Chem. C 121 (6) (2017) 3540–3548. [23] R.-D. Sun, A. Nakajima, A. Fujishima, T. Watanabe, K. Hashimoto, J. Phys. Chem. B 105 (10) (2001) 1984–1990. [24] H.P. Chang, E.D. Chu, Y.T. Yeh, Y.C. Wu, F.Y. Lo, W.H. Wang, M.Y. Chern, H.C. Chiu, Langmuir 33 (34) (2017) 8362–8371. [25] S. Lany, A. Zunger, Phys. Rev. B 72 (3) (2005). [26] C.E. Kim, P. Moon, S. Kim, J.-M. Myoung, H.W. Jang, J. Bang, I. Yun, Thin Solid Films 518 (22) (2010) 6304–6307. [27] C. Drouilly, J.M. Krafft, F. Averseng, S. Casale, D. Bazer-Bachi, C. Chizallet, V. Lecocq, H. Vezin, H. Lauron-Pernot, G. Costentin, J. Phys. Chem. C 116 (40) (2012) 21297–21307. [28] S.M. Evans, N.C. Giles, L.E. Halliburton, L.A. Kappers, J. Appl. Phys. 103 (4) (2008), 043710. . [29] H. Kaftelen, K. Ocakoglu, R. Thomann, S. Tu, S. Weber, E. Erdem, Phys. Rev. B 86 (1) (2012). [30] L. Zhang, L. Yin, C. Wang, N. Lun, Y. Qi, D. Xiang, J. Phys. Chem. C 114 (21) (2010) 9651–9658. [31] L. Jing, Z. Xu, J. Shang, X. Sun, W. Cai, H. Guo, Mater. Sci. Eng. A 332 (1–2) (2002) 356–361. [32] N.G. Kakazey, T.V. Sreckovic, M.M. Ristic, J. Mater. Sci. 32 (17) (1997) 4619–4622. [33] S. Liang, H. Sheng, Y. Liu, Z. Huo, Y. Lu, H. Shen, J. Cryst. Growth 225 (2–4) (2001) 110–113.
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Systematic investigations on the effect of prolong UV illumination on optoelectronic properties of ZnO honeycomb nanostructures Jitesh Agrawal a, Tejendra Dixit b, I.A. Palani c, Vipul Singh a,⁎ a b c
Molecular and Nanoelectronics Research Group (MNRG), Department of Electrical Engineering, IIT Indore, Indore 453552, Madhya Pradesh, India Department of Physics and Nano Functional Materials Technology Centre, IIT Madras, Chennai 600036, India Mechatronics and Instrumentation Lab, Department of Mechanical Engineering, IIT Indore, Indore 453552, Madhya Pradesh, India
a r t i c l e
i n f o
Article history: Received 13 November 2018 Accepted 25 December 2018 Available online xxxx Keywords: Zinc oxide Honeycomb structure XRD Deep UV photodetector Lattice defects
a b s t r a c t Herein, the effect of prolong UV illumination over ZnO optoelectronic characteristics has been investigated. The photoluminescence analysis has shown significant enhancement in deep level emission (DLE) after sample being exposed to UV radiations. The formation of photo-induced oxygen vacancies (VO) over the ZnO surface was found to be responsible for such noteworthy enhancement in DLE. The observed phenomenon was further utilized for controlled incorporation of VO in ZnO via UV illumination, towards obtaining optimal device performance. The UV treated photo-detector has shown significantly high photo-responsivity and photo-sensitivity in the deep UV region. © 2018 Published by Elsevier Ltd on behalf of Acta Materialia Inc.
ZnO has attracted a lot of interest of researchers for past two decades because of its wide bandgap of 3.37 eV and various intriguing optical and electronic applications, i.e. LEDs, photo-detectors [1–4], gas sensors [5], piezoelectric transducers [6], transparent conducting electrodes [7] and many more [8,9]. ZnO based deep UV photo-detectors find application in flame detection, UV radiation detection, missile launching systems, etc. [10]. Therefore, various efforts have been made towards the development of high sensitivity UV photo-detectors viz. growth of complex nanostructures to obtain high surface to volume ratio and doping of certain impurities i.e. Ge, Cd, Be etc. [11–14]. However, doping is a complex process, which increases the complexity of the fabrication process and also the cost involved in device development. Notably, recent studies have suggested that the intrinsic lattice defects viz. VO, Zni etc. could significantly improve the device performance. Additionally, high VO concentration over the surface would encourage more atmospheric Oxygen to get adsorbed over the ZnO nanostructures which in turn leads to reduction in the dark current [15]. Therefore, a lot of efforts have been made towards the development of VO rich ZnO nanostructures by controlling growth ambient in PLD and sputtering systems and by postgrowth annealing [16–18]. However, limited success has been achieved and the development of high responsivity and sensitivity deep UV ZnO based photodetector is still a challenging task. Furthermore, in various optoelectronic applications like solar cells, photodetectors etc., the devices were subjected to high energy UV radiations which could significantly modulate the device characterstics. ⁎ Corresponding author. E-mail address: [email protected] (V. Singh).
https://doi.org/10.1016/j.scriptamat.2018.12.029 1359-6462/© 2018 Published by Elsevier Ltd on behalf of Acta Materialia Inc.
Therefore, it is desirable to investigate the effect of UV irradiation over the ZnO properties. Notably, the formation energy of VO at the surface is comparatively lower than the bulk [19]. Therefore, there are high possibilities of the formation of VO defect states over the surface, as the device exposed to UV radiation. Hence, intentional prolong UV illumination could also pave the way for tuning of defect states which in turn significantly affect optoelectronic characteristics of ZnO nanostructures. However, to the best of our knowledge, no such investigation has been reported till now. Moreover, along with high VO concentration, incorporation of Zni could not only increase the n-type behavior but also improve deep UV absorption which will extend the applicability of ZnO in the deep UV region [15]. In this work, we have performed hydrothermal growth of Zni rich and thinner ZnO honeycomb nanostructures (NSs), which could provide a large surface to volume ratio for enhanced deep UV sensitivity. Further, the effect of UV illumination has been methodically analyzed using photoluminescence (PL), electron paramagnetic resonance (EPR) and diffuse reflectance analysis (DRS) analysis. The photodetector has shown significant enhancement in photo-sensitivity and photo-responsivity in deep UV region. Fig. 1(a) shows FESEM images of honeycomb NSs obtained via citrate assisted hydrothermal growth of ZnO (see supplementary material for detailed experimental procedure). The growth mechanism of the honeycomb nanostructure can be explained as follows: The citrate ions are negatively charged which act as a chelating agent, therefore get adsorbed over the polar plane of ZnO lattice which consists of positively charged Zn ions. As a consequence, the growth along polar c-axis inhibited and the same is along nonpolar a-axis [11,15]. The XRD plot in
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J. Agrawal et al. / Scripta Materialia 163 (2019) 1–4
Fig. 1. (a) FESEM image of ZnO honeycomb nanostructures, inset shows high magnification FESEM image; (b) XRD plot of ZnO honeycomb nanostructures.
Fig. 1(b) shows the diffraction peaks at 31.72°, 34.38° and 36.20° which is in good agreement with the JCPDS file of ZnO (JCPDS 01-089-7102) and were accordingly assigned to (100), (002) and (101) planes of ZnO Wurtzite structure. XRD plot also shows intense (101) and (100) peaks, whereas (002) peak intensity was almost suppressed, which has further confirmed that the growth along the polar c-axis has been inhibited and promoted along nonpolar a-axis. In order to investigate the types of defects present in the sample, PL analysis has been performed. The PL spectra in Fig. 2(a) show a sharp emission peak centered around 385 nm, which is the characteristic peak of ZnO. However, the Gaussian peak extended towards the higher wavelength region. The deconvolution of the peak has shown two separate peaks centered at 380 and 420 nm. The peak centered at 420 nm can be assigned to the Zni defects present in the crystal lattice [15]. Further, a low intensity deep level emission (DLE) peak in the visible region has been observed, which indicates towards the lower Oxygen vacancy related defects concentration in the sample. Further, towards analyzing the effect of UV illumination over the sample, PL characterization of the UV illuminated sample has been performed. Interestingly, a shoulder peak in the near band emission (NBE)
region starts dominating as the UV illumination time has been increased. The shoulder peak corresponds to the increasing Zni defect states. Notably, significant enhancement in the DLE has been observed which was attributed the formation of new defect states in the sample. The DLE peak was centered nearly at 580 nm that has been assigned to the doubly ionized oxygen vacancies (VO++). Henceforth, increasing DLE peak intensity with UV illumination can be assigned to the formation of the doubly ionized oxygen vacancies [20–22]. Moreover, DLE was observed to increase for continuous illumination of the sample by 254 nm light for duration of 1 h. Therefore, it indicated that the intrinsic defects viz. Zni and VO, concentration can be efficiently tuned by controlling the UV illumination time. The formation of the doubly ionized oxygen vacancies on UV illumination can be explained via following equations [23–25]: þ
hv→h þ e− þ
ðGeneration of excitonsÞ þ
O2− þ h ↔O− þ h ↔1
2
O2 þ V O
ðFormation of VO Þ
ð1Þ ð2Þ
Fig. 2. (a) PL spectra of UV illumination sample for different durations; (b) Normalized PL spectra (inset shows magnified image of NBE peak) of sample for different durations of UV illumination; (c) Diffuse reflectance analysis of pristine and UV illuminated samples; (d) EPR analysis of pristine and UV illuminated sample.
J. Agrawal et al. / Scripta Materialia 163 (2019) 1–4
3
Fig. 3. (a) Device schematic; (b) I–V photo response in dark and after 30, 90, 150 and 210 s. of UV exposer; (c) UV switching response of ZnO honeycomb nanostructures.
− V O →V 2þ O þ 2e
ðPhoto−excitation of Oxygen vacanciesÞ
ð3Þ
As the ZnO sample surface comes in contact with atmospheric Oxygen, the Oxygen molecules get chemically adsorbed over the nanostructure surface and trap the free electron from the surface of ZnO which leads to the formation of space charge region near the surface [15]. When the sample was exposed to UV light, excitons were generated over the surface (Eq. (1)). The built-in electric field at the surface space charge region was high enough to surmount the coulombic attraction between the excitons, therefore increasing the dissociation rate to photo-excited charge carriers. Further, some of the holes reach to lattice oxygen at the surface and release the lattice Oxygen which results in the formation of VO (Eq. (2)). Some of the VO defect states were photoexcited to metastable defect states in conduction band i.e. VO2+ (Eq. (3)), resulting in the enhanced DLE emission [19]. Furthermore, confocal measurement of the samples by sub bandgap excitation i.e. 559 nm and 635 nm has been recorded (see supplementary material S1 for confocal images). A significant enhancement in the luminescence of photo-excited sample has been observed, which was assigned to the formation of VO defects states in the bandgap. Moreover, 2-photon analysis has been performed for the confirmation of the formation of new energy states in the mid-bandgap due to the photo-induced VO defects states. For the 2-photon analysis samples were photo-excited with different wavelengths ranging from 720 to 820 nm (see supplementary material S1 for 2-photon images). The pristine samples have shown feeble emission which clearly suggests the absence of defect states in 1.5–1.72 eV energy region above the vacancy band. However, the prolong UV irradiated sample showed significant enhancement in the luminescence intensity for the same excitation wavelength which confirmed the incorporation of new defects i.e., VO in the samples. In order to probe in more detail about the formation of defect states after UV illumination DRA analysis has also been performed. As shown in Fig. 2(c), UV illuminated samples showed significant blue shift in the near UV absorption peak from 372 to 360 nm which was assigned to the high electron concentration in the conduction band that resulted in bandgap widening. Therefore, in order to estimate the variation in the bandgap, Tauc plot has been used (see supplementary material S2 for Tauc plot image). A significant enhancement in the bandgap from 3.21 to 3.32 eV has been observed which could be assigned to the BursteinMoss effect [26]. Furthermore, UV illuminated samples have shown notable enhancement in the absorption in visible region, indicating the formation of new defect states after UV exposure. The obtained results were completely correlated with PL analysis. To further analyze the effect of UV illumination, EPR analysis has been performed (Fig. 2(d)). A sharp EPR peak at 1.96 g-factor has been observed. However, the origin of the peak is still unclear; several research groups have assigned this peak to the hole trapped at neutral VO i.e. VO+ [27–29]. The peak intensity of the pristine ZnO sample was quite low which has been significantly increased after 1 h of UV illumination. Therefore, the EPR results show a correlation with the PL
analysis and confirm the formation of oxygen vacancies after UV illumination. Furthermore, another peak close to free electron value (g ≈ 2.005) has also been observed which was attributed to the electron trapped at oxygen vacancies [30–32]. A slight enhancement in the peak intensity has been assigned to the formation of oxygen vacancies. In order to analyze the deep UV sensing performance, I–V characterization has been performed (shown in Fig. 3(b)). The device active area was 0.001 cm2. Under the dark conditions, a significantly small current of 5 nA at 20 V bias has been observed which can be explained as follows: when the sample left in the air, electrons were trapped by adsorbed oxygen molecule resulting in the formation of the depletion region. As the thickness of the nanostructures was comparable to the width of the depletion region (~20 nm), NSs got wholly depleted from free electrons. Therefore low free charge carrier concentration resulting in the low dark current. Additionally, the Schottky junction at Ag/ZnO interface has further reduced the dark current [33]. As the sample was illuminated with 254 nm, 2.11 mW·cm−2 light source, excitons were generated. Large electric field in the surface space charge region would surmount the attractive Coulomb potential between the excitons and dissociate electron-hole pairs. As a result, electrons move towards the inner zone of the nanostructures whereas photo-excited holes move towards the surface, releasing the adsorbed oxygen molecules. As a consequence, NSs were again populated with free electrons, resulting in significant enhancement in the current from 5 nA to 2.45 mA. The measured device photo-sensitivity and responsivity was around 106 and 1150 A/W, respectively. Further, as shown in the Fig. 3 (c), enhancement in the photo-current with UV illumination time has been observed that can be attributed to increasing free electron concentration due to the formation of photo-induced n-type defects i.e. Zni and VO, in the sample. Iph reach to its maximum (Iph(max)) in 210 s, thereafter rapid decrease in the Iph has been observed. The abrupt enhancement in the defects concentration due to prolong UV illumination were assigned to the substantially degraded device performance. In the next switching cycle, noteworthy suppression in the Iph(max) has been observed, which indicates the permanent damage to the sample caused by prolong UV illumination. In conclusion, the effect of photo-induced defects viz. Zinc interstitials and Oxygen vacancies have been investigated. Photoluminescence has shown a shoulder peak and increasing DLE with increasing UV illumination time. A systematic analysis of the photo-induced n-type intrinsic defect states has been performed. The device has shown a significantly low dark current of 5 nA, whereas photocurrent increased to 2.45 mA. The resulting photo-current to dark current ratio was around of 6 orders. The prepared device was a potential candidate for the future optoelectronic application. Acknowledgments This work was supported by UGC, India (3526(NET-DEC. 2014)). J. A. acknowledges the Sophisticated Instrument Centre, IIT Indore for
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providing required facilities. Authors would further like to acknowledge Dr. Pankaj R Sagdeo for allowing the usage of UV Visible DRS facility. Appendix A. Supplementary data The supplementary material contains the experimental section (explaining the device fabrication steps) and Figs. S1 and S2. Supplementary data to this article can be found online at https://doi.org/10. 1016/j.scriptamat.2018.12.029. References [1] P. Ivanoff Reyes, C.-J. Ku, Z. Duan, Y. Xu, E. Garfunkel, Y. Lu, Appl. Phys. Lett. 101 (3) (2012), 031118. . [2] S. Hullavarad, N. Hullavarad, D. Look, B. Claflin, Nanoscale Res. Lett. 4 (12) (2009) 1421–1427. [3] A. Bilgaiyan, T. Dixit, I.A. Palani, V. Singh, Phys. E. 86 (2017) 136–141. [4] A. Kumar, T. Dixit, I.A. Palani, D. Nakamura, M. Higashihata, V. Singh, Phys. E. 93 (2017) 97–104. [5] A.P. Chatterjee, P. Mitra, A.K. Mukhopadhyay, J. Mater. Sci. 34 (17) (1999) 4225–4231. [6] T. Yamamoto, T. Shiosaki, A. Kawabata, J. Appl. Phys. 51 (6) (1980) 3113–3120. [7] H. Nanto, T. Minami, S. Shooji, S. Takata, J. Appl. Phys. 55 (4) (1984) 1029–1034. [8] T. Dixit, A. Bilgaiyan, I.A. Palani, D. Nakamura, T. Okada, V. Singh, J. Sol-Gel Sci. Technol. 75 (3) (2015) 693–702. [9] A. Janotti, C.G. Van de Walle, Rep. Prog. Phys. 72 (12) (2009), 126501. . [10] M. Razeghi, A. Rogalski, J. Appl. Phys. 79 (10) (1996) 7433–7473. [11] J. Agrawal, T. Dixit, I.A. Palani, M.S.R. Rao, V. Singh, J. Phys. D. Appl. Phys. 51 (18) (2018), 185106. . [12] C. Li, Y. Bando, M. Liao, Y. Koide, D. Golberg, Appl. Phys. Lett. 97 (16) (2010) 161102. [13] J. Zúñiga-Pérez, V. Muñoz-Sanjosé, M. Lorenz, G. Benndorf, S. Heitsch, D. Spemann, M. Grundmann, J. Appl. Phys. 99 (2) (2006), 023514. .
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