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Characteristics of ZnO MSM UV photodetector with Ni contact electrodes on poly propylene carbonate (PPC) plastic substrate
Current Applied Physics 10 (2010) 1452e1455
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Characteristics of ZnO MSM UV photodetector with Ni contact electrodes on poly propylene carbonate (PPC) plastic substrate N.N. Jandow*, F.K. Yam, S.M. Thahab, H. Abu Hassan, K. Ibrahim Nano-Optoelectronics Research and Technology, Laboratory School of Physics, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 April 2010 Received in revised form 22 May 2010 Accepted 24 May 2010 Available online 1 June 2010
In this study, we report the fabrication of ZnO metal-semiconductor-metal UV photodetector (MSM UV PD) by deposition ZnO thin film on poly propylene carbonate (PPC) plastic substrate using direct current (DC) sputtering technique, and Nickel (Ni) contact as electrodes. The structural, optical and electrical properties of the ZnO thin film were investigated by using atomic force microscopy (AFM), X-Ray diffraction (XRD) measurement, and photoluminescence (PL). The electrical characteristics of the detector were investigated using the currentevoltage (IeV) measurements, the dark- and photo-currents were found to be 1.04 and 93.80 mA, respectively. Using forward dark conditions at 5 volt; the barrier height FB was calculated to be 0.675 eV. Under incident wavelength of 385 nm, it was found that the maximum responsivity (R) of the Ni/ZnO/Ni MSM PD was found to be 1.59 A/W. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: ZnO DC sputtering Barrier height Metalesemiconductoremetal (MSM) photodiodes
1. Introduction Detection of ultraviolet (UV) radiation is an important feature for many applications such as environmental, military, and industrial fields, for instance: flame monitor, water purification system, money counting, photochemical phenomena detection, etc. [1]. In recent years, many researchers have been focusing on semiconductor-based ultraviolet (UV) photodiodes, ZnO is one of the prominent semiconductor materials which can be used for UV light detection because of its wide band gap (Eg) (3.37 eV), large exciton binding energy of 60 meV, low cost and ease of manufacturing, in addition, it is also sensitive to the UV region [2], these properties make ZnO a promising photonic material for optoelectronic applications, including light-emitting diodes, laser diodes, and photodetectors for the UV spectral range [3]. ZnO can be grown by several methods such as radio-frequency magnetron sputtering (RFMS) [4], pulse laser deposition (PLD) [5], molecular beam epitaxy (MBE) [3], metal-organic chemical vapor deposition (MOCVD) [6], electrostatic spray deposition [7] and spray pyrolysis [8], SoleGel method [9]. ZnO Schottky diodes and MSM PDs detecting in the UV region have been investigated by many researchers like for example, Young et al. [10] fabricated MSM UV sensor by growing ZnO
* Corresponding author. E-mail address: [email protected] (N.N. Jandow). 1567-1739/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.05.012
epitaxial films on sapphire (0001) substrates with Ni contact electrodes by using RF plasma-assisted molecular beam epitaxy. MSM PD is a device with two back-to-back Schottky junctions formed on the same semiconductor surface. Under the application of a bias voltage, one of the contacts becomes reverse biased, forming a depletion region that will tend to sweep out photocarriers; the other becomes forward biased, allowing the collected photocurrent to flow out just as an ohmic contact. Under sufficiently high bias voltage, the depletion region extends and touches the small space-charge region under the forward biased electrode. To produce high-performance MSM UV PDs, it is important to achieve a large Schottky barrier height at the metalesemiconductor interface that leads to a small leakage current and high breakdown voltage which could result in improving responsivity and photocurrent to dark current contrast ratio [11]. To achieve a large Schottky barrier height on ZnO, one can choose metals with highwork functions such as gold (Au), platinum (Pt), palladium (Pd), and Nickel (Ni). Ni is an interesting metal that has recently been used as a stable Schottky contact due to its high metal work function. However, there are few publications on using Ni as ZnO MSM PD contacts. Many studies on ZnO-based MSM photodetectors using silicon [12], glass [8]. Sapphire substrates [10] or flexible substrates [13] such as polyethylene terephthalate (PET) [14], polytetrafluoroethylene (Teflon) [15] or other plastics have been reported, nevertheless the used of PPC plastic as a substrate has not been reported in the literature, to the best of our knowledge, we are the first group
N.N. Jandow et al. / Current Applied Physics 10 (2010) 1452e1455
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to used the PPC plastic as a substrate for fabrication ZnO MSM UV detector with Ni contacts. PPC has been selected as a substrate in this study because it is cheap, has low chemical reactivity. In addition, it also has a high dielectric strength over wide frequencies and high surface resistivity; these properties are attractive for PPC to serve as a good substrate for a variety of microelectronic applications. In this work, we report the deposition of ZnO thin film on the PPC plastic sheet, subsequently ZnO MSM PD with Ni electrodes is fabricated, eventually the responsivity of the detector is determined. The study shows that the fabrication of ZnO PD on PPC plastic with using Ni contacts is encouraging. Fig. 2. AFM image of ZnO on PPC substrate (RMS ¼ 10 nm).
2. Experiment work The ZnO thin films were deposited on PPC plastic by sputtering system (model Edwards Auto 306). The sputtering system operated with base pressure of about 5 105 torr; the target material used in this work was ZnO disk having a diameter of about 76.2 mm; it is thickness was around 6.35 mm; with purity 99.99%. The distance between the target and the substrates was around 7 cm. Prior to the deposition, the plastic substrates were cleaned thoroughly. The cleaning procedures started by immersing the plastic sheets in the active cleaning liquid (DECON90) for 5 min, then rinsed with distilled water, after that they were dried by N2 gas. Before the deposition process, the target was pre-sputtered for 20 min to remove any contamination from the target surface. After deposition, the thin film thickness was measured by using surface optical system Filmetric F20-VIS. The microstructure of the films was studied by imaging the morphology of the films surface using AFM. The crystalline structure of the film was studied by XRD using High resolution (XRD (PANalytical X’pert Pro MRD)) with a Cu-Ka1 A), while the photoluminescence radiation source (l ¼ 1.5406 spectrum (PL) was recorded by using photoluminescence spectroscopy system Model: Jobin Yvon HR 800 UV. The line of HeeCd laser of 320 nm having a power of 1 mW is used to excite the sample, and the transmission spectrum of the sample was measured using a Hitachi U-2000 Double-beam spectrophotometer and. Finally ZnO MSM PD was fabricated by sputtering the Ni with a thickness of about 130 nm on the ZnO thin film by using a metal MSM mask, The structure of ZnO MSM PD consists of two interdigitated Schottky contact (electrodes) with finger width of 0.2 mm; the length of each electrode is about 5.4 mm and the spacing between the electrodes is 0.4 mm, each electrode has five fingers as shown in Fig. 1. As we mentioned before, typically, the device is biased such that one of the two contacts is grounded while the other is reversed biased. At sufficiently high bias the depletion regions beneath the Schottky gates can be connected. The MSM device can be used as a PD by shining light on the top surface of the structure [16].
3. Results and discussion The thickness of the prepared film was found to be about 1mm; the film surface was examined by AFM as shown in Fig. 2. A typical AFM image shows that ZnO thin film consists of some columnar structure grains, The grain height was found to be in the range of 100 nm and the root mean square (RMS) is equal to 10 nm; this nanostructure of the film surface may be useful in the absorption of the UV light when this material is used as a UV detector. Fig. 3 shows the XRD diffraction pattern with 2q from 30 to 70 of ZnO thin film deposited on PPC plastic substrate. The strong peak observed at 2q ¼ 34.25 with a full width at half maximum intensity (FWHM) of 0.36 corresponds to ZnO (002) plane. And lower intensity (110) peak was also observed at 2q ¼ 52.7. The PL of the prepared ZnO films on PPC plastic was observed at room temperature. The result is given in Fig. 4. We can find two luminescence peaks. One is the UV emission of ZnO thin films at 380 nm corresponding to the near band edge emission (NBE) due to the electronic transition from near conduction band to valence band according to Gao and Li [17]; another luminescence peak is the blueegreen emission at 490 nm, which may be due to the defect related deep level emission [18] and in agreement with Wei et al., which is attributed to the transition of electron from defect level of Zn interstitial atoms to top level of valence band [19]. Fig. 5 shows the currentevoltage (IeV) characteristics of the Ni/ ZnO/Ni UV PD on PPC plastic measured without and with UV illumination. Under dark environments, the current at 5 V was found to be 1.04 mA, on the other hand, when the sample was illuminated with UV lamp with power of 58.40 mW the current (same voltage) was observed to be 93.80 mA. The UV light can effectively create electronehole pairs in ZnO and greatly increase the carrier density. Under UV illumination, light impinges onto the thin film, the highenergy photons will be absorbed by the ZnO layer producing more 3000
intensity(a.u.)
Z n O /P P C
(0 0 2 )
2500 2000 1500 1000 500
(1 1 0 )
0 30
40
50
60
2 θ (d e g .) Fig. 1. Schematic drawing of MSM interdigitated photodetector.
Fig. 3. X-ray diffraction for ZnO thin film on PPC substrate.
70
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N.N. Jandow et al. / Current Applied Physics 10 (2010) 1452e1455
50000
R(A/W)
Intensity (a. u.)
40000
ZnO/PPC
30000 20000 10000 0 300
400
500
600
700
1.8 1.6 Ni/PPC 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 320 340
Wavelength (nm)
360
380
400
420
440
λ (nm)
800
Fig. 6. Spectral responsivity of the fabricated Ni/ZnO/Ni MSM PD on PPC substrate.
Fig. 4. Room temperature PL spectrum of DC- sputtered ZnO on PPC substrate.
electronehole pairs, the application of a bias voltage to the Ni contact creates an electric field within the underlying ZnO thin film that sweeps the photogenerated carriers out of the depletion region resulting to increase the photocurrent with voltage. The contrast ratio of this photo to dark currents was 90.19 times. This electrical result is very encouraging for further study of photodetector. The MSM-PD performance is critically dependent on the quality of the Schottky contacts. Therefore, it is necessary to measure the Schottky barrier height of the actual contact, which is a constituent part of the MSM-PD under investigation. As we mentioned, the MSM- PD essentially consists of two Schottky contacts connected back-to-back. When a bias is applied, one of the Schottky contacts is forward-biased and the other is reverse-biased. Only the reverse currentevoltage (IeV) characteristics of these Schottky contacts can be measured. The Schottky barrier height can be determined by using forward-biased IeV measurements as in Eq. (1) [20]
I ¼ Io expðqV=fnKTgÞ½1 expð qV=fKTgÞ
(1)
I+ ¼ A* AT 2 expf qFB =ðKTÞg
(2)
where Io is the saturation current, n is the ideally factor, K is the Boltzmann’s constant, T is the absolute temperature, FB is the barrier height, A is the area of the Schottky and A* is the effective Richardson coefficient w32 A/cm2K2 [21]. Based on equation (1), the plot of lnfIexpðqV=fKT gÞ=½expðqV=fKT gÞ 1g vs. V will give a straight line; Io is derived from the intercept with y-axis when V ¼ 0, and from Eq. (2) we found that Schottky barrier height FB at the Ni/ZnO interface is equal to 0.675 eV. 1e-3
Ni-PPC 1e-4
One of the important parameters to evaluate a photodetector is the responsivity R, which is the ratio of the device photo-current to the incident optical power which is given by Eq. (4) [22]:
Rl ¼ Iph =Pl ðA=WÞ
(4)
where Iph is the photocurrent and Pl is the incident optical power. Fig. 6 shows the measured spectral responsivity of the fabricated Ni/ZnO/Ni MSM PD fabricated on the PPC plastic. As shown in the figure, the photodetector responsivity was nearly constant in a gradual leasing with values which were very close together in the below UV region w320e365 nm; while it was an obvious response in the UV region, peaking at 385 nm and began decreasing whenever became close to the visible region, we could explain these results as follow; when ZnO detector is irradiated by UV light with energy higher than the bandgap (3.37 eV for ZnO), electronehole pairs will generate, as a result these excess charge carriers contribute to photo current and result in the response to the UV light. The cut-off at wavelength of 385 nm is nearly close to the ZnO energy bandgap of 3.37 eV; the responsivity decreased at the shorter wavelength range due to the decrease of the penetrating depth of the light, resulting in an increase of the surface recombination [11]. With an incident UV wavelength of 385 nm and 5 V applied bias, it was found that the maximum responsivity of the fabricated Ni/ZnO/Ni MSM PD is 1.59 A/W. This high responsivity may be attributed to its high internal gain. According to Liu [1], this indicated that a Schottky barrier at the metal electrodeesemiconductor interface can exhibit hole-trapping in the reversed-bias junction that shrinks the depletion region and allows tunneling of additional electrons into the photoconductor; if electrons pass multiple times, this mechanism yields photoconductive gain greater than unity. The responsivity of our detector is comparable to other UV- detector reported by Liang et al. [21] who also found that responsivity of their MSM photodiode could reach 1.5 A/W by fabricated Ag/ZnO Schottky UV photodetector on Rplane sapphire substrate.
I (A)
1e-5
4. Conclusion
1e-6 1e-7 1e-8
dark current photo current 385 (nm )
1e-9 0
1
2
3
4
5
V(volt) Fig. 5. IeV characteristics of Ni/ZnO/Ni MSM PD on PPC substrate measured in (dark current) and (photocurrent).
In brief, we have fabricated metalesemiconductoremetal UV photodetector by deposition ZnO thin film on PPC plastic substrate. The characteristics of ZnO thin film were investigated by various tools. AFM image which shows that the ZnO thin film has a nanostructure surface with RMS of 10 nm, XRD spectrum showed that ZnO has preferential orientation along the c-axis with dominant peak observed at 2q ¼ 34.25 . PL measurement exhibited a strong UV emission peak at 380 nm. IeV measurements showed that the dark- and photo-current were observed to be1.04 and 93.80 mA at 5 V respectively and the contrast ratio of this photo to dark currents
N.N. Jandow et al. / Current Applied Physics 10 (2010) 1452e1455
was 90.19 times. In addition, the maximum responsivity of the Ni/ ZnO/Ni MSM PD was determined to be 1.59 A/W. The results showed that the fabrication of ZnO MSM UV photodetector on PPC plastic by using DC-sputtering technique is possible. Acknowledgments This work was conducted under the grant no.: 100/PFIZIK/8/ 4010 support from Universiti Sains Malaysia is gratefully acknowledged. References [1] K.W. Liu, J.G. Ma, J.Y. Zhang, Y.M. Lu, D.Y. Jiang, B.H. Li, D.X. Zhao, Z.Z. Zhang, B. Yao, D.Z. Shen, Solid-State Electron. 51 (2007) 757e761. [2] S.J. Young, L.W. Ji, S.J. Chang, Y.P. Chen, K.T. Lam, S.H. Liang, X.L. Du, Q.K. Xue, Y.S. Sun, IET Optoelectron. 1 (3) (2007) 135e139. [3] S.P. Chang, S.J. Chang, Y.Z. Chiou, C.Y. Lu, T.K. Lin, Y.C. Lin, C.F. Kuo, H.M. Chang, Sensor. Actuat. A 140 (2007) 60e64. [4] F.S. Mahmood, R.D. Gould, A.K. Hassan, H.M. Salih, Thin Solid Films 770 (1995) 376e379. [5] F. Danie Auret, W.E. Meyer, P.J. Janse van Rensburg, M. Hayes, J.M. Nel, Holger von Wenckstern, H. Schmidt, G. Biehne, H. Hochmuth, M. Lorenz, M. Grundmann, Phys. B 401e402 (2007) 378e381. [6] Y. Liu, C.R. Gorla, S. Liang, N. Emanetoglu, Y. Lu, H. Shen, M. Wraback, J. Electron. Mater. 29 (No. 1) (2000) 69e74.
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[7] Kyu-Seog Hwang, Ju-Hyun Jeong, Young-Sun Jeon, Kyung-Ok Jeon, ByungHoon Kim, Ceram. Int. 33 (2007) 505e507. [8] P. Nunes, E. Fortunato, R. Martins, Int. J. Inorg. Mater. 3 (2001) 1125e1128. [9] Ziqiang Xu, Hong Deng, Juan Xie, Yan Li, Yanrong Li, J. SoleGel Sci. Technol. 36 (2005) 223e226. [10] S.J. Young, L.W. Ji, S.J. Chang, Y.K. Su, J. Cryst. Growth 293 (2006) 43e47. [11] S.J. Young, L.W. Ji, T.H. Fang, S.J. Chang, Y.K. Su, X.L. Du, Acta Mater. 55 (2007) 329e333. [12] R. Ondo-Ndong, G. Ferblantier, F. Pascal-Delannoy, A. Boyer, A. Foucaran, Microelectr. J. 34 (2003) 1087e1092. [13] Kaixin Chen, Kin Seng Chiang, Hau Ping Chan, Pak Lim Chu, Opt. Mater. 30 (2008) 1244e1250. [14] A.N. Banerjee, C.K. Ghosh, K.K. Chattopadhyay, Hideki Minoura, Ajay K. Sarkar, Atsuya Akiba, Atsushi Kamiya, Tamio Endo, Thin Solid Films 496 (2006) 112e116. [15] Yun-yan Liu, Yu-zhen Yuan, Xu-tuan Gao, Shi-shen Yan, Xin-zhong Cao, Gongxiang Wei, Mater. Lett. 61 (2007) 4463e4465. [16] Kevin F. Brennan, Joe Haralson II, Joseph W. Parks Jr., Ali Salem, Microelectron. Reliab. 39 (1999) 1873e1883. [17] Wei Gao, Zhengwei Li, Ceram. Int. 30 (2004) 1155e1159. [18] S.S. Tneh, Z. Hassan, K.G. Saw, F.K. Yam, H. Abu Hassan, Phys. B 405 (2010) 2045e2048. [19] X.Q. Wei, Z.G. Zhang, M. Liu, C.S. Chen, G. Sun, C.S. Xue, H.Z. Zhuang, B.Y. Man, Mater. Chem. Phys. 101 (2007) 285e290. [20] F.K. Yam, Z. Hassan, J. Optoelectron. Adv. M. 10 (2008) 545e548. [21] S. Liang, H. Sheng, Y. Liu, Z. Huo, H. Shen, J. Cryst. Growth 225 (2001) 110e113. [22] C.Y. Liu, B.P. Zhang, W. Lu, N.T. Binh, K. Wakatsuki, Y. Segawa, R. Mu, J. Mater. Sci. 20 (2009) 197e201.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Characteristics of ZnO MSM UV photodetector with Ni contact electrodes on poly propylene carbonate (PPC) plastic substrate N.N. Jandow*, F.K. Yam, S.M. Thahab, H. Abu Hassan, K. Ibrahim Nano-Optoelectronics Research and Technology, Laboratory School of Physics, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 April 2010 Received in revised form 22 May 2010 Accepted 24 May 2010 Available online 1 June 2010
In this study, we report the fabrication of ZnO metal-semiconductor-metal UV photodetector (MSM UV PD) by deposition ZnO thin film on poly propylene carbonate (PPC) plastic substrate using direct current (DC) sputtering technique, and Nickel (Ni) contact as electrodes. The structural, optical and electrical properties of the ZnO thin film were investigated by using atomic force microscopy (AFM), X-Ray diffraction (XRD) measurement, and photoluminescence (PL). The electrical characteristics of the detector were investigated using the currentevoltage (IeV) measurements, the dark- and photo-currents were found to be 1.04 and 93.80 mA, respectively. Using forward dark conditions at 5 volt; the barrier height FB was calculated to be 0.675 eV. Under incident wavelength of 385 nm, it was found that the maximum responsivity (R) of the Ni/ZnO/Ni MSM PD was found to be 1.59 A/W. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: ZnO DC sputtering Barrier height Metalesemiconductoremetal (MSM) photodiodes
1. Introduction Detection of ultraviolet (UV) radiation is an important feature for many applications such as environmental, military, and industrial fields, for instance: flame monitor, water purification system, money counting, photochemical phenomena detection, etc. [1]. In recent years, many researchers have been focusing on semiconductor-based ultraviolet (UV) photodiodes, ZnO is one of the prominent semiconductor materials which can be used for UV light detection because of its wide band gap (Eg) (3.37 eV), large exciton binding energy of 60 meV, low cost and ease of manufacturing, in addition, it is also sensitive to the UV region [2], these properties make ZnO a promising photonic material for optoelectronic applications, including light-emitting diodes, laser diodes, and photodetectors for the UV spectral range [3]. ZnO can be grown by several methods such as radio-frequency magnetron sputtering (RFMS) [4], pulse laser deposition (PLD) [5], molecular beam epitaxy (MBE) [3], metal-organic chemical vapor deposition (MOCVD) [6], electrostatic spray deposition [7] and spray pyrolysis [8], SoleGel method [9]. ZnO Schottky diodes and MSM PDs detecting in the UV region have been investigated by many researchers like for example, Young et al. [10] fabricated MSM UV sensor by growing ZnO
* Corresponding author. E-mail address: [email protected] (N.N. Jandow). 1567-1739/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.05.012
epitaxial films on sapphire (0001) substrates with Ni contact electrodes by using RF plasma-assisted molecular beam epitaxy. MSM PD is a device with two back-to-back Schottky junctions formed on the same semiconductor surface. Under the application of a bias voltage, one of the contacts becomes reverse biased, forming a depletion region that will tend to sweep out photocarriers; the other becomes forward biased, allowing the collected photocurrent to flow out just as an ohmic contact. Under sufficiently high bias voltage, the depletion region extends and touches the small space-charge region under the forward biased electrode. To produce high-performance MSM UV PDs, it is important to achieve a large Schottky barrier height at the metalesemiconductor interface that leads to a small leakage current and high breakdown voltage which could result in improving responsivity and photocurrent to dark current contrast ratio [11]. To achieve a large Schottky barrier height on ZnO, one can choose metals with highwork functions such as gold (Au), platinum (Pt), palladium (Pd), and Nickel (Ni). Ni is an interesting metal that has recently been used as a stable Schottky contact due to its high metal work function. However, there are few publications on using Ni as ZnO MSM PD contacts. Many studies on ZnO-based MSM photodetectors using silicon [12], glass [8]. Sapphire substrates [10] or flexible substrates [13] such as polyethylene terephthalate (PET) [14], polytetrafluoroethylene (Teflon) [15] or other plastics have been reported, nevertheless the used of PPC plastic as a substrate has not been reported in the literature, to the best of our knowledge, we are the first group
N.N. Jandow et al. / Current Applied Physics 10 (2010) 1452e1455
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to used the PPC plastic as a substrate for fabrication ZnO MSM UV detector with Ni contacts. PPC has been selected as a substrate in this study because it is cheap, has low chemical reactivity. In addition, it also has a high dielectric strength over wide frequencies and high surface resistivity; these properties are attractive for PPC to serve as a good substrate for a variety of microelectronic applications. In this work, we report the deposition of ZnO thin film on the PPC plastic sheet, subsequently ZnO MSM PD with Ni electrodes is fabricated, eventually the responsivity of the detector is determined. The study shows that the fabrication of ZnO PD on PPC plastic with using Ni contacts is encouraging. Fig. 2. AFM image of ZnO on PPC substrate (RMS ¼ 10 nm).
2. Experiment work The ZnO thin films were deposited on PPC plastic by sputtering system (model Edwards Auto 306). The sputtering system operated with base pressure of about 5 105 torr; the target material used in this work was ZnO disk having a diameter of about 76.2 mm; it is thickness was around 6.35 mm; with purity 99.99%. The distance between the target and the substrates was around 7 cm. Prior to the deposition, the plastic substrates were cleaned thoroughly. The cleaning procedures started by immersing the plastic sheets in the active cleaning liquid (DECON90) for 5 min, then rinsed with distilled water, after that they were dried by N2 gas. Before the deposition process, the target was pre-sputtered for 20 min to remove any contamination from the target surface. After deposition, the thin film thickness was measured by using surface optical system Filmetric F20-VIS. The microstructure of the films was studied by imaging the morphology of the films surface using AFM. The crystalline structure of the film was studied by XRD using High resolution (XRD (PANalytical X’pert Pro MRD)) with a Cu-Ka1 A), while the photoluminescence radiation source (l ¼ 1.5406 spectrum (PL) was recorded by using photoluminescence spectroscopy system Model: Jobin Yvon HR 800 UV. The line of HeeCd laser of 320 nm having a power of 1 mW is used to excite the sample, and the transmission spectrum of the sample was measured using a Hitachi U-2000 Double-beam spectrophotometer and. Finally ZnO MSM PD was fabricated by sputtering the Ni with a thickness of about 130 nm on the ZnO thin film by using a metal MSM mask, The structure of ZnO MSM PD consists of two interdigitated Schottky contact (electrodes) with finger width of 0.2 mm; the length of each electrode is about 5.4 mm and the spacing between the electrodes is 0.4 mm, each electrode has five fingers as shown in Fig. 1. As we mentioned before, typically, the device is biased such that one of the two contacts is grounded while the other is reversed biased. At sufficiently high bias the depletion regions beneath the Schottky gates can be connected. The MSM device can be used as a PD by shining light on the top surface of the structure [16].
3. Results and discussion The thickness of the prepared film was found to be about 1mm; the film surface was examined by AFM as shown in Fig. 2. A typical AFM image shows that ZnO thin film consists of some columnar structure grains, The grain height was found to be in the range of 100 nm and the root mean square (RMS) is equal to 10 nm; this nanostructure of the film surface may be useful in the absorption of the UV light when this material is used as a UV detector. Fig. 3 shows the XRD diffraction pattern with 2q from 30 to 70 of ZnO thin film deposited on PPC plastic substrate. The strong peak observed at 2q ¼ 34.25 with a full width at half maximum intensity (FWHM) of 0.36 corresponds to ZnO (002) plane. And lower intensity (110) peak was also observed at 2q ¼ 52.7. The PL of the prepared ZnO films on PPC plastic was observed at room temperature. The result is given in Fig. 4. We can find two luminescence peaks. One is the UV emission of ZnO thin films at 380 nm corresponding to the near band edge emission (NBE) due to the electronic transition from near conduction band to valence band according to Gao and Li [17]; another luminescence peak is the blueegreen emission at 490 nm, which may be due to the defect related deep level emission [18] and in agreement with Wei et al., which is attributed to the transition of electron from defect level of Zn interstitial atoms to top level of valence band [19]. Fig. 5 shows the currentevoltage (IeV) characteristics of the Ni/ ZnO/Ni UV PD on PPC plastic measured without and with UV illumination. Under dark environments, the current at 5 V was found to be 1.04 mA, on the other hand, when the sample was illuminated with UV lamp with power of 58.40 mW the current (same voltage) was observed to be 93.80 mA. The UV light can effectively create electronehole pairs in ZnO and greatly increase the carrier density. Under UV illumination, light impinges onto the thin film, the highenergy photons will be absorbed by the ZnO layer producing more 3000
intensity(a.u.)
Z n O /P P C
(0 0 2 )
2500 2000 1500 1000 500
(1 1 0 )
0 30
40
50
60
2 θ (d e g .) Fig. 1. Schematic drawing of MSM interdigitated photodetector.
Fig. 3. X-ray diffraction for ZnO thin film on PPC substrate.
70
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50000
R(A/W)
Intensity (a. u.)
40000
ZnO/PPC
30000 20000 10000 0 300
400
500
600
700
1.8 1.6 Ni/PPC 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 320 340
Wavelength (nm)
360
380
400
420
440
λ (nm)
800
Fig. 6. Spectral responsivity of the fabricated Ni/ZnO/Ni MSM PD on PPC substrate.
Fig. 4. Room temperature PL spectrum of DC- sputtered ZnO on PPC substrate.
electronehole pairs, the application of a bias voltage to the Ni contact creates an electric field within the underlying ZnO thin film that sweeps the photogenerated carriers out of the depletion region resulting to increase the photocurrent with voltage. The contrast ratio of this photo to dark currents was 90.19 times. This electrical result is very encouraging for further study of photodetector. The MSM-PD performance is critically dependent on the quality of the Schottky contacts. Therefore, it is necessary to measure the Schottky barrier height of the actual contact, which is a constituent part of the MSM-PD under investigation. As we mentioned, the MSM- PD essentially consists of two Schottky contacts connected back-to-back. When a bias is applied, one of the Schottky contacts is forward-biased and the other is reverse-biased. Only the reverse currentevoltage (IeV) characteristics of these Schottky contacts can be measured. The Schottky barrier height can be determined by using forward-biased IeV measurements as in Eq. (1) [20]
I ¼ Io expðqV=fnKTgÞ½1 expð qV=fKTgÞ
(1)
I+ ¼ A* AT 2 expf qFB =ðKTÞg
(2)
where Io is the saturation current, n is the ideally factor, K is the Boltzmann’s constant, T is the absolute temperature, FB is the barrier height, A is the area of the Schottky and A* is the effective Richardson coefficient w32 A/cm2K2 [21]. Based on equation (1), the plot of lnfIexpðqV=fKT gÞ=½expðqV=fKT gÞ 1g vs. V will give a straight line; Io is derived from the intercept with y-axis when V ¼ 0, and from Eq. (2) we found that Schottky barrier height FB at the Ni/ZnO interface is equal to 0.675 eV. 1e-3
Ni-PPC 1e-4
One of the important parameters to evaluate a photodetector is the responsivity R, which is the ratio of the device photo-current to the incident optical power which is given by Eq. (4) [22]:
Rl ¼ Iph =Pl ðA=WÞ
(4)
where Iph is the photocurrent and Pl is the incident optical power. Fig. 6 shows the measured spectral responsivity of the fabricated Ni/ZnO/Ni MSM PD fabricated on the PPC plastic. As shown in the figure, the photodetector responsivity was nearly constant in a gradual leasing with values which were very close together in the below UV region w320e365 nm; while it was an obvious response in the UV region, peaking at 385 nm and began decreasing whenever became close to the visible region, we could explain these results as follow; when ZnO detector is irradiated by UV light with energy higher than the bandgap (3.37 eV for ZnO), electronehole pairs will generate, as a result these excess charge carriers contribute to photo current and result in the response to the UV light. The cut-off at wavelength of 385 nm is nearly close to the ZnO energy bandgap of 3.37 eV; the responsivity decreased at the shorter wavelength range due to the decrease of the penetrating depth of the light, resulting in an increase of the surface recombination [11]. With an incident UV wavelength of 385 nm and 5 V applied bias, it was found that the maximum responsivity of the fabricated Ni/ZnO/Ni MSM PD is 1.59 A/W. This high responsivity may be attributed to its high internal gain. According to Liu [1], this indicated that a Schottky barrier at the metal electrodeesemiconductor interface can exhibit hole-trapping in the reversed-bias junction that shrinks the depletion region and allows tunneling of additional electrons into the photoconductor; if electrons pass multiple times, this mechanism yields photoconductive gain greater than unity. The responsivity of our detector is comparable to other UV- detector reported by Liang et al. [21] who also found that responsivity of their MSM photodiode could reach 1.5 A/W by fabricated Ag/ZnO Schottky UV photodetector on Rplane sapphire substrate.
I (A)
1e-5
4. Conclusion
1e-6 1e-7 1e-8
dark current photo current 385 (nm )
1e-9 0
1
2
3
4
5
V(volt) Fig. 5. IeV characteristics of Ni/ZnO/Ni MSM PD on PPC substrate measured in (dark current) and (photocurrent).
In brief, we have fabricated metalesemiconductoremetal UV photodetector by deposition ZnO thin film on PPC plastic substrate. The characteristics of ZnO thin film were investigated by various tools. AFM image which shows that the ZnO thin film has a nanostructure surface with RMS of 10 nm, XRD spectrum showed that ZnO has preferential orientation along the c-axis with dominant peak observed at 2q ¼ 34.25 . PL measurement exhibited a strong UV emission peak at 380 nm. IeV measurements showed that the dark- and photo-current were observed to be1.04 and 93.80 mA at 5 V respectively and the contrast ratio of this photo to dark currents
N.N. Jandow et al. / Current Applied Physics 10 (2010) 1452e1455
was 90.19 times. In addition, the maximum responsivity of the Ni/ ZnO/Ni MSM PD was determined to be 1.59 A/W. The results showed that the fabrication of ZnO MSM UV photodetector on PPC plastic by using DC-sputtering technique is possible. Acknowledgments This work was conducted under the grant no.: 100/PFIZIK/8/ 4010 support from Universiti Sains Malaysia is gratefully acknowledged. References [1] K.W. Liu, J.G. Ma, J.Y. Zhang, Y.M. Lu, D.Y. Jiang, B.H. Li, D.X. Zhao, Z.Z. Zhang, B. Yao, D.Z. Shen, Solid-State Electron. 51 (2007) 757e761. [2] S.J. Young, L.W. Ji, S.J. Chang, Y.P. Chen, K.T. Lam, S.H. Liang, X.L. Du, Q.K. Xue, Y.S. Sun, IET Optoelectron. 1 (3) (2007) 135e139. [3] S.P. Chang, S.J. Chang, Y.Z. Chiou, C.Y. Lu, T.K. Lin, Y.C. Lin, C.F. Kuo, H.M. Chang, Sensor. Actuat. A 140 (2007) 60e64. [4] F.S. Mahmood, R.D. Gould, A.K. Hassan, H.M. Salih, Thin Solid Films 770 (1995) 376e379. [5] F. Danie Auret, W.E. Meyer, P.J. Janse van Rensburg, M. Hayes, J.M. Nel, Holger von Wenckstern, H. Schmidt, G. Biehne, H. Hochmuth, M. Lorenz, M. Grundmann, Phys. B 401e402 (2007) 378e381. [6] Y. Liu, C.R. Gorla, S. Liang, N. Emanetoglu, Y. Lu, H. Shen, M. Wraback, J. Electron. Mater. 29 (No. 1) (2000) 69e74.
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