Keywords: Thin films, Zinc Oxide, Sol-gel, XRD, Optical properties

Transcription

1 Advanced Materials Research Vol. 895 (2014) pp Online available since 2014/Feb/13 at (2014) Trans Tech Publications, Switzerland doi: / Structural and Optical Properties of Nickel-doped and Undoped Zinc Oxide Thin Films Deposited by sol-gel method Siti Hajar Basri 1, a, Mohd Arif Mohd Sarjidan 1,b and Wan Haliza Abd Majid 1,c 1 Low Dimensional Materials Research Centre, Physics Department, University of Malaya, Malaysia a hajarbasri@gmail.com, b mohd_arif5005@yahoo.com.my, c q3haliza@um.edu.my Keywords: Thin films, Zinc Oxide, Sol-gel, XRD, Optical properties Abstract ZnO thin films with and without Ni-doping were successfully deposited by sol-gel method with zinc acetate dihydrate as inorganic precursor, and nickel (II) acetate tetrahydrate as dopant. The solutions were prepared by dissolving zinc acetate and nickel (II) acetate in ethanol and diethanolamine (DEA) as its chelating agent. Thin films were fabricated by using spin-coating method on glass substrates. ZnO films were obtained by pre-heating and post-heating at 300 C for 10 minutes and 500 C for 1 h respectively. The films were analyzed by X-ray diffraction (XRD), UV-Vis transmittance and photoluminescence (PL). All samples exhibit high transparency in visible. Ni dopant does not alter so much ZnO structure, which due to the ion substitution between Ni and Zn. However, the Ni tends to create a dopant energy interlayer in ZnO energy band gap which cause significant change in PL intensity. Introduction Zinc oxide is a direct band gap semiconductor with a wide energy band gap of 3.3 ev and large binding energy of 60 mev. The major interest of this material is that they are highly transparent at visible range and highly conductive in electrical [1]. Thus, this makes them very potentially useful in various optoelectronic applications such as optical sensors, transparent electrode material and light emitter [2-4]. ZnO thin film can be fabricated using various techniques such as magnetron sputtering [5], metalorganic chemical vapor deposition (MOCVD)[6], atomic layer deposition [7], sol-gel method [8], and thermal evaporation [9].Of the various methods, the sol gel method is known as a simple technique, easy to control stoichiometry accurately over a large area, and relatively a low process temperature [10] with an acceptable cost. Recently, many groups have done research on effects of doping on properties of ZnO thin films [11, 12]. Doping with metal ions results an increased in conductivity and modify the energy band of the ZnO thim film [12]. Commonly, the aluminum (Al) ion was used as a dopant to produce AZO thin film [13]. In this present work, the effects of Ni-doping in ZnO on the structural and optical properties of the thin films prepared using sol-gel method were investigated. Experimental Sol solution are prepared with zinc acetate dihydrate as the zinc precursor, ethanol and diethanolamine (DEA) as solvent and additive respectively. The solutions were then doped with Ni + sources from nickel (II) acetate tetrahydrate of mol percentage varied from 0% to 3%. In 0.5 M of concentration, the solution was prepared by stirring at 60 C for one hour and aged for at least 24 hours prior deposited onto cleaned glass substrates. The glass substrates were ultrasonically cleaned in soap water, and then rinsed with water, acetone and ethanol, subsequently, and finally purged with nitrogen to dry. The Ni-doped and undoped ZnO sol-gel solution were then spin-coated onto the substrates with spin rate of 3000 rpm in 20 s. As-fabricated thin films were then preheated at All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: /03/14,05:45:21)

2 Transmittance (%) PL intensity (a.u) PL Ratio Advanced Materials Research Vol C for 10 minutes to remove residual organic solvent. The coating and heating procedures were repeated for five times to produce a thin film thickness of about nm. Post heating were then been carried out at 500 C for one hour in conventional furnace to obtain crystallized ZnO. The structures of thin films were studied by Siemens D5000 X-ray diffractometer (XRD), optical transmittance measurements were done with Perkin Elmer Lambda 750 UV/VIS/NIR Spectrometer and Perkin Elmer Luminescence Spectrometer LS50B for the emission characteristic. Results and Discussion Figure XRD patterns of the undoped and Ni-doped ZnO thin films Figure 1 shows the XRD patterns of undoped zinc oxide and nickel doped zinc oxide thin films. All samples give strong diffraction peaks at 2θ=31.70, and which corresponds to (100), (002) and (101) peaks of ZnO hexagonal wurtzite structure. The crystalline sizes of the undoped and ni-doped ZnO were determined by using the Scherrer formula, where λ is the X-ray wavelength of 1.54 Å, θ B is the Bragg diffraction angle of (101) peak, and β is the FWHM of θ B. The crystallite sizes are 42.47, 56.63, and nm respectively for 0%, 1%, 2% and 3% of nickel doping. Most diffraction peaks intensities increase with increasing Nidoping concentration. This shows an increase in crystalline quality especially in -doped ZnO. No diffraction peaks of NiO or any other impurities were observed in the 2θ region displayed indicating nickel substituting zinc in the hexagonal crystal lattice, leave no changes in the lattice structure (ahv) 2 (ev/cm) 3 3E+15 2E+15 1E E E E % of Ni E(eV) Wavelength l (nm) 5.00E E Wavelength, l (nm) (a) (b) Figure 2 (a) Transmittance spectra of undoped ZnO and Ni-doped ZnO thin films. Insert is plot of photon energy dependence of (ahv)2 of the thin films. (b) Photoluminescence spectra of undoped ZnO and Ni-doped ZnO thin films. Insert is ratio of PL intensity at 486 nm over at 410 nm

3 252 Solid State Science and Technology IV The transmittance spectra of undoped ZnO and Ni-doped ZnO thin films in Fig. 2(a). is clearly shows that all samples demonstrate a high transmittance (>85%) at wavelength higher that 400 nm. The number of fringe pattern of the samples is three and the peak-to-peak high is very small which is attributed to low in reflective index. However, transmittance peak of ZnO thin films were observed shifted to higher wavelength when doped with Ni. A rapid decrease of the spectra at wavelength lower that 380 nm related to the band edge absorption. Optical energy gap, E g can be estimated from Tauc relation: hv=b(hv-e g ) n where a, h, v E g and n are absorption coefficient, Planck s constant, frequency, respectively. Interception of an extrapolation plot of (αhv) 2 versus photon energy, hv (see insert Fig. 2(a).) gives the value of the E g of the thin film. All samples exhibit similar E g value of 3.28 ev, indicates that the energy band diagram was not significantly altered by the existing of such amount of Ni element in ZnO matrix, prepared by this sol-gel method. Fig. 2(b) shows the emission characteristic of the samples after exposed with excitation wavelength of 300 nm. All samples demonstrate a strong blue emission at 410 nm, however, the intensity was reduced by increased with dopant percentage. The emission at 486 nm is increased relatively (see insert Fig. 2(b)) as compared to emission at 410 nm which indicates that the Ni atom may create a secondary emission level where several amount of energy was transferred from exited state to this level before emission is released. Conclusion The effects of Ni doping on structural and optical properties of ZnO thin film have been investigated. Ni dopants up to 3% do not change the crystal structure on ZnO thin films where the 1% of Ni gives the best crystallinity. The pure and Ni-doped ZnO thin films exhibit a high transmittance films in the visible region. The sub-level created in the energy gap of the Ni-doped ZnO thin films were result on significant reduction on PL emission in blue light region and the increase in ratio of PL emission at 486 nm to 410 nm of wavelength. The conductivity study will be the future work. Acknowledgement This work was supported by ERGS ER A, UM.C/625/1/HIR/041 and UM/MOHE High Impact Research Grant Allocation (F ). References [1] Minami, T., H. Nanto, and S. Takata, Highly conductive and transparent zinc oxide films prepared by rf magnetron sputtering under an applied external magnetic field. Applied Physics Letters, (10): p [2] Service, R., Materials Science: Will UV Lasers Beat the Blues? Science, (5314): p [3] Jeong, S., et al., Deposition of aluminum-doped zinc oxide films by RF magnetron sputtering and study of their structural, electrical and optical properties. Thin Solid Films, (1): p [4] Makino, T., et al., Room-temperature luminescence of excitons in ZnO/(Mg, Zn) O multiple quantum wells on lattice-matched substrates. Applied Physics Letters, (7): p

4 Advanced Materials Research Vol [5] Lee, J.B., S.H. Kwak, and H.J. Kim, Effects of surface roughness of substrates on the< i> c</i>-axis preferred orientation of ZnO films deposited by rf magnetron sputtering. Thin Solid Films, (2): p [6] Gorla, C., et al., Structural, optical, and surface acoustic wave properties of epitaxial ZnO films grown on (011 2) sapphire by metalorganic chemical vapor deposition. Journal of Applied Physics, (5): p [7] Yamada, A., B. Sang, and M. Konagai, Atomic layer deposition of ZnO transparent conducting oxides. Applied Surface Science, : p [8] Alam, M. and D. Cameron, Preparation and properties of transparent conductive aluminumdoped zinc oxide thin films by sol gel process. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, (4): p [9] Fouad, O., et al., Zinc oxide thin films prepared by thermal evaporation deposition and its photocatalytic activity. Applied Catalysis B: Environmental, (1): p [10] Cheng, H.C., C.F. Chen, and C.Y. Tsay, Transparent ZnO thin film transistor fabricated by sol-gel and chemical bath deposition combination method. Applied Physics Letters, (1): p [11] Nunes, P., et al., Effect of different dopant elements on the properties of ZnO thin films. Vacuum, (3): p [12] Majumder, S., et al., Investigations on solution derived aluminium doped zinc oxide thin films. Materials Science and Engineering: B, (1): p [13] Kim, K.-T., et al., Characteristics of Nickel-doped Zinc Oxide thin films prepared by sol gel method. Surface and Coatings Technology, (22 23): p

5 Solid State Science and Technology IV / Structural and Optical Properties of Nickel-Doped and Undoped Zinc Oxide Thin Films Deposited by Sol-Gel Method /