Accepted Manuscript. Radio frequency magnetron sputtered ZnO/SiO 2 /glass: role of ZnO thickness on structural and optical properties

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1 Accepted Manuscript Radio frequency magnetron sputtered ZnO/SiO 2 /glass: role of ZnO thickness on structural and optical properties Alireza Samavati, Hadi Nur, Ahmad Fauzi Ismail, Zulkafli Othaman PII: DOI: S (16) /j.jallcom Reference: JALCOM To appear in: Journal of Alloys and Compounds Received Date: 27 December 2015 Revised Date: 10 February 2016 Accepted Date: 11 February 2016 Please cite this article as: A. Samavati, H. Nur, A.F. Ismail, Z. Othaman, Radio frequency magnetron sputtered ZnO/SiO 2 /glass: role of ZnO thickness on structural and optical properties, Journal of Alloys and Compounds (2016), doi: /j.jallcom This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

2 Radio frequency magnetron sputtered ZnO/SiO 2 /glass: role of ZnO thickness on structural and optical properties Alireza Samavati a*, Hadi Nur a, Ahmad Fauzi Ismail b, Zulkafli Othaman a a Centre for Sustainable Nanomaterials, Ibnu Sina Institute for Scientific and Industrial Research, Universiti Teknologi Malaysia, UTM Skudai, Johor, Malaysia b Advanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi Malaysia, Skudai, Johor, Malaysia * Corresponding alireza.samavati@yahoo.com Abstract Optically transparent zinc oxide (ZnO) thin films having thickness of 70, 150, 200 and 270 nm are deposited on heated glass substrates using the radio frequency (rf) magnetron sputtering method to examine their structural and waveguide properties. The XRD analyses show that the ZnO films on SiO 2 /Glass substrate are poly-oriented and have a hexagonal wurtzite structure. All thin films exhibit optical transmittance of >80% in the visible range. The increase in film thickness to 270 nm leads to growth of crystallinity and grain size to 35 nm, increase in the transmittance and refractive indexes for both TM and TE modes, and decrease in the bandgap energy. The room temperature photoluminescence (PL) exhibit UV (~ ev) and green (~2.37 ev) emission for all samples. However, the peaks intensity for UV emission gradually decreases which is attributed to decrease in the concentration of photo-carriers, their aggregations and larger amount of energy transfer to the down layer. Achieving the high optical transmittance of 95%, high refractive indexes and high level of crystallinity for 270 nm ZnO films prepared by socio economic rf magnetron sputtered make it appropriate transparent conductors for waveguide applications. The excellent features of the results suggest that our systematic growth and analysis methods may establish a basis for the tunable growth of ZnO/SiO 2 /glass hetero-structure thin films suitable in nanophotonics particularly waveguide application. Keywords: ZnO thin films, Optical properties, Structural analysis, Transmittance

3 1. Introduction ZnO is used in many optoelectronic devices such as light-emitting diodes [1], ultraviolet lasers [2], ultraviolet photoconductive detectors [3], optical waveguides [4] and optical storages [5] due to its unique properties such as direct band gap, wide band gap of 3.37 ev and a large exciton binding energy of 60 mev. In recent years, the ZnO thin film has attracted lots of attention due to its excellent optical and electrical properties. The possibility of controlling and tuning the optical properties such as changing the optical band gap, increasing the transmittance in the visible region, enhancing the excitonic emission of ZnO thin films by adjusting the growth condition make it potential candidate for various nanophotonic devices [6-8]. However, the structural and morphological evolution of ZnO/SiO 2 /glass and their optical properties for waveguide application at different thicknesses of ZnO top layer is not thoroughly scrutinized. Varieties methods such as sol-gel [9], spin coating [10], MOCVD [11], thermal evaporation [12], and pulsed laser deposition [13] are used for fabrication of ZnO thin film on different kind of substrates. However, in comparison, rf and dc magnetron sputtering among the above methods are more economical and safer methods for large scale production. In the present communication, we have fabricated ZnO/SiO 2 thin films on glass substrate by an efficient and easy fabrication method using rf magnetron sputtering as a prerequisite for a waveguide structure. The aim of this study is to provide some insight on the ZnO thickness dependent growth morphology, structural and optical properties of samples, applicable for waveguide devices. 2. Experimental ZnO/SiO 2 films are prepared by rf magnetron sputtering (HVC Penta Vacuum) on glass substrates using high purity ZnO and SiO 2 target of 3 inches diameter. Before deposition, the substrates have been rinsed by acetone, ethanol and deionized following by water ultrasonic bath for 10 min. The hetero-structure is grown as follows: initially a ~50 nm thickness layer of SiO 2 on substrate, and then a ZnO layer with different thicknesses of ~ 70, 150, 200 and 270 nm of ZnO on top of the SiO 2 layer are sputtered. The thickness of films are measured by spectroscopic reflectometry (Filmetrics).The rf power and deposition time for SiO 2 are 250 W and 60 min and for ZnO are 175 W and 18, 38, 50 and 60 min for 70, 150, 200 and 270 nm thickness respectively. The Ar flow rate and substrate temperature are kept constant at 10 sccm and 350 C

4 respectively. The working pressure in the growth chamber is Torr. The substrates are fixed at a distance of 12 cm above the targets. Mechanical rotary pump and turbo pump are used to evacuate the sputtering chamber to its ultimate pressure of about Torr. The samples are kept inside the chamber to cool to room temperature after growth. An AFM built by Seiko Instrument Inc. (SPI3800) is used to study the surface morphology, whereas a structural details of the samples are studied by X-ray diffraction (XRD) (D8 Advance Diffractometer, Bruker, USA ) using Cu-Kα radiation (0.154 nm) at 40 kv and 100 ma. The 2θ range is set to with a step size of 0.02 and a resolution of 0.011ᵒ. The field-emission scanning electron microscope (FESEM, JEOLJSM 6380LA) attached with enegy dispersive X- ray spectroscopy (EDX) are employed for observing layer formation and elemental analysis. The transmittance is recorded by a spectrophotometer (UV-3600). Optical waveguide properties are studied by prism coupling method (Metricon Model 2010 prism coupler). In the prism coupling experiment, a TiO 2 (rutile) prism and Otto configuration are used. Photoluminescence (PL) spectra are recorded at room temperature using a luminescence spectrometer (LS 55, Perkin Elmer, USA) under 239 nm excitation wavelenght. 3. Results and discussion The structural analysis of ZnO thin films having different thicknesses is carried out using XRD, varying the diffraction angle, 2θ from 20 to 80. The XRD patterns of the films are depicted in figure 1. The XRD patterns of the films indicate the existence of a ZnO single phase with a hexagonal wurtzite structure found in the standard reference data (JCPDS ). All the films have polycrystalline structure with orientation along with different planes. The most intense peaks are related to (002), (100) and (101). The same crystal structure is observed for all the films but the full width at half maximum (FWHM) and intensities values of these peaks changed with different film thicknesses. By increasing the film thickness the diffraction peak intensities increases and the FWHM decreases which indicates the improvement of crystallinity and relaxation of tensile strain. The best crystallinity is seen in 270 nm film thickness. For different film thicknesses, the observed d values, lattice parameters, (h,k,l) miller indices, FWHM for the most intense peak (101), dislocation density, tensile strain, grain size and diffraction angle (2θ) of the films are tabulated in table 1. The Williamson-Hall (W-H) plot is used to estimate the grain size and tensile strain of samples (see following equation) [14].

5 β exp erimentcosθ = λk + 4ε sinθ D Where β experiment is the FWHM of the XRD peaks, k is a constant equal to 0.94, λ is the X-ray wavelength used, β and θ are the angular line width at half-maximum intensity in radians and Bragg's angle respectively. However, due to space constraints in this article, those plots are not presented here, rather the calculated strain and grain size are presented in table 1 and method is explained in our previous study [15]. For the ZnO SiO 2 interface the Zn atoms bind with two O atoms. However, the O- atoms of down sites cannot bind with Zn atoms and instead may create defect sites in the interface between the SiO 2 and ZnO together with the lattice mismatch between them causes the large amount of compressive strain for thinner films. At higher thickness the influence of epitaxial strain could be minimal and leads to the lattice relaxation process which causes the peaks shifted to higher angles close to bulk value. This is interpreted in terms of change in interatomic distances and lattice contracted. Furthermore, the grain size of the thin films is also calculated by XRD patterns using Debye Scherrer's formula [14], D = 0.9λ β cos θ D, λ, β, and θ value are explained above. We calculate the grain size and dislocation density of the films using the FWHM of (101) peak obtained through the Scherrer's method. Additionally, to have more information on the amount of defects in the films, the dislocation density (δ) is estimated by the following formula [16], δ = 1 D 2 Therefore, larger film thickness resulted in appearance of larger grain size leads to achieve smaller β and δ values. This indicates the better crystallization of the films, lower defect and miner dislocation density which is clearly seen in table 1. The AFM images scanned in contact mode over a surface area of nm 2 for the ZnO thin films prepared at different thicknesses are represented in figure 2. For ZnO thickness of 70 nm, a relatively 2-D film consist of a few dots in compare with other samples is formed. This might be occurred due to insufficient ZnO atoms and nucleation sites to form dots but for ZnO thickness of 270 nm, dots coalesced together and it could be considered due to excess ZnO

6 atoms. The dependence of RMS surface roughness and the occupied area on ZnO thickness is shown in table 2. Increasing the thickness to 270 causes to increase the area occupied by grains and decrease the rms roughness after getting maximum at 200 nm thickness sample. The material deposition and thin film growth are referred to the process of generation of the desired atomic species at a target, transportation through the surrounding medium and condensation of the deposited material at the substrate. The trend of dots formation with increasing ZnO thickness is explained in schematic diagram in figure 3. Initial stage is the condensation and surface diffusion of atoms and nucleation of isolated clusters lead to formation of very low density QDs on the surface with average size of ~8 nm (figure 2a). Further increase of deposited material reduces the strain in the interface area of QDs and SiO 2 sub-layer which is explained earlier. According to Li et al. reducing the strain energy causes to reduce the total free energy E and improves thermodynamic stability [17] of growth clusters (figure 2 b). Continuing the sputtering process resulted in coalescence of clusters with close proximity and possible formation of polycrystalline QDs having ~ 25 nm size which is clearly seen in corresponding AFM images of figure 2c. Then, continuous development of the film and local epitaxy in columnar grains are taken placed. Finnaly, by increasing the ZnO thickness to 270 nm grain coarsening and competitive grain growth is occurred and renucleation on previously developed grains cause to enhance the QDs density and occupied area as well as increase the size of QDs (~ 35 nm) as can be seen in figure 2d. The surface morphologies of the ZnO films having different thicknesses of 70, 150, 200 and 270 nm are illustrated in figure 4a, b, c and d, respectively. The inset of figures 4b and d indicate the high magnification of selected area and the inset figure 4c reveal the EDX spectra for 200 nm thickness sample. It is clearly seen that with the thickness increasing the morphology of ZnO thin films have a large difference. The average grain sizes of the films measured by FESEM observation are also listed in table 1. The average grain sizes analyzed by XRD method (Scherer and W-H) are slightly smaller than those by FESEM observation because the lattice intrinsic dislocation and defect causes broadening of the XRD peaks. The EDX spectrum indicates that the layers are composed of Zn, O, Si, Au and C. A weak peak for C and Au are attributed to the supporter carbon tape and Au thin coating for FESEM image purpose. Figure 5 illustrates the transmittance spectrum as a function of wavelength for the samples with different ZnO thicknesses. The average transmittance is observed to be more than 80% in

7 the visible range for all the samples. Due to the band gap absorption, the transmission decreases sharply near low wavelength region. As seen from this figure, the optical transmittance of the films increases with thickness increases, which can be explained by the fact that the crystallite size increases significantly with increasing the thickness leads to reducing the grain boundary scattering and increasing the optical transmittance. These findings are in good agreement with the findings of Meng et al. [18]. It is also detected that the cut-off wavelength shifts towards higher wavelength and optical band gap changes towards a smaller energy value when the thickness increases. It is possible to describe this phenomenon by Burstein-Moss (B-M) effect [19, 20]: E B M g = 2 h 2m c 1+ m 3 m v v 2 ( π ne) 2 3 where n e is the carrier concentration, m c and m v are the conduction and valence band effective masses, respectively. The B-M effect results from Pauli Exclusion Principle. With the increment of the thickness, carrier concentration increases, resulting in the Fermi energy level is increased in the conduction band and the forbidden band is narrowed, consequently the cut-off wavelength of the films shift to the red side. The relationship between absorption coefficient (α) and optical transmittance (T) of the films is determined by the following formula [21]: Ln α = ( 1 ) l T where l is the film thickness, then the curves of (αhv) 2 as a function of hν is plotted for 70 and 270 nm thickness samples and shown as inset figure 5. Since ZnO film is direct transition semiconductor, the optical energy gap (E g ) could be estimated by extrapolating the straight line portion at (αhν) 2 = 0 [22], as displayed in inset figure 5. The E g values are ~3.23, ~3.21 ev for the 70 nm and 270 nm samples, respectively. The derivative of the transmittance with respect to energy (dt/de) which experimentally validated is used for further confirmation of bandgap energy [23, 24]. Figure 6 shows the dt/de curves of ZnO thin films with different thicknesses. Bandgap values of 3.23 and 3.21 are obtained for thin films with 70 and 270 nm thickness respectively which is in consistent with transmittance results. This decrease in the bandgap by increasing thickness is attributed to

8 widening of the valence and conduction bands along with an increase of grain size as revealed by FESEM micrographs and AFM images. The schematic diagram of the prism coupling experimental setup is demonstrated in figure 7a. The small air gap between the film and the base of a rutile prism is created. A laser beam incidences the base of the prism and normally is reflected totally at the prism base on to a photodetector. At certain incident angles, photons can tunnel across the air gap into the film and enter in to a guided mode, causing a sharp drop (corresponding to a dark mode) in the intensity of reflected light. Figures 7b and c show the relative intensity of the TE and TM polarized light at 633 nm respectively. They are reflected from the prism as a function of effective refractive index of the dark modes for the sputtered film waveguide at different thicknesses. Here, TM and TE polarization corresponding to extraordinary refractive index (n e ) and the ordinary refractive index (n o ) of the ZnO film waveguide, respectively. For the two cases, a sharp dips accompanied by small dips are observed and their effective refractive indices are found to be higher than that of the SiO 2 sub-layer (n sub = 1.468). This suggests that the dips correspond to real guided modes, where the light is well confined. The measured effective refractive indices of the TM and TE modes for sputtered ZnO film waveguides at different thicknesses are shown in figure 7d. The effective refractive indices of the guided mode increase with the increase of ZnO thickness. The reason is the higher the thickness, the better the crystallization; therefore, the refractive indices of the film are more close to those of bulk ZnO crystal (n o bulk = 1.988, n e bulk = 2.005). The room temperature photoluminescence (PL) emission measurements are performed to exhibit the ability of the sputtered ZnO thin film to serve as thin film photonic devices. Figure 8 shows the room-temperature PL spectrum of the ZnO thin film at different thicknesses. The spectra clearly presents UV centered at ~ 3.23 ev along with green emissions at ~2.37 ev. The free exciton transitions from the bottom of the conduction band to the valence band [25] are responsible for UV emission. The green emission is originated from the recombination of photo carries at local level in band gap. Therefore, the origin of these peaks may be resulted from the emission trough variation of the intrinsic defects in ZnO film, such as zinc vacancy V Zn, oxygen vacancy V O, interstitial zinc Zn i, interstitial oxygen O i, and anti-site oxygen O Zn which is shown as inset figure 8. The energy interval from the bottom of the conduction band to the anti-site defect O Zn level ~2.38 ev is approximately consistent with the energy of the green emission observed in our experiment. The energy interval between the bottom of the conduction band and the Oi level ~2.28 ev also conforms to the green emission, but the probability of forming O i is little due to large diameter of oxygen atom.

9 The PL intensity for UV emission spectra shows continuous decrement going from 70 to 270 nm thickness. Larger thickness leads to decrease of the photo generated carriers due to a higher probability of recombination. In addition, aggregation of QDs is increased by increasing layer thickness and causes to increase light absorption. Moreover, agglomeration decreases the ZnO/SiO 2 interfacial area, thus charge transfer is decreased due to increasing the exciton recombination [26]. Therefore, all phenomena explained above results in decrement of UV emission intensity. ZnO QDs with different shapes and size have several energy levels and different confinement energies. The broadening of the UV emission (see FWHM in inset figure 8) upon increasing thickness could be attributed to increasing the QDs size inhomogeneity having different confinement energies. The observed red shift ~0.03 ev in the PL peak for 70 nm thickness compare to 270 nm one is attributed to weak quantum confinement effect of photo carrier related to increase of average width and height of ZnO QDs on the surface. There is no significant change in the green emission when the thickness increases which confirms the different origination of this peak with UV. 4. Conclusion The effect of film thickness on the structural and optical properties of rf magnetron sputtered ZnO thin films is investigated. XRD results reveal that all the ZnO films have polyoriented hexagonal wurtzite structure. In addition, increasing the film thickness causes increasing the crysttalinity and grain size which also confirmed by AFM and FESEM images. The increase of the thickness tends to reduce the tensile strain in the film. The appearance of UV and green emissions are related to the free exciton transitions from the bottom of the conduction band to the valence band and energy interval from the bottom of the conduction band to the interstitial level of anti-site defect O Zn respectively. The photoluminescence results indicate that the ZnO thickness can tune the luminescence efficiency of the samples. ZnO/SiO 2 /Glass hetero-structure thin films show excellent optical waveguiding properties. The effective refractive indices of the 270 nm film are n o =1.985 and n e =1.998 close to those of bulk ZnO crystal. Optical waveguides with such properties can already be practically used in systems of integrated optics.

10 Acknowledgments The authors gratefully acknowledge funding from Universiti Teknologi Malaysia (UTM) through Research Grant Vote number R. J F626. Dr. Samavati is thankful to TNCPI for visiting research grants. References [1] S. Chu, J. Zhao, Z. Zuo, J. Kong, L. Li, J. Liu, Enhanced output power using MgZnO/ZnO/MgZnO double heterostructure in ZnO homojunction light emitting diode, Journal of Applied Physics, 109 (2011) [2] Y.N. He, C. C. Zhu, J.W. Zhang, The study on mechanism of ultraviolet laser emission at room temperature from nanocrystal thin ZnO films grown on sapphire substrate by L-MBE, Microelectronics journal, 35 (2004) [3] Q. Xu, J. Zhang, K. Ju, X. Yang, X. Hou, ZnO thin film photoconductive ultraviolet detector with fast photoresponse, Journal of Crystal Growth, 289 (2006) [4] N. Mehan, M. Tomar, V. Gupta, A. Mansingh, Optical waveguiding and birefringence properties of sputtered zinc oxide (ZnO) thin films on glass, Optical Materials, 27 (2004) [5] K. Hui, C. Lai, H. Ong, Electron-beam-induced optical memory effect in metallized ZnO thin films for the application of optical storage, Thin Solid Films, 483 (2005) [6] L. Xu, G. Zheng, J. Miao, J. Su, C. Zhang, H. Shen, L. Zhao, Regulating effect of SiO 2 interlayer on optical properties of ZnO thin films, Journal of Luminescence, 136 (2013) [7] J.-H. Kim, M. Lee, T.Y. Lim, J.-H. Hwang, E. Kim, S.H. Kim, Fabrication of transparent superhydrophobic ZnO thin films by a wet process, Journal of Ceramic Processing Research, 11 (2010) [8] L. Zhao, J. Lian, Y. Liu, Q. Jiang, Structural and optical properties of ZnO thin films deposited on quartz glass by pulsed laser deposition, Applied surface science, 252 (2006) [9] M. Kamalasanan, S. Chandra, Sol-gel synthesis of ZnO thin films, Thin Solid Films, 288 (1996)

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12 [24] C. Parker, J. Roberts, S. Bedair, M. Reed, S. Liu, N. El-Masry, L. Robins, Optical band gap dependence on composition and thickness of InxGa 1-x N (0< x< 0.25) grown on GaN, Applied Physics Letters, 75 (1999) [25] A. Samavati, Z. Othaman, S. Ghoshal, M. Mustafa, The influence of growth temperature on structural and optical properties of sputtered ZnO QDs embedded in SiO 2 matrix, Superlattices and Microstructures, 86 (2015) [26] J. Yan, P.D. McNaughter, Z. Wang, N. Hodson, M. Chen, Z. Cui, P. O'Brien, B.R. Saunders, Controlled aggregation of quantum dot dispersions by added amine bilinkers and effects on hybrid polymer film properties, RSC Advances, 5 (2015) Figures: Intensity (a. u.) (002) (100) (101) (102) ZnO JCPDS (110) (103) (200) (112) (201) 270 nm 200 nm 150 nm 70 nm Fig θ (degree)

13 (a) (b) Fig. 2 (c) ZnO (d) Fig. 3

14 (a) 18 nm (b) O C KeV Fig. 4 Transmittance (%) O Zn 270 nm nm 150 nm 70 nm Si At% Mass% Si Zn O Au C Au (αhν) 2 (10 10 cm -2 ev 2 ) (c) 8.0x x x x10-4 Wavelength (nm) 18 nm 34 nm 38 nm hν (ev) 270 nm 70 nm 20 nm 100 nm (d) 100 nm Fig. 5

15 dt/de Fig. 6 Laser 270 nm 200 nm 150 nm 70 nm 3.23 ev 3.21 ev Energy (ev) Sample Reflected intensity (a. u.) Prism TM 0 Modes θ Glass Reflected intensity (a. u.) TE 0 Modes Effective index 270 nm 200 nm 150 nm 70 nm Effective index ZnO SiO 2 Coupling head Photodetector 270 nm 200 nm 150 nm 70 nm (c) (a) Refractive index TM TE Thickness (nm) (b) (d) Fig. 7

16 Fig. 8 Tables: Table 1 Thickness (nm) Table 2 PL Intensity (a. u.) hkl (100) (002) (100) (002) (100) (002) (100) (002) λ Excitation =239 nm 3.23 ev 3.20 ev 0.03 ev (Violet) Zn i (IR) 2.90 ev ± Wavelength (nm) FWHM (deg.) hkl (101) d hkl (nm) ± VO 1.62 ev 270 nm 200 nm 150 nm 70 nm (Green) O i 2.28 ev a, c (nm) ± a= c= a= c= a= c= a= c= O Zn 2.38 ev 2.37 ev V Zn 3.06 ev FWHM (nm) (UV) 3.23 ev Ec Ev δ ε ± ± Thickness (nm) rms(nm) Number density cm -2 Ratio of grain area (%) Grain size (nm) Scherrer W-H AFM FESEM

17 Figure captions: Fig. 1 X-rad diffraction patterns of ZnO thin film at different thicknesses. Fig. 2 3D AFM images of ZnO thin film having 70 (a), 150 (b), 200 (c) and 270 nm thickness (d). Fig. 3 Schematic diagrams of various stages for ZnO thin film formation by increasing deposition time (ZnO thickness). Fig. 4 Top view FESEM images of 70 (a), 150 (b), 200 (c) and 270 nm (d) ZnO thickness. Inset figures a and d shows high magnification of selected area. Inset figure c indicates EDX spectra of sample having 200 nm ZnO thickness. Fig. 5 Spectral transmission characteristics of a ZnO thin film having different thicknesses. Inset shows the plot of (αhν) 2 ~hv for obtaining optical band gaps of the 70 and 270 nm thickness samples. Fig. 6 Plots of the derivative of the transmittance with respect to energy of ZnO thin films having different thicknesses. Fig.7 Schematic diagram of the prism coupling experimental setup (a) relative intensity of the TE (b) and TM (c) polarized light at 633 nm reflected from the prism versus the effective refractive index of the dark modes for the ZnO film having different thicknesses and the measured effective refractive indices of TE and TM guided modes in the film having different thicknesses (d). Fig. 8 Room temperature PL spectra of samples at different ZnO thicknesses. Inset indicates the schematic band diagram based on PL data. Table captions: Table 1 The thickness dependent of XRD peak position, lattice parameters, FWHM of (101), strain, density of dislocation and grain size obtained with different methods. Table 2 rms roughness, number density and ratio of grail area as a function of ZnO thickness.

18 1. The role of ZnO thickness on structural and waveguide properties of ZnO/SiO 2 /glass 2. Increasing the grain size, crystallinity, transmittance and refractive indices through increasing the ZnO top layer thickness 3. The appearance of UV and Green photoluminescence band accompanied by shift for UV emission through increasing the grain size