Journal of Alloys and Compounds 588 (2014) 170–176
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Nanostructured CexZn1xO thin films: Influence of Ce doping on the structural, optical and electrical properties R. Mariappan a,b,⇑, V. Ponnuswamy a, P. Suresh c, R. Suresh a, M. Ragavendar d, A. Chandra Bose e a
Department of Physics, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore 641 020, Tamil Nadu, India Department of Physics, Adhiyamaan College of Engineering, Hosur 635 109, Tamil Nadu, India c Materials Research Centre, Indian Institute of Science, Bangalore 560 012, India d Department of Physics, KPR Institute of Engineering and Technology, Coimbatore 641 407, Tamil Nadu, India e Department of Physics, National Institute of Technology, Tiruchirappalli 620 015, Tamil Nadu, India b
a r t i c l e
i n f o
Article history: Received 30 March 2013 Received in revised form 23 October 2013 Accepted 28 October 2013 Available online 1 November 2013 Keywords: Nebulizer spray pyrolysis X-ray diffraction Scanning electron microscopy Optical properties Electrical properties
a b s t r a c t Thin films of CexZn1xO thin films were deposited on glass substrates at 400 °C by nebulizer spray pyrolysis technique. Ce doping concentration (x) was varied from 0 to 10%, in steps of 2.5%. X-ray diffraction reveals that all the films have polycrystalline nature with hexagonal crystal structure and high preferential orientation along (0 0 2) plane. Optical parameters such as; transmittance, band gap energy, refractive index (n), extinction coefficient (k), complex dielectric constants (er, ei) and optical conductivity (rr, ri) have been determined and discussed with respect to Ce concentration. All the films exhibit transmittance above 80% in the wavelength range from 330 to 2500 nm. Optical transmission measurements indicate the decrease of direct band gap energy from 3.26 to 3.12 eV with the increase of Ce concentration. Photoluminescence spectra show strong near band edge emission centered 398 nm and green emission centered 528 nm with excitation wavelength 350 nm. High resolution scanning electron micrographs indicate the formation of vertical nano-rod like structures on the film surface with average diameter 41 nm. Electrical properties of the Ce doped ZnO film have been studied using ac impedance spectroscopy in the frequency range from 100 Hz–1 MHz at different temperatures. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Zinc Oxide (ZnO) semiconductor thin film has great potential for optoelectronic devices fabrication. It has direct wide band gap energy 3.3 eV and hexagonal wurtzite structure with large exciton binding energy 60 meV. ZnO thin films have high conductivity and transparency in the visible region. The one-dimensional (1D) ZnO nanomaterials, such as nanowires or nanorods, are especially attractive due to their tunable optical [1], electrical [2], structural [3], gas sensing [4], piezoelectric [5] and photo electrochemical properties [6]. Rare earth materials have been immensely used to modify the properties of functional materials due to their unique physicochemical activity [7]. Cerium is a major element in the useful rare earth family. Pure and Ce doped ZnO thin films can be used to develop devices like; ultraviolet (UV) light emitters [8], piezoelectric devices [9], chemical sensors [10], heat mirrors [11], gas sensors [12–14], Schottky diodes [15], optoelectronic devices [16,17], solar cells [18] and civil and military applications [19].
⇑ Corresponding author at: Department of Physics, Adhiyamaan College of Engineering, Hosur 635 109, Tamil Nadu, India. Tel.: +91 4344261016; fax: +91 4344260573. E-mail address:
[email protected] (R. Mariappan). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.10.210
Thin films of Ce doped ZnO have been grown using a variety of deposition techniques, such as; sol–gel spin coating [20], electrodeposition [21], sputtering [22] and spray pyrolysis techniques [23–25]. The technique used in this work is known as Nebulizer Spray Pyrolysis (NSP) technique which is one of the most widely used techniques. It is a very easy, low cost, safe, and vacuum less system of deposition technique for preparing transparent conducting oxides compared with other techniques. It can be adapted easily for production of large area uniform film coatings. The aim this work is to study the structural, optical, surface and electrical impedance properties of CexZn1xO films deposited by NSP technique at an optimized substrate temperature 400 °C with different Ce doping concentrations. The main objective is to employ the nebulizer spray pyrolysis technique to obtain ZnO nano-rods which provide appropriate characteristics required for the fabrication of optoelectronic devices.
2. Experimental details Thin films of CexZn1xO were prepared at different concentrations (x) from 0% to 10% with varied film thickness as 226 nm, 232 nm, 245 nm, 252 and 263 nm through Nebulizer Spray Pyrolysis (NSP) technique. The nebulizer spray pyrolysis experimental set-up and the details of the procedure for the deposition CexZn1xO thin films have been described elsewhere [26]. Thin films of CexZn1xO were
171
R. Mariappan et al. / Journal of Alloys and Compounds 588 (2014) 170–176 chemically deposited using aqueous solutions of zinc acetate dehydrate and cerium nitrate hexahydrate. The spray solution was prepared with 0.1 of M zinc acetate dehydrate dissolved in 25 ml of de-ionized water and stirred for 10 min using magnetic stirrer. 5 ml of acetic acid and 25 ml of methanol were mixed in separate beakers and the solutions were added to the starting solution slowly from a burette held vertically with zinc acetate solution until the pH value reached 7. Similarly, 0.1 M cerium nitrate hexahydrate solution was prepared. Cerium nitrate hexahydrate solution was added to the above solution with (Ce/Zn) nominal volume proportions 0%, 2.5%, 5%, 7.5% and 10%. Stirring was continued for 30 min to get a clear and homogeneous solution. The prepared solution was sprayed onto the ultrasonically cleaned glass substrates. The thickness of CexZn1xO films was measured using alpha step D-120 stylus profilometer. The structural properties of the nebulizer-sprayed CexZn1xO films were studied by X-ray diffraction (Bruker D8 advance X-ray diffractometer) with Cu Ka radiation (k = 0.154 nm) in the h/2h and the angle 2h ranged from 20° to 80° in steps 2°. The optical studies of the films were done with a Ultra Violet– Visible–Near Infra Red (UV–Vis–NIR) spectrophotometer (Model JASCO-V-570) in the range from 300–2500 nm. The room temperature photoluminescence (PL) spectrum was recorded with spectrofluorimeter (Fluorolog Model FL3-11). Surface morphology for the films were characterized via high resolution scanning electron microscopy (HRSEM, FEI Quanta FEG 200) applying 30.0 kV operating voltage. The composition of the films was analyzed by an energy dispersive analysis by X-rays (EDAX) equipped in HRSEM, for which a 30 kV accelerating voltage was employed with a 10 mm working distance. Impedance measurements were performed using a Frequency Response, Solartron, Model 1360 coupled with the Solartron Dielectric Interface, 1296. X-ray photoelectron spectroscopy (XPS) (VG ESCALAB Mark II), was used (the X-radiation source A1 Ka (hm = 1486.6 eV), target voltage 13 kV, target power 300).
3. Results and discussion Fig. 1a shows the X-ray diffraction (XRD) profiles of the nebulizer-sprayed CexZn1xO films deposited at an optimized substrate temperature 400 °C with different doping concentrations (x). It is observed that (Fig. 1a) pure ZnO films are polycrystalline in nature with hexagonal crystal structure having preferential orientation growth along (0 0 2) plane and other peaks are associated with the (1 0 0), (1 0 1), (1 0 2) and (1 0 3) planes. For Ce doped ZnO, no diffraction peaks of Ce or other impurity phases were found in our samples, suggesting that Ce4+ ions would uniformly substitute into the Zn2+ sites or interstitial sites in ZnO lattice. The observed diffraction peak patterns were compared with standard diffraction values of JCPDS (card No. 89-7102). Fig. 1b shows the XRD peak shift in the sprayed CexZn1xO thin films. It is very important to note that a slight increase of Ce concentration shift the position of (0 0 2) peaks to lower angles (Fig. 1b). It can be seen from (Fig. 1b) the intensity of the peak increases with increase of Ce doping concentration (x) from 0% to 10%. Taking the ionic radius into account, the radius of Zn2+ (0.074 nm) is lower than that of Ce4+ (0.102 nm) and the doping of Ce leads to the increase of the structural parameters [19]. The Ce has an important role in the growth of CexZn1xO nano-rods due to the specific interactions
Fig. 1. (a) XRD patterns of CexZn1xO films. (b) XRD peak position of the sprayed Ce doped ZnO films.
Table 1 Calculated the structural parameters of sprayed CexZn1xO thin films. S. No.
Doping concentration
2h
d-Spacing [Å]
FWHM [°2Th.]
(hkl) Crystal system
1
x = 0% Ce
31.783 34.418 36.264 56.560 62.879
2.815 2.603 2.475 1.910 1.466
0.112 0.114 0.136 0.091 0.159
100 002 101 102 103
31.749 34.386 34.489 36.209 47.502 62.820
2.818 2.605 2.604 2.478 1.912 1.478
0.187 0.091 0.068 0.114 0.114 0.159
100 002 021 101 102 103
2
x = 2.5% Ce
Lattice constant (a Å)
3.24977
3.24929
Lattice constant (c Å)
5.20181
5.20993
(Scherrer) Crystallite size (nm)
Dislocation density, d (1014 lin/m2)
Micro strain, e (104 lin2m4)
Standard intensity (Is)
Observed intensity (Io)
Texture coefficient (Tc)
77 76 64 99 61
1.692 1.723 2.456 1.012 2.694
4.390 4.100 4.653 2.305 2.908
55.755 39.339 100.00 19.719 25.325
04.700 100.00 75.510 30.380 14.390
0.07677 2.31503 0.68768 1.40604 0.51747
46 95 127 76 79 61
4.701 1.103 0.620 1.706 1.582 2.696
7.325 3.283 2.454 3.884 2.886 2.911
55.755 39.339 100.00 100.00 19.719 25.325
02.970 100.00 54.570 30.220 06.840 05.330
0.07989 3.81252 0.81845 0.45324 0.52022 0.31565
(continued on next page)
172
R. Mariappan et al. / Journal of Alloys and Compounds 588 (2014) 170–176
Table 1 (continued) S. No.
Doping concentration
2h
d-Spacing [Å]
FWHM [°2Th.]
(hkl) Crystal system
3
x = 5% Ce
31.766 34.403 34.507 46.226 47.527 62.816
2.816 2.604 2.603 2.477 1.911 1.478
0.130 0.091 0.068 0.228 0.114 0.136
100 002 021 101 102 103
31.742 34.392 34.496 36.209 47.530 62.835
2.819 2.605 2.404 1.247 1.911 1.477
0.112 0.068 0.045 0.114 0.159 0.364
100 002 021 101 102 103
31.764 34.392 34.497 36.214 47.525 62.806
2.817 2.605 2.604 2.478 1.911 1.478
0.093 0.068 0.045 0.136 0.228 0.225
100 002 021 101 102 103
4
5
x = 7.5% Ce
x = 10% Ce
Lattice constant (a Å)
3.30772
3.24862
3.2505
Lattice constant (c Å)
5.36201
5.20893
5.20974
(Scherrer) Crystallite size (nm)
Dislocation density, d (1014 lin/m2)
Micro strain, e (104 lin2m4)
Standard intensity (Is)
Observed intensity (Io)
Texture coefficient (Tc)
66 95 127 38 79 71
2.303 1.103 0.620 6.824 1.581 1.981
5.124 3.281 2.453 7.776 2.884 2.495
55.755 39.339 100.00 100.00 19.719 25.325
07.040 100.00 57.680 25.860 05.340 05.460
0.18987 3.82249 0.86735 0.38886 0.40720 0.32419
77 127 190 77 58 26
1.692 0.620 0.275 1.706 3.100 14.08
4.396 2.462 1.636 3.884 4.037 6.653
55.755 39.339 100.00 100.00 19.719 25.325
02.090 100.00 54.920 08.980 01.630 02.740
0.06596 4.47359 0.96652 0.15037 0.14546 0.19040
92 127 190 64 40 43
1.175 0.620 0.275 2.456 6.328 5.303
3.661 2.462 1.636 4.660 5.769 4.160
55.755 39.339 100.00 100.00 19.719 25.325
00.420 100.00 52.160 01.910 00.410 00.710
0.01439 4.85877 0.99699 0.03650 0.03974 0.05358
between the nuclei and the Ce. It is indicated that the films prepared at higher Ce doping concentration (10%) have higher crystalline quality with (0 0 2) preferred orientation. The structural parameters such as lattice constant, crystallite size, dislocation density, micro strain, and texture coefficient were calculated from XRD studies and listed in Table 1. It is observed that (Table 1) the dislocation density and micro strain decrease with increase of Ce doping concentration whereas the crystallite size increases along (0 0 2) plane. The lower dislocation density and micro strain indicate the crystallanality improvement in the film. The texture coefficient (Tc) values of CexZn1xO films calculated for (1 0 0), (0 0 2), (0 2 1), (1 0 1), (1 0 2) and (1 0 3) planes are listed in Table 1. The calculated texture coefficient values give some important structural information: (i) Tc[hi ki li] has to be bigger than 1 to determine the preferential orientation, (ii) if Tc[hi ki li] is approximately 1 for all the [hi ki li] planes considered at X-ray diffraction patterns then the films are randomly oriented, (iii) Tc[hi ki li] values higher than 1 indicate the abundance of grains in [hi ki li] direction and (iv) 0 < Tc[hi ki li] < 1 values indicate the lack of grains oriented in that direction [26]. Ce doped ZnO films have more grains along [0 0 2] direction, and there is lack of grains along [1 0 0], [0 2 1], [1 0 1], [1 0 2] and [1 0 3] directions. The increase in texture coefficient along (0 0 2) plane with doping concentration (x) from 0% to 10% indicates the crystalline nature improvement in the films. Fig. 2a shows optical transmittance spectra of the sprayed CexZn1xO films deposited at 400 °C with different Ce doping concentrations. All the films exhibit a transmittance above 80% in the UV–Vis–IR region from 330 to 2500 nm and a sharp absorption edge around 380–410 nm. It is observed that (Fig. 2a) the transmittance decreases with increase in Ce doping concentrations and film thickness which is similar to Ce doped ZnO thin films [27]. There is a shift in the absorption edge to longer wavelength side for CexZn1xO films. The absorption coefficient ‘a’ is calculated using the Beer–Lambert law [28].
a¼
2:303 log t
1 T
ð1Þ
The optical band gap energy of the CexZn1xO films was calculated from the relation, (aht) = A(ht Eg)1/2. The equation gives the band gap energy Eg, when the straight portion of (aht)2 versus (ht) plot is extrapolated to the point a = 0. The variation of (aht)2
Fig. 2. (a) Optical transmittance spectra of the sprayed CexZn1xO films. (b) Plots of (aht)2 versus (ht) of the sprayed Ce doped ZnO films.
versus (ht) plot of the CexZn1xO films deposited at 400 °C with Ce different doping concentration is shown in Fig. 2b. The optical band gap energy evaluated from the intercepts of the plots was
173
R. Mariappan et al. / Journal of Alloys and Compounds 588 (2014) 170–176
Fig. 4. PL spectra of the sprayed CexZn1xO films.
where TM and Tm are the maximum and minimum transmission values at a particular wavelength and
2
!1=2 3 1 1 5 s¼4 þ 1 Ts T 2s
ð4Þ
where s is the refractive index of the glass substrate is calculated using the above relation [29]. Ts is the transmission spectrum of glass substrate. The variation of refractive index with wavelength of the CexZn1xO films is shown in Fig. 3a. The refractive index (n) value increases with the increasing Ce concentration. It decreases from 3.64 to 1.9 over the spectral range from 450 nm to 770 nm and then become nearly constant with wavelength increase. The calculated refractive index values in the UV region are approximately close to the values reported [31,32]. The extinction coefficient (k) of the CexZn1xO films is calculated from following equation [33]. Fig. 3. The variation of (a) refractive index and (b) extinction coefficient with wavelength of the CexZn1xO films.
found to be decreased from 3.25 to 3.12 eV with increase of Ce doping concentration from 0% to 10%, which is similar to Ce doped ZnO thin film [29]. The refractive index (n) plays a very important role in optical communication and designing of the optical devices. Refractive index (n) can be calculated from transmission data using the following relation [30].
1=2 1=2 n ¼ N1 þ N21 s2
ð2Þ
2 TM Tm s þ1 þ N1 ¼ 2s 2 TMTm
ð3Þ
k¼
ak 4p
ð5Þ
where ‘a’ is the absorption coefficient and k is the wavelength. The wavelength dependence of extinction coefficient (k) of the films is shown in Fig. 3b. It is observed that (Fig. 3b) the extinction coefficient (k) is found to be minimum at low energy and increases with the wavelength. It has been concluded that when the incident light interacts with a material which has large amount of particles, the refraction will be high and hence the refractivity of the films will be increased. The frequency dependence of the complex dielectric constant is known to be a fundamental material property. The real (er) and imaginary (ei) parts of the complex dielectric constant are related to ‘n’ and ‘k’ values as follows [34].
er ¼ n2 k2
ð6Þ
Table 2 The variations of dielectric constant and optical conductivity for sprayed CexZn1xO thin films. S. No.
Doping concentration (%)
Reflectance (%)
Dielectric constant Real part (er)
1 2 3 4 5
x = 0.0% Ce x = 2.5% Ce x = 5.0% Ce x = 7.5% Ce x = 10.0% Ce
1 1.5 2 4 6
4.1 5.0 8.1 8.7 14.2
Optical conductivity Imaginary part (ei) 0.150 0.177 0.360 0.964 2.007
Real part (rr) (X-m)1 6
0.15 10 0.23 106 0.29 106 0.32 106 0.51 106
Imaginary part (ri) (X-m)1 0.42 104 0.51 104 1.03 104 1.66 104 3.43 104
174
R. Mariappan et al. / Journal of Alloys and Compounds 588 (2014) 170–176
Fig. 5. SEM images of the sprayed CexZn1xO films (a) EDAX spectrum of the sprayed Ce2.5Zn7.5O films.
175
R. Mariappan et al. / Journal of Alloys and Compounds 588 (2014) 170–176 Table 3 Atomic percentage of CexZn1xO thin films. S. No.
Doping concentration
Elements Weight%
1 2 3 4 5
ei ¼ 2nk
x = 0.0% Ce x = 2.5% Ce x = 5.0% Ce x = 7.5% Ce x = 10.0% Ce
Atomic%
O
Si
Ca
Ce
Zn
O
Si
Ca
Ce
Zn
22.90 20.92 21.71 19.00 17.27
33.24 25.17 21.88 17.00 16.14
08.20 05.84 06.47 05.47 05.83
– 02.62 03.76 05.82 07.05
35.66 45.45 46.19 52.71 53.71
45.64 44.50 43.55 41.36 36.93
32.27 30.36 25.29 19.26 18.46
06.09 04.96 05.18 04.95 05.28
– 01.82 03.31 5.28 6.84
15.99 18.36 22.67 29.15 32.49
ð7Þ
Table 2 shows the variation of the real and imaginary parts of the dielectric constant as a function of wavelength for CexZn1xO films. It is also observed that (Table 2) the values of real part are higher than that of the imaginary part and the complex dielectric constant values are increasing with the Ce concentration. The real and imaginary parts of the complex dielectric constant provide information about the electronic band structure [35]. Besides, the real (rr) and imaginary (ri) parts of optical conductivity values are determined by the following relations [34].
rr ¼ xei eo
ð8Þ
ri ¼ xer eo
ð9Þ
where ‘x’ is the angular frequency and ‘eo’ is the free space dielectric constant. The variation of the real and imaginary parts of optical conductivity with wavelength of the films deposited at different concentration (x) is shown in Table 2. It is seen from Table 2, the optical conductivity of CexZn1xO films increases with the increasing optical energy and which is due to the electrons excited by photon energy. It is well known that optical properties of polycrystalline films are strongly influenced by their structural characteristics. Fig. 4 shows the room temperature Photoluminescence (PL) spectra of the sprayed CexZn1xO films. For x = 0.0% Ce, the film indicates a PL peak in the broad UV emission peak centered at 398 nm (3.10 eV). When the doping concentration increases from 2.5% to 10%, the film exhibits three emission bands in the spectra: a sharp ultra-violet (UV) near-band emission peak centered around 398 nm (3.10 eV), a blue emission peak around 467 nm (2.65 eV) and a sharp visible deep-level green emission around 528 nm (2.34 eV) for the CexZn1xO films. The UV peak corresponds to emission due to band excitons, while the green emission mainly attributed to different intrinsic defects such as; zinc vacancy and oxide antisite defect forming local deep levels in the band gap. The intensity of peaks decreases with increase the doping concentration. Fig. 5 shows the high resolution scanning electron microscopy (HRSEM) images of the sprayed CexZn1xO films deposited at 400 °C with different Ce doping concentration. It is apparent from the electron micrograph that the change in morphology is strongly dependent on the Ce doping concentration of the films. It is also observed that (Fig. 5) the granular surface of ZnO films is smooth, dense and tiny porous with uniform plate-like nano-walls structures. The film with doping concentration 5% consists of irregularly shaped grains with average thickness 30 nm. Further increase of Ce doping concentration (7.5–10%), apparent smooth and uniform vertically grown isolated nano-rods like structures with average diameter 41 nm is obtained (Fig. 5). Energy dispersive analysis by X-rays (EDAX) spectra of the sprayed CexZn1xO films are shown in Fig. 5a. These spectra show the presence of expected elements Ce, Zn and O in the solid films with Si and Ca elements. These
Fig. 6. TEM image of 10% Ce doped ZnO thin film.
Fig. 7. (a) XPS survey spectrum of CexZn1xO films, (b) XPS Zn 2p3/2 and Zn 2p1/2 peaks and (c) XPS O 1s peaks.
elements Si and Ca may probably include from the glass which is used as substrate. The atomic percentage of CexZn1xO elemental compositions are presented in Table 3. The structure of the asgrown films in selected samples was also analyzed by TEM. Fig. 6 shows the TEM micrographs was prepared at 10% Ce doped ZnO thin films. TEM image of 10% Ce doped ZnO thin films consist of well dispersed hexagonal nanoparticles with diameter in the range 48–62 nm. The chemical composition and valance states of element present in the samples are analyzed by XPS. The survey spectrum of 10% Ce doped ZnO films deposited at 400 °C are shown in insert
176
R. Mariappan et al. / Journal of Alloys and Compounds 588 (2014) 170–176
indicates that the emission peaks are centered at 398, 467 and 529 nm. More electrons can be contributed by cerium dopants so that the radiative recombination of these excess excitons will lead to a green shift and broaden the UV emission peak. The low intensity green emission properly indicates the presence of less number of defects. HR-SEM shows that the Ce0.1Zn0.9O film is smooth with uniform nano-rod like structures having an average diameter 41 nm. It has been concluded that the highly textured CexZn1xO nano-rods show good optical properties at room temperature and these optical materials will find wide applications in the future. Acknowledgements The authors are very much grateful to the Sophisticated Test and Instrumentation Centre, IIT, Madras, Department of Physics, National Institute of Technology, Tiruchirappalli and Material Research Centre, Indian Institute of Science, Bangalore. References Fig. 8. Complex impedance of the CexZn1xO film.
Fig. 7a. The XPS peaks of Ce, Zn and O are clearly seen. The binding energy of Ce 4d5/2 level is located at 108.5 respectively. There is only very small amount of C on the surface of the films which may be absorbed from air. The high resolution scans of Zn 2p and O 1s shown in Fig. 7b–c. The peaks located at 1021 and 1044 eV correspond to Zn 2p3/2 and Zn 2p1/2, respectively. For 1S, two XPS peaks at 529.8 eV corresponds to the oxygen bonded as CeO2 and ZnO, the second peak at 531.5 is probably due to oxygen chemisorbed on the surface in the form of CO, CO2, etc. Compared with standard XPS energy peak location of Ce 4d5/2 at 108.2, the peak Ce 4d5/2 for the Ce4+ doped ZnO nanorods have a slightly blue shift, indicating that the distance between Ce and O in CeO2 has changed due to Ce4+ ions doped into ZnO lattices, which is similar to ZnO:Tb thin films [36]. Impedance measurements were done in the frequency range 100 Hz to 1 MHz at different temperatures. Fig. 8 shows the complex impedance spectroscopy of Ce0.1Zn0.9O film as a function of temperature. The impedance spectroscopy was carried out at different temperatures and at fixed AC voltage 100 mV. The bulk resistance is found to be decreased when the temperature is increased from 70 to 175 °C which suggests the lowering of barrier height due to the injection of charge carriers that enhances the electrical conduction at higher temperature. The decrement of resistance with the increase in temperature may be caused by the increase in thermal movement of the defects, especially the oxygen vacancies [37]. 4. Conclusions Pure ZnO films have hexagonal wurtzite structures with preferential orientation along (0 0 2) plane and stronger c-axis orientation perpendicular to the substrate. However, all doped films exhibit mixed phases of hexagonal and orthorhombic crystal structure with orientations along (0 0 2) and (0 2 1) planes. The optical transmittance of the layers is above 80% in the visible region, and the optical band gap energy values is found to decreased from 3.25 to 3.12 eV with an increase in Ce doping concentration. Optical properties show that the refractive index (n), extinction coefficient (k), complex dielectric constants, optical conductivity increase with Ce concentration in the visible region. The band gap energy of the films decreases as the crystallite size increases in the UV region where the CexZn1xO phase controls their optical properties. The optical properties study based on PL spectrum
[1] H.S. Kang, J.S. Kang, J.W. Kim, S.Y. Lee, J. Appl. Phys. 95 (2004) 1246. [2] S. Yoon, I. Huh, J.-H. Lim, B. Yoo, Curr. Appl. Phys. 12 (2012) 784. [3] A.M. Torres-Huerta, M.A. Dominguez-Crespo, S.B. Brachetti-Sibaja, H. Dorantes-Rosales, M.A. Hernandez-Perez, J.A. Lois-Correa, J. Solid State Chem. 183 (2010) 2205. [4] G. Agarwal, R.F. Speyer, J. Electrochem. Soc. 145 (1998) 2920. [5] T. Mitsuyu, S. Ono, K. Wasa, J. Appl. Phys. 51 (1980) 2464. [6] K. Keis, L. Vayssieres, H. Rensmo, J. Electrochem. Soc. 148 (2001) A149. [7] M. Anbia, S.E.M. Fard, J. Rare Earth. 30 (2012) 38. [8] A.C. Mofor, A.S. Bakin, B. Postels, M. Suleiman, A. Elshaer, A. Waag, Thin Solid Films 516 (2008) 1401. [9] S. Tuzemen, E. Gur, Opt. Mater. 30 (2007) 292. [10] Y. Li, J. Gong, Y. Deng, Sens. Actuators A: Phys. 158 (2010) 176. [11] D.P. Norton, M. Ivill, Y. Li, Y.W. Kwon, J.M. Erie, H.S. Kim, K. Ip, S.J. Pearton, Y.W. Heo, S. Kim, B.S. Kang, F. Ren, A.F. Hebard, J. Kelly, Thin Solid Films 496 (2006) 160. [12] N.H. Al-Hardan, M.J. Abdullah, A. Abdul, Aziz, Int. J. Hydrogen Energy 35 (2010) 4428. [13] P. Nunes, E. Fortunato, A. Lopes, R. Martins, Int. J. Inorg. Mater. 3 (2001) 1129. [14] G. Chunqiao, X. Changsheng, C. Shuizhou, Mater. Sci. Eng., B 137 (2007) 53. [15] E.V. Lavrov, Physica B 340–342 (2003) 195. [16] W.I. Park, G.C. Yi, J.W. Kim, S.M. Park, Appl. Phys. Lett. 82 (2003) 4358. [17] J. Yang, M. Gao, L. Yang, Y. Zhang, J. Lang, D. Wang, Y. Wang, H. Liu, H. Fan, Appl. Surf. Sci. 255 (2008) 2646. [18] E. Fortunato, A. Goncalves, A. Pimentel, P. Barquinha, G. Goncalves, L. Pereira, I. Ferreira, R. Martins, Appl. Phys. A – Mater. 96 (2009) 197. [19] D. Fangli, W. Ning, Z. Dongmei, S. Yingzhong, J. Rare Earth. 28 (2010) 391. [20] M. Yousefi, R. Azimirad, M. Amiri, A.Z. Moshfegh, Thin Solid Films 520 (2011) 721. [21] X.Y. Chen, F. Fang, A.B. Djurisic, W.K. Chan, H.F. Lui, P.W.K. Fong, C. Surya, K.W. Cheah, Thin Solid Films 520 (2011) 1125. [22] M. Bender, E. Fortunato, P. Nunes, I. Ferreira, A. Marques, R. Martins, N. Katsarakis, V. Cimalla, G. Kiriakidis, Jan. J. Appl. Phys. 42 (2003) L435. [23] M.E. Jouad, M.A. Lamrani, Z. Sofiani, M. Addou, T.E. Habbani, N. Fellahi, K. Bahedi, L. Dghoughi, A. Monteil, B. Sahraoui, S. Dabos, N. Gaumer, Opt. Mater. 31 (2009) 1357. [24] S.R.C. Vivekchand, G. Gundiah, A. Govindaraj, C.N.R. Rao, Adv. Mater. 16 (2004) 1842. [25] G. Adamopoulos, S. Thomas, P.H. Wobkenberg, D.D.C. Bradley, M.A. Mclachlan, T.D. Anthopoulos, Adv. Mater. 23 (2011) 1894. [26] R. Mariappan, V. Ponnuswamy, P. Suresh, Superlattices Microst. 52 (2012) 500. [27] M. Yousefi, M. Amiri, R. Azimirad, A.Z. Moshfegh, J. Electroanal. Chem. 661 (2011) 106. [28] T. Prasada Rao, M.C. Santhosh Kumar, J. Alloys Comp. 506 (2010) 788. [29] A. Ali Fatima, Suganthi Devadason, J. Pure Appl. Ind. Phys. 1 (2011) 115. [30] R. Swanepoel, J. Phys. E 16 (1983) 1214. [31] A. Shour, M.A. Kaid, N.Z. El-Sayed, A.A. Ibrahim, Appl. Surf. Sci. 252 (2006) 7844. [32] T. PrasadaRao, M.C. SanthoshKumar, S. Anbumozhi, Angayarkanni, M. Ashok, J. Alloys Comp. 485 (2009) 413. [33] R. Mariappan, M. Ragavendar, V. Ponnuswamy, J. Alloys Comp. 509 (2011) 7337. [34] Y. Caglar, S. Ilican, M. Caglar, F. Yakuphanoglu, Spectrochim. Acta A 67 (2007) 1113. [35] E. Fatas, P. Herrasti, F. Arjona, E. Garciacamarero, J.A. Medina, Electrochem. Acta 32 (1987) 139. [36] X.M. Teng, H.T. Fan, S.S. Pan, C. Ye, G.H. Lia, J. Appl. Phys. 100 (2006) 053507. [37] M.A.L. Nobre, S. Lanfredi, Mater. Lett. 47 (2001) 362.