Vol. 126 (2014) ACTA PHYSICA POLONICA A No. 6

Inuence of Thermal Process on Physical Properties

of ZnO Films Prepared by Spray Pyrolysis

O. Gencyilmaza,*, F. Atayb and I. Akyuzb

aDepartment of Physics, CankrKaratekin University, Cankr, TurkeybDepartment of Physics, Eskisehir Osmangazi University, Eskisehir, Turkey

(Received January 6, 2014; in nal form July 22, 2014)

ZnO lms were deposited on glass substrates by ultrasonic spray pyrolysis technique at a substrate temperatureof 300±5 C. All of the lms have been annealed at 500 C temperature for dierent time (1, 2, and 3 h) to improvethe optical, electrical and surface properties. The eect of annealing time on the lms of physical properties has beeninvestigated. UV-Vis spectrophotometer has been used for transmittance measurements. Also, band gap values ofthe lms have been determined by optical method. Atomic force microscopy has been used to have information thesurface morphology and roughness values of the lms. Thicknesses, refractive index and extinction coecient valuesof the lms have been determined by spectroscopic ellipsometry technique. The electrical conduction mechanismsand resistivity of the lms were investigated using two probe technique. After all the investigations it was concludedthat annealing time has a dramatic eect especially on the surface, optical properties and electrical resistivity valuesof ZnO lms. From the results of these investigations, the application potential of the lms for solar cell devicesas transparent electrode was searched.

DOI: 10.12693/APhysPolA.126.1331

PACS: 81.05.Dz, 78.66.Hf, 73.61.Ga, 68.37.Ps

1. Introduction

ZnO is one of the few metal oxides which can be used asa transparent conducting oxide in many applications [1].Thin lms of ZnO have been used for a variety of applica-tions such as display devices, heat mirror, photovoltaicsand electronic transducers because of their high stability,electrical conductivity and transparency [2, 3]. On theother hand, this oxide has been applied in light emittingdiodes [4], photo detectors [5], piezoelectric cantilever [6],gas sensors [7], buer layer in CIGS solar cells [8], anddye sensitized solar cells (DSSC) [9]. ZnO is an attractiveoxide semiconductor which has wide band gap (3.3 eV atroom temperature) and direct band transition. ZnO thinlms may be prepared by dierent techniques includingmetalorganic chemical vapor deposition [10, 11], mag-netron sputtering [12, 13] and chemical spray pyroly-sis [1417]. Among these methods, spray pyrolysis is anattractive method to obtain thin lms of ZnO, since it islow cost, simple method and it does not require a highvacuum apparatus.Thermal annealing is widely used to improve crystal

quality, which aects electrical, optical and structuralproperties by reducing defects in materials. The anneal-ing process, especially at the surface regions, inuencethe structure of the material. Detailed the eects ofannealing processes on surfaces properties of ZnO lmscan be investigated by analysis of surface properties suchas surface roughness or optical properties by spectro-scopic ellipsometry. Spectroscopic ellipsometry technique

*corresponding author; e-mail: eren_o@hotmail.com

is a very sensitive measurement technique that uses po-larized light to characterize thin lms, surfaces, and ma-terial microstructure. There is almost no study abouteect of annealing time on physical properties of ZnOlms in the literature. Aim of this work is to see theeect of annealing time on the optical, surface and elec-trical properties of ZnO lms.

2. Experimental details

ZnO lms were grown onto glass substrates (1×1 cm2)at 300± 5 C temperatures by spray pyrolysis technique.The spray solution was prepared from a mixture 0.1 Mzinc acetate [Zn(CH3COO)2 · 2H2O] and distilled wa-ter. Totally, 100 ml of solution was used and sprayedfor 20 min. The solution ow rate was kept at 5 ml/minand controlled by a owmeter. Details of the spray py-rolysis technique were published elsewhere [18, 19]. Afterspraying process, the deposited ZnO lms were annealedin ambient air at 500 C temperature for dierent time(1, 2, and 3 h).Optical transmission measurements were performed

with a Perkin Elmer Lambda 2S UV-VIS spectropho-tometer over the wavelength range of 300900 nm.The thicknesses, refractive index and extinction coe-cient values of ZnO lms were obtained from spectro-scopic ellipsometry (SE) measurements.PHE-102 Spectroscopic Ellipsometer (spectral range

2502300 nm) was used to determine the ∆ parame-ters, refractive indices (n), extinction coecient (k) andthicknesses (t) of the lms. The CauchyUrbach disper-sion model was used to t the experimental ∆ param-eters. This model is a modied type of Cauchy model.In the Cauchy-Urbach dispersion model, the refractiveindex n(λ) and the extinction coecient k(λ) as a func-tion of the wavelength are given by

(1331)

1332 O. Gencyilmaz, F. Atay, I. Akyuz

n(λ) = A+B

λ2+C

λ4, (1)

kλ = α expβ

[12400

(1

λ− 1

γ

)], (2)

where A, B, C, α, β and γ are model parameters [20, 21].The CauchyUrbach model may be used for samples withlow absorption. For this reason, the measurements weretaken between the wavelength ranges of 12001600 nmwhere the lms have low absorption. For the sampleshaving depolarization eect, the incident angle is an im-portant factor. This will aect the intensity and phase ofthe reected light which then goes to the analyzer. Fivedierent incident angles (55, 60, 65, 70, 75) weretried to take the measurements. The best angle was de-termined to be 65 using experimental ∆ spectra. Then,the parameters (A, B, C, α, β) related to the CauchyUrbach model were determined.The surface morphology was studied by using Park

System XE 70 model atomic force microscopy. The mea-surements were taken in non-contact mode, ≈ 300 kHzfrequency and 0.75 Hz scan rate in air at room tempera-ture. A silicon cantilever which has a spring constant of40 N/m was used. Also, average (Ra) roughness valueswere obtained using XEI version 1.7.1 software. All theimages were taken from an area of 2×2 µm2. The rough-ness values belong to all scanned area.The electrical properties of the lms were investigated

by using a two-probe system, Fluke Voltage/Current Cal-ibrator Model 382a and Keithley 619 Electrometer.

3. Results and discussion

Ellipsometric angles ∆ were investigated by spectro-scopic ellipsometry. The optical constants (n), (k) andthe thickness of ZnO lms were investigated. Ellipso-metric data of ZnO lms were tted according to theCauchyUrbach exponential model and reection spec-tra taken from spectroscopic ellipsometry. The techniqueitself is based on the fact that linearly polarised lightbeam, which is cast on the sample surface, is reectedand its polarization state is changed. The ellipsometricangles are denoted by ∆, which are related to complexreectance ratio ρ:

ρ =RP

RS

= tan Ψe i∆, (3)

where RP and RS are complex reection coecientswhich are parallel and perpendicular to the plane of in-cidence, respectively [22]. The measured SE data for ∆of ZnO lms on glass substrates in the wavelength rangeof 12001600 nm is shown in Fig. 1. The experimentaldata were analyzed by using the CauchyUrbach expo-nential model. In spite of a good correlation providedbetween model and experimental data, there are somesmall deviations on tted ∆ values. We think that thepossible reason may be the roughness, grain boundaries,and morphology of the sample.Fitting the experimental ellipsometric spectra of ∆ al-

lowed the determination of the lm thickness (t), spec-

tra of refractive index (n) and extinction coecient (k)of all lms. The thickness values of ZnO lms and ellip-sometric data are given in Table I. The thickness valuesof the lms were varied from 259 to 276 nm and valuesof MSE were fairly low. When the ZnO lms were an-nealed for more than 2 h, thickness of lms was found toincrease.

Fig. 1. SE spectra of as-deposited at 300 C and an-nealed ZnO lms at 500 C: (a) as deposited, (b) an-nealed for 1 h, (c) annealed for 2 h, (d) annealed for 3 h.

TABLE I

Thickness and ellipsometric data values of ZnO lms, asa function of annealed time, obtained by a spectroscopicellipsometry study.

Materialt

[nm]An

Bn

[nm2]

Cn

[nm4]α

β

[nm]MSE

as deposited 259 1.97 0.04 0.01 0.048 0.074 0.43

annealed for 1 h 236 2.04 0.01 0.06 0.086 0.036 0.89

annealed for 2 h 243 2.01 0.01 0.01 0.075 0.042 0.81

annealed for 3 h 276 1.88 0.29 0.05 0.124 0.063 0.6

Optical properties of unannealed and annealed ZnOlms were investigated with the help of transmissionspectra in the UVVisible region and recorded in therange 300-900 nm. Figure 2 shows the optical trans-mission spectra of ZnO lms prepared on the glass sub-strates at a temperature of 300 C followed by anneal-ing at various time (1, 2, 3 h). It is clear from Fig. 2that annealing the samples caused an increase in trans-mittance values. Decrease of thickness may be a reasonfor the increase in transmittance values of samples an-nealed for 1 and 2 h. Similar results have been reportedby Nadarajah et al. [23]. There is another eect of an-nealing on the optical properties of ZnO lms. Samplesannealed for 1 and 2 h have shown interference fringes intransmittance spectra. It is well known that thin lmswith uniform and continuous surfaces may probably showthese fringes. So, we think that improved surface textureand decreasing roughness values of the samples annealedfor 1 and 2 h caused interference fringes to occur in trans-mittance spectra.Annealing process has also been an important eect

on the surface properties of ZnO lms as can be seenfrom our atomic force microscopy (AFM) images (Fig. 3).

Inuence of Thermal Process on Physical Properties of ZnO Films. . . 1333

Fig. 2. UV-Vis transmittance spectra of as-depositedat 300 C and annealed ZnO lms at 500 C: (a) asdeposited, (b) annealed for 1 h, (c) annealed for 2 h,(d) annealed for 3 h.

So, the improvement of the transmittance for annealedsamples could be related to decrease in optical scatteringprocesses after annealed. This kind of eect has beenreported by Senadm et al. [24], Ma et al. [25] and Mah-mood et al [26].

ZnO lm that is annealed during 2 h has the highestoptical transmittance (≈ 86%). The high annealing timeabove 2 h leads to decrease of the transmittance in thevisible range. It suggests that annealing ambient has agreat inuence on the optical transmittance of ZnO lm.Aly et al. [27] found that thermal annealing in air couldimprove the optical transmittance of ZnO lms, attribut-ing to the oxygen reaction with ZnO. In addition, we cansee from Fig. 2 that the absorption edge has been changedafter the annealing. The sharp absorption edges can beclearly observed with the increases of the annealing timeas shown in the inset of Fig. 2. In addition, sharp ab-sorption edges can be clearly observed and shift to theshorter wavelength as the increases of the annealing time(below 3 h) as shown in the inset of Fig. 2. We thinkthat our as-grown sample has deformation and defectsnear band edges. Annealing may probably lead to de-crease of such deformations and resulted with a sharperabsorption edge. This will also cause the optical bandvalues to increase as can be seen from Fig. 4.

Reection (R) spectra of ZnO lms are shown inFig. 5. It was determined that the average reectionvalue of lms is below 6% in the visible region of thespectrum. Especially, low reectance value of long du-ration annealed lm (3 h) makes this lm a promisingmaterial for photovoltaic applications as window mate-rial or front contact.

Figure 6 shows the dependence of the refractive indicesof ZnO lm on the wavelength. It can be seen that the re-

Fig. 3. AFM images of ZnO lms for as-deposited at300 C and annealed ZnO lms at 500 C: (a) as de-posited, (b) annealed for 1 h, (c) annealed for 2 h,(d) annealed for 3 h.

fractive index increases while increasing annealing time.When we look at Fig. 6, refractive indices of all lms arealmost constant with increasing wavelength. This is anexpected result for this wavelength range because all lmsare nearly transparent in this region. There is a little de-crease in refractive index value for ZnO which is annealedfor 3 h. Also, refractive index values of the samples arenearly constant (≈ 1.95−2.15) at long wavelengths.

Figure 7 shows the extinction coecients (k) of ZnOlms which are derived from model tting the experimen-tal spectroscopic ellipsometric data. It can also be seenthat the extinction coecient increases when increasingthe annealing time.

The optical band gap energy Eg was calculated fromthe absorption spectra. We used the variation of the ab-

1334 O. Gencyilmaz, F. Atay, I. Akyuz

Fig. 4. (αhν)2 versus (hν) plots of ZnO lms for as-deposited at 300 C and annealed ZnO lms at 500 C:(a) as deposited, (b) annealed for 1 h, (c) annealedfor 2 h, (d) annealed for 3 h.

Fig. 5. Reection (R) spectra of as-deposited at 300 Cand annealed ZnO lms at 500 C: (a) as deposited,(b) annealed for 1 h, (c) annealed for 2 h, (d) annealedfor 3 h.

sorption coecient (α) with photon energy, the relation

αhν = A(hν − Eg)n, (4)

where A is a constant, Eg the band gap of the mate-rial and n is a constant which determines type of theoptical transition (n = 1/2 for allowed direct transi-tions and n = 2 for allowed indirect transitions). Heren = 1/2, corresponding to the allowed direct transi-tion [28]. The optical gap was determined from the plotof (αhν)2 versus photon energy.

Fig. 6. Refractive index (n) spectra of as-deposited at300 C and annealed ZnO lms at 500 C: (a) as de-posited, (b) annealed for 1 h, (c) annealed for 2 h,(d) annealed for 3 h.

Fig. 7. Extinction coecient (k) spectra of as-deposited at 300 C and annealed ZnO lms at 500 C:(a) as deposited, (b) annealed for 1 h, (c) annealedfor 2 h, (d) annealed for 3 h.

Figure 4 shows the plots of (αhν)2 versus (hν) for var-ious ZnO lms. Extrapolation of the linear portions ofthe plots onto the energy axis was used to estimate theband gap values. The extrapolated values of the opti-cal energy gap Eg are between 3.2 and 3.3 eV for thesamples that were as grown and annealed for 1, 2, 3 h,respectively. It can be seen that the optical absorptionedge exhibits slight blue shift with increasing annealingtime below 3 h. However, it exhibits evident red shift at3 h of the annealing time. Xuea et al. [29] have also re-ported a blue shift and a red shift with annealing processin their studies.As we mentioned before, the shifts of the optical ab-

sorption edge may be attributed to the decreasing de-fects and deformation of the ZnO lms with increasingannealing time. Also, this may be due to the extension ofelectronic states of the impurity phase, precipitates andclusters, into the band gap of ZnO. Also, all lms werefound to be direct band gap materials, which is a desired

Inuence of Thermal Process on Physical Properties of ZnO Films. . . 1335

property for photovoltaic solar cell applications.It is known that the absorption coecient near the

band edge shows an exponential dependence on photonenergy [30],

α(ν) = α0 exp(hν/E0), (5)

where E0 is the Urbach energy which corresponds to thewidth of the band tail and can be evaluated as the widthof the localized states. α0 is a constant. Thus, a plotof ln[α(λ)] versus photon energy should be linear andUrbach energy can be obtained from the slope.The ln[α(λ)] versus photon energy plots for the lm

annealed at dierent hours are shown in Fig. 8. The Ur-bach energy was calculated from the reciprocal gradientof the linear portion of these curves.

Fig. 8. The Urbach plots of for as-deposited at 300 Cand annealed ZnO lms at 500 C: (a) as deposited,(b) annealed for 1 h, (c) annealed for 2 h, (d) annealedfor 3 h.

Both Urbach energy and optical band gap alterationis shown in Fig. 9. The Urbach energy increases slightlywith increase of the annealing time. After that, the widthof the band tail increases with increase of the annealingtime above 2 h. In this long annealing time, we thinkthat thermal induced structural disorder of the lm in-creases with the increase of annealing time, which leadsto a degradation of the ZnO lm. These results funda-mentally agree with the analysis results of the shift of theoptical absorption edge.AFM has been used to have information on the surface

morphology and roughness values of the lms. The AFMimages of the lms are shown in Fig. 3. Figure 3 showsthat there are dierent regions on the surface of as-grownZnO lms with various heights. There are randomly dis-

Fig. 9. Urbach energy and optical band gap of the ZnOlm as a function of the annealing time.

tributed mount type formations and regions with dier-ent height and width. Annealing caused that the lmshave dierent surface morphology with decrease of heightand width. Also, cluster-like formations and discontinu-ity on lm surface need to be noted. But, with increaseof annealing time, surface texture of the lms starts tochange. Especially the sample annealed for 2 h has a uni-form surface with distinguishable particles. This granularstructure may probably have positive eect on the opticalproperties of this sample, as we have mentioned before.We can conclude that annealing process has a dramaticeect on the surface properties of the lms, especiallyfor the sample annealed for 2 h. Similar eect has beenreported by Nehru et al. [31]. Consequently, the surfacemorphology of ZnO lms that were annealed for 2 h hasthe best granular structure.

Annealing process has also changed the roughness val-ues of ZnO lms. Average (Ra) roughness values of theZnO lms were examined and these values are given inTable II. It is clear that annealing process aected theroughness values of ZnO lms. The roughness value ofthe lm slightly decreases with the annealing that hasbeen reported in the literature [31, 32]. We think thatthis decrease is manly related to the surface combinationand surface form. Also, the average roughness (Ra) val-ues of the lms are between 3063 nm which are lowerthan that of the results of the report given by Nehruet al. [31]. Samples annealed for 1 and 2 h have lowerRa values than other samples. Annealing may probablycause the atoms to migrate energetically favorable sitesand cause a uniform distribution on the surface. As wehave mentioned before, this type of texture caused inter-ference fringes in transmittance spectra to occur.

The I−V characteristics of lms were investigated andthe electrical resistivity of ZnO lms which were unan-nealed and annealed at 1, 2, and 3 h was measured. TheI−V plots of all lms in these conditions are shown inFig. 10. For the ZnO lms, the current changed in the10100 V voltage range. Also, the resistivity values of alllms were calculated and were presented in Table III.It was seen that the resistivity of ZnO lms decreasewith increase of annealing time. A similar behavior with

1336 O. Gencyilmaz, F. Atay, I. Akyuz

TABLE II

The average roughness values of the unan-nealed and annealed ZnO lms.

Material Ra [nm] (2 × 2 µm2)

as deposited 63

annealed for 1 h 46

annealed for 2 h 30

annealed for 3 h 59

the change in electrical resistivity has been reported byNunes et al. [33], Lee and Park [34] and Ma and Shim intheir studies [35].

Fig. 10. IV plots of as-deposited at 300 C and an-nealed ZnO lms at 500 C: (a) as deposited, (b) an-nealed for 1 h, (c) annealed for 2 h, (d) annealed for 3 h.

TABLE III

Electrical resistivity values of ZnO lms.

Material Electrical resitivity [Ωcm]

as deposited 218

annealed for 1 h 18.5

annealed for 2 h 27.9

annealed for 3 h 11.1

We think that the reason for this annealing of the ZnOlm can lead to desorption and chemisorption of oxygenmolecules which are chemisorbed in the lm both at grainboundaries and also on the surface. Because the resistiv-ity behavior on annealing of lms at dierent ambienttemperature and time can be explained on the basis ofchemisorption and desorption of oxygen at grain bound-aries of polycrystalline lms [36]. Annealing process

may cause the desorption of oxygen from these regions.We think that this will not only lower the potential bar-rier but also increase carrier concentration. In addition,it is well known that oxygen tends to site at grain bound-aries in polycrystalline lms. These oxygen spaces mayproduce potential barrier for charge carriers [3739]. Fi-nally, these will cause the resistivity values of annealedZnO lms to decrease, as shown in Table III.

4. Conclusion

The ZnO thin lms have been fabricated by spray py-rolysis method at 300 ± 5 C substrate temperature andannealed for 1, 2, and 3 h. Eects of the annealing timeon optical, surface and electrical properties of ZnO thinlms were investigated. Optical properties of the lmswere analyzed by transmittance, absorbance spectra andthe optical method was used for determining the bandgaps of the lms. The optical constants (n), (k) andthe thickness of ZnO lms were tted according to theCauchyUrbach exponential model by reection spectrataken from spectroscopic ellipsometry. A good t is foundbetween model and experimental data but there are somedeclinations on tted ∆ values. Also, the optical bandgap values between 3.2 and 3.3 are changed. The highesttransmittance was observed in 2 h annealing. As shownfrom Fig. 3, maximum reduction in refractive index hasbeen at 3 h annealing. The surface morphology of ZnOlms were investigated by using atomic force microscopy(AFM) images; also these pictures showed grain bound-aries and roughness values of the lms. The surface mor-phology characterizations of ZnO lms with results ofspectroscopic ellipsometry are compatible. The electricalresistivity values were calculated using the I−V graph-ics. The electrical resistivity of ZnO lms decrease withincrease of annealing time. As shown from Table III,minimum electrical resistivity of ZnO lms has been at3 h annealing. Consequently, 1 and 2 h annealing ZnOlms can be used as transparent electrode in photovoltaicsolar cell. However, 3 h annealing ZnO lm can be usedin dierent applications.

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