Effect of post-annealing temperature on structural and optical properties of nano-ZnO synthesised from ZnO2 by laser ablation method

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Applied Surface Science 258 (2011) 1881– 1887

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

j our nal ho me p age: www.elsev ier .com/ loc ate /apsusc

ffect of post annealing temperature on structural and optical properties ofnCdO thin films deposited by sol–gel method

manpal Singha,b,∗, Dinesh Kumara, P.K. Khannab, Bhubesh Chander Joshia,b, Mukesh Kumara

Department of Electronic Science, Kurukshetra University, Kurukshetra 136119, IndiaCentral Electronics and Engineering Research Institute, Council of Scientific and Industrial Research, Pilani 333031, India

r t i c l e i n f o

rticle history:eceived 20 August 2011eceived in revised form 17 October 2011ccepted 18 October 2011vailable online 23 October 2011

eywords:inc oxideol–gelhotoluminescence

a b s t r a c t

Ternary ZnCdO thin films oriented along c-axis have been successfully deposited on p-Si (1 0 0) sub-strates using sol–gel spin coating route. To optimize most suitable annealing temperature for theZn1−xCdxO thin films; these films with selected cadmium content x = 0.10 were treated at annealingtemperatures from 300 ◦C up to 800 ◦C in oxygen ambient after deposition. The structural and opticalproperties of deposited thin films have been characterized by X-ray diffraction, energy dispersive spec-troscopy, atomic force microscopy, UV–Vis spectroscopy, and photoluminescence spectra. The resultsshow that the obtained films possess high crystallinity with wurtzite structure. The crystallite size, lat-tice parameters, lattice strain and stress in the deposited films are determined from X-ray diffractionanalysis. The band gap energy increased as a function of annealing temperatures as observed from opti-

V–Vis spectroscopy cal reflectance spectra of samples. The presence of Cd in the deposited films is confirmed by energydispersive spectrum and it is observed that Cd re-evaporate from the lattice with annealing. The pho-toluminescence measurements as performed at room temperature did not exhibit any luminescencerelated to oxygen vacancies defects for lower annealing temperatures, as normally displayed by ZnOfilms. The green yellow luminescence associated to these defects was observed at higher annealingtemperatures (≥700 ◦C).

. Introduction

The increasing interest on low power consummating optoelec-ronic devices such as white light-emitting diodes, blue lasers,V detectors and emitters has stimulated the research on wideandgap semiconductors [1]. The GaN is well established mate-ial in this field. As an alternative to GaN, the ZnO is an option withimilar hexagonal wurtzite crystal structure, lattice parameters andirect wide bandgap of 3.34 eV [2]. The fundamental absorptionhows excitonic recombination due to its comparatively high exci-onic binding energy (∼60 meV) than GaN (∼25 meV) even at roomemperature [3]. This property makes ZnO much more interestingn the research field for future optoelectronic devices. Moreover,nO is commercially available with advantages such as compar-tively low cost, environment-friendly non-toxic material, easyet chemical processes, high resisting to radiation damage, and

igh thermal and chemical stability [4]. In modern optoelectronicevices, it is the realization of bandgap engineering to create quan-um wells in device hetero-structures. To achieve such devices in

∗ Corresponding author at: Department of Electronic Science, Kurukshetraniversity, Kurukshetra 136119, India.

E-mail address: [email protected] (A. Singh).

169-4332/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rioi:10.1016/j.apsusc.2011.10.096

Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

ZnO the challenges are to accomplish p-type behavior and to tai-lor the bandgap. Hence, the bandgap modulation in ZnO is veryimportant to realize the functional device with minimum disloca-tion densities. The crystal structure of ZnO easily allows the foreignelement for alloying and ZnO exhibit the possibility of tuning itsproperties such as bandgap and emission energies. The bandgap ofZnO can be tailored by alloying of the group II elements i.e. Be, Mg,Ca, Cd, and Sr [5,6].

The Cd is known to reduce the bandgap. A large Cd contentallows a large change in the bandgap and is desired to emit the lightin the visible region. The ZnCdO thin films can be deposited usingvarious techniques such as dc reactive magnetron sputtering [3],molecular-beam epitaxy [7], pulse laser deposition (PLD) [8], metaloxide chemical vapor deposition (MOCVD) [9], and electrochemi-cal deposition [10]. Majority of the previous reports on ZnO and itsalloys are focused on deposition by expensive and time consum-ing physical methods. The sol–gel route offers many advantageslike easy for compositional modification, simple and inexpensiveequipment, and minimum variables to control the growth of film,excellent control on stoichiometry [11].

The Cd content in lattice, crystallinity, morphological and opti-cal properties is affected by post annealing temperature. A fewof research articles related to cadmium incorporation in ZnO thinfilms deposited by sol–gel route like blue luminescence study in

ghts reserved.

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1882 A. Singh et al. / Applied Surface Science 258 (2011) 1881– 1887

0O thi

CpcmbetapMpmohsdeoTaobifw

Fig. 1. XRD patterns recorded for Zn0.90Cd0.1

dZnO films by Ogawa and Fujihara [12], ZnCdO films as a trans-arent conducting oxide by Choi et al. [13], photovoltaic solarell properties in CdZnO films by Ilican et al. [14], effect of cad-ium content on morphological, optical and electrical properties

y Caglar et al. [15] have been reported. In all these articles,fforts are made to vary the cadmium content and its effect onhe physical properties is demonstrated. A limited literature isvailable on the detailed study related to effect of annealing tem-erature on optical and structural properties of ZnCdO thin films.a et al. [3] investigate the effect of post annealing on lattice

arameter ‘c’ of ZnCdO thin films deposited by dc sputtering, Mah-oud et al. [16] have described the influence of heat treatment

n the structures of ZnCdO nanopowder, recently Pillai et al. [8]ave investigated the effect of annealing temperature on grainize and band gap for ZnCdO thin films grown by pulse lasereposition. In this paper, a detail investigation about the influ-nce of post annealing on morphological and optical propertiesf ZnCdO thin films deposited by sol–gel route is carried out.his detailed investigation suggests a suitable annealing temper-ture window to achieve better quality ZnCdO thin films in termsf better crystallinity with minimum defects and strain as the

and gap tailored with doping of Cd. Cadmium content x = 0.10

s selected for this study to avoid any phase separation as it isound in the form of CdO in the films at higher Cd content. Theork has been carried out in a number of terms, and authors found

n films at different annealing temperatures.

repeatability in results with fine control on stoichiometry of filmsby this method.

2. Experimental

The Zn1−xCdxO thin films were deposited on p-Si (1 0 0) sub-strates by sol–gel route. The precursor solution of 0.2 mol/lconcentration was prepared by dissolving the Zinc acetate dihy-drate [Zn(CH3COO)2·2H2O, 99.5% purity] in 2-methoxy ethanol[CH3–O–CH2–CH2–OH, 99.5% purity] with the help of magneticstirrer at 60 ◦C. While the solution became transparent, theethanolamine [C2H7NO, 99.5% purity] was added in 2:1 molarratio as a sol stabilizer. To deposit Zn1−xCdxO (x = 0.10) thin filmsthe appropriate calculated amount of cadmium acetate-di-hydrate[Cd(CH3COO)2·2H2O, 99.97% purity] was mixed in this solution.This solution was kept on continuous stirring for an hour then fil-tered by Whatman’s grade GF/A glass microfiber filter papers. Thesol was transparent, clear, homogeneous and stable at room tem-perature. p-Si wafer substrates were ultra-cleaned through RadioCorporation of America (RCA1 and RCA2) standard cleaning process[17].

After aging the stable solution up to 48 h for necessary hydroly-sis process [17], the solution was spun coated on the substrates. Theprecursor sol was dropped on the Si substrate and spun at 3000 rpmfor 1 min and dried with blowing hot air during the spinning in last

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ce Science 258 (2011) 1881– 1887 1883

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Fig. 2. (a) Variation in position and FWHM of characteristic diffraction peak(0 0 2) with annealing. (b) Variation in stress and strain of Zn0.90Cd0.10O films with

A. Singh et al. / Applied Surfa

ew seconds, so that films can be prevented from cracks due to sur-ace tension contraction. Each layer was preheated at 250 ◦C withhe help of hot plate for 30 s. This process was repeated 15 times tobtain multilayer and dense film. The thin film Zn0.90Cd0.10O wasliced in many equal pieces. The films were then annealed at variousemperatures from 300 to 800 ◦C with the help of tubular furnacen oxygen ambient for an hour.

The crystal phase structure was analyzed with a XPERT-RO diffractometer from Panalytical, equipped by a Giono-meterW3050/60 working with Cu K� radiation of wavelength

= 1.54060 A. The Giono scan was performed at the scan ratef 0.01◦ s−1. The voltage was set at 45 kV with 40 mA flux. Theicrostructure was analyzed by Atomic Force Microscope (AFM),odel Pro 47, NT MDT. The chemical composition of deposited

amples was determined by using energy dispersive spectrumEDS) system of oxford instruments. Optical reflectance spec-ra were observed with the help of Perkin-Elmer lambda-650V–Vis spectrophotometer. The photoluminescence (PL) measure-ents were carried out by RPM 2000 photoluminescence mapper

quipped with Nd-YAG laser employing an excitation wavelengthf 266 nm.

. Results and discussion

The X-ray diffraction (XRD) patterns of cadmium doped ZnOZn1−xCdxO; x = 0.10) thin films annealed at different temperaturesn oxygen ambient is shown in Fig. 1. The XRD patterns revealedhat all the annealed samples exhibit three characteristic peaksf orientation (1 0 0), (1 0 1) and (0 0 2). However, the reflectionlane (0 0 2) is dominated in deposited structures. Apart fromnO characteristic peaks, no other peak corresponding to eitheradmium, zinc or their complex oxide could be detected, which areormally expected to appear upon the mixing of these two oxides.

t is observed that the characteristic peak (0 0 2) moved towardsigher angle from 34.051◦ to 34.389◦ while the Full width at Halfaxima (FWHM) decreased from 0.922◦ to 0.258◦ with annealing

s demonstrated in Fig. 2(a). It is an indication of reduction inhe lattice parameter ‘c’ of ZnCdO hexagonal structure. The latticearameters ‘c’ and ‘a’ are given in Table 1 and calculated from (002)nd (100) peaks by using c = �/sin � and a = (2/

√3)d, respectively.

here, � is the wavelength of X-ray radiation, ‘�’ is the Bragg’sngle of the dominant peak and ‘d’ is inter plane distance for latticetructure. The shrinkage observed in c-axis length is probably dueo the Cd re-evaporation in view of smaller lattice energy of CdO3806 kJ mol) than that of ZnO (4142 kJ mol) [3]. The replacementf Zn ions (ionic radii 0.060 nm) by Cd is responsible for extensionn c-axis length due to its larger ionic radii (0.074 nm) [10] butt higher annealing temperature possibly cadmium disappearedrom the lattice. The discontinuity in pattern at 350 ◦C may beue to the effective replacement of Zn sites by Cd. The drop inWHM attributed to improvement in crystallinity and/or enlarge-ent in grain sizes. The grain sizes are calculated by Scherrer

ormula [17]

= 0.9�

cos �

here is FWHM in radians. As expected, the grain size is increasedith annealing from 9.01 nm (300 ◦C) to 32.11 nm (800 ◦C).

The lattice strain is expected in the deposited films is due to theattice mismatch with substrate, entry of the guest element having

different size than host elements and/or thermal treatment. It is

alculated by using following expression [18]:

train = afilm − abulk

abulk× 100%

annealing.

where abulk, afilm are the lattice constants of standard ZnO powder(abulk = 3.250 nm) and deposited films, respectively. The variation instrain for ZnCdO thin films with annealing temperature is shownin Fig. 2(b). The positive and negative signs for estimated strainfor the films indicated that the crystallites are under a state oftensile and compressive strain, respectively. Replacement of thehost element Zn by Cd having larger radii introduced tensile strain.However, the annealing tends to readjust the lattice. It could beascribed to Cd evaporation from the lattice with annealing. Anneal-ing at higher temperatures causes the films undergo tensile strainas already noticed for undoped ZnO thin films deposited on Siwafers [18]. It is clearly observed from data displayed in Table 1that the Zn0.90Cd0.10O films annealed at 550 ◦C exhibited minimumstrain. The residual stress in the deposited ZnCdO thin films is cal-culated by using the expression which is valid for hexagonal lattice[19,20]

stress = 2C213 − C33(C11 + C12)

2C13× cfilm − cbulk

cbulk

where C11, C12, C13, C33 stand for the elastic constants of ZnO film in

different directions with the values 208.8, 119.7, 104.2, 213.8 GPa[19,20]. cbulk, cfilm are the lattice constants of standard ZnO pow-der (cbulk = 5.205 nm) and deposited films, respectively. Putting the

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1884 A. Singh et al. / Applied Surface Science 258 (2011) 1881– 1887

Table 1The observed experimental results for ZnCdO thin films as a function of annealing temperatures.

Sample Peak position(2�◦) (±0.001◦)

FWHM (◦)(±0.001◦)

L.P. ‘a’ (A)(±0.001)

L.P. ‘c’ (A)(±0.017)

Strain (%)(±0.031)

Residual stress(GPa) (±0.077)

Grain size (nm) Band gap (eV)(±0.03)

300 ◦C 34.051 0.922 3.253 5.262 +0.092 −2.494 09.01 ± 0.03 3.09350 ◦C 33.985 0.691 3.253 5.272 +0.092 −2.936 12.01 ± 0.04 2.96400 ◦C 34.177 0.864 3.257 5.243 +0.215 −1.649 09.61 ± 0.03 3.08450 ◦C 34.209 0.691 3.256 5.238 +0.185 −1.434 12.02 ± 0.04 3.10500 ◦C 34.233 0.475 3.252 5.235 +0.062 −1.278 17.48 ± 0.07 3.11550 ◦C 34.238 0.432 3.247 5.234 −0.092 −1.242 19.23 ± 0.08 3.23600 ◦C 34.271 0.302 3.241 5.228 −0.277 −1.027 27.48 ± 0.13 3.27650 ◦C 34.281 0.259 3.232 5.227 −0.554 −0.958 32.06 ± 0.16 3.28700 ◦C 34.306 0.259 3.233 5.224 −0.523 −0.791 32.09 ± 0.16 3.29750 ◦C 34.355 0.258 3.234 5.216 −0.492 −0.467 32.12 ± 0.16 3.29

−0.492 −0.247 32.11 ± 0.60 3.31

vf

s

TZis(sgr(its≥

Ca

Table 2EDS analysis showing the element contents in the deposited samples.

Sample Zn (at.%) Cd (at.%) O (at.%)

300 ◦C 21.13 ± 0.09 2.09 ± 0.05 76.82 ± 0.11400 ◦C 22.05 ± 0.11 2.04 ± 0.07 75.86 ± 0.09500 ◦C 22.18 ± 0.08 1.96 ± 0.06 75.86 ± 0.08600 ◦C 20.21 ± 0.13 1.63 ± 0.02 78.16 ± 0.10

800 ◦C 34.389 0.258 3.234 5.212

alues of elastic constants in above expression, we can derive theollowing formula [8,10,19,20]:

tress = −233(

cfilm − cbulk

cbulk

)GPa

he variation in calculated stress with annealing temperature fornCdO thin films is shown in Fig. 2(b). The residual (internal) stressn the deposited films could be due to differences in thermal expan-ion (thermal stress) and/or from the microstructure of the filmsintrinsic stress). Initially, the stress observed in the films is of ten-ile type, which may be due to entry of the comparatively large radiiuest element (Cd) in host lattice. As the annealing temperature isaised the effect of the thermal expansion coefficients mismatches56%) between the substrate and deposited film should be dom-nated [21]. It is also considerable that Cd re-evaporating fromhe lattice. A combined effect appeared as the decrease in ten-ile stress, approximate linearly with annealing for temperatures

400 ◦C.

EDSs were recorded to determine the chemical composition ofd in ZnCdO thin films at different annealing temperatures. Zn, Cdnd O are the dominating elements in films. The Cd content in the

Fig. 3. AFM pictures for Zn0.90Cd0.10O thin films at different annealing temperat

700 ◦C 20.71 ± 0.06 1.59 ± 0.09 77.69 ± 0.18800 ◦C 21.07 ± 0.14 0.27 ± 0.12 78.66 ± 0.18

sample annealed at 300 ◦C is found on the same at.% as it was mixedin Sol. It confirms the presence of Cd in ZnO. The Cd content inZnCdO thin films exhibited a decreasing trend with annealing tem-perature as shown by Table 2. It gives strong evidence that the Cddwindled with annealing from lattice with annealing as evapora-tion of Cd with heating was guessed from XRD results.

Fig. 3 displays the AFM images of Zn1−xCdxO (x = 0.10) samples

annealed at different temperatures. The grain size observed near10 nm for the samples annealed at 300 ◦C. It is observed that thegrain sizes are increased with annealing and found to be in the rangeof 10–15 nm for 400 ◦C, inexact 20 nm for 500 ◦C, while observed

ures: (a) 300 ◦C, (b) 400 ◦C, (c) 500 ◦C, (d) 600 ◦C, (e) 700 ◦C and (f) 800 ◦C.

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A. Singh et al. / Applied Surface Science 258 (2011) 1881– 1887 1885

0.10O t

nwio8t

astlitFbs(ttsoieo

o

Fig. 4. Optical reflectance spectra for Zn0.90Cd

ear 30 nm for 600, 700 and 800 ◦C. The grain sizes are consistentith the results obtained from XRD. The grains are observed spher-

cal in shape up to 500 ◦C, but no definite shape for 600 ◦C, whileval shapes of grains are observed for samples annealed at 700 and00 ◦C. The pores can be sited in each sample as the characteristicexture problem in films deposited through sol gel route.

The optical reflectance spectra of ZnCdO thin films for variousnnealing temperatures are displayed in Fig. 4. The excitonic tran-ition peak is observed in the visible region for lower annealingemperatures (≤500 ◦C) and afterwards shifted to UV zone, whicheads to the widening of band gap. The excitonic peak with anneal-ng becomes more intense and sharp. The band gap is calculated athe energy value for which Kubelka–Munk [17] remission function

= ((1 − R)2/2R) starts to increase linearly. The calculated values ofand gap are expressed in Table 1. It is found that the band gaplightly decreased with annealing from 3.09 eV (300 ◦C) to 2.96 eV350 ◦C) may be due to effectively doping of Cd in host lattice onhis annealing temperatures as discussed in XRD results. Later on ashe annealing temperature is increased the band gap energy valehift towards the intrinsic values for undoped ZnO (3.36 eV); asbserved 3.31 eV for sample annealed at 800 ◦C. The presence of Cdn lattice was the key reason for reduction in the band gap. This

nhancement in the band gap could be ascribed to re-evaporationf Cd from lattice as confirmed by EDS analysis.

Fig. 5 displays the PL spectra of Zn0.90Cd0.10O thin filmsbserved at room temperature. The spectra exhibited a visible

hin films annealed at different temperatures.

blue luminescence for samples annealed at lower temperatures(≤550 ◦C) and UV emission for samples annealed at higher tem-peratures (≥600 ◦C). These emissions are observed due to radiativerecombination of free excitons at near band edge (NBE). The visibleNBE emission is noticeable effect from reduction in the band gapof ZnO due to Cd doping. The intensity of this NBE peak extremelyenhanced with annealing temperature indicating the improve-ment in crystal lattice arrangement and optical quality. It is clearlyobserved that as the annealing temperature increased the NBEpeak shifted towards lower wavelengths signifying enhancementin the band gap (inset of Fig. 5a). The shift appears prominentlyas annealing performed higher than 500–550 ◦C. The only possiblereason for this blue shift is enhancement in band gap; more clearlythe properties of ZnCdO tend to undoped ZnO as the Cd leaving thelattice. Annealing at the temperature higher than 700 ◦C createsoxygen vacancies defects in crystal and the emission related tothis defect appears in the visible yellow-green region (Fig. 5b).One more visible emission in infra red region around 780 nm[1.60 eV] appeared at such higher annealing (Fig. 5c), intensityof this emission increased with heat treatment. This emissioncannot be related with defects generated by Cd doping, since thisphotoluminescence observed also for undoped ZnO thin films

around 738 nm [1.68 eV] [22]. The comparative shift may be dueto band gap shrinkage in ZnCdO thin films. As suggested by firstprinciple study of native point defects in ZnO by Kohan et al. thisemission may be due to oxygen anti site defect (OZn) [23].

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1886 A. Singh et al. / Applied Surface Science 258 (2011) 1881– 1887

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[3] D.W. Ma, Z.Z. Ye, J.Y. Huang, L.P. Zhu, B.H. Zhao, J.H. He, Effect of post annealingtreatments on the properties of Zn1−xCdxO films on glass substrates, Mater. Sci.

ig. 5. PL spectra of Zn0.90Cd0.10O thin films observed at room temperature: (a) n

. Conclusions

The Zn1−xCdxO (x = 0.10) thin films have been deposited by solel method and annealed at various temperatures in the range of00–800 ◦C. The obtained films showed good crystallinity and didot show any phase segregation for the Cd concentration usinghis route. All the films are exhibited the wurtzite crystal struc-ure. The systematic study of ZnCdO thin films at various annealingemperatures exposed that the crystallinity, grain sizes, bandgapnd photoluminescence enhanced with annealing. The Cd/Zn ratioecreased with heat treatment due to Cd evaporation. A large Cdontent presence in host lattice of ZnO desired to emit the lightn visible region and it seems possible at lower annealing tem-eratures (350 ◦C). Following this we have to compromise withrystallinity, which is also one of the important factors in functionalptoelectronic devices. The annealing around 500 ◦C is appropri-te for ZnCdO thin films as the crystallinity is the sufficiently high,train and defects are the minimum as well as the band gap is

ccomplished with value 3.11 eV (∼400 nm) for Zn0.90Cd0.10O thinlms in comparison to 3.36 (∼370 nm) of pure ZnO.

nd edge emission, (b) yellow-green luminescence, (c) infra red luminescence.

Acknowledgements

The authors wish to thank all the members of Hybrid micro-circuits group, CEERI, Pilani and Department of Electronic Science,K.U.K. for their technical support. The authors are extremely thank-ful to Dr. Chandra Shekhar, Director, CEERI, Pilani for his permissionto utilize the facilities in his institution. One of the authors, Aman-pal Singh, is also thankful to Council of Scientific and IndustrialResearch (CSIR), India for funding support.

References

[1] S. Yoshida, T. Ito, A. Hiraki, H. Saito, S. Fujita, Y. Ishitani, S. Sakai, T. Miyajima,Y. Yamada, Y. Kawakami, Fundamental Properties of Wide Bandgap Semicon-ductors, Springer Publications, 2007.

[2] G. Dhanaraj, M. Dudley, D. Bliss, M. Callahan, M. Harris, Growth and processinduced dislocations in zinc oxide crystals, J. Cryst. Growth 297 (2006) 74–79.

Eng. B 461 (2001) 250–255.[4] A. Mohanta, R.K. Thareja, Photoluminescence study of ZnCdO alloy, J. Appl. Phys.

103 (2008) p24901.

Page 7

ce Sci

[

[

[

[

[

[

[

[

[

[

[

[

[22] A. Kumar, S. Jeedigunta, I. Tarasov, S. Ostapenko, Photoluminescence studies ofepitaxial ZnO thin films on Si (1 0 0) substrates by pulsed laser deposition, AZo

A. Singh et al. / Applied Surfa

[5] A. Ohtomo, M. Kawasaki, T. Koida, K. Masubuchi, H. Koinuma, Y. Sakurai, Y.Yoshida, T. Yasuda, Y. Segawa, Mg1−xZnxO as widegap semiconductor alloy,Appl. Phys. Lett. 72 (1998) 2466–2468.

[6] A. Singh, D. Kumar, P.K. Khanna, A. Kumar, M. Kumar, Dielectric anomaly inMg doped ZnO thin films deposited by sol–gel method, J. Electrochem. Soc. 158(2011) G9–G12.

[7] S. Sadovef, S. Blumstengel, J. Cui, J. Puls, S. Rogaschewski, P. Schafer, Visibleband-gap ZnCdO heterostructures grown by molecular beam epitaxy, Appl.Phys. Lett. 89 (2006) p201907.

[8] R. Vinodkumar, K.J. Lethy, P.R. Arunkumar, R. Krishnan, N.V. Pillai, V.P.M. Pillai,R. Philip, effect of cadmium oxide incorporation on the microstructural andoptical properties of pulsed laser deposited nano structured zinc oxide thinfilms, Mater. Chem. Phys. 121 (2010) 406–413.

[9] J. Ishihara, A. Nakamura, S. Shigemori, T. Aoki, J. Temmyo, Growth and charac-terization of Zn1−xCdxO films using remote plasma MOCVD, Appl. Surf. Sci. 244(2005) 381–384.

10] T. Singh, D.K. Pandya, R. Singh, Synthesis of Cadmium oxide doped ZnOnanostructures using electrochemical deposition, J. Alloys Compd. 509 (2011)5095–5098.

11] A. Singh, A. Kumar, N. Suri, S. Kumar, M. Kumar, P.K. Khanna, D. Kumar,Structural and optical characterization of ZnO thin films deposited by sol–gelmethod, J. Optoelectron. Adv. Mater. 11 (2009) 790–793.

12] Y. Ogawa, S. Fujihara, Blue luminescence of MgZnO and CdZnO films depositedat low temperatures, J. Electrochem. Soc. 154 (2007) J283–J288.

13] Y.S. Choi, C.G. Lee, S.M. Cho, Transparent conducting Zn1−xCdxO thin films pre-pared by the sol–gel process, Thin Solid Films 289 (1996) 153–158.

14] S. Ilican, Y. Caglar, M. Caglar, M. Kundakci, A. Ates, Photovoltaic solar cell prop-erties of CdxZn1−xO films prepared by sol–gel method, Int. J. Hydrogen Energy34 (2009) 5201–5207.

[

ence 258 (2011) 1881– 1887 1887

15] Y. Caglar, M. Caglar, S. Ilican, A. Ates, Morphological, optical and electrical prop-erties of CdZnO films prepared by sol–gel method, J. Phys. D: Appl. Phys. 42(2009) p065421.

16] W.E. Mahmoud, A.A. Al-Ghamdi, S. Al-Heniti, S. Al-Ameer, The influence of tem-perature on the structure of Cd-doped ZnO nanopowders, J. Alloys Compd. 491(2010) 742–746.

17] A. Singh, D. Kumar, P.K. Khanna, A. Kumar, M. Kumar, M. Kumar, Anomalousbehavior of ZnMgO thin films deposited by sol–gel method, Thin Solid Films519 (2011) 5826–5830.

18] R. Ghosh, D. Basak, S. Fujihara, Effect of substrate-induced strain on the struc-tural, electrical and optical properties of polycrystalline ZnO thin films, J. Appl.Phys. 96 (2004) 2689–2692.

19] R. Cebulla, R. Wendt, K. Ellmer, Al-doped zinc oxide films deposited by simul-taneous rf and dc excitation of a magnetron plasma: relationships betweenplasma parameters and structural and electrical film properties, J. Appl. Phys.83 (1998) 1087–1095.

20] C. Wang, D. Xu, X. Xiao, Y. Zhang, D. Zhang, Effects of Oxygen pressure on thestructure and photoluminescence of ZnO thin films, J. Mater. Sci. 42 (2007)9795–9800.

21] W.R. Liu, Y.H. Li, W.F. Hsieh, C.H. Hsu, W.C. Lee, Y.J. Lee, M. Hong, J. Kwo, Domainmatching epitaxial growth of high quality ZnO film using a Y2O3 buffer layeron Si (1 1 1), Cryst. Growth Des. 9 (2009) 239–242.

J. Mater. 6 (2010) 1–10.23] A.F. Kohan, G. Ceder, D. Morgan, G. Chris, V. Walle, First principal study of native

point defects in ZnO, Phys. Rev. B 61 (2000) 15019–15027.