ratha, S. Belgacem a,b

2 Tuia

Cu doped ZnO

hinl sphean

02)FM

change in optical constants. From the spectroscopy impedance analysis we investigated the frequencyrelaxation phenomenon and the circuit equivalent circuit of such thin layers. Finally, all results have been

iconduthe ioe oxyg the

lms are studied as a function of copper doping concentration.In the same way, this work reported the electrical of impedance

spectroscopy measurements to model the microstructure of suchoxide lms in terms of Cu content and determine the activationenergy evolution with doping.

60 C by thered using zinstarting solut

a 102 mol/l concentration. Consecutively and under similar experimenttions, copper-doped (ZnO:Cu) thin lms solution have been prepared bhydrated copper chloride anhydrous (CuCl2, 99.9% purity) to the precursorwhile maintaining acidity level at 4.7 acetic acid. In the different elaboratedsamples, the ratio in the starting solution between Cu and Zn ([Cu]/[Zn]) elementsvaries from 1% to 3%. X-ray diffraction data of Cu doped ZnO lms were performedby a copper-source diffractometer (Analytical X Pert PROMPD) with the wavelengthk = 1.54056 A

0. Morphological aspects and surface topography of the lms were

examined by atomic force microscopy (AFM) (Park Scientic Instrument) in contactmode. The optical measurements in the UVVisible range were carried out using aShimadzu UV 3100 double-beam spectrophotometer within 3001800 nmwavelength range. Finally, the electrical measurements of real and imaginary

Corresponding author. Tel.: +216 97797763.E-mail address: mhaammar@gmail.com (A. Mhamdi).

Journal of Alloys and Compounds 610 (2014) 250257

Contents lists availab

Journal of Alloys a

.e lacoustic devices (LEDs) [36]. This paper deals with the synthesisof microcrystalline Cu-doped ZnO thin lms an appropriate atlow substrate temperature by simple and inexpensive spray pyro-lysis technique. The structural and optical properties of ZnO thin

2. Experimental details

ZnO thin lms were deposited onto glass substrate at 4spray technique [9,10]. Undoped ZnO thin lms were prepa(C4H6O4Zn, 2H2O) dissolved in isopropyl alcohol to obtain ahttp://dx.doi.org/10.1016/j.jallcom.2014.04.0070925-8388/ 2014 Elsevier B.V. All rights reserved.chemicalc acetateion withal condi-y addingsolutionis the addition of certain dopants. Transition metal elements havebeen successfully employed as dopants in ZnO such as V, Ni, Mnand Cu. In particular, the group IB transition element, copper withsimilar ionic radius to that of Zn2+ ion and electronic shell struc-ture, has many physical and chemical properties similar to thoseof Zn [2]. Zinc oxide is a very promising for optoelectronic applica-tions in UV region, especially as TCO in solar cells, gas sensors and

ZnO lms as gas sensors as reported by Gong et al. [8] for thesensitivity of CZO regarding CO toxic gas at 6 ppm. That is whywe focus our study to reach more information on structural andelectrical investigations by means of conductivity measurementsat various temperatures of CZO sprayed lms.StructureOptical constantsDielectric propertiesImpedance spectroscopy

1. Introduction

Zinc oxide ZnO is an n-type semconductor behaviour is originated byatoms at interstitial positions and thone of the efcient ways of improvindiscussed in terms of the copper doping concentration. 2014 Elsevier B.V. All rights reserved.

ctor. The n-type semi-nization of excess zincgen vacancies [1]. Also,properties of ZnO lms

This study aims to use these CZO thin lms in sensitivity appli-cations. It is reported that this type of doping may be of interest tochange native n-type of ZnO to p-type character as reported byChung et al. [7]. Moreover, this behavior leads indeed to someinteresting change in electrical properties especially the resistivityof such lms. Thus, Cu doped element can in principle reinforceKeywords:Thin lms

Doping by copper resulted in a slight decrease in the optical band gap energy of the lms and a noticeablyStudy of copper doping effects on structuand electrical properties of sprayed ZnO

A. Mhamdi a,, R. Mimouni a, A. Amlouk a, M. AmloukaUnit de physique des dispositifs semi-conducteurs, Tunis EL MANAR University, 209bCollege of Sciences and Art at ArRass, Qassim University, PO Box 53, 51921, Saudi Arab

a r t i c l e i n f o

Article history:Received 27 February 2014Received in revised form 1 April 2014Accepted 1 April 2014Available online 13 April 2014

a b s t r a c t

Copper-doped zinc oxide tsubstrates using a chemicaand optical properties of tconsist of single phase ZnOtially oriented towards (0atomic force microscopy (A

journal homepage: wwwnis, Tunisia

lms (ZnO:Cu) at different percentages (13%) were deposited on glassray technique. The effect of Cu concentration on the structural, morphologyZnO:Cu thin lms were investigated. XRD analysis revealed that all lmsd were well crystallised in wrtzite phase with the crystallites preferen-direction parallel to c-axis. The Film surface was analyzed by contact) in order to understand the effect of the doping on the surface structure.l, opticalin lms

le at ScienceDirect

nd Compounds

sevier .com/locate / ja lcom

of the sample using silver paste.

that the results shown in Table 2 are largely matched with stan-

.u)

(002)

ZnO:Cu 2%

(002)ZnO:Cu 3% Results JCPDS

(pure powderExperiment: Cu content Cu/Zn (%)

(002)

A. Mhamdi et al. / Journal of Alloys and3. Results and discussion

3.1. Structural propertiescomponents of impedance parameters (Z0 and Z00) were made over a wide range oftemperature 618708 K and frequency 10 Hz13 MHz by means of a HewlettPack-ard HP 4192 impedance analyzer. The conguration for electrical measurementswas performed using two-electrodes which were applied on the two extremities

40393837363534333231302 ()

(002)

(101)undoped

Inte

nsity

(a

(002)ZnO:Cu 1%

Fig. 1. X-ray diffraction spectra conned to the range h = [15, 20].Fig. 1 shows the XRD patterns of CZO thin lms of d = 0.2 lmthickness prepared at different Cu doping concentrations: 13%.For the undoped ZnO, it can be seen that thin lm is polycrystallinewith several strong diffraction peaks located at 2h = 34.46 and36.30 corresponding respectively to (002) and (101) directionsof ZnO in hexagonal wurtzite structure. For further claricationand comparison we have included in Fig. 1 all spectra with limitedangles range over the constructive diffraction of X-rays. In all lms(undoped and Cu doped) the crystallites have preferential c-axisorientation along (002) direction perpendicular to the plane ofthe glass substrate, this is indicated by the values of the orientationdegree of the (002) line given in Table 1. Furthermore, no phasescorresponding to Cu oxides were detected, which indicate thatcopper ions occupy substitutional positions and did not changethe hexagonal wurtzite structure in CZO lms. Also, the orientationdegree of the peak representing (002) plane was lower in the ZnO

Table 1The constructive diffraction angle and the orientation degree I002I101 values of the sprayedCu doped ZnO thin lms.

Results JCPDS(pure powderZnO)

Experiment: Cu contentCu/Zn (%)

0 1 2 3

Constructive diffractionangle 2h ()

34.43 34.46 34.47 34.51 34.5136.24 36.30 36.31 36.25

(002)(1 0 1)

Orientation degree I002I1010.56 3.43 2.53 12.67 3.75dhkl 143

h2k2hka2

2c2

r 1

d002 122

c2

q c2d101 1

43

12

a2

12

c2

r8>>>>>>>:

)c 2d002a 2cd101

3c2d2101p

(2

We note that the parameter c decreases from 5.200 in theundoped ZnO to 5.192 in ZnO:Cu 3% lms, Table 2. This decreasein c lattice parameter may be due to a possibly substitution of Zn2+

by Cu ions. Indeed, if most Cu atoms replace Zn, (002) peak posi-tion of ZnO:Cu moves to a slightly high angle as compared to that

+dard data [11]. In the same way, by using the relationship of dhklfor the hexagonal system and inclusion of Miller indices for thetwo crystallines planes (002) and (101), we deduced the appropri-ate lattice constants a and c as follows:lm than in all ZnO:Cu lms. By increasing the Copper doping con-centration, the reected intensity at (002) plane appears toincreases at 2% at the expense of those at 1% and 3%. This indicatesthat dopant incorporation affects the crystallinity of the lms, pos-sibly because copper ion Cu+ has slightly greater ionic radius(0.096 nm) than Zn2+ (0.074 nm). In addition, the position of(002) peak shifted slightly (0.05) from 2h = 34.46 in undopedZnO lm to higher angles as Cu content increases (2h = 34.51 forCZO 3%).

From these spectra, we calculated the interplanar spacing dhkl ofthe crystalline planes families (hkl) using Braggs law. We found

dhkl () 2.48 2.47 2.47 2.47(101)

a () 3.25 3.24 3.24 3.25c () 5.20 5.20 5.19 5.19 5.19ca 1.60 1.60 1.60 1.59ZnO) 0 1 2 3

Interplanar spacing 2.60 2.60 2.60 2.59 2.59Table 2The interplanar spacing dhkl and the lattice constants a and c values of the sprayedZnO:Cu thin lms.

Compounds 610 (2014) 250257 251of undoped ZnO because the ionic radius of Cu has a greater ionicradius than does Zn2+. This result is also reported by Drmosh et al.on ZnO:Cu prepared using pulsed laser deposition [12].

On the other hand, the values of full width at half maximum(FWHM) for (002) diffraction peak for CZO lms were carriedout and used in order to estimate the mean grain size by theDebyeScherrers formula [13]:

D Kkbcosh

3

where K = 0.9 and b is its FWHM. The results is reported in Table 3where it can be seen that the doping ZnO:Cu 2% exhibits the bestcrystallinity with D = 48.97 nm. A broadening in X-rayon peak withincreasing copper concentration is especially observed at 3% of Cucontent beyond which the disorder appears. Similar results wereobtained elsewhere [14].

3.2. Surface morphology

An atomic force microscope (AFM) was used to measure thesurface roughness of the lms over a 2 lm 2 lm area in contactmode. The three-dimensional (3D) images of AFM micrographs areshown in Fig. 2 where we note that all lm surfaces are rough. Theperturbed surfaces are probably due to very small droplets ofthe aerosol spray of started liquid solution which is vaporized onthe substrates for growing germs giving rise microcrystallites. Infact the thin layers are distinguished by rounded clusters separatedby the depressions that can be interpreted as crystallized zoneslimited by grain joints. We observe also nanoncrystallites on theseclusters whose density decreases especially at 2% Cu content. Thisobservation is in agreement with those found by X-ray diffractionanalysis.

We note a root-mean square (RMS) of average surfaceroughness 14.5 < dRMS < 25.2 nm in thin layers, In particular, for

Cu-doped ZnO lms the lower value corresponding to the doping2%, so that there is a compromise between the improvement ofthe crystallinity and the surface roughness. However a slightly tor-mented surface is quite favorable for the use of such componenttype in the photovoltaic conversion eld.

3.3. Optical study

3.3.1. Optical band-gapIn order to correlate the optical behavior of lms with results

obtained from X-ray and AFM analysis, the optical transmittanceand reectance measurements of undoped and doped ZnO thinlms were made in the wavelength range of 3001800 nm usingUVVis spectroscopy, Fig. 3. These lms show a high transparencywithin the visible range with an average transmittance lyingbetween 80% and 90%. This suggests that these lms indicate agood optical quality due to low scattering or absorption losses.The reectance decreases while the transmittance increases,depending on the Cu doping.

From Fig. 3, it is clear that all the samples have sharp absorptionedges in the wavelength region between 360 and 400 nm. This sug-gests that the optical band gap has shifted toward the blue regionwith incorporation of Cu. These blue shifts may be caused by achange of c lattice parameter in ZnOmatrix due to doping. An anal-ysis of the optical band-gap energy Eg of the thin lms has been

Table 3The grain size D and the roughness values of the sprayed Cu doped ZnO thin lms.

Cu content Cu/Zn (%)

0 1 2 3

FWHM b(002) () 0.12 0.17 0.10 0.17The grain size D (nm) 69.37 48.97 83.25 48.97Roughness RMS (nm) 14.69 25.49 18.68 22.35

252 A. Mhamdi et al. / Journal of Alloys and Compounds 610 (2014) 250257ZnO undopedZnO :Cu 2%Fig. 2. AFM 3D micrographs of sprayed ZnO:CuZnO :Cu 1%ZnO :Cu 3%thin lms. (x: 0.5 lm/div; z = 200 nm/div).

The calculated variation of refractive index varying with thephonon energy for all lms is shown in Fig. 5. For a given doping,we note a decrease of the refractive index values with the wave-length in the range [1.95; 2.67] whereas the extinction coefcientof lms is characterized by an increase but remains relatively lowin the domain [0.015; 0.095], Fig. 5. In fact, the slight increase inthe values of k at the long wavelengths is due to the contributionof the absorption of the free carriers which is much stronger inthe doped layers. In addition, the great values of k in the funda-400 600 800 1000 1200 1400 1600 1800

0

20

40

60

80

100

ZnO pur ZnO:Cu 1% ZnO:Cu 2% ZnO:Cu 3%T

%

R%

Wave length (nm)

Table 4Calculated values of the optical band gap Eg of the sprayed Cu doped ZnO thin lms.

Cu content Cu/Zn (%)

0 1 2 3

Eg (eV) 3.28 3.27 3.26 3.25

A. Mhamdi et al. / Journal of Alloys and Compounds 610 (2014) 250257 253made using the optical absorption coefcient a given in meanabsorption domain by the approximate formula [15]:

a 1dLn

1 R 2T

!4

As shown in Fig. 4, the (ahm)2 plots with the photon energy hmaxis are linear in mean absorption range indicating that the elec-tronic transitions are direct. From the intersection of these linearparts slope, the optical band gap Eg values were deduced to accord-ing the following relation [16]:

ahm 2 Ahm Eg 5in which A is a constant characteristic of the semiconductor. Theobtained Eg values are given in Table 3 where it can be seen thatthe value of Eg decrease with Cu doping 2%. In fact, theoretical calcu-lations based on the density functional theory (DFT) show well thatthe band gap is narrowed by Cu doping in ZnO [17] (see Table 4).

3.3.2. Refractive index and extinction coefcientThe refractive index and the extinction coefcient of lms have

been obtained by tting from the transmittance and reectancespectra of ZnO:Cu thin lms in the spectral domain varying from

Fig. 3. Transmission and reection spectra of doped sprayed ZnO:Cu thin lms.300 to 1800 nm. For these calculations, we have used the methodof Belgacem et al. based on the Muller numerical method of reso-lution of non-linear equations [15].

2,0 2,2 2,4 2,6 2,8 3,0 3,2 3,40

1x1013

2x1013

3x1013

4x1013

5x1013

()2

(eV.

cm-1)

h (eV)

ZnO pur : Eg=3.28 eV ZnO:Cu 1%: Eg=3.24 eVZnO:Cu 2%: Eg=3.16 eVZnO:Cu 3%: Eg=3.18 eV

Fig. 4. Variation of the absorption (ahm)2 as a function of the light energy hm ofsprayed ZnO:Cu thin lms.mental absorption region (low wavelength k < 4 lm) are due tothe intrinsic absorption for the higher energy gap. In the visible,the low value of k implies that these layers are transparent asshowed in the transmission spectra.

We also observe a dependence on wavelength of the refractiveindex and the extinction coefcient with the doping concentration.This phenomenon can be explained on the basis of the contributionfrom both lattice parameter change and the presence of defaults inthe prepared lms. In particular, we note a decrease in the extinc-tion coefcient when Cu content increases, which may be due tothe increase in the average crystallite size in the ZnO lms afterCu doping and a decrease in the surface roughness effect as shownin Fig. 2.

3.3.3. Dielectric constantFrom the refractive index and the extinction coefcient results,

we have analyzed the real and imaginary parts of complex dielec-tric constant e(x) of pure and doped ZnO with copper at differentconcentrations using the following relations:

ek nk ikk2 e1k ie2k 6

e1 k n k 2 k k 2 7

e2 k 2n k k k 8For all samples, it is found that in infrared range the dispersion of

e1 is linear function of the square of the wavelength k2 while theabsorption e2 is linear with k3, Fig. 6. This behavior is in good agree-ment with the classical theory of the dielectric constant whichexpressed by the following system in the near infrared (xs 1) [15]:

0,3

0,4

0,5

0,6

0,7

k

2,2

2,3

2,4

2,5

2,6

2,7

n200 400 600 800 1000 1200 1400 1600 1800

0,0

0,1

0,2

Wave length (nm)

1,8

1,9

2,0

2,1

Fig. 5. Refractive index n(k) and extinction coefcient k(k) of ZnO doped Cu thinlms.

e1 e1 e1x2p4p2c2

k2 9

e2 e1x2p8p3c3s

k3 10

where e1 is the dielectric constant at high frequencies, xp the pul-sation plasma and the relaxation s time.

The extrapolation of the curve of e1 at low wavelengths pro-vides the dielectric constant e1 which shows a decrease from5.22 to 4.52, Table 5. We note the same variation of the plasmapulse xp with doping deduced from the slope of the same curves.In particular, the different values of xp correspond to the wave-lengths k higher than 1.8 lm, this explains the non decay of thetransmission in the area of relative transparency to visible.

From the slope of the linear dependence of the e2(k3) and theknowledge of xp we have determined the relaxation time swhose the values are gathered in Table 5. In the same table weintroduced the values of the free carrier concentration to effectivemass ratio Nm which is calculated from the following well-knownequations:

0 1x109 2x109 3x109 4x109 5x1090,0

0,1

0,2

0,3

0,4

0,5

0,6

ZnO pur ZnO:Cu 1% ZnO:Cu 2% ZnO:Cu 3%

2

3(nm3)0,0 5,0x105 1,0x106 1,5x106 2,0x106 2,5x106 3,0x10

6 3,5x1062

3

4

5

6

7

8

9

10

ZnO pur ZnO:Cu 1% ZnO:Cu 2% ZnO:Cu 3%

1

2(nm2)

Fig. 6. Variation of the real and imaginary parts e1 and e2 of dielectric constant of sprayed Cu doped ZnO as a function of k2 and k3 respectively.

Table 5Calculated values of e1, xp and other constants.

e1 xp (1014 rad s1) s (1014 s) Nm (1047 g1 cm3)

ZnO undoped 5.22 5.95 7.79 6.38ZnO:Cu 1% 4.71 4.25 0.73 2.95ZnO:Cu 2% 4.47 4.67 1.01 3.37ZnO:Cu 3% 4.09 3.13 0.34 1.39

0

100

200

300

400CuZN 1% T=638 K

T=648 K T=658 K T=668 K T=678 K T=688 K T=698 K T=708 K

Z'' (

Koh

m)

8 10 12 14 16 186 8 10 12 14 16 180

100

200

300

400

500

600 T=628 K T=638 K T=648 K T=658 K T=668 K T=678 K T=688 K T=698 K T=708 K

Z'' (

Koh

m)

ZnO undoped

254 A. Mhamdi et al. / Journal of Alloys and Compounds 610 (2014) 250257500

600

700

CuZNO 2% T=618 K T=628 K T=638 K T=648 K

Ln ()8 10 12 14 16 180

100

200

300

400 T=658 K T=668 K T=678 K

Z'' (

Koh

m)

Ln ()

Fig. 7. Angular frequency dependence of Z00 of sprayLn ()

8 10 12 14 16 180

100

200

300

400

500

600CuZNO 3% T=618 K

T=628 K T=638 K T=648 K T=658 K T=668 K T=678 K

Z'' (

Koh

m)Ln ()

ed ZnO:Cu thin lms at different temperatures.

x2p Ne2

11

When k decrease away from the region of the near infrared, weremark a deviation from the evolution of two parts of the dielectricconstant under the classical theory particularly the quadratic lawof e1. This difference arises from the fact that this region need tobe involved in the relationships (9) and (10) the relaxation times which depends on the conduction mechanism of carriers inthe optical and acoustic phonons, lattice defects and ionizedimpurities.

3.4. Complex impedance analysis

3.4.1. Impedance analysisThe ColeCole plots of imaginary part Z00 with frequency for CZO

at different working temperatures are plotted in Fig. 7. It is stillremarkable that the imaginary part Z00 increases with frequencyreaching a maximum peak Z00max at the relaxation frequency xmafter what it decreases as the temperature increases. Moreover,the position of the relaxation peak shifts towards higher frequen-cies with increasing temperature while Z00max values decrease.

In fact, the angular relaxation frequency obeys to the well-known Arrhenius law [18]:

xm xoeEaKT 12

wherexo is a constant and Ea is the thermal activation energy of thecarriers charge. In this case, Ea represents the difference betweenthe trap level and the conduction band. As shown in Fig. 8, theexpression of Lnxm f 1000T

leads to a linear function, in good

agreement with expression Eq. (12). The calculated values of theactivation energy vary slightly around the value 0.78 eV with aslight increase for 2% Cu content, Table 6. A similar frequency

1,40 1,44 1,48 1,52 1,56 1,60

13,2

13,6

14,0

14,4

14,8

15,2

15,6

16,0ZnO undopedZnO:Cu 1%ZnO:Cu 2%ZnO:Cu 3%fit ZnO undopedfit ZnO:Cu 1%fit ZnO:Cu 2%fit ZnO:Cu 3%

Ln (

m)

1000/T (K-1)

Fig. 8. Temperature dependence of angular frequency relaxation of sprayed ZnO:Cuthin lms for different doping.

Table 6Calculated values of activation energy Ea.

Cu content Cu/Zn (%) Ea (eV)

0 0.87 0.0081 0.65 0.0452 0.90 0.0343 0.71 0.028

A. Mhamdi et al. / Journal of Alloys and Compounds 610 (2014) 250257 255eoe1m

where eo is the vacuum dielectric constant.

6000 200 400 600 800 1000 1200 14000

100

200

300

400

500 T=628 K T=638 K T=648 K T=658 K T=668 K T=678 K T=688 K T=698 K T=708 K

Z' (kohm)

Z'' (

Koh

m)

ZnO undoped

(a)

0 200 400 600 800 1000 1200 14000

100

200

300

400

500

600 T=618 K T=628 K T=638 K T=648 K T=658 K T=668 K T=678 K

Z'' (

Koh

m)

Z' (kohm)

CZO 2%

(c)Fig. 9. Complex impedance spectra of sprayed Zdependence of relaxation time in Fe-doped ZnO thin lms has beenobserved by Hassan et al. [19].

0 200 400 600 800 10000

50

100

150

200

250

300

350

400 T=638 K T=648 K T=658 K T=668 K T=678 K T=688 K

Z'' (

Koh

m)

Z' (kohm)

CZO 1%

(b)

0 200 400 600 800 1000 1200 14000

100

200

300

400

500

600 T=618 K T=628 K T=638 K T=648 K T=658 K T=668 K T=678 K

Z'' (

Koh

m)

Z' (kohm)

CZO 3%(d)nO:Cu thin lms at different temperatures.

)and1,0

1,2ZnO undopedZnO:Cu 1%ZnO:Cu 2%ZnO:Cu 3%

)

256 A. Mhamdi et al. / Journal of Alloys3.4.2. The equivalent ac circuitThe Nyquist plots for CZO with working temperature are dis-

played in Fig. 9. The plots drawn between imaginary Z00 and realparts Z0 of the impedance show semicircular arcs whose theirradius decreases with increase in temperature. The proper inter-pretation of the given impedance spectra allowed us to determinethe equivalent ac circuit of this polycrystalline CZO thin lmdeposited on glass substrate composed by a parallel resistor R

(a)

0,2

0,4

0,6

0,8

R (M

ohm

T (K)620 640 660 680 700 720

620 640 60,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

C (p

F)

Fig. 10. The variation of the capacitance and parallel resistanc

0 200 400 600 8000

50

100

150

200

250

300

350

400

ZnO undopedZnO:Cu 1%ZnO:Cu 2%ZnO:Cu 3%

Z" (K

ohm

)

Z' (Kohm)

Fig. 11. Complex impedance spectra of sprayed ZnO:Cu thin lms at T = 648 K.1,0 T=638 K T=648 K T=658 K T=668 K T=678 K

Compounds 610 (2014) 250257and capacitor C network connected. This parallel circuit RC repre-sents the contribution of the grain boundaries delineating the ori-ented columnar microcrystallites along c-axis.

The experimental values of the above parameters were found asfollow: the capacitance was determined from the associated fre-quency at the maximum data point Z00max of the curve on the realaxis (Fig. 10), the second intercept gave the value of resistance R.

(b)

0,0

0,5

1 2 3

R (M

ohm

0Cu content Cu/Zn (%)

60 680 700 720

ZnO undopedZnO:Cu 1%ZnO:Cu 2%ZnO:Cu 3%

T (K)(c)e of sprayed ZnO:Cu thin lms at different temperatures.

0 200 400 6000

50

100

150

200

250

300

350

400

ZnO:Cu 2% Fit curve ZnO:Cu 2%

Z" (K

ohm

)

Z' (Kohm)

Fig. 12. The complex impedance spectra theoretical and experimental of ZnO:Cu 2%at T = 648 K.

It was noted that R value decreased dramatically with the compo-sition as shown in Fig. 10. Essentially, it appears from this studythat the low value of R corresponds to the doping of 2% indicatingthe homogeneity of the layer. We observe this clearly in Fig. 11where the Nyquist plots were compared for the various percent-ages of doping at the same temperature T = 648 K.

To check the validity of this approach, we have introduced theexperimental values of R and C in the known relationships of thereal and imaginary parts of parallel circuit RC complex impedanceand calculated the theoretical variation of Z00(Z0) which shows agood agreement with experiment for example as illustrated inFig. 11 at 2% doping. These observations are consistent with theresults of XRD (see Fig. 12).

4. Conclusion

We have investigated the structural and the optical propertiesof Cu-doped ZnO thin lms deposited by the spray pyrolysis tech-nique. It has been found that these properties depend mainly onthe Copper-to-Zinc ratio. All lms were crystallized in the hexago-nal wurtzite structure with preferential c-axis orientation along(002) direction which is quite dominant especially for 2% Cucontent. On the other hand, the optical study showed that thedeposited lms have a relatively high absorption coefcient(aP 104 cm1) and exhibit a direct transition gap. The resultsobtained by impedance measurements of sprayed ZnO thin lmsare discussed in terms of the copper content and it found that theyconrm those obtained by XRD particularly the structure type. Thesame again this study has provided us the electro-optical parame-ters consistent with the results of structure and interesting for theuse of such materials as a window of light in solar cells and sensors

electromagnetic radiation. Finally the behaviour of the spectra ofthe components of the dielectric constant needs more investiga-tions particularly in infra-red region but we can consider before-hand that this result is quite encouraging since a costless andsimple used depositing technique.

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Study of copper doping effects on structural, optical and electrical properties of sprayed ZnO thin films1 Introduction2 Experimental details3 Results and discussion3.1 Structural properties3.2 Surface morphology3.3 Optical study3.3.1 Optical band-gap3.3.2 Refractive index and extinction coefficient3.3.3 Dielectric constant

3.4 Complex impedance analysis3.4.1 Impedance analysis3.4.2 The equivalent ac circuit

4 ConclusionReferences