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8/19/2019 3. Arif Wahya Handoko_The Crystalline Structure, Conductivity and Optical Properties of Co-doped ZnO Thin Films

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8/19/2019 3. Arif Wahya Handoko_The Crystalline Structure, Conductivity and Optical Properties of Co-doped ZnO Thin Films

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5866 S. Benramache et al. / Optik 125 (2014) 5864–5868

Fig.3. Thevariationof crystallitesize,the FWHMand theresidualstressof undoped

and Co-doped ZnOthin films with doping level.

tures [19,20]. The average crystallite sizeof the films fromthe XRD

was calculated using the Scherer’s equation [21]:

D =0.9

ˇ cos  . (2)

where D is the crystallite size, is the X-ray wavelength

(=1.5406 A), ˇ is the full width at half-maximum (FWHM), and

 is Bragg angle of (002) peak.According to thehexagonal symmetry of the ZnOstructure, the

lattice constant can be calculated by the following formula [22]:

dhk l  =

4

3

h2+ hk+ k2

a2  +

l2

c 2

−1/2

. (3)

wherea, c arethelatticeparameters,(h,k, l) aretheMiller indices

of the planes and dhk l is the interplanar spacing.

Additional informationon structural properties canbe obtained

from the residual stressmeasurementscalculated by the following

formula [23].

 = 450×c 0 − c 

c   . (4)

where   in (GPa) is the mean stress, c and c 0   are the latticesconstant of the ZnO thin film and the bulk material (standard

c 0 = 0.5206nm). The calculated data of average mean stress   issummarized in Table 1.

In Fig. 3 we have reported the variation of the crystallite size,

the full width-at-half-maximum (FWHM) and the residual stress

asa functionof dopinglevel. As itcan beseen, the variation of crys-

tallite size is opposite to the fullwidth-at-half-maximum(FWHM).

From 0 to 1wt%Co dopinglevels the crystallite size decreases from

63.99 to 29.61 nm (see Table 1) then increases to reach 59.42nm

as maximum value of 2 wt% and decreases further than the doping

level increase. Thevaluesof residual stressdecreaseswith increas-

ing of cobalt concentration from 0 to 1wt% then increases with

3wt% whereas the lattice parameter of the films decreases indi-

cating that the stress is done along the c -axis. The increase in thecrystallite size reveals the enhancement of the crystallinity under

c -axis orientation of ZnO thin films.

 3.2. The optical properties of Co-doped ZnO thin films

Fig.4 showstheoptical transmissionspectraofundopedandCo-

doped ZnO as a function of doping level. As can be seen, a region

of strong transparency is located between 400–800nm; the value

of the average transmission is about 70–95%, then decreases in

the region of the absorption edge, in the layers (360–400nm) the

decrease is dueto thetransition between the valence band andthe

conduction band revealing the onset fundamental absorption. At

nakedeye,we arenotingthat thedopingeffectwasclearlyobserved

in the layer quality; hence the smoothness and homogeneityof the

Fig. 4. Transmission spectra of Co-doped ZnO thin films as a function of cobalt

concentration Co/Zn:(a) 0, (b) 1, (c) 2 and (d) 3wt%.

Fig. 5. The typical variation of ( Ahv)2 vs. photon energy hv for undoped ZnO thin

film. The inset shows the drawn of ln  A as a function of photon energy (hv) for

deducing Urbachenergy.

layers reached an optimal state for theelaborated doped filmwith

2 wt%.

In order to investigate theeffectof Co concentration on ZnO:Cofilms further, the optical band gap energy (E  g ) was measured from

the transmission spectra using the following relations [24]:

 A = ˛d = − lnT, (5)

( Ah)2 = C (h − E  g ), (6)

where A is theabsorbance,d is the filmthickness;T is the transmis-

sion spectra of thin films; ˛ is the absorption coefficient values; C 

is a constant, h is the photon energy and E  g  the band gap energy

of the semiconductor. As it was shown in (Fig. 5) a typical varia-

tion of( Ah)2 as a function of photon energy (h) of undoped ZnO

thin film, the optical band gap was determined by extrapolation

of the linear region to (Ah)2 =0 [25]. Besides, we have used the

Urbachenergy (Table 2), which is related to the disorder in the filmnetwork and expressed as [15]:

 A = A0 exp

h

E u

, (7)

 Table 2

The band gap energy E  g , the Urbach energy E u   and electrical conductivity   for

ZnO:Co thin films were measured as a function of doping level.

Doping level (wt%) E  g  (eV) E u  (meV)   ( cm)

0 3.367 085 7.71

1 3.278 267 7.19

2 3.319 040 8.33

3 3.295 166 7.86

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S. Benramache et al. / Optik 125 (2014) 5864–5868 5867

Fig. 6. The variation of optical band gap and Urbach energy of undoped and Co-

doped ZnOthin films with doping level.

where  A0   is a constant and E u   is the Urbach energy, the latter

decreased with increasing the band gap indicating the decrease of 

defects in the elaborated films as shown in (Table 2).

Fig. 5 showsa plot of absorbance ( Ah)2 vs.photonenergy (h)

for undoped ZnO thin films. The linear dependence of  A on h at

higherphotonenergiesindicates thatthefilmsareessentiallydirect

transitionn-type semiconductors. Extrapolationof linearportionof the graph to the energy axis at A= 0 in the range between 360 and

380 nm giving the band gap energy E  g . The inset show the drawn

of ln A as a function of photon energy (hv) fordeducing the Urbach

energyE u. Thedetermined values of E  g  and E u are showninTable2.

In Fig. 6 we have reported the variation of band gap (E  g ) for

the elaborated films as a function of the doping level. It could be

noticedthatthebandgapenergyof the filmsdecreasefrom 3.367to

3.278 eV with increasing Co doping from 0 to 1 wt%, then increase

to 3.319eV for 2wt%. This variation in band gap with the cobalt

concentration may be explained in terms of Moss–Burstein effect

(band values should increase with Co concentration, but E  g  value

decreasing with doping of 3wt%). This may be explained by the

increasing of thecrystallite size (see Fig. 3). Ascanbe seenin Fig. 6,

a minimum Urbach energy was reached with Co-doped ZnO thin

films at 2wt%, which means that this doping level (2wt%) leads to

a less disorder; we have as consequence large band gap and vice

versa as it was expressed in the literatures [15,26,27].

 3.3. The electrical conductivity of Co-doped ZnO thin films

Fig.7 shows thevariationofelectricalconductivity ofundoped

andCo-dopedZnOfilmsasafunctionofdopinglevel.Ascanbeseen,

the conductivity decreases from 7.71 to 7.19( cm)−1 when the

Co concentration increases from 0 to 1wt% then increase to reach

8.33( cm)−1 for2 wt% dopinglevel. Theslight variation(increase)

in the conductivityof thefilmshas been explainedby displacement

of electrons[28] which arecoming from the ions Co2+ donorsin the

Fig. 7. Electrical conductivity  ofundoped and Co-dopedZnO thinfilmsas a func-

tion of cobalt concentrations.

substitutional sites of Zn2+, as a result thecarrier density increases.

Moreover, thedecrease of the electrical conductivity with increas-

ing of the doping level may be explained by a segregated part of 

cobalt atoms into the grain boundaries which acts in increasing of 

the potential barriers, this interpretation is consistent with similar

results obtained elsewhere [26–31].

4. Conclusions

Inconclusion,highly transparentconductiveCo-dopedZnOthin

films have been deposited on glass substrate by ultrasonic spray

at a substrate temperature of 350 ◦C. The structural, optical and

electrical properties were investigated; these properties of thin

films were improved by annealing temperature at 500 ◦C. All the

films are nanocrystalline-hexagonal structure wurtzite and with

strong (00 2) orientation; the maximum value of crystallite size

G=63.99nm is attained with undoped ZnO film. The value of the

average transmission is about 70–95% in the visible region. The

band gap energydecreased from E  g = 3.367 to3.319eV forundoped

andCo-doped ZnOthinfilmsat2wt%. Theelectrical conductivityof 

thefilmsincreased from 7.71 to8.33 ( cm)−1. ImprovedCo-doped

ZnO films are achieved with 2wt% as amount of Co doping level.

 Acknowledgments

This work was supported in part by the National Project

Research (PNR) andVTRS laboratory of El-OuedUniversity, Algeria.

X-ray diffraction data in this work were acquired with an instru-

ment supported by the University of Biskra. We thank Prof. S.

Rahman and B. Gasmi (Biskra Universty) for the assistance in XRD

data acquisition.

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