Journal of Luminescence

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Effects of stabilizer ratio on photoluminescence properties of sol-gel ZnO nano-structured thin ﬁ lms

F. Boudjouan

^a

, A. Chelouche

^a,n

, T. Touam

^b

, D. Djouadi

^a

, S. Khodja

^b

, M. Tazerout

^a

, Y. Ouerdane

^c

, Z. Hadjoub

^b

aLaboratoire de Génie de l'Environnement, Université de Bejaia, 06000 Bejaia, Algerie

bLaboratoire des Semi-conducteurs, Université Badji Mokhtar, BP 12, Annaba 23000, Algerie

cLaboratoire Hubert Curien, Université Jean Monnet, 42 000 Saint-Etienne, France

a r t i c l e i n f o

Article history:

Received 26 June 2014 Received in revised form 7 September 2014 Accepted 12 September 2014

Keywords:

ZnO thinﬁlms Sol–gel Stabilizer ratio Photoluminescence

a b s t r a c t

Nanostructured ZnO thinfilms with different molar ratios of MEA to zinc acetate (0.5, 1.0, 1.5 and 2.0) have been deposited on glass substrates by a sol–gel dip coating technique. X-ray diffraction, Scanning Electron Microscopy, UV–visible spectrophotometry and photoluminescence spectroscopy have been employed to investigate the effect of MEA stabilizer ratio on structural, morphological, absorbance and emission properties of the ZnO thin films. Diffraction patterns have shown that all the films are polycrystalline and exhibit a wurtzite hexagonal structure. Thecaxis orientation has been enhanced with increasing stabilizer ratio. SEM micrographs have revealed that the morphology of the ZnOfilms depend on stabilizer ratio. The UV–visible absorption spectra have demonstrated that the optical absorption is affected by stabilizer ratio. The photoluminescence spectra have indicated one ultraviolet and two visible emission bands (green and red), while band intensities are found to be dependent on stabilizer ratio. ZnO thinfilms deposited at MEA ratio of 1.0 show the highest UV emission while the minimum UV emission intensity is observed in thinfilms deposited at ratio of 0.5 and the maximum green has been recorded forfilms deposited at MEA ratio of 2.0.

1. Introduction

Zinc oxide (ZnO) is considered as a promising semiconductor owing to its interesting properties including the stable wurtzite structure, wide and direct band gap (3.37 eV), large exciton binding energy (60 meV) and high thermal and chemical stability.

These properties made ZnO thinﬁlms an excellent candidate for various technological applications such as transparent electrodes [1], light-emitting diodes in the blue and UV regions[2,3], thin ﬁlms transistor[4], ultraviolet photoconductive detectors[5], solar cells[6–9], piezoelectric transducers[10]and gas sensors[11].

ZnO thinﬁlms have been prepared using numerous methods such as molecular beam epitaxy (MBE)[12], pulsed laser deposition (PLD)[13], metal-organic chemical vapor deposition (MOCVD) [14], magnetron sputtering[15], electron beam evaporation [16], spray pyrolysis [17], electrochemical deposition [18,19], hydro- thermal method [20] and sol–gel process [21–25]. The sol–gel route is the most widely used for its simplicity, low cost and

capability to produce thin, transparent and homogenousﬁlms of different compositions on silicon and glass substrates. However, the properties of theﬁlms prepared by the sol–gel process might be affected by a number of factors including sol aging time[26,27], nature and concentration of the precursor[28,29], type of solvent [30], preheating and post-annealing temperature[31,32].

The use of additives in thin films preparation promotes the dissolution of the precursor, controls the rate of sol–gel and supportsfilm formation during the coating process. Furthermore, the nature of the stabilizer has different effects on structural, morphological and optical properties of ZnO thinfilms. Few works have been reported in the literature regarding the influence of amino-alcohols on ZnO nanostructures properties [33–37]. For example, Yahia et al.[35]have reported the dependence of ZnO thinfilms optical constants on MEA to Zn ratio (r) and Thongsur- iwong et al. [37] have studied the influence of MEA ratio on morphological properties of ZnO powder. However, to the best of our knowledge, the effect of MEA ratio on the photoluminescence (PL) properties of ZnO thinfilms has not yet been reported.

In this paper, ZnO thinﬁlms were prepared by the sol–gel method and deposited on glass substrates by the dip-coating technique. The inﬂuences of stabilizer molar ratio on the structural, morphological Contents lists available atScienceDirect

journal homepage:www.elsevier.com/locate/jlumin

Journal of Luminescence

nCorresponding author. Tel.:þ213 556 51 98 68.

E-mail address:azeddinechelouche@gmail.com(A. Chelouche).

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and photoluminescence properties of the prepared ZnO thinﬁlms are investigated.

2. Experiments

ZnO thin films were prepared by the sol–gel process. As a starting material, zinc acetate dihydrate (Zn(CH3COO)22H2O) was dissolved in a mixture of ethanol and mono-ethanolamine (MEA, C₂H₇NO). The MEA to Zn ratios (r) were varied between 0.5 and 2.0. Thefinal concentration of Zinc acetate in the solution was kept at 0.75 M. All prepared ZnO sols were maintained under continuous magnetic stirring at 501C for 1 h. The substrates were washed with a liquid detergent, rinsed with distilled water, and then immersed in 4 M nitric acid for 24 h. They were then ultrasonically cleaned and rinsed in ethanol and distilled water during 15 min at 601C and,finally, were dried at 1001C for 2 h.

The deposition was carried out using a KSV dip-coater with a withdrawal speed of 1.5 cm/min and the number of deposited layers on the glass substrates was set to eight. The deposited ﬁlms were preheated at 2001C for 10 min after each coating. The ﬁlms were subsequently heated up to 5001C for 4 h in order to obtain crystallized ZnO.

The prepared thinﬁlms were characterized by X-ray diffraction (XRD) using a PANalytical's X-ray diffractometer operating at 40 kV and 30 mA using Cu K_αradiation (

λ

¼1.54 Å). The character- ization by Scanning Electronic Microscopy (SEM) was performed with a Raith PIONEER System. The optical absorption spectra were obtained using a Safas UVmc²UV–visible spectrophotometer and the PL measurements were carried out at room temperature using a He–Cd laser 325 nm.

3. Results and discussion

Fig. 1 shows XRD spectra of ZnO thin ﬁlms prepared with different molar ratios of MEA to zinc acetate. All visible peaks are indexed as characteristic peaks of a polycrystalline ZnO hexagonal wurtzite structure. The broad peaks observed in the XRD patterns indicate small size particles. It is noticeable that increasing the amount of MEA, the intensity of (0 0 2) increases which demonstrates the enhancement in c-axis preferred orientation. This preferred orientation can be explained by the improvement of the formation of zinc oxy-acetate by the large amount of MEA in the sol. It is well known that in general the role of complexing agent (such as MEA) is to facilitate the Zn^2þ condensation and

promote the formation of ZnO due to the presence of amine which increases the pH of the solution. Since the (0 0 2) plane of ZnO has the minimum surface energy, higher ratios of MEA leads to higher condensation rate of Zn²^þions along the (0 0 2) plane resulting in higherc-axis orientation. The enhancement ofc-axis orientation with increasing MEA ratios has been also reported by Hosseini Vajargah et al.[34]and Sagar et al.[36]. Moreover, the increase in XRD peaks intensity is due to the increase in thefilm thickness with increasing MEA ratio as confirmed by the thickness investiga- tions carried out by mechanical surface profiling. The measured thickness was of the order of 180, 220, 225 and 240 nm for the thin films deposited at ratio of 0.5, 1.0, 1.5 and 2.0, respectively. It has been also reported that the pH of the sol increases with increasing MEA ratio[38]which may lead to an increase infilm thickness[39].

The crystallite size may be estimated from the full width at half-maximum of (0 0 2) diffraction peak using Scherer's formula [40]:

D¼ 0:89

λ

β

^cos

θ

^ð¹^Þ

where

λ

is the X-ray wavelength,

β

is the full width at half- maximum of the XRD peak and

θ

is the Bragg diffraction angle.

The XRD analysis revealed that the calculated crystallite size is between 17.6 and 12.4 nm. As shown inFig. 2the crystallite size decreases with increase in the stabilizer molar ratio.

The scanning electron micrographs of the ZnOﬁlms deposited at different molar ratios of MEA to zinc acetate are shown inFig. 3.

All the samples have a smooth and uniform morphology. The grains dispersion in the films is homogeneous. We can observe also the presence of some pores in thefilms deposited using sols containing MEA to Zn ratios lower than 2.0. In the case of the ZnO thinfilm deposited at MEA ratio of 2.0, there are no pores and the surface seems to be well compact. The micrographs reveal that the grains forming the ZnO films deposited at a stabilizer ratio of 0.5 are statistically bigger than those forming the other ZnO thin films. However, we can note some large grains in ZnO film deposited using a sol prepared with a MEA ratio of 2.0. The grains forming the films deposited at r¼1.0 and r¼1.5 are relatively small. We can explain this behavior by the fact that when a low amount of stabilizer is used, the number of stabilizer molecules remains insufficient to fully interact with the growth species; then some ZnO nuclei can grow further. It is well established that MEA prevents uncontrollable growth of particles and also avoids particles aggregation. Thus, once the ratio is less than the unity, the ZnO crystallites agglomerates to form large

30 35 40 45 50 55 60

(101) (002)

(100)

Intensity (a.u.)

2 (°)

r=0.5 r=1.0 r=1.5 r=2.0

θ

Fig. 1.XRD patterns of ZnO thinﬁlms prepared with different stabilizer molar ratios.

Crystallite size (nm)

Stabilizer molar ratio

Fig. 2.Crystallite size of ZnO thinﬁlms prepared with different stabilizer molar ratios.

F. Boudjouan et al. / Journal of Luminescence 158 (2015) 32–37 33

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grains. However, at moderate concentration of MEA in the sol (r¼1.0 andr¼1.5), stabilizer molecules can fully interact with all growth species and hence control growth rate and particle size.

The result of these interactions leads to the formation of the small grains. In the case of higher ratio (r¼2.0), the MEA retarded the condensation process. However, it promotes the formation of ZnO since the presence of amine increases the pH of the solution[36].

We believe that the delayed condensation results in the develop- ment of few large crystallites because the nuclei's are not formed at the same time. From XRD patterns and SEM micrographs, it is clearly seen that ZnO deposited thin films arefine grained and consisted of agglomeration of several crystallites. Therefore, the obtained samples contain very developed grain boundaries and free surfaces. These defects are sensitive to the stabilizer molar ratio which may affect ZnO thin films properties. It has been recently demonstrated that the physical properties of nanograined ZnO strongly depend on the presence of interphase and grain boundaries[41,42].

Fig. 4displays the optical absorption spectra of ZnO thinfilms deposited at different stabilizer molar ratios. It is clearly seen that all ZnO thinfilms exhibit an excitonic absorption at about 375 nm. It is also noticeable that the films show a maximum absorption at 350 nm. However the intensity of this maximum is dependent on the ratio: thinfilms deposited atr¼0.5 shows the lowest absorption values and the one deposited at MEA ratio of 1.0 demonstrates the highest ones at lower wavelengths. We can also observe that the absorption of the thinfilms deposited atr¼1.5 is greater than those prepared atr¼2.0. It has been recently demonstrated by Zawadzka et al.[43]that the assignments of the exciton peaks may be also verified by the temperature evolution of the PL spectra.

Fig. 5(a) displays PL spectra in wavelengths ranging from the ultraviolet (UV) to the near infrared for all the prepared thinﬁlms.

In order to better see the effect of stabilizer ratio on the UV emission, the emission spectra in the UV region only is depicted in Fig. 5(b). All the PL spectra consist of sharp emission in the UV

range and a broad emission bands in the visible region ranging from 1.45 to 2.75 eV (450 to 850 nm). It is clear that the widest visible emission is recorded for MEA ratio of 2.0. We can also note that the UV emission is maximum forr¼1.0 and is minimum for r¼0.5. A slight red-shift of the emission forr¼0.5 is also observed at 3.19 eV (388 nm). In addition, the UV emission of the sample deposited atr¼2.0 seems to be a superposition of two bands, the ﬁrst band is centered at 3.26 eV (380 nm) while the second one is centered at 3.19 eV. Such a red-shift may be due to the increase of the particle size. The dependence of grain size on ratio is in agreement with XRD patterns and SEM micrographs.

Fig. 6(a) shows the dependence of the UV and visible emission intensity on MEA ratio. The increase of MEA ratio from 0.5 to 1.0, greatly enhance the UV emission peak. Its full width at half- maximum (FWHM) is increased, then the UV peak is weakened again and its FWHM is decreased. However, the latter remains higher than of the one corresponding to the ratio of 0.5. The visible Fig. 3.SEM micrographs of ZnO thinﬁlms prepared with different stabilizer molar ratios: (a)r¼0.5, (b)r¼1.0, (c)r¼1.5 and (d)r¼2.0.

Fig. 4.Optical absorption spectra of ZnO thinﬁlms deposited at different stabilizer molar ratios.

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emission seems to have an opposite behavior. It is largely accepted that the UV emission belongs to the near band edge emission (NBE) and originate from the recombination of free excitons.

While the visible emission arises from the recombination of holes with electrons trapped at defect levels. A method generally used for evaluating the concentration of defects in ZnO thinfilms is based on the ratio of the PL intensity of UV emission to the visible one. Fig. 6(b) depicts the intensity ratio of the UV to visible emission (Iuv/Ivisible) of the ZnO thin films as a function of MEA ratio. This intensity ratio reaches a maximum at an MEA ratio of 1.0 after it decreased at MEA ratios of 1.5 and 2.0. The larger Iuv/Ivisibleratio implied the higher quality of ZnOfilms[44], which means that the best ZnO stoichiometry is obtained for the ratio of MEA to Zn of 1:1.

In order to clarify the different visible emission bands and study their evolution behavior with the MEA ratio, we carry out a Gaussian curveﬁtting for these visible emission spectra as shown inFig. 7. All the samples show two emission bands including green (2.3 eV) and red (1.9 eV). A small emission peak is observed in the red range centered on 1.6 eV. In the literature, it is well established that the visible emission due to the defect emission arises from the recombination of holes with electrons trapped at level defects in the gap. However, there is still no consensus on the origin and the energy levels within the ZnO gap. In addition, it is widely admitted that inn-type ZnO, the most favorable defects are oxygen vacancy ðVoÞ and zinc vacancyðVznÞ [45], yet; the majority of researchers consider that the green emission (centered on 2.3 eV) can be

attributed to the transition related withVo [18,46], while the red emission maybe related the transition between single and double ionized oxygen vacancies ðV_o^þandV_o^{þ þ}Þ and zinc vacancy (Vzn) acting as a deep acceptor [47]. Based on these works, we may consider that the emission at 1.6 eV results from transition between V_o^{þ þ} and Vzn, while the one at 1.9 eV originates from transition between V_o^þ andVzn. The V_o^þ state is unstable but can acts as a metastable state during optical illumination[45].

From Fig. 7(a), it can be seen that when the red emission intensity increases, the green one decreases indicating thatVoand Vznare greatly affected by MEA ratio. The MEA ratio of 0.5 favors the formation of large density ofVznbecause the number of MEA molecules is insufﬁcient to fully interact with the growth species and consequently not stabilized. The result is the formation of large ZnO crystallites with a large density of zinc vacancies. For MEA ratio equal to 1.0, all growth species are stabilized and the resultingﬁlm is of higher quality (Fig. 6(b)). However, when a large amount of MEA is used, the organic molecules ones evaporated increases the oxygen vacancy density which may explain the enhancement of the green light emitted by the sample deposited at MEA ratio of 2.0.

4. Conclusion

In this work, effects of stabilizer molar ratios on the structural, morphological, optical and photoluminescence properties of sol–gel

0.5 1.0 1.5 2.0

400 600 800 1000 1200 1400

MEA ratio (r)

UV Green Red

0.5 1.0 1.5 2.0

0.0 0.3 0.6 0.9 1.2 1.5 1.8

Iuv / Ivisble

MEA ratio (r)

Fig. 6.Plots of (a) UV and visible emission intensities, and (b) intensity ratio of the UV to the visible emission versus stabilizer molar ratios.

1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00 3.25 3.50

Energy (eV) r=0.5 r=1.0 r=1.5 r=2.0

2,990 3,055 3,120 3,185 3,250 3,315 3,380

Energy (eV) r=0.5

r=1.0 r=1.5 r=2.0

Fig. 5.ZnO thinﬁlms prepared with different stabilizer molar ratios: (a) room temperature photoluminescence spectra and (b) magniﬁcation of the UV emission region.

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nanostructured ZnO thin films prepared from zinc acetate solution were investigated. XRD measurements have revealed that all diffraction patterns are typical of the ZnO wurtzite hexagonal structure. The preferablyc-axis orientation increases as a function of MEA to zinc acetate ratio. SEM micrographs have shown that ZnO thinfilms have a smooth and uniform morphology with particularly large grains for ratio of 0.5. Optical characterizations in the UV–visible range have indicated that the absorption of ZnO thinfilms is dependent on the stabilizer molar ratio. Room temperature PL results have demonstrated that thinfilm emissions are clearly influenced by MEA to Zn ratio. The red emission has shown an opposite behavior and the maximum green emission intensity has been obtained from thefilm deposited at MEA ratio of 2.0.

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1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8

1.9 eV

1.6 eV

2.3 eV

Energy (eV)

1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8

1.6 eV 1.9 eV

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Energy (eV)

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1.6 eV 1.9 eV

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Energy (eV)

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1.9 eV

1.6 eV

2.3 eV

Energy (eV)

Fig. 7.Fitting curves of visible emission spectra of ZnO thinﬁlms prepared with different stabilizer molar ratios: (a)r¼0.5, (b)r¼1.0, (c)r¼1.5 and (d)r¼2.0.

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