Optik 126 (2015) 5548–5552
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Effect of silver doping on the structural, morphological, optical and electrical properties of sol–gel deposited nanostructured ZnO thin ﬁlms T. Touam a,∗ , F. Boudjouan b , A. Chelouche b , S. Khodja a , M. Dehimi a , D. Djouadi b , J. Solard c , A. Fischer c , A. Boudrioua c a
Laboratoire des Semi-conducteurs, Université Badji Mokhtar-Annaba, BP 12, Annaba 23000, Algeria Laboratoire de Génie de l’Environnement, Université de Bejaia, 06000 Bejaia, Algeria c Laboratoire de Physique des Lasers, Université Paris 13, 93430 Villetaneuse, France b
a r t i c l e
i n f o
Article history: Received 24 September 2014 Accepted 6 September 2015 Keywords: Silver-doped zinc oxide Sol–gel thin ﬁlm Optical transmittance Electrical properties Optoelectronic applications
a b s t r a c t In this paper, we report the study of physical and optical properties of undoped and silver-doped zinc oxide (SZO) thin ﬁlms prepared by sol–gel dip-coating process onto glass substrates. The structural, morphological, optical and electrical properties of the thin ﬁlms as a function of silver concentration have been investigated using X-Ray Diffraction (XRD), Scanning Electronic Microscopy (SEM), Atomic Force Microscopy (AFM), UV–visible spectrophotometry and four points probe technique. XRD spectra have shown that undoped thin ﬁlm exhibit (1 0 1) orientation while doped ones display a strong c-axis orientation. SEM micrographs and AFM images have revealed that grain sizes and surface roughness decrease with increasing silver concentration. The UV–visible transmittance results show that Ag-doped ZnO thin ﬁlms exhibit better transparency than undoped ones. A change in the optical bandgap of ZnO thin ﬁlms is also revealed. The electrical measurements have shown that the resistivity of the thin ﬁlms decrease with increasing Ag doping. The present results show that ZnO thin ﬁlms doped with 4 at.% Ag have more suitable properties of high transmittance and low resistivity for optoelectronic device applications. © 2015 Elsevier GmbH. All rights reserved.
1. Introduction Zinc oxide (ZnO) is one of the most important and promising materials for a large number of areas. It has distinct advantages over its competitors such as non-toxicity, low material costs, chemical stability, high transparency in the visible and near infrared spectral region, direct wide band gap (Eg = 3.37 eV at 300 K), large exciton binding energy (60 meV). Furthermore, its related quantum well may have 100% internal quantum efﬁciency and most of its dopants are readily available [1–3]. These advantages are of considerable interest for practical uses in transparent conductive ﬁlms, solar cell windows, surface acoustic wave devices, optoelectronics and gas sensing applications [4–10]. The structural, optical and electrical properties of ZnO thin ﬁlms were governed by synthesis parameters, deposition conditions and dopants. Doping in ZnO with selective elements is the effective
∗ Corresponding author. Tel.: +213 557308598.. E-mail address: [email protected]
(T. Touam). http://dx.doi.org/10.1016/j.ijleo.2015.09.066 0030-4026/© 2015 Elsevier GmbH. All rights reserved.
alternative to modify its electrical, optical, magnetic, and chemicalsensing properties without any change in the crystalline structure [11,12]. Among dopants, the silver is one of the most promising to enhance optoelectronic ZnO properties, in particular, the electrical ones. Undoped and silver-doped ZnO (SZO) nano-structured thin ﬁlms have been prepared using different fabrication methods such as pulsed laser deposition [13,14], sputtering technique [15–19], spray pyrolysis [20,21], oxidation method [22,23], vapor–liquid–solid mechanism [24,25], e-beam evaporation techniques [26,27], and sol–gel methods [28–33]. The sol–gel process has attracted a great attention due to its unique advantages including low cost, simple deposition equipment, easy adjusting composition and dopants, lower crystallization temperature, and it can be used to deposit ﬁlms over a large area with a very uniform thickness. Despite the fact that some results have been reported on the effect of Ag-doping on physical properties of ZnO thin ﬁlms, there are still some discrepancies between them [15,17,19,28] and further study is needed. Moreover, in the majority of research works
T. Touam et al. / Optik 126 (2015) 5548–5552
dealing with electrical properties, most authors concentrated on the p-type conductivity. However, to the best of our knowledge the effect of Ag contents on resistivity of the SZO thin ﬁlms prepared by sol–gel process has not been reported. In this paper, undoped and Ag-doped ZnO thin ﬁlms were prepared on glass substrates by sol–gel dip-coating process. The effect of silver doping concentration on structural, morphological, optical and electrical properties was investigated. 2. Experimental All commercial reagents were used as received. Pure ZnO and silver doped ZnO (SZO) thin ﬁlms were prepared by the sol–gel process. As a starting material, zinc acetate dihydrate (Zn(CH3 COO)2 ·2H2 O; Sigma–Aldrich) was dissolved in a mixture of absolute ethanol (EtOH, 100%, BioChem) and monoethanolamine (MEA; Sigma–Aldrich) yielding to a precursor concentration of 0.75 mol L−1 . MEA acts as bidentate ligands to Zn2+ to stabilize the solution against any precipitation thus producing clear solution for coating procedure . The MEA to zinc acetate molar ratio was set to 1. For doped ﬁlms, silver nitrate (AgNO3 ) was added to the mixture with an atomic percentage ﬁxed at 1, 2, 3, 4 and 5 at.% Ag. The resulting ZnO and SZO sol were stirred at 50 ◦ C for 1 h, and then aged at room temperature for 36 h to get a clear and transparent homogeneous ZnO and SZO aqueous solutions. The commercial glass substrates (Esco Optics) were cleaned with detergent, rinsed in ﬂowing deionized water and immersed in 4 M nitric acid for 1 h. Rinsing with acetone and isopropanol, drying with nitrogen and in an oven at 100 ◦ C for 30 min complete the procedure. The substrates were dipped in the prepared sols and then withdrawn at a constant dip-coating speed of 15 mm/min. Synthesized ﬁlms, doped and undoped, were preheated at 200 ◦ C for 10 min after each coating. This procedure was repeated twelve times to increase the thickness. The ﬁlms were subsequently heated up to 500 ◦ C for 1 h with a heating rate of 5 ◦ C/min in order to obtain crystallized ZnO. The prepared nano-structured thin ﬁlms were characterized for the crystalline structure by X-ray Diffraction (XRD) with a PANalytical X´ıPert diffractometer, operating at 40 kV and 30 mA using Cu ˚ at a grazing incidence angle ω equal K␣1 radiation ( = 1.54056 A) to 0.54◦ . Scanning Electronic Microscopy (SEM) characterizations were carried out by means of a Raith PIONEER System. Surface morphology of thin ﬁlms in terms of root mean squared roughness (Rrms ) was explored from the images collected in contact mode by Atomic Force Microscopy (AFM, Nanosurf easyScan 2) operated at room temperature. The AFM images were collected in four different regions on the surface of each sample. The Rrms values used throughout this paper were calculated with the SPIPTM analysis software (Image Metrology). The optical transmission spectra were analyzed at room temperature by a Safas UVmc2 UV–Visible spectrophotometer and the optical band gap energy data was then derived from the transmission spectra. Electrical resistivity characterizations of the thin ﬁlms have been carried out using the four-point probe method. The ﬁlm thickness of the all samples was measured by using a Veeco Dektak 150 Surface Proﬁler. 3. Results and discussion 3.1. Structural and morphological properties The crystallinity and the preferred crystal orientation of the undoped and Ag-doped ZnO thin ﬁlms were analyzed by the X-ray diffraction. Fig. 1 represents the diffraction patterns of the pure, 1, 2, 3, 4 and 5 at.% Ag-doped 12-layer ZnO samples. The diffracted peaks have been identiﬁed using standard ZnO cards indicating that all the deposited ﬁlms have a polycrystalline wurtzite hexagonal
Fig. 1. XRD patterns of pure, 1, 2, 3, 4 and 5 at.% Ag-doped ZnO thin ﬁlms.
structure, and no other crystallized phases are observed. This means Ag-incorporation does not change the crystal structure. The results show that all the pure ZnO thin ﬁlms have (1 0 1) as the preferred orientation. The intensity of the (0 0 2) peak of Ag-doped samples indicates that all the SZO thin ﬁlms exhibit preferential orientation growth along (0 0 2) direction, that is, perpendicular to the substrate surface. We have to note that broadening of the diffracted peaks and overlapping of the (0 0 2) and (1 0 1) peaks are usually a result of a smaller crystallite size and lattice. It is clearly seen that the peak intensities of the Ag-doped ZnO thin ﬁlms increase when the silver content were increased from 1 to 5 at.%. This feature demonstrates that the crystallinity of the samples is enhanced when Ag concentration was increased. An apparent increase in crystallinity has been also observed by Li et al.  as suggested by the increase of the (0 0 2) peak intensity in the XRD patterns of the sol–gel Li:ZnO thin ﬁlms with increasing the Ag co-doping contents. Morphological study of the nano-structured samples has been investigated using SEM imaging. Fig. 2 presents high-magniﬁcation scanning electronic micrographs of the undoped and SZO thin ﬁlms on glass substrates. The ﬁlms show uniform and dense morphology overall the surface; this morphology consists of spherical aggregate grains in the 50–90 nm range in the vicinity of which one can see much ﬁner nanoparticles, with a mean diameter around 15–25 nm. Since the average crystallites size (different orientations) was estimated by the Scherrer method  in the 4–8 nm range, this means that each grain consists of several crystallites with different orientations. It is clear from these micrographs that the surface morphology of SZO ﬁlms was slightly modiﬁed with doping. It can be seen that grain size seems to be sensitive to the Ag content as ﬁner particles and aggregate grains are observed. The present results seem to be in agreement with those reported by Thongsuriwong et al.  who showed from AFM images that the grain size of the Ag-doped ZnO thin ﬁlms slightly decreased with increasing Ag concentration. Surface features of the all samples have also been studied by AFM in contact mode to examine the quality of ﬁlm deposition on glass substrate. Fig. 3 displays the three-dimensional AFM images of the undoped and Ag-doped ZnO thin ﬁlms scanned over a surface area of 5 × 5 m2 . From Fig. 3, it can be seen that all the samples have uniform and dense ZnO grains. Furthermore, the grains have columnar shapes which grow preferentially along the c-axis orientation. This observation is in good agreement with the results of XRD and SEM analyses. The ﬁlms exhibit different surface roughness which seems to be dependent on the Ag-doping. The root mean squared roughness was calculated and found to be as 19.57, 18.87, 17.76, 12.64, 10.52 and 15.44 nm for 0, 1, 2, 3, 4 and 5 at.%
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Fig. 4. Transmittance spectra of pure and Ag-doped ZnO thin ﬁlms. The inset shows the magniﬁcation of absorption edge region.
It is possibly connected with the decrease of ZnO grain as revealed by SEM micrographs. Our results and observations agree with previous work , where it has been demonstrated that the doping of Ag can reduce the scattering or absorption loss at the surface and thus change the properties of ZnO thin ﬁlms signiﬁcantly.
Fig. 2. SEM micrographs of Ag-doped ZnO thin ﬁlms: (a) 0 at.% Ag, (b) 1 at.% Ag, (c) 2 at.% Ag, (d) 3 at.% Ag, (e) 4 at.% Ag and (f) 5 at.% Ag.
Ag-doped ZnO thin ﬁlms, respectively, which shows that good quality of ﬁlms was achieved by our present sol–gel process. Undoped ZnO thin ﬁlm has the largest surface roughness, but when Agdoping concentration is increased, the surface roughness decreases gradually reaching its minimum value at 4 at.% silver contents, then slightly increase when the Ag concentration is increased to 5 at.%.
Fig. 3. AFM images of Ag-doped ZnO thin ﬁlms: (a) 0 at.% Ag, (b) 1 at.% Ag, (c) 2 at.% Ag, (d) 3 at.% Ag, (e) 4 at.% Ag and (f) 5 at.% Ag.
3.2. Optical and electrical properties Fig. 4 shows the UV–Vis–IR transmittance spectra of Ag-doped ZnO thin ﬁlms for various Ag-doping concentrations. The maximum optical transmissions in the visible region were observed higher than 80% and SZO ﬁlms show better transparency than the undoped ZnO, but the transmission decreases substantially at short wavelengths near the ultraviolet range. With increasing silver dopants up to 4 at.%, the maximum transmission increases to 91.64% but then decreases to 82.7% when the Ag concentration is increased to 5 at.%. The increase in the transmittance of the thin ﬁlms with increase of Ag-doping may be attributed to the enhancement of the crystallinity and the decrease of the surface roughness. From the magniﬁcation of absorption edge region shown by inset Fig. 4, it is found that by increasing silver contents up to 4 at.%, the absorption edge and maximum transmittance peak position shift toward shorter wavelengths which may indicate an increase in the bandgap and a decrease in the ﬁlm thickness. However, at 5 at.% doping, opposite behavior is observed by a slight shift to a longer wavelength region. In order to determine the allowed direct optical bandgaps of undoped and Ag-doped ZnO thin ﬁlms, we have used the technique based on the derivative of the transmittance with respect to energy, dT/dE. This very accurate method, experimentally validated, has been successfully used for the analysis of bandgaps by several authors [39–41]. According to the transmittance spectra measured, the dT/dE curves of ZnO ﬁlms with different Ag-doping concentration are presented in Fig. 5. Bandgap values of 3.20, 3.21, 3.22, 3.23, 3.24 and 3.22 eV were obtained for pure ZnO, 1, 2, 3, 4 and 5 at.% of Ag, respectively. It can be seen that the band gap initially increases as Ag contents increase then slightly decreases when the Ag concentration is increased to 5 at.%. Several reports indicated controversial results concerning the energy gap (Eg ) change of silver doped ZnO. Some authors have reported that the optical bandgap increased with increasing Agdoping concentration in ZnO nanocrystals [15,17], while others have found that optical bandgap of ZnO thin ﬁlms decreased with increasing Ag-doping content [19,28]. For example, Xue et al.  have investigated the inﬂuence of Ag-doping on the optical properties of ZnO ﬁlms prepared by rf magnetron sputtering technique.
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Table 1 Root mean squared roughness, maximum transmittance, peak position, bandgap energy, ﬁlm thickness and electrical resistivity of undoped and Ag-doped ZnO thin ﬁlms. Ag-doping percentage (%)
RMS roughness (nm)
Maximum transmittance (%)
Peak position (nm)
Bandgap energy, Eg (eV)
Film thickness (nm)
Film resistivity ( cm)
0 1 2 3 4 5
19.6 18.8 17.7 12.6 10.5 15.4
80.1 80.6 80.7 85.9 91.6 82.7
757 754 743 702 656 677
3.204 3.212 3.221 3.229 3.238 3.221
450 412 382 372 371 382
0.0456 0.0416 0.0395 0.0327 0.0273 0.0096
Fig. 6. Measured electrical resistivity of pure and Ag-doped ZnO thin ﬁlms.
Fig. 5. Plots of the derivative of the transmittance with respect to energy of pure and Ag-doped ZnO thin ﬁlms.
It was demonstrated that the optical band edge shifted to a shorter wavelength ﬁrst as Ag is incorporated, and then to a longer wavelength with the increasing of Ag doping concentration. Sahu  has studied the effect of Ag-doping on the structural, optical and electrical properties of ZnO thin ﬁlms prepared by simultaneous rf magnetron sputtering of ZnO and dc magnetron sputtering of Ag on glass substrate. It was found that the absorption edge shifted slightly to a longer wavelength and the band gap narrowed with increasing Ag content. The present results seem to be in very good agreement with the work of Xue et al. . According to our obtained results, the increment in the band gap by silver doping may be attributed, on the one hand, to the quantum size effect as indicated by the work of Marotti et al. , where it was suggested that the band gap and the absorption edge of nano-structured materials shift due to quantum size effects. On the other hand, this widening of the optical band gap is well known as Burstein–Moss effect [43,44] and was discussed in many semiconducting materials of low density of state effective mass and doped semiconductors [45,46]. This increment in the bandgap of the Ag-doped ZnO thin ﬁlms is considered as advantage property since the optical transmittance window is broadened. To enrich the present investigation, the electrical resistivity was measured at room temperature by four-probe method. The thickness of all thin ﬁlms was determined from surface proﬁle analysis. Thickness values of 450, 412, 382, 372, 371 and 382 nm were obtained for pure ZnO, 1, 2, 3, 4 and 5 at.% of silver, respectively. Fig. 6 depicts the variation of resistivity of Ag doped ZnO ﬁlms with doping concentration. The resistivity of undoped ZnO is about 4.56 10−2 cm; this value decreases with increasing Ag concentration to reach its lowest value of 9.6 10−3 cm for Agdoping concentration of 5 at.%. This behavior can be attributed to an increase of not only in carrier concentration but also in mobility. All the obtained results from the different characterization techniques of undoped and Ag-doped ZnO thin ﬁlms prepared by the sol–gel process are summarized in Table 1; it can readily be noticed
that ZnO thin ﬁlms doped with 4 at.% Ag exhibited interesting properties of high transmittance and low resistivity that could be very suitable for optoelectronic applications. 4. Conclusions Pure and Ag-doped ZnO thin ﬁlms were successfully prepared by the sol–gel dip-coating method. The effects of Ag-doping concentration (1–5 at.% Ag) on the structural, morphological, optical and electrical properties of the obtained ﬁlms were investigated by X-ray diffraction, scanning electronic microscopy, atomic force microscopy, UV–visible spectrophotometer and four points probe technique. It was found that pure ZnO thin ﬁlms exhibit (1 0 1) orientation while doped ones display a strong c-axis orientation perpendicular to the substrates with enhanced crystallinity. SEM micrographs and AFM images have revealed that the grain size and surface roughness tends to decrease with increasing silver concentration. The maximum optical transmissions in the visible region were found to be higher than 80% and Ag-doped ZnO thin ﬁlms show better transparency with a maximum value of 91.6% obtained at 4 at.% concentration. Moreover, it is found that the absorption edge is shifted toward UV region indicating a widening of optical bandgap with silver doping. The measured resistivity of thin ﬁlms decreases with increasing Ag doping. Hence, it can be concluded that all obtained results put into evidence that ZnO thin ﬁlms doped with 4 at.% Ag have the best physical properties that may be well suited for optoelectronic applications. Acknowledgements The authors are grateful to Mr. L. Benidiri for his help in the experimental characterization. The authors would like to express their thanks to Prof. A. Doghmane for his careful reading of the manuscript and English language corrections. References  M. Hass, H. Weller, A. Henglein, Photochemistry and radiation chemistry of colloidal semiconductors. 23. Electron storage on ZnO particles and size quantization, J. Phys. Chem. 92 (1988) 482–487.
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