STRUCTURAL AND OPTICAL PROPERTIES OF ZnO THIN FILMS PREPARED BY SOL-GEL PROCESS
N. Ouafek, H. Dehdouh,W. Bedjaoui, R. Zellagui
Division des Matériaux et surfaces structures / Unité de Développement des Couches Minces et Applications (U.D.C.M.A) Sétif / Le Centre de Recherche en Technologies Industrielles (CRTI) / BP 64 Cheraga /Algérie.
n.ouafak@csc.dz
Abstract : Undoped zinc oxide (ZnO) thin films have been Spin-Coated on glass substrates by sol-gel process. The structural and optical properties of ZnO thin films have been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and spectroscopy UV-visible. The obtained films are composed of nano- crystalline grains with diameter range between 14 to 29 nm. ZnO films have a transmittance over 85% in visible range and band gap of 3.14 eV.
Keywords : thin film, ZnO, XRD, Spin-Coating.
1. Introduction
For many years, zinc oxide (ZnO) and its alloys are the materials of choice because of their attractive properties. The main applications of this material are in the fields of chemical and pharmaceutical industry, in the photovoltaic field [1], ceramics [2], Biomedical[3], and optic devices. Currently new optoelectronics research reveals a great interest for pure and doped ZnO thin films because of their important properties : dielectric constant, high resistivity, high thermal conductivity, high heat capacity, high absorption of ultraviolet radiation. ...etc. It is a wide band gap semiconductor about 3.3 eV and a large exciton binding energy (60meV). This last can vary depending on method of preparation and the doping level ; it is possible to change widely the zinc oxide properties by doping.
The electrical properties of ZnO make it a good candidate for applications in optoelectronics as : an ultraviolet detectors or laser diodes emitting in the blue or ultraviolet. Several methods were chosen to prepare thin films ; the choice of a particular technique of synthesis depends on several factors.
The present work is devoted to synthesize undoped ZnO thin films using sol-gel process, deposited by spin coating method and annealed at 400°C. The structural and optical characterizations of deposited films are performed.
2. Experimental procedure
Precursor solutions were prepared from zinc acetate dehydrate (Zn(CH3COO)2·2H2O), ethanol as solvent and diethanolamine (HN(CH2CH2OH)2) (DEA) as stabilizer (in 2:1 molar ratio to zinc acetate dehydrate), using sol-gel process at 60°C with the help of a magnetic stirrer. The obtained precursor were transparent and homogeneous and kept safe up to 24 h for hydrolysis process. ZnO thin films were deposited on precleaned glass substrates using Spin-Coating (1500 tr/min).Finally, the samples were annealed for 1 h30 at 400°C in air. The structure was analyzed with an Emperean Panalytical working with Cu Kα radiation of wavelength λ=1.54060 Å, the morphology study by scanning electronic microscopy VEGA TS 5130 MM and the optical study was investigated by UV-Visible called OPTIZEN 3220.
3. Results
Fig. 1 shows the X-Ray diffraction pattern of ZnO thin films annealed at 400°C for 1h 30, it exhibits a single würtzite crystal structure (hexagonal) with lattice parameters a = 3.249 A° c = 5.205A°( from PDFWIN [4] n° 890511). The average size of the particles is 21.46 nm. We calculated the size of the grains from theXRD diffraction spectra using the Debye -Scherrer formula [5].
400 600 800 0
20 40 60 80 100
Transmittance (%)
(nm)
1,0 1,5 2,0 2,5 3,0 3,5
h (cm-1 . eV)
h (eV)
The micrographs using scanning electron microscopy (SEM) of theZnO thin films surface (Fig. 2), obtained by spin coating heat-treated at400 °C, show a wrinkle structure with small spherical agglomerated particles, the surface exhibits a porous structure.
2θ (°) (hkl) D (nm)
31.71 (100) 19.69
34.48 (002) 27.26
36.24 (101) 21.04
47.60 (102) 22.09
56.71 (110) 28.90
62.94 (103) 16.95
68.04 (112) 14.28
69.21 (201)
Fig. 1Left XRD patterns of ZnO thin film prepared by sol gel and deposited by spin coating annealed at 400°C for 1h 1/2, RightTab2θ,
hkl, and grain size values for ZnO thin film.
Fig. 2SEM images of ZnO prepared by sol gel and deposited by spin coating annealed at 400°C for 1h 1/2
Fig. 3 Left Transmittance spectra of ZnO thin film prepared by sol gel and deposited by spin coating annealed at 400°C for 1h 1/2. RightThe plots (αhν)2 vs. phonon energy of the ZnO thin film
20 30 40 50 60 70
0 50 100 150 200 250 300 350
(201)(112)
(103)
(110)
(102)
(101)(002)
Intensité
2 (°)
(100)
Fig. 3 left transmittance spectra shows a strong absorption located at λ<350 nm due to the electronic transition interband, it is used to determine the optical gap of the films. Beyond 350 nm a large transmittance (from 70 to 90%) on a wide range of wave length from 350 to 800 nm ( ZnO does not absorb light in the visible range there for it is considered as transparent). This increased transparency is one of the properties, which explains the interest in ZnO thin films.
Fig. 3 right presents the (αhν)2 versus hν plot, this last determinates the optical band gap by extrapolating the straight line portion of the spectrum to the energy axis, starting from the transmittance spectra and using the following equation related to direct transitions in ZnO crystals [6]
(𝛼ℎ𝜈)2 = 𝐴(ℎ𝜈 − 𝐸𝑔) (1) α : absorption coefficient. hν the photon energy
Eg :optical band gap
A: constant depending on electron-hole mobility
The thickness d of the film was calculated via the formula (Swanepoel’s interferometric envelop method) [7] based on the optical transmission spectra T (%):
d = λ1λ2
2(λ1n2−λ2n1) (2)
Where n1 and n2 are the refractive indices of the films measured at two different wavelengths λ1 and λ2.
The band gap is about 3.14 eV.
4. Conclusion
Good quality undoped ZnO thin films were deposited on glass substrates using sol-gel spin coating method with high Transparency in the visible region. Grain size is in nanometric scale with diameter range between 14 to 29 nm and crystallized in the würtzite structure with any preferential direction.
The thin films exhibit a porous structure shown by SEM. All these properties make them good candidates for optoelectronic applications.
References
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