Fabrication and characterization of pure ZnO thin films deposited by Sol – gel method
S.Haya *, O.Halimi, M.Sebais, B. Boudine Exact Sciences Faculty, department of Physics
Laboratoire de Cristallographie Constantine 1, Algeria [email protected]
O.BRAHMIA
Exact Sciences Faculty, department of Chemistry Laboratoire des Techniques Innovantes de Préservation de
l’Environnement Constantine 1, Algeria
Abstract—ZnO thin films were prepared via Sol-gel method and were deposited on an ordinary glass substrate using dip coating technique. These films undergo optical annealing using UV irradiation during 2 hours. The starting materials used were zinc acetate dehydrate, 2-methoxyethanol which was used as solvent as well as the mono-ethanolamine (MEA) as stabilizer. X- ray diffraction study shows that all the films prepared in this work have a preferential orientation situated at 34° and correspond to (002) plan of the hexagonal Wurtzite structure, with lattice constants a = b = 3.02 Å, c = 5.20 Å. The optical band gap energy of the thin films was found to be a direct allowed transition ~3.23 eV. These values belong to the blue shift absorbance. Moreover, the photoluminescence measurement reveals that the prepared samples exhibit intense emission band in the visible and near UV. This observation led us to practical applications in the area of optoelectronic.
Keywords—ZnO;sol-gel method; photoluminescence; band gap I. INTRODUCTION
Significant research efforts have been made for developing highly oriented and transparent ZnO thin film, because of their potential application in the transparent electrode in display, window layers in solar cells, field emitters, ultraviolet laser emission, photo-detectors, piezoelectricity, and biosensors [1]. ZnO thin film reached an important place in new technologies due to its unique optical, electrical and semiconducting properties, which are used in various applications. It has a wide band gap (3.3 eV at room temperature) and large excitonic binding energy 60 meV [2].
Despite several approaches adopted for making these ZnO thin films, controlling the size, shape, crystallinity and various parameters affecting the size and shape of these materials still need to be investigated. Therefore, it is essential to investigate optimum conditions for fabrication of highly oriented and transparent ZnO thin films. The main concern of researcher is to get a better quality of material stoichiometry. Many methods have been employed to prepare ZnO thin films like spray pyrolysis [3], chemical vapor deposition, molecular beam epitaxy [4] and the Sol-gel technique. The latter method was widely adopted due to its comparatively simple procedure as there is no need of costly vacuum system and it has a wide- range advantage of large area deposition and uniformity of the
films thickness [5]. The structural and optical properties of ZnO thin films prepared by Sol-gel technique using various inorganic and organic precursors at different deposition conditions have been reported in the literature [6]. In the present work, we studied the growth of ZnO thin films on glass substrate by Sol-gel method using zinc acetate dehydrate precursor and undergo optical annealing using UV irradiation with a UV lamp during different time of irradiation and we also investigated the structural, optical and photoluminescence properties of ZnO thin films
.
II. EXPERIMENTAL DETAIL
The pure ZnO solution was prepared by dissolving 1.5g of zinc acetate dehydrate [Zn(CH3COO)2 , 2H2O, Merck] in 40 mL (2-methoxyethanol,Merck) with continuous stirring at the ambient temperature, when the solution turned milky, an equimolar amount of MEA was added drop by drop to obtain a clear solution after stirring at 60°C for 2hours . The final solution was transparent, clear and homogeneous. The sol was deposed onto cleaned ordinary glass substrates using the Dip- Coating technique. The layers were dipped in the sol for one minute and then drawn from it at the speed of 6 cm/min. The obtained thin films on the substrates were then kept at room temperature to dry for 10 min. Finally, the films were heated by optical annealing with UV lamp (24Watt).
III. RESULTS AND DISCUSSION
30 35 40 45 50
0 10 20 30 40 50
ZnO 2H UV lamp
(002)
Intensity (a.u)
2 theta degree
Fig. 1.XRD pattern of the ZnO thin films irradiated for 2 hours with UV lamp
30 40 50 0
10 20
30 (002)
---ZnO:2H ---ZnO:4H ---ZnO:6H ---ZnO:8H
Intensity(a.u)
2theta degree
Fig. 2. XRD patterns of ZnO thin films irradiated for: 2, 4,8,12 hours with UV lamp
TABLEI. CRYSTALLITES SIZE, FULL WIDTH HALF MAXIMUM AND LATTICE PARAMETERS CALCULATED FROM XRD
Irradiation time (hour)
2θ (deg)
FWHM (deg)
Lattice constants
D (nm) a (A°) c (A°)
2H 4H 6H 8H
34.006 34,107 34.296 34.201
1,447 1,351 1.023 0.783
3.035 3.031 3.015 3.023
5.258 5.251 5.223 5.237
5.890 6.309 8.337 10.891
To investigate the crystalline structure of the ZnO thin films, XRD patterns were scanned in the 2θrangedfrom 20°- 50°. Figure1 shows the XRD pattern of ZnO thin film as function of optical annealing using UV lamp (24Watt) irradiated for 2 hours. It clearly appears that the film have a preferential orientation of crystallites situated at 34, 6° that correspond to (002) plan of the Wurtzite structure with a lattice constants a=3.24982Å, c=5.20661Åaccording to the (JCPDS Data Card no 36-1451). This intense diffraction peak suggests that ZnO thin films grew along the (002) direction which is perpendicular to the glass substrate. In Figure 2, we readily observed that the intensity of the (002) diffraction peak increases with increasing in optical annealing time, resulting in enhanced crystallization of ZnO thin films. Similar results have also been observed and reported with other ZnO films, but using thermal annealing [7], where the crystallinity was generally improved with the temperature increasing. The lattice constants ‘a’ and ‘c’ of the Wurtzite structure of ZnO can be calculated using the (1) & (2) [8].
The crystallite size (D) of the ZnO thin films were estimated using the Debye-Scherer’s formula (3) [9]. Where D is the
crystallites size, β is the full-width at half maximum (FWHM) of the diffraction peaks, θ is the diffraction angle.
D=
Table 1 presents the average sizes of crystallites corresponding to (002) plan. By comparing calculated crystal sizes of optically annealed samples at different time, we observed that the crystal size of these samples increase with increasing optical annealing time from 2 to 8 h which, could be attributed to ZnO thin films growth during the annealing process.
2,36 2,37 2,38 2,39 2,40 2,41 100
120
140 2.38eV
Intensity(a.u)
Energy(eV)
Fig. 3. PL of ZnO thin film irradiated for 2 hours with UV lamp The optical properties of a semiconductor are related to both intrinsic and extrinsic effects. PL is a suitable technique to determine the crystalline quality and the exciton fine structure. The PL spectrum pattern of the ZnO thin films irradiated for 2 hours with UV lamp (Figure 3) has a strong green emission band located at 2.38eV. In the past several decades, the green luminescence mechanisms of ZnO have been studied and various models have been proposed [10, 11].
According to some research [12], it was proved that the singly ionized oxygen vacancy is responsible for the green emission in the ZnO and the emission results from the recombination of a photogenerated hole with an electron occupying the oxygen vacancy [13].
300 320 340 360 380 400
0 1 2 3
2H ZnO 4H ZnO 6H ZnO 8H ZnO
Absorbance (a.u)
wavelength(nm) ---2H ZnO ---4H ZnO ---6H ZnO ---8H ZnO
Fig. 4a. UV-Vis absorption of ZnO thin films irradiated for 2:4:6:8 hours with UV lamp
3,0 3,2 3,4 0,1
0,2 0,3 0,4 0,5
---2H ZnO ---4H ZnO ---6H ZnO ---8H ZnO
(h)2 (a.u)
hv(eV)
Fig. 4b. Optical band gap ZnO films irradiated for 2:4:6:8 hours with UV lamp
TABLE II. Energy band gap calculated from Absorbance spectra Irradiation time (hours)with UV lamp of
ZnO thin films
Energy gap, Eg (eV)
2H 4H 6H 8H
3.195 3.204 3.216 3.230 To investigate the influence of the optical annealing on the of absorbance ZnO thin films deposited on glass substrates in the wavelength range 300 –400 nm, the samples absorbance spectra were illustrated in Figure 4(a).The optical band gap could be obtained by analyzing the absorption edge as well as by applying the Tauc’s (4).
Where n is a constant that depends on the transition probability; it takes the values 1/2, 1/3, 2 and 3 for direct allowed, direct forbidden, indirect allowed and indirect forbidden transition respectively. Figure (4b) shows the variation of (αhν)2 in terms of hν, which is the straight line in the higher energies domain, indicating a direct optical transition. The band gap energy Eg is obtained by extrapolating the linear portion of the graph to energy axis at α=0.
In Figure 4a we observed the blue-shift in absorption edge when the irradiation time increases. So that, the wavelength of absorption edge was observed irradiated at 394 nm, 392 nm,390 nm, 387 nm of 2, 4,6,8, hours respectively. The Eg values of ZnO thin films were reported in Table 2. The band gap of ZnO thin films increase from 3.195 to 3.230 eV for films irradiated 2 to 8 hours. The band gap increase could be attributed to the quantum confinement effect [14].Hence, it is confirmed that the optical band gap of the ZnO thin films strongly depends on the optical annealing time.
IV. CONCLUSION
We have successfully synthesized ZnO thin films deposited on glass substrates by dip-coating technique, heated by optical annealing using UV irradiation. Films have been characterized
using optical and structural measurements. All the films exhibit high transmittance in the visible region, thus making the films suitable for optoelectronic devices, such as window layers in solar cells. The UV-Vis studies showed that ZnO films shift the absorption edge to lower wavelengths (blue shift). The X-ray diffraction analysis revealed that all the samples have hexagonal Wurtzite structure with a preferential orientation of crystallites situated at (002) that appear after two hours of irradiation with UV lamp (24 Watts). The crystallites size as measured using XRD data were found to be in the range of 5.890 to 10.891 nm. The film has the strong emission band at 2.38 eV.
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