Annealing Effect on Structure and Optical Properties of ZnO Thin Films Prepared by Spray Pyrolysis

Polycrystalline ZnO has been grown onto glass substrates by chemical spray pyrolysis (CSP) method. They were given heat treatment at different temperature and constant time and for different time with constant temperature in air. The change in structural and optical properties was studied by means of X-ray diffraction (XRD), SEM, and optical absorption measurements. Structural analysis by X-ray diffraction pattern showed annealed ZnO film has high-orientation along c - direction (0 0 2), which remained the same with different heat treatment. The lattice constants of ZnO thin films were also obtained from XRD data. It is found that, with the increase of different heat treatment, the lattice constant a increases from 3.208 Å to 3.254 Å, and c increases from 5.125 Å to 5.219 Å. where at higher annealing temperature and time the lattice constant c and a approach from bulk value.

V.R. Shinde et al. prepare textured ZnO thin films have been deposited using CBD method from aqueous alkaline medium, and are annealed in air at 623 K for 2 h [14]. Y. F. Lu et al. investigate the effects of thermal annealing on the film composition and the chemical bonding [8]. Harish Kumar Yadav et al. study of the influence of postdeposition annealing treatment on the structural and optical properties of rf sputtered Zn 1−x Mn x O thin films deposited on glass substrate at room temperature [25]. In this paper, we report on the systematic study of the influence of post deposition annealing treatment (at different annealing temperature for constant time and for different time at constant temperature) on the structural and optical properties of chemical spray pyrolysis (CSP) thin films deposited on glass substrate at 350 o C.

Experimental
The substrate used for deposits of ZnO thin films in this work is the microscopic glass slides ( boro-silicate glass ) with dimensions ( 76 x 21x 1 ) mm . It was cleaned by diluted HF , ethanol and distilled water , then dried and wept by optical cleaning paper.
The ZnO films were prepared by using an aqueous solution of Zinc Chloride ( ZnCl 2 2H 2 O ) with molarity (0.1) M . The aqueous solution was diluted in distilled water and mixed by a magnetic stirrer, and in each deposition the volume used was (100 ml) .
The ZnO thin films were deposited by spray pyrolysis technique. The deposition method involves the decomposition of an aqueous solution of zinc chloride. The spray solution was sprayed onto heated substrates held at (573 ± 5 o K) . Compressed air was used as a gas carrier and it was fed with the solution into a spray nozzle at a pre-adjusted constant atomization pressure.
The nozzle-to-substrate distance was 25 cm and the spraying period was (5 s) with flow rate as ( 3 ml/min ) . Aluminum electrodes were evaporated on the surface of ZnO thin films using thermal evaporation equipment through a mask giving sensitive area (0.5*0.5)cm 2 . Ellipsometer equipped with a He-Ne laser source (λ= 632.8 nm) were conducted to calculate film thickness.
To determine the nature of the growth films and the structural characteristics of ZnO films, X -ray diffraction measurement has been done and compared with the ASTM (American Society of Testing Materials) cards, using Philips PW 1840 Xray diffractometer of λ α = 1.54 °A from Cu -Kα. The average grain size (GS) of the polycrystalline material can be calculated from the X -ray spectrum by means of Full Width at Half Maximum (FWHM) method (Scherrer relation) [26].
where θ ∆ is the full -width at half maximum of the XRD peak appearing at the diffraction angle θ , A the shape factor, the value of which depends on the crystalline shape, and generally it is 1. For the (100) orientation the lattice constant a was calculated by [10] for the (002) orientation the lattice constant c was calculated by (4) The morphology of the films was studied by Scanning Electron Microscopy (SEM) type VEGA TE SCAN equipment operated at 30 keV. UV-VIS, Phoenix-2000V device was used to record the optical transmission for ZnO/glass thin films annealing in the range (300 -1100 nm). The data from transmission spectrum can be used in the calculation of the absorption coefficient (α) for ZnO films, according to the following equation [27]: Where d is the thickness of thin film, and T is the transmission. In the direct band gap structure or direct transition semiconductors (present case), the absorption coefficient and optical band gap (Eg) are related by [28].
Where h is Plank ' s constant and υ is the frequency of the incident photon. The strain along the c axis, ε zz is given by the following equation [17] θ λ sin = c And the stress (σ f ) in the plane of the film has been calculated from the estimated value of lattice parameter of film (c f ) using the expression, [25] (8)

Structures properties
The X-ray diffraction patterns of ZnO thin film deposited at 573P P K are shown in figure (1) for different annealing temperature with constant time and for different time with constant temperature. Polycrystalline ZnO thin films show a preferred orientation in the (002) direction perpendicular to the substrate. Diffraction patterns preliminary recorded on the film indicated that all investigated films were polycrystalline for a range of 2θ from 20P o P to 80P o P at 1P o P glancing angle. The film was crystallized in the wurtzite phase and presents a preferential orientation along the caxis, the strongest peak observed at 2θ = 34.390 P o P ( d = 0.2605nm ), Other orientations corresponding to (1 0 0) and (1 0 1) are present with very low relative intensities as compared to that of (0 0 2) plane. These results are close to those reported in the literature [14]. Also the intensity of (002) peak was much stronger than that of (101) orientation. This indicates that the c axis of the grains became uniformly perpendicular to the substrate surface, suggesting that the surface energy of (002) plane was the lowest in ZnO crystals. Some additional peaks with small intensities were detected with orientation 110, and 004 with mainly be due to heat treatment. As seen from figure, with increasing annealing temperature and time, the orientation along (0 0 2) has remained the same, but other orientations (1 0 0), (1 0 1), (110), and (004) have increased in their relative intensities.
The average grain size was calculated using Scherrer's formula (2), the values of average grain size show increases with increasing annealing temperature and time as shown in figure (2) it may be due to decrease in growth rate. The calculated grain size is smaller than grain size for ZnO deposited by pulsed laser deposition [8]. This result is consistent with the researcher Harish Kumar Yadav et al. [25]. The lattice constant a and c are obtained from equations (3) and (4) respectively. These are shown in Figure  (3) as a function of annealing temperature and time, it is clear from figure that the lattice constant c continuously increases with an increase in the postdeposition annealing temperature and time. The defects in the form of interstitial oxygen present in the films annealed out at a higher annealing temperature, and the compressive forces become weaker, resulting the c-axis value approach from bulk of c-axis value with an increase in annealing temperature and time. The small value of lattice constant for the as grown and at low annealed samples compared to the unstressed powder value shows that the unit cell is elongated along the c axis, and compressive forces act in the plane of the film. Also we found at higher annealing temperature and time the lattice constant c and a approach from bulk value. The stress f &Tech.Journal, Vol. 33,Part (B), No.1, 2015  The negative sign of estimated stress for all the samples indicates that the crystallites are in a state of compressive stress. However, the presence of interstitial oxygen has an expansive effect on the lattice, which results in the compressive strain, normally observed to occur along the c axis [25]. The table (1) show the obtained results. Figure (5) shows the scanning electron microscopy SEM micrographs for the films deposited on a glass substrate by using spray pyrolysis technique for ZnO films deposited at substrate temperature 573 k. ZnO forms 'blocks', tilted image gives more information about"blocks" shape.

Optical properties
Optical transmission spectra depend on the chemical and crystal structure of the films, and also on the film thickness and on films surface morphology. The effect of different annealing temperature and time on these spectra is shown in Figure (6). We have found that the films have high transmission at long wave lengths approximately (90%), and decreasing transmission to approximately (20%) at short wave lengths. The transparent is increasing with increasing annealing temperature and time due to decreasing in the films thickness with increasing annealing temperature and time. These results are consistent with other published results such as results of Jong Hoon Kim et al. [7] who attributed the increasing in transmission for the preservation of the stoichiometry of the deposited polycomponent films. The crystallization of thin films strongly depends on the RTA temperature. At higher temperature, films have less impurities and it is the origin of varying of the electrical and optical properties. The absorption coefficient (α) of ZnO films with different annealing temperature and time, determined from transmittance measurements using equation (5) shown inset figure (6). From this figure, the absorption coefficient of ZnO thin films decreased not sharply in the UV / VIS boundary and depends directly on the cut-off wavelength, and then decreased gradually in the visible region because it is inversely proportional to the transmittance. The optical energy gap (Eg) value is calculated by extrapolation of the straight line of the plot of (αhυ) 2 versus photon energy for different annealing temperature and time as shown inset in Figure (7). The linear dependence of (αhυ) 2 with (hυ) indicates direct band gap. The annealed samples show a relative decrease in optical band gap with both annealing temperature and time Figure (7) the shift of band gap energy is related to the compressed lattice will provide a wider band gap because of the increased repulsion between the oxygen 2p and the zinc 4s bands. On the other hand, the stretched lattice can result in the narrowing of the band gap. This result is consistent with other researchers such as V.R. Shinde [14], Hong Seong Kang [9], and Harish Kumar Yadav [25]. The results of energy gap as a function of annealing temperature and time show in table (2).

Conclusions
ZnO (0 0 2) textured films with wurtzite crystal structure (confirmed from XRD) were obtained using chemical spray pyrolysis (CSP) method. The postdeposition annealing of ZnO film increases the grain size and reduces the presence of compressive stress. The band gap of ZnO films decreases slightly with annealing temperature and time. The improvement in disorder and the reduction in stress with postdeposition annealing of ZnO film gives a good optical and structural property. &Tech.Journal, Vol. 33,Part (B), No.1, 2015