Investigation on the effect of annealing period on the structural and optical properties of ZnO thin films prepared using a polymer precursor

Numerous synthesis strategies including sputtering,molecular beam epitaxy,chemical vapour deposition, pulsed laser deposition etc., have been reported to grow high quality ZnO thin films[1]. However among these mounted synthesis strategies, simple and price powerful path to develop ZnO thin films by way of usage of cheaper, non poisonous and environmentally benign precursors are nevertheless the important issues. This work advise a simple and novel technique to the fabrication of ZnO thin films with the aid of poly vinyl alcohol (PVA) as precursor. This article also investigate the impact of annealing period on the structural and optical properties of grown films by analyzing XRD, UV absorption and photo luminescence spectra. The XRD pattern reveals poly crystalline hexagonal wurtzite structure. The particle size and stress values varies with changes in annealing period. The values of UV absorption spectra are utilized to plot Tauc plot to get the band gap of the films. The band gap shows significant decrease from 3.35ev to 3.10ev with increase in annealing period. The films shows good transmittance >75% and transmittance increase as the annealing parameter increases. The PL spectra have peaks in UV and visible regions,. the mechanism behind these emission peaks are also discussed.


1.Introduction
In recent years ZnO attracts the attention of researchers due to its specific properties inclusive of high band gap energy (3.37eV) high excitonic binding energy(60meV), availability of large area single crystals, second and third order non linear optical behavior, piezoelectric behavior etc.These properties makes ZnO a suitable material in the field of optoelectronics, phosphor applications,solar cells, transducers and sensors [1][2][3][4].Number of reports on the fabrication of ZnO thin films are to be had but a low cost environment friendly synthesis strategires are still the key issue.The prevailing work reports the synthesis of ZnO thin films by means of low cost dip coating method by way of employing Polyvinyl alcohol (PVA) as a precursor.To have a look at the effect of annealing duration on the morphological and optical properties the grown films are subjected to annealing at 4500C for 0.5hour,1 hour, 1.5 hour, 2 hour.

2.Experimental
Sol-gel dip coating approach is utilized to synthesis ZnO thin films.The coating solutions have been obtained by means of mixing suitable quantity of zinc acetate dihydrate [Zn(CH3COO)2.2H2O] in 2.5wt% of poly vinyl alcohol solution.The concentration of the films was made constant at x=0.06.
The received solution is stirred for 1 hour to yield a clear solution.The coating is made through dipping precleaned glass substrate to the solution with a withdrawal speed of 10cm/sec.Since the boiling point of PVA is 230 0 C, heat treatment at 250 0 C can be required for the solvent to vaporize absolutely out of the film.So,the received film was dried at 250 0 C for 10 min in a furnace to evaporate the organic residuals.The heat treatment procedure was repeated 10 times to get the desired thickness and to vaporize PVA molecules.To have a knowledge about the effect of annealing period on the structural and optical properties of the film, the films were subject for an annealing process at, 450 0 C for 0.5hr,1 hr, 1.5hr, and 2 hr.
The X Ray Diffraction patterns have been obtained with XPERT-PRO PANalytical CuKα radiation(λ=1.5406Å).The obtained diffractograms have been analyzed using JCPDS data card .The UV measurements had been performed at room temperature with the use of T90 UV-VIS spectrometer (PG Instruments) in the wavelength region 300nm-700nm.PL measurements have been executed using the Shimadzu 530RFPC photometer.

3.1Structural Studies
Figure 1 suggests XRD spectrum of the obtained samples annealed at different annealing periods.The XRD pattern reveals poly crystalline hexagonal wurtzite structure growth.The reflections from (100), (002), (101), (102), (110), ( 103) and (200) planes are observed.The peak intensities of the films first decrease as the annealing period changes from 0.5 hour to 1 hour; then these increase for the film annealed for 1.5 hour, and again decrease for the film annealed for 2 hours.The observed shrinkage of the peaks of ( 100) and(101) planes may be due to substrate effects since the films are grown on glass substrates..The Scherer formula [5] was employed to determine the grain size.
.9 cos The particle size were found to be 42.35 nm, 28.76 nm, 43.81 nm and 42.30 nm for the films having annealing periods 0.5hr, 1 hr, 1.5 hr and 2 hr respectively.It is interesting to note that variation in peak intensity and grain sizes were in the same manner.As the intensity varies, FWHM of the films also varies in a reverse manner results in the irregular pattern observed in grain size [6,7] .
The lattice parameters a and c are calculated from the XRD data and found to be less than the normal powder values [3].After the annealing procedure, the films show a shift in angular peak positions, indicating that the films are in a uniform state of stress with tensile component parallel to c axis; ie all the films exhibit tensile stress [1].The existence of extrinsic stress in the films are due to the misatch between the thermal expansion coefficient of ZnO film (αZnO=4x10 -6 K -1 ) and glass substrate (αglass=9x10 -6 K -1 ) [1].Because of this large thermal mismatch the glass substrate exerts a resultant tensile stress during cooling from higher temperatures [6,7].The calculated structural parameters and of prepared samples are given in table 1.The Tauc plot for ZnO thin films annealed at different annealing periods are given in figure 3(c).The band gap values were found to be decreasing from 3.35eV to 3.10eV as the annealing period increases from 0.5hr to 2hr.As reported by researchers, the lowering behavior of band gap energy with annealing duration is due to the to the decrease in the number of defects such as oxygen vacancies and or grain boundaries.Increase in annealing duration may result in the decrease in oxygen vacancies/ grain boundaries which lead to decrease in carrier concentration in conduction band [9][10][11].The observed band gap values for ZnO thin films subjected to different annealing period is given in table 1.The dependence of photoluminescence property of ZnO thin films having subjectd to different annealing duration is illustrated in figure 4. For the films annealed at 0.5 hr and 1hr we can observe peaks at UV, violet and blue regions.But if we increase the annealing period to 1.5 hr or 2 hr it is quite exciting that, the films have peaks only in UV and blue emission regions.
The near band edge (NBE) transition is the reason for the observed strong UV peaks and originate from the recombination of the free exciton through collision process [12] .
The study of the energy levels of different intrinsic defects in ZnO suggests that, the visible luminescence in ZnO is due to the presence of different defect states such as oxygen vacancies (Vo), zinc vacancies (VZn), oxygen interstitials (Oi), zinc interstials (Zni) and antisite oxygen (OZn) [13].The energy levels of these defect levels can be calculated by using Full Potential Linear Muffin Tin Orbital (FP LMTO) method [14,15] and shown in figure 5. From the above figure it is clear that the interface levels at 3.06eV and 2.9 eV are due to the existence of VZn and Zni in the band gap of ZnO.Therefore it is reasonable to conclude that the peaks observed at violet region may be due to the transition between these interface levels with conduction band or valence band of ZnO [16].
The emission peaks obtained at blue regions can be assigned to electron transition from Zni to VZn [17][18][19].The photon energy of blue emission occurring from the prepared film is consistent with the energy interval between these two intrinsic defects levels about 2.8eV.So, it is possible to conclude that the blue emission in the grown samples are due to the presence of these defects [17].

4.Conclusion
Thin films of ZnO were synthesized by a low cost environmentally benign dip coating method.The impact of annealing period on the structural and optical properties of the films were studied in detail.The XRD studies reveals hexagonal wurtzite structure.The structural parameters were calculated and shows variations with variation in annealing period.The SEM studies also reveals the dependency of surface morphology on annealing period.The crystalline nature of the films increases with changes in the annealing duration, the optical band gap of the films also shows significant variation.The PL emission spectra have peaks in UV,violet and blue region and shows significant variations in peak intensity and peak positions with the change in annealing period.

Figure 1 .
Figure 1.XRD pattern of synthesized samples annealed for different duration.

Figure 2
SEM image of the samples annealed at 450 0 C for (a) 0.5 hour (b) 1 hour (c) 1.5 hour (d) 2 hr. Figure 2(a)-2(d) show the surface morphological study of ZnO samples annealed at 450 o C being subjected to different annealing periods.As observed in the figure the surface morphology of the films greatly depends upon the annealing periods.The SEM image of the films annealed for 0.5 hour shows almost flat rough surface.For the first two samples uniformly distributed grains are seen in the surface of the films.As observed in the SEM micrographs, further increase in annealing period may deform the surface morphology of the film.

Figure 3 (
Figure 3 (a).Transmittance spectra of ZnO films having different annealing periods.

Figure 4 .
Figure 4. Photo luminescence spectra of the prepared samples

Figure 5 .
Figure 5. Energy levels of intrinsic defects in ZnO.

Table 1 .
The calculated structural and optical parameters of synthesized thin films subjected to different annealing periods.