Author: Islam, M R
Date published: November 1, 2011
(ProQuest: ... denotes formulae omitted.)
Zinc oxide (ZnO) is one of the most promising materials due to its unique and attractive properties like non-toxicity, good electrical, optical and piezoelectric behavior and its low cost [1-2]. ZnO occurs naturally as an ?-type semiconductor with hexagonal wurtzite structure and has a wide band gap of about 3.3 eV . The excellent optical properties of ZnO make it suitable for use in surface acoustic wave devices , gas sensors , electrodes in solar cells , flat panel displays  and for energy conversion and storage . Recently, nano scale single crystalline zinc oxide has attracted much by the nano-materials technologists and crystallographers because its properties are expected to depend not only on the structure, but also on its shapes, sizes and distributions of grains (e.g., nanorods and nanowires) .
Unlike many other film deposition techniques, such as chemical vapor deposition , plasmaenhanced chemical vapor deposition , r. f. magnetron sputtering , molecular beam epitaxy , pulsed laser deposition , metal organic chemical vapor deposition , etc., spray pyrolysis represents a very simple and relatively cost-effective processing method (especially with regard to equipment costs). It offers an extremely easy technique for preparing films of any composition. Spray pyrolysis does not require high-quality substrates or chemicals or any vacuum system. The aim of the present study is to see the influence of annealing temperature on the structural and optical properties of as deposited nano size ZnO grains thin film.
2. Experimental Details
In this work, a home-made spray pyrolysis unit was used and the method was described elsewhere . ZnO thin films were deposited on commercial glass substrate kept at suitable temperature 200 °C. 0.1 M aqueous solution of zinc acetate [Zn(CH^sub 3^COO)^sub 2^.2H^sub 2^O] was used as precursor of ZnO. The pH of the solution was measured to be 6.45. Before deposition the substrate was ultrasonically cleaned in acetone solution and then rinsed in de-ionized water. Air was used as carrier gas and air pressure was kept at constant 0.5 bar. During deposition the distance between the spray nozzle and substrate was 25 cm and the solution flow rate was kept constant at 0.5 mL/min. The solution was sprayed onto the substrate with a deposition time of 10 minutes. The films were annealed at 300, 400, 500 and 600 °C for 30 minutes each in air atmosphere. The possible chemical reaction that takes place on the heated substrate to produce ZnO thin film may be as follows: when the droplets of the solution reach the heated substrate, chemical reaction of the zinc acetate water solution takes place under stimulated temperature and provides the formation of ZnO film.
Scanning Electron Microscope (HITACHI S -3 400N model) attached with an EDX was used to examine the surface properties and to measure the composition of the elements of the film. The optical transmission spectra for as-deposited thin films were obtained by UV-VIS spectrophotometer (Model: UV-1201 V, Shimazdu). A Philips PW3040 X'Pert PRO X-ray diffractometer was used to characterize the materials and to determine the lattice parameters. The monochromatic (using Ni filter) CuK0 radiation was used with primary beam power of 40 kV and 30 mA. All the samples were irradiated over 20 range from 30° to 70° to get possible fundamental peaks of the sample with the sampling pitch of 0.02° and time for each step data collection was 1.0 sec. All the data of the samples were analyzed by using computer software "X'PERT HIGHSCORE" from which structural parameters was determined.
3. Results and Discussions
3.1. Compositional Studies
Fig. 1 shows EDX result of as-deposited ZnO film. Two strong peaks corresponding to Zn and O are found in the spectrum, which confirms the high purity of the ZnO thin film. An average atomic percentage of Zn: O is 49.99: 49.26. It is noteworthy that the as-deposited ZnO film is stoichiometric. Si peak is observed in the spectrum as the electron beam penetrates the film and reaches the glass surface due to high operating voltage.
3.2. Surface Morphology by SEM
Fig. 2 (a, b and c) show the surface morphology of as-deposited ZnO film under 10K magnifications. In our study we have observed that after annealing, the fibers become clear and narrower (Fig. 3.c). The average diameter of the fibers is around 600 nm. This is for the first time we have observed nano fiber structure on undoped ZnO thin films deposited by spray pyrolysis method .
3.3. X-ray Diffractogram
Fig. 3 shows the X-ray diffraction (XRD) patterns of as deposited ZnO thin films and annealed at different temperatures conditions. Eight different fundamental peaks were identified as (100), (002), (101), (102), (1 10), (103), (1 12) and (201) which indicates the hexagonal structure of ZnO with space group P63mc . This is a kind of Wuztrite type structure which is formed in the present case during pyrolysis of ZnO. No extra peaks were observed which obviously indicates the single phase of ZnO thin film.
Grain size of the prepared ZnO thin film was determined from the stronger peaks of (002) and (101) from each XRD patterns using Scherrer formula,
where Dg is the average grain size, λ is the wavelength of the radiation used as the primary beam of CuK^sub α^ (λ = 1.54178 Å), θ is the angle of incidence in degree and ? is the full width at half maximum (FWHM) of the peak in radian, which was determined experimentally after correction of instrumental broadening (in the present case it is 0.05°). Fig. 3. (b) shows the XRD pattern of ZnO thin films, where two strong peeks (002) and (101) were shown in expanded form to understand the variation of FWHM and peak shift of Bragg peaks with temperature. Lattice parameters a and c were determined from the 20 value. All the values of lattice parameters (a and c), grain size and peak shift (20 value) of the film at different temperatures for the two reflections of (002) and (101) are listed in table 1. It is seen that the value of lattice parameter 'c' was observed 5.2625 Å in as-deposited condition and 5.1938 Å after heat treated at 600 °C. Similarly the value of 'a' was observed 3.2779 Å in as deposited condition and 3.2426 Å after heat treatment at 600 °C. Both the values of a and c decrease with increasing temperature. Average grain size of ZnO thin film was determined in the range of 2 1 to 27 nm, which indicate the nanometric size of ZnO grains developed on the film. Peak shift was observed very clearly in Fig. 3. (b) and in Table 1. It is found that peak has been shifted to higher angle side with increasing temperature (from 34.072° to 34.537° and from 35.943° to 36.368°), which indicate the stabilization of peak after heat treatment via atomic rearrangement.
3.4. Optical Properties
The transmittance and absorbance spectrums of ZnO thin films annealed at different temperatures are shown in Fig. 4. It is seen from Fig. 4 that transmittance decreases with annealing temperature. On the other hand, it is seen from the figure that the absorption coefficient increases up to certain values of wavelength in the UV region and then it decreases exponentially and finally becomes constant.
It is also found from this figure that absorption coefficient increases with the annealing temperature. In Fig. 5, A^sup 2^ is plotted against the photon energy in order to achieve the band gap for ZnO films. The band gap of the films varied between 3.17 eV to 3.33 eV and the band gap decreases with the increase of the annealing temperature. This suggests that the absorption edge shifts to the lower energy as a consequence of the thermal annealing on the film and the fundamental absorption edge corresponds to a direct energy gap.
From Table 1 it is observed that higher annealing temperature causes a decrement in the band gap energy. The temperature dependent parameter that affects the band gap is re-organization of the thin film. By filling the voids in the film one expects denser film and lower energy gap. Another possibility is perhaps due to annealing process, atoms rearrange to more energetic and suitable position in the valance band. In this way, the photo current can be increased by the longer mean free path electrons. So that less energy is needed for an electron to jump from valance band to the conduction band.
Fig. 6 shows the variation of the refractive index with wavelength for ZnO thin film annealed at different temperatures. Refractive index were calculated using the formula
where k is the extinction coefficient and R is the optical reflectance.
From Fig. 6 it is evident that the refractive indexes of the films increase as we annealed it at higher temperature. This increase may be attributed to higher packing density and change in crystalline structure. Moreover, the XRD data indicates that the lattice parameters increase with the annealing temperature which means that the compactness of the film increases. This may be the cause for the increase of refractive index in our samples.
The structural and optical properties of ZnO nano fiber thin films annealed at different temperatures have been studied. SEM micrograph shows that the nano fibers become more prominent and visible with the increase of temperature. Film composition data have demonstrated that all films contain Zn and O with stoichiometric ratio. The direct band gap was found in the range 3.28 - 3.32 eV and it decreases with the annealing temperature. X-ray diffraction study reveals that all the films are in nature having wurtzite crystal structure with strong c-axis orientation. It has found that peak has shifted to higher angle side with increasing temperature which indicates that the crystallinity of ZnO was improved with temperature. The experimental result shows that this obtained nano fiber material may be suitable for using as transparent and conducting window materials in solar cells as well in sensing of gases and vapours.
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1 M. R. Islam, '1J. Podder, 2S. F. U. Farhad and 3D. K. Sana
1 Department of Physics, Bangladesh University of Engineering & Technology, Dhaka- 1000, Bangladesh
2 Industrial Physics Division, BCSIR Laboratories, Dhaka- 1205, Bangladesh
3 Materials Science Division, Atomic Energy Centre, Dhaka- 1000, Bangladesh
* E-mail: firstname.lastname@example.org
Received: 3 May 2011 /Accepted: 21 November 2011 /Published: 29 November 2011