Structural, optical and electrical studies on spray deposited highly oriented ZnO films
Introduction
Zinc oxide (ZnO) is the semiconducting material having wide band gap and high transparency, also stable for chemical as well as thermal fluctuations. Generally, optical transparent oxides tend to be electrical insulators by virtue of their large optical band gap (≥3.1 eV), but ZnO is one of them having high transparency and conductivity like FTO, SnO2 films. Its transparency and conductivity prove that it is one of the promising candidate in the number of optoelectronic applications like energy windows, liquid crystal displays [1], solar cells, gas sensors, ultrasonic oscillators, transducers [2], [3], [4], [5], [6], etc. Thin films of ZnO have been prepared by number of techniques such as chemical bath deposition [7], reactive evaporation [8], sputtering [9], SILAR [10], spray pyrolysis [11]. Among these methods, spray pyrolysis is better for large area thin film formation.
Many researchers have prepared the thin films of ZnO by spray pyrolysis technique through aqueous and partly non-aqueous media using different sources as ingredients. In the present investigation, thin films of ZnO are prepared by spray pyrolysis technique using 0.025 M solution of zinc acetate prepared in purely double distilled water because it is observed that zinc acetate decomposes earlier than nitrate. Structural, optical and electrical characterization of the optimized sample have been carried out.
Section snippets
Experimental
Using zinc acetate (A.R grade), 0.025 M aqueous solution was prepared in double distilled water. Ultrasonically cleaned amorphous microslides were used as substrates. The substrates were kept over the heater at the constant temperature 698 K controlled by electronic temperature controller connected to the heater. Prepared solution was sprayed through specially designed glass nozzle on to the substrate using air as a carrier gas at the spray rate of 6 cm3/min. To and fro motion of the nozzle was
Characterization
Structural properties of the film were studied by using X-ray diffraction (XRD) and it was performed on Philips (PW-1710) Cu Kα diffractometer by varying the diffraction angle 2θ from 10° to 100° with step width of 0.02°. Optical absorption and transmittance was measured in the wavelength range 350–850 nm by using Hitachi (UV–VIS–NIR) Japan model 330 spectrophotometer. Electrical resistivity and thermoelectric power measurements were carried out using two-probe method within the temperature
Result and discussion
When aerosol droplets arrive close to the heated substrates, a pyrolytic process occurs and highly adherent ZnO films were produced. Possible reaction proposed by Paraguay et al. [12] is as followsFilms formed were uniform, transparent and well adherent to the substrates.
Film characterization
Fig. 1 shows the diffraction pattern of the ZnO film deposited on the glass substrates at the temperature 698 K. XRD studies reveal that the deposited film is polycrystalline having hexagonal wurtzite type lattice. Strongest peak corresponds to (100) plane indicating all grains have x-axis normal to the surface. The observed d value (2.793) is in good agreement with standard d value (2.816) taken from ASTM diffraction data [13]. Slow scan of XRD having step size 0.002° was used to calculate the
Conclusions
From the results reported here, it is concluded that ZnO film preparation through aqueous medium by spray pyrolysis technique is possible. XRD study shows that ZnO films deposited by using 0.025 M solution at the substrate temperature 698 K are polycrystalline and shows strong orientation along (100) plane. The electrical resistivity is found to be of the order of 10−1 Ω cm and exhibits n-type conductivity.
Acknowledgements
One of the authors, BJL, would like to thank UGC New Delhi for awarding teacher fellowship.
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