Effect of the Deposition Times on the Properties of ZnO Thin Films Deposited by Ultrasonic Spray Pyrolysis for Optoelectronic Applications

In the present work, undoped ZnO thin films have been deposited on glass substrates at 300 °C by ultrasonic spray pyrolysis technique. Zinc acetate, acetic acid and methanol have been used as precursors and solvents. The prepared thin films have a thickness ranging from 216 to 680 nm. Structural transformation has been observed as a function of deposition time. Thin films obtained at low deposition time are polycrystalline in nature, they crystallized under hexagonal wurtzite structure and presented a preferential orientation along the c-axis perpendicular to the substrate, where the maximum crystallite size was found to be 14.4 nm for a smallest deposition time of 5 min. However, those obtained at deposition time higher than 15 min exhibit amorphous nature and we observed a disappearance of diffraction peaks. Scanning electron microscopy has been applied for a morphology characterization of the films. A growth of nanowires in a length scale of few microns can be seen clearly for a deposition time of 5 min and enhanced at 10 min. An amorphous nature of ZnO thin films was obtained at deposition time of 15 and 20 min. The average transmittance of all films is over 80 % in the visible region and the band gap energy for crystalline phase increases from 3.17 to 3.45 eV with the increase of the deposition time. Low resistivities in the range of 6.91×10 – 3 and 6.60 (Ω.cm) – 1 were produced when the deposition time increases from 5 to 10 min, and more than 1.90 ( 105 (Ω.cm) – 1 for deposition time greater than 10 min. The Urbach energy decreases from 324 to 314 meV with increasing deposition time from 1 to 4 min due to the strain and disorder in the thin films. The optical and electrical properties of deposited thin films revealed that these thin films have potential applications in optoelectronic devices especially in solar cells.