Effect of the pH on the physical properties of ZnO:In thin films deposited by spray pyrolysis

The optical, electrical and structural properties of polycrystalline ZnO : In thin films grown from starting solutions with different pH values are reported. The films were deposited on glass substrates by spray pyrolysis at temperatures below 500 degrees C. Zinc acetate dissolved in an aqueous solution was used as a precursor. Different amounts of acetic acid were added to change the pH of the solution to two different values. The average optical transmission of the films was above 85% and the film thickness was about 0.65 mu m. The resistivity of the films decreased as the pH of the solution decreased. Concentration and mobility values, measured from Hall effect, were slightly lower for pH = 3.2 grown films. pH = 3.8 grown films showed textured surfaces with the texture changing from rounded to rough and angular uniform grains as the substrate temperature increased. pH = 3.2 grown films had smooth surfaces. X-ray diffraction spectra showed a (0 0 2) preferential orientation for pH = 3.2 grown films, and (0 0 2) and (1 0 1) orientations for pH = 3.8 grown films. These results have been discussed on the basis of the chemical species involved in the solution. A two-dimensional predominance zone diagram was constructed in order to define the thermodynamic distribution of chemical species in both systems. (C) 1999 Elsevier Science B.V. All rights reserved.