Preferential regions of growth of chemical bath deposited ZnO and Zn(OH)₂ thin films at room conditions

Zinc oxide (ZnO) and zinc hydroxide (Zn(OH)₂) films were deposited onto glass substrates at room conditions (25 °C and atmospheric pressure) by the chemical bath deposition technique. ZnSO₄, KOH, and NH₄NO₃ were used as chemical reagents for the chemical bath. The species distribution diagrams and the solubility curves were used to find the most adequate chemical conditions for the selective film deposition according to the concentrations and type of chemical reagents. The selective deposition of ZnO and/or Zn(OH)₂ was achieved by adjusting the pH of the chemical bath in the range from 11.35 to 12.30, where three regions of growth were determined. The optical bandgap energy measured on the films ranged from 3.31 eV for Zn(OH)₂ to 3.41 eV for ZnO. Lower values of Urbach energy were found when the ZnO formation was dominant. X-ray diffraction analysis showed that the pH of the chemical bath influences on the crystallinity of the films, where better crystallinity was observed for films with higher presence of ZnO. An aging experiment to evaluate the stability of the ZnO 15 weeks after the deposition process was carried out.