Effect of heat treatment on the properties of ZnO thin films prepared by successive ion layer adsorption and reaction (SILAR)

Abstract

ZnO thin films prepared by the chemical deposition technique, SILAR (successive ion layer adsorption and reaction) exhibit zincite (hexagonal) structure, are transparent in the visible and infrared region (∼ 90% optical transmittance), and photoconductive. Heat treatments significantly modify the stoichiometry and crystal structure of as-prepared ZnO and therefore its optical and electrical properties. The dark conductivity of ZnO thin films changes according to the gas atmosphere employed during the treatment. As-prepared samples (of thickness 667Å) show a dark conductivity of 1.50 × 10−6 [Ω ·cm]−1, while a maximum dark conductivity of 2.70 × 10−2 [Ω ·cm]−1 was achieved after consecutive heat treatments in O2 and H2 at 350°C. The optical bandgap, 3.38 eV, of the as-prepared ZnO decreases by 0.125 eV after annealing. The activation energy for the dark conductivity is much less, 0.65 eV (as prepared) to 0.11 eV (annealed in O2) indicating that the defect structure in the film plays an important part in the charge carrier transport.