Effects of the deposition condition on the microstructure and properties of ZnO thin films deposited by metal organic chemical vapor deposition with ultrasonic nebulization

Abstract

ZnO thin films were deposited on glass substrates by metal-organic chemical vapor deposition with ultrasonic nebulization at different substrate temperatures and source solution concentrations, and the effects of the deposition conditions on the microstructure and properties of the ZnO films were investigated. The deposition rate was controlled by the rate of the surface reaction at low substrate temperatures and by the rate of mass transfer of the reactants at high substrate temperatures. The transition temperature between the surface-reaction-controlled condition and the mass-transfer-controlled condition increased with increasing concentration of the source solution. The microstructure and crystallographic properties of the films were largely affected by the deposition conditions. The ZnO thin films grown under the surface-reaction-controlled conditions were composed of columnar grains with a plate–pillar structure and some open spaces between the columnar grains. The crystal direction of the grains was oriented randomly. On the other hand, under the mass-transfer-controlled condition, the ZnO films adopted a dense rock-like structure composed of very large grains, whose crystal directions were highly [001] oriented perpendicular to the substrate surface. The resistivity of the films depended mainly on the mobility of the charge carriers, which varied significantly with the microstructure. The resistivity of the films with the columnar structure was much higher than that of the films with the rock-like structure because of the open space between the columnar grains. The mean transmittance of the films in the visible range was more than 80%. The optical band gap of the films was approximately 3.3eV.