Thickness dependence of microstructure and properties of ZnO thin films deposited by metal-organic chemical vapor deposition using ultrasonic nebulization

ZnO thin films were deposited on soda lime glass substrates at 275 degrees C by metal-organic chemical vapor deposition using ultrasonic nebulization and the dependence of their microstructures and properties on film thickness were investigated. Deposition rate was controlled by mass transfer rate at flow rates lower than 6 l/min and by surface reaction rate above 6 l/min. The microstructures of the films were affected largely by the process controlling deposition rate. At the surface-reaction-controlled conditions, the ZnO films grew to a narrow columnar structure composed of columnar grains and open spaces between the grains and the crystal directions of the grains were somewhat randomly oriented. On the contrary at the mass-transfer-rate-controlled conditions, the ZnO films grew to 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 with the columnar structure was much higher than that of the films with the rock-like structure due to the open space between the columnar grains, and it gradually decreased with increasing film thickness. The resistivity of the films with the rock-like structure rapidly decreased with increasing film thickness up to 400 nm, but it became constant over 400 nm. The average transmittance of all films in the visible range was over 80% up to film thickness of 400 nm and then rapidly decreased with increasing film thickness. The optical band gap of the films was about 3.3 eV. (c) 2013 Elsevier B.V. All rights reserved.