We have reported a low-cost and fast formation of highly efficient Er centers in ZnO thin films. As a high sensitivity tool for the detection of trace of Er dopant in ZnO film, spectroscopic ellipsometry is employed to disclose the systematic interrelationship of the crystallinity, dielectric function and optical band structure. Pure ZnO thin film shows very sharp band structure. The films with 0.05 at.% Er dopant, annealed at 600 degrees C and 800 degrees C, exhibit the similar tendency where the dopant level appears at the band tail. The band structure of the films with 0.05 at.% Er dopant, annealed at 400 degrees C, is very close to that of pure ZnO. While the samples annealed at 1000 degrees C are on the verge of amorphousness, and the flat curve of photon energy dependent epsilon(i)(E) is observed. The strain effect caused by the formation of ErO6 pseudo-octahedron structure greatly affects the value of dielectric constants. Therefore, SE analyses reveal significant effect of Er doping and annealing temperatures on the modification of optical band structure, dielectric property and optically active center in ZnO films. (C) 2013 Elsevier B.V. All rights reserved.