Effect of Nucleation and Growth Kinetics on the Electrical and Optical Properties of Undoped ZnO Films

Physical properties of ZnO thin films are of great importance for their potential application. In this paper, we reported the electrical and optical properties of undoped ZnO films with a high dependence on the nucleation and growth kinetics. These ZnO films were deposited on glass substrates within the temperature range of 250−350 °C by pulsed-spray evaporation chemical vapor deposition (PSE-CVD). X-ray diffraction (XRD) shows that ZnO films grow in the c -axis orientated hexagonal structure. The inspection of the surface morphology with scanning electron micrographs (SEM) shows a substantial effect of the growth temperature, which was directly correlated with the nucleation kinetics. A comprehensive nucleation and growth process was also proposed for the interpretation of changeable film morphology and corresponding electrical conductivity. The films deposited at intermediate growth temperatures of 310−340 °C show compact structures and consequent high electrical conductivity, which was still improved with increase of film thickness. In contrast to the electrical conductivity, the optical band gap rather shows a correlation with the size of the crystals forming the films rather than with their arrangement. The band gap energy decreases from 3.56 to 3.42 eV toward the bulk value upon the increase of the crystallite size from 15 to 30 nm.