A comparative analysis of effect of annealing condition on structural, optical and electrical properties of fluorine-doped ZnO thin film prepared by spray-pyrolysis technique

Abstract

This study is focused on the deposition of Fluorine-doped ZnO (FZO) thin film on borosilicate glass substrate by spray pyrolysis technique and the influence of post-deposition annealing condition (air and vacuum) on its structural, morphological, optical and electrical characteristics. F concentration is maintained at 15 at.% with respect to Zn in the precursor solution. The presence of F in the film along with Zn and O is confirmed by energy dispersive X-ray analysis. There is distinguishable change in the surface morphology of the films with variation in annealing condition. Although annealing imparts a significant growth of crystallites along other planes like (100), (101), (110) etc., vacuum-annealing preserves the strong (002) preferred orientation of as-deposited FZO film, while air annealing leads to reorientation along the (100) plane, highlighting the role of oxygen in the crystallite growth process. No secondary phase corresponding to ZnF2 is observed in the X-ray diffraction analysis. As-deposited film shows the highest transmittance (similar to 80% at 550 nm wavelength), while a slight reduction is observed for the air-annealed film. In contrast, the vacuum-annealed film exhibits comparatively lower transmittance. The optical band gaps are estimated to be 3.23 eV, 3.15 eV and 3.06 eV in case of as-deposited, air-annealed and vacuum annealed FZO films. As-deposited film exhibits better electrical conductivity at room temperature (RT) compared to the annealed films, although X-ray photoelectron spectroscopic analysis indicates better structural ordering of FZO films with annealing. This may be linked to the loss of Fluorine during annealing process which act as donor. Better figure of merit at room temperature combined with high, stable and continuous rise of conductivity at elevated temperatures up to 200 degrees C demonstrates that as-deposited FZO film can be a good choice for use in optoelectronic devices.