Annealing and biasing effects on the structural and optical properties of PECVD-grown TiO2 films from TTIP/O2 plasma

Abstract

This work presents a study of the structural, morphological, and optical properties of titanium dioxide thin films prepared via plasma enhanced chemical vapour deposition at the floating potential (Vf) or substrate bias (Vb) of − 10 and − 50 V, after being submitted to annealing processes varying the temperature from 450 to 850 °C. By tuning the annealing temperature and substrate bias voltage respectively, which could be helpful for the explanation about the thermal effect and the influence of ion bombardment on TiO2 films obtained from low-temperature plasma deposition. The results have shown that the TiO2 thin films grown at Vf and − 10 V persist in the anatase phase even after annealing at 850 °C. The phase transformation from anatase to rutile occurred in the case of − 50 V, and the crystallization enhancement has been identified as the annealing temperature increased. From scanning electron microscopy measurements, the formation of gradient columnar morphology was also perceived in the cases of Vf and − 10 V, with increasing annealing temperature at 850 °C, these structures disappear transforming into homogeneous columns with larger size at Vf and granular structure at − 10 V. In the case of − 50 V, a well-organized columnar morphology has been found in the as-deposited film, and it also can be modified into granular structure with the post-annealing effect at 850 °C but without thickness reduction. Spectroscopic ellipsometry (SE) study was used to determine the effect of annealing temperature on the thickness and on the optical constant of TiO2 thin films. SE shows that the band gap of TiO2 thin films was found to decrease when the annealing temperature increases. Meanwhile, the annealing temperature of 450 °C leads to a decrease of refractive indices of the films grown at Vf and − 10 V, but has no effect on the film of − 50 V. Then further increase of annealing temperature at 850 °C, an improvement of refractive indices can be identified for all the films deposited with various bias voltages.