Optical properties of amorphous Ta2O5 thin films deposited by RF magnetron sputtering

Optical properties including refractive indices, extinction coefficient, thickness and both optical direct and indirect band gaps of the Ta2O5 thin films deposited by RF magnetron sputtering were determined in this work. The influence of sputtering power, which varies from 70 to 130 W, on microstructure and optical properties of the films were investigated as well. Phase structure, surface morphology, optical transmittance and optical constants of the samples were characterized by X-ray diffraction, atomic force microscopy, spectrophotometry and ellipsometry, respectively. The results show that deposition rate of the films increases with increasing sputtering power, and the maximum deposition rate, 2.1 nm/min, is achieved at 130 W, while the minimum one, 0.84 nm/min, is obtained at 90 W. The as-deposited thin films are amorphous according to the X-ray diffraction. Good surface quality featured by dense and compact microstructure is obtained at lower and higher sputtering powers while some micro-pores are found in the film deposited at medium sputtering powers. The optical constants, such as refractive index, extinction coefficient, and thickness of the Ta2O5 thin films were determined as a function of wavelength based on the ellipsometry measurements and fittings, and the dispersive refractive index was obtained for each sputtering power. The optical constants have been verified by comparing the measured transmittance spectra with the simulated ones, as they are consistent well with each other. Direct bandgap ranging from 4.05 to 4.18 eV and indirect bandgap ranging from 4.53 to 4.66 eV were acquired for the films deposited with the aforementioned sputtering powers.