Spectroscopic ellipsometry study of the effect of illumination and thermal annealing on the optical constants of thin Ge-As-S films

The effects of illumination and post-illumination thermal annealing on the optical properties of chalcogenide thin (similar to 150 nm) films were studied by spectroscopic ellipsometry. The films were thermally evaporated from Ge30.8As5.7S63.5 and Ge32As5S63 glasses. They were exposed to illumination with an HBO 500 lamp and to subsequent thermal annealing at a temperature of 350 degrees C. Ellipsometric measurements in the spectral range 300-820 nm were carried out after each technological step. From the ellipsometric data analysis the optical constants (n, k, epsilon), optical band gap energy E-og and film thickness have been determined, while the oscillator energies E-0 and E-d have been estimated applying the single-oscillator approximation theory. In the epsilon(2) spectra three peaks, denoted by E-1, E-2 and E-3, have appeared, which are attributed to interband transitions. By illumination, the values of the complex refractive index ((n) over tilde = n - ik), dielectric function ((epsilon) over tilde = epsilon(1) - i epsilon(2)) and dispersion energy E-d decrease, whereas the band gap energy (E-og) and the oscillator energy (E-0) values increase. All these are accompanied by a similar to 12-13% enhancement of film thickness. Thermal annealing leads to a further increase of the energetic parameters values, but causes a similar to 14-15% decrease of film thickness in comparison to that of illuminated films. The E-1 and E-2 peaks diminish on illumination and post-illumination annealing, whereas the magnitude of the E-3 peak decreases on illumination and increases by annealing, approaching its initial value for the as-deposited state.