Photo-Induced Evolution of Randomly Rough Surfaces of Amorphous Chalcogenide Films

Photoinduced (PI) evolution of statistically rough surfaces of amorphous chalcogenide films As₂₀Se₈₀ at room temperature has been studied by measuring the angular dependence of the intensity of light scattered from a surface illuminated by CW laser (λ = 660 nm). The interpretation of the scattering data based on the resonant scattering theory enables to confirm unequivocally the diffusion mechanism of PI mass transfer. It is detected that the change of the amplitude of a spatial harmonic in the roughness spectra strongly depends on its period ∧. During illumination, the amplitude increases at ∧ > ∧^*, whereas harmonics with ∧ < ∧^* decreases by ∧^*, which corresponds to zero evolution rate, is found to be 6.7 μm. In accordance with our theoretical prediction, both growth and decrease are exponential with the rates depending on ∧. As the result, the roughness with initial rms height of 50–70 nm transforms into quasiperiodic surface grating with the average amplitude of about 400 nm and average period close to 15 μm. From the kinetics of time variation of the scattered intensity, the PI diffusion coefficient D is calculated. When the laser intensity changes from 5.6 to 14 W cm⁻², D is found in the range 1 × 10⁻¹³–3.4 × 10⁻¹³ m² s⁻¹.