Photo induced self-diffusion and viscosity in amorphous chalcogenide films

Abstract

Acceleration of the mass transport in amorphous chalcogenide films under band gap light illumination is usually attributed to the decrease of the film viscosity. However, our direct measurements of the film viscosity at various temperatures and light intensities, made by flattening of surface relief gratings, have shown that the viscosity did not vary under illumination and the acceleration of the mass transfer was caused by the contribution of photo-induced (PI) self-diffusion. The PI diffusion coefficient is not related to the viscosity coefficient by the Stokes-Einstein relation and PI diffusion should be considered as an additional mechanism of the overall mass transport. In this paper, using well-known models of self-trapped excitons, we present the first atomic interpretation of PI diffusion coefficients, explain their dependence on temperature and light intensity, and compare with our experimental data. For characterization of PI acceleration of the mass transfer we introduce the term 'diffusional viscosity', like it is used for description of diffusion creep in crystalline solids. We estimate the temperature dependence of diffusional viscosity and show that it noticeably depends on the distance over which the material is redistributed. Taking into account the diffusional viscosity allows an adequate general interpretation of many photo-induced phenomena observed in the literature.

Laser Recording in Chalcogenide Glass Films: Driving Forces and Kinetics of the Mass Transfer

“…2) The effective diffusion coefficients of atoms, D, which define the kinetics of the PI mass transfer, [20] are proportional to the local density of free volumes, whose concentration depends mainly on the local concentration of radiation defects, N d (ρ). Dependence D(ρ) is defined by relations [34] DðρÞ % 1 4…”

“…2) The effective diffusion coefficients of atoms, D , which define the kinetics of the PI mass transfer, [ 20 ] are proportional to the local density of free volumes, whose concentration depends mainly on the local concentration of radiation defects, N d ( ρ ). Dependence D ( ρ ) is defined by relations [ 34 ] Here, Γ is the frequency of atomic jumps, a is the average jump length, c ( ρ ) ≈ Ω N d ( ρ ), k is the Boltzmann constant, Q m is the atomic migration energy, T ( ρ ) is defined by Equation (), and N d ( ρ ) by a Gaussian function with the numerical fitting coefficients k 1 and k 2 to get the carrier flux equal to that defined by Equation ().…”

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

“…In contrast to above‐mentioned mechanism of viscous flow and optical driving force, we suggest that PI mass transfer occurs by atomic diffusion motion of chalcogens and pnictides. [ 7 ] The Stokes–‐Einstein relation, which for many materials links diffusion and viscosity coefficients, fails for ACFs, [ 8 ] and thus the diffusion is an independent mechanism of mass transfer.…”

Photo-induced bending of chalcogenide glass fiber

“…It could be assumed as a kind of opto-mechanical effects, which appear in dye-polymers [15][16][17] and chalcogenide glasses. 16,[18][19][20][21] Regarding the latter, Krecmer et al 18,22) have discovered transitory deflection of silicon-nitride AFM cantilevers, which are overlayed by As-S-Se films, upon changing the direction of electric fields of linearly-polarized light. Asao and Tanaka have reproduced the phenomenon using As 2 S 3 (and Se) films deposited on mica substrates.…”