Stimulated Surface Relief Erasing in Amorphous As‐Se Layers: Thermal‐ and Light Induced Effects

Abstract

Here the kinetics of erasing surface relief gratings (SRGs) in amorphous As20Se80 thin films are studied under various temperatures (in the range 298–408 K) and continuous‐wave illumination by band gap light of various intensities (0–2.5 W cm−2) and polarizations. The SRGs with 3.5 and 15 μm periods and 300 nm amplitudes are recorded by two beam interference. By the measurements of erasing kinetics on the grating of two the different periods, the authors are able to separate contributions of viscous flow and photo‐induced (PI) diffusion flow, which the authors consider as independent mechanisms of mass transfer. The authors detect that band gap light illumination does not change the viscosity of chalcogenide films. In contrast to previous results, the authors suggest that mass transport is accelerated by light due to contribution of PI diffusion flow, not by decrease of “pure” viscous flow. Contribution of PI diffusion flow, as well as anisotropy of mass transport under illumination by polarized light, drops with increasing temperature, whereas contribution of viscous flow grows with temperature. By separation of contributions of PI diffusion and viscous flow in mass transfer, the authors can determine PI diffusion coefficients and their temperature dependences.

Photo induced self-diffusion and viscosity in amorphous chalcogenide films

“…According to our direct measurements of the viscosity coefficients in the dark and under illumination by a band gap light [24], the 'pure' viscosity coefficients practically do not depend on the illumination. Acceleration of the mass transfer under band gap illumination is caused only by the contribution of PI diffusion in the mass transfer process.…”

“…It was found that D linearly increased with the intensity, I, and the coefficient β=D/I was equal 2.5×10 −18 m 4 /J [22]. Later, the PI diffusion coefficients were quantitavely estimated by the kinetics of growth of surface relief gratings (SRGs) at 77 and 300 K [23] for the same film composition, and recently, the PI coefficients were measured [24] in these films in a wide temperature range (308-398 K) and light intensities (0-2.5 W/cm 2 ). Using the method of capillary flattening of SRGs with two different grating periods, Λ, the authors could separate mechanisms of viscous flow (which was detected near and above glass transition temperature T g in the dark) and PI self-diffusion (at lower temperatures under band gap laser illumination).…”

“…All known data on PI diffusion coefficients for As 20 Se 80 films are summarized in figure 1 for light intensities at about 1.45 W/cm 2 . In [24], the PI diffusion coefficients were presented as non-Arrhenius dependence with the effective diffusion activation energy changing from 0.1 to 0.7 eV as temperature increased (see continuous line in the inset). However, the addition of our low-temperature data [23] led to the temperature dependence of D in the form of two straight lines in the Arrhenius plot, given by equations:…”

“…Temperature dependence of PI diffusion coefficients in As 20 Se 80 films measured in [23] and [24] for 1.45 W/cm 2 light intensity and estimated by equation (7). Inset: Data obtained in [24] (continuous green line) presented taking into account data of [23] (red dotted lines).…”

“…As it was mentioned above, our results on PI diffusion were obtained by measurements of the erasing kinetics for gratings previously recorded on the film surface. According to the theoretical analysis [22], which accounted for both the viscous flow and the bulk diffusion mechanisms, the effective smoothing constant can be expressed as [24]…”