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Thin structures Ag–thin film of chalcogenide glasses (ChG) have attracted the attention of researchers for 60 years, ever since the effect of light stimulated diffusion of silver into ChG was discovered by M. T. Kostyshyn, K. V. Michailovska and P. F. Romanenko [1]. This is due to the rich physicochemical nature of the phenomena occurring within these structures, resulting in a wide variety of practical applications (information recording, inorganic resists, the fabrication of optical elements and holograms, photovoltaic absorbers, among others). Recently, researchers have become increasingly interested in photo- and thermostimulated diffusion of Ag into superthin (tens of nanometers thick) chalcogenide layers [2, 3] to form the elements of radiation-hard non-volatile memory, which is in high demand, particularly in the space industry.
The porosity of superthin ChG layers affects both the photo- and thermally stimulated diffusion of Ag, as well as structural transformations within them. Due to the small thickness of these layers and the sensitivity of Ag–ChG structures to irradiation, studying photo- and thermally stimulated processes in such structures using traditional methods presents challenges. We have applied a method based on surface plasmon-polariton (SPP) resonance, commonly used in optical sensors, to study Ag–ChG structures with chalcogenide layer thicknesses of 10–20 nm [4,5]. In this approach, a substrate with a profiled surface in the form of a diffraction grating was used to excite SPPs at the interface between silver and ChG layer.
This study examines the thermally stimulated diffusion of Ag into superthin porous As₄₀S₆₀ films thermally deposited onto the silver grating. Annealing of the samples was conducted in the temperature range of 100–130 °C. Changes in the film due to silver diffusion were registered by tracking the position and shape changes of the SPP resonance in the angular dependence of specular reflection of p-polarized He-Ne laser radiation (λ = 632.8 nm).
The detection of SPP resonance shifts enabled the determination of changes in the refractive index of the As₂₀S₃₀ layer due to silver doping, and subsequently, the calculation of the kinetics of thermally stimulated Ag concentration growth in the As₄₀S₆₀ film, as well as the corresponding decrease in silver layer thickness ($\delta d_{Ag}$). At all annealing temperatures, the $\delta d_{Ag}$ kinetics exhibited linear segments with respect to $t^{1/2}$ (t – annealing time), allowing estimation of the Ag diffusion coefficients. It was found that the temperature dependence of the diffusion coefficient follows the Arrhenius equation with activation energy $E_a$ = 1.5 eV. The obtained value of $E_a$ coincides with the activation energy for thermal diffusion of silver in thick, nonporous As₄₀S₆₀ films. This means that the key stage in the process of thermally stimulated diffusion of silver into ultrathin porous As₄₀S₆₀ films is also interatomic diffusion into the chalcogenide matrix.
Acknowledgements. The authors express their gratitude to the National Academy of Sciences of Ukraine for support within the framework of the project 0125U000799 "New physical principles and technologies for the development of the element base of modern infrared photoelectronics".
References
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