Andriy Kovalskyy
The Condensed Matter & Surface Sciences Colloquium Series presents Andriy Kovalskyy on “Modification of surface structure in chalcogenide glass thin films: mechanisms and practical applications” on Thursday, March 12, at 4:10 p.m. in Walter Lecture Hall 245.
Abstract: Chalcogenide glass thin films (ChGTF) have a number of applications (such as optical gratings, memory devices, microoptical elements etc.) which are based on the structural modification of the film by light, electrons, ions, gamma-quanta, chemical reagents, or other external factors. Surfaces play an important role in the performance of the mentioned devices and therefore a thorough understanding of structural modifications on the surface of ChGTF is particularly important.
We have employed high-resolution XPS and low energy ion scattering (LEIS) to characterize the surface of binary and ternary ChGTF under the influence of sub-, super- and bandgap light radiation of various intensities, electron beam at different fluences, sputtering by different types of ions, chemical etching in several solvents and plasma etching in controlled environment. Studies of photostructural transformations have been performed in air, nitrogen and vacuum on ChGTF thermally deposited or spin coated from solution. Comparison of structural environment on the surface and in the interior of ChGTF by high resolution XPS and Raman spectroscopy methods demonstrates significant differences between the concentration of homopolar and heteropolar bonds for thermally deposited films and a quite similar structure for the spin-coated layers. Reactivity of the surface layers towards oxygen is studied at ambient atmosphere.
Photoinduced transformations on the surface of ChGTF caused by super-bandgap irradiation are found to be very intense. The change in chemical composition of the surface is observed by XPS for stoichiometric and S-rich thermally deposited films within As-S system. Band gap light irradiation of freshly prepared films leads only to the redistribution of hompolar and heteropolar bonds without change in the composition.
XPS shows that low dose electron beam irradiation activates the surface of ChGTF, which contributes to the oxidation of As atoms in “wrong” As–As bonds, thereby decreasing the S concentration in the very top parts of thin ChG film. Electron irradiation with high doses alters the structure drastically. On subsequent exposure to air, the under-coordinated As atoms react with oxygen and decrease S/As ratio.
Depth profile of surface oxidation is studied for phase change memory Ge-Sb-Te (GST) thin film using LEIS method. Structural mechanisms of chemical etching of ChGTF surface in different solvents are discussed. Examples of applications of surface modification of ChGTF are presented.
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