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Karishmae Kadrager1 Al-Amin Ahmed Simon1 Maria Mitkova1

1, Boise State University, Boise, Idaho, United States

Chalcogenide glasses (ChGs) are widely used in electrical and optical memory because of their high refractive index, transparency in infrared (IR) region and thermally driven amorphous-to-crystalline phase change. Recent advancements in ChGs application in electronics and photonics devices demand glasses with high crystallization temperature (Tc) and high refractive index (n). In that aspect, Ge containing binary ChGs are promising candidates because of their high coordination number and relatively strong chemical bonds. In this paper, thermally induced phase change of GexSe100-x (x = 40, 33, 20) glasses from amorphous to crystalline condition and temperature dependency of their optical properties (refractive index, optical band gap, extinction coefficient) have been studied. The studies were carried out on thermally evaporated thin ChG films over silica substrates. Differential Scanning Calorimetry for different heating rates (10, 20, 40 K/min) have been carried out in order to obtain data about the crystallization temperature (Tc) of the bulk glasses, which was used as a benchmark for the annealing temperature for crystallization of the films. The heating of the thin films was performed in a rapid thermal annealing system at various target temperatures up to the Tc. The temperature was raised at a rate of 150 K/min and the samples were kept at the target temperature for 1 min and then cooled naturally. Refractive indices and extinction coefficients of annealed samples were measured in an ellipsometer using light of 270 nm to 1650 nm at three different incident angles 50°, 60°, 70°. From the ellipsometer data, optical bandgaps (E) have been calculated using Tauc procedure. In amorphous ChG thin films, the refractive indices at their transparent zone (> 600nm light) are observed to follow the order n40>n33>n20 (nx = refractive index at x) and optical bandgaps are observed to follow a similar trend E40>E33>E20. The Se rich compositions show a decrease in n with increasing of the temperature up to some minimum, after which at further increase of the temperature n rises in parallel with the temperature. An exception of this dependence are the data obtained for the Ge rich Ge40Se60 films, for which only the decrease of n with temperature has been observed. The surface morphology study of Ge40Se60 films using scanning electron microscopy (SEM) confirmed formation of crystals on the thin film surface. The obtained results are discussed based on the structural specific of the studied glasses and the possibility for application of the observed effects for optical recording.

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