The isothermal crystal growth in Se 100−x Te x bulk glasses (x = 10, 20, and 30) was studied directly using infrared microscopy. The crystals grew spherulitically and linearly in the course of time, which is typical for crystal growth controlled by liquid-crystal interface kinetics. An operative growth model was found using a combination of growth and viscosity data, and using two different approaches for calculations of the Gibbs free energy change between the undercooled melt and crystalline phase. The study shows that the exact knowledge of the Gibbs free energy change calculated from both, heat capacities, and the simple approximation proposed by Turnbull, can provide comparable results regarding determination of an operative crystal growth model. A detailed discussion about the relationship between the kinetic coefficient of crystal growth rate and viscosity (u kin ∝ η −ξ ) is presented. Moreover, the activation energies of crystal growth were found to be higher than the activation energies of the overall crystallization process obtained by differential scanning calorimetry. The relation between these two quantities is considered under the experimental conditions. ■ INTRODUCTIONChalcogenide glasses and thin films are very interesting materials that exhibit unique physical properties. Because of their diverse active properties, chalcogenide materials can be used in various optical and optoelectronical devices or in various electronic thresholds and switches. 1−3 Interesting applications of chalcogenide materials also are found in modern high-tech memory devices. 4−6 Thermal stability and crystallization play a key role in processing and usage of the materials. In particular, the crystallization process needs to be considered from two points of viewin order to obtain an ideal glass the crystallization has to be prevented, and, on the other hand, the controlled amorphous-to-crystalline transformation is a fundamental process of considered technology (modern phase change materials, PCM). Thus, knowledge and understanding of crystal growth kinetics and nucleation in such materials are essential for their future applications. The crystallization studies 7−11 focus on evaluation of mechanism of crystal growth and nucleation, and on prediction of crystallization behavior in a wide temperature range. Such a description can be useful for tailoring and optimization of new high-tech materials as they provide a possibility to predict the crystallization behavior in similar materials by revealing the basic mechanisms and properties of the material.Selenium-based materials have been commercially applied in photoreceptors, photovoltaic materials, etc. 12 The properties of pure selenium can be significantly improved by alloying with other elements such as tellurium, germanium, antimony, arsenic, etc. Se−Te glasses and thin films are very attractive materials that exhibit an intermediate behavior between pure selenium and tellurium. The Se−Te mixtures exhibit numerous advantages in comparison with pure Se, for example, great...

The crystal growth velocity of spherulitic As2Se3 in a supercooled melt of the same composition was studied by optical microscopy and thermoanalytical methods in isothermal and nonisothermal conditions. The time dependence of crystal size is linear, which suggests the crystal growth is controlled by interface kinetics. Crystal growth velocity was determined as the slope of these linear dependences. The experimental results presented in this paper considerably extend the previously reported range of crystal growth velocity. All isothermal crystal growth velocity data can be well described by the standard two-dimensional surface nucleated growth model (2Dsg) including crystal growth viscosity decoupling (ξ = 0.647). The activation energy of crystal growth for microscopic experiments is in a good agreement with values obtained from thermoanalytical experiments, and the ratio of the activation energy of crystal growth and the activation energy of viscous flow well corresponds to an independently determined decoupling parameter. The same model successfully describes also crystalline layer thickness and growth pattern at the amorphous As2Se3 surface in nonisothermal conditions.

Crystal growth rates in GeSbSe bulk glass and thin film were measured using optical and scanning electron microscopy under isothermal conditions. The studied temperature region was 255-346 °C and 254-286 °C for bulk glass and thin film, respectively. The compact crystalline layer growing from the surface into the amorphous core was formed in bulk glasses and no bulk crystallization was observed. In the case of thin films, needle-shape crystals were formed. The crystalline layer and needle-shape crystals grew linearly with time that corresponds to a crystal growth controlled by the crystal-liquid interface kinetics. In the narrow temperature range, crystal growth rates exhibit simple exponential behavior, so the activation energies of crystal growth for the studied temperature regions were estimated (E = 294 ± 6 kJ/mol for bulk glass and E = 224 ± 12 kJ/mol for thin film). Viscosity of GeSbSe material was measured in the region of the undercooled melt and glass. The extrapolation of viscosity data into the immeasurable, but important, temperature range is discussed. The experimental growth data were combined with melting and viscosity data and the appropriate growth models were proposed to describe crystal growth in a wide temperature region. The standard crystal growth models are based on a simple proportionality of the crystal growth rate to the viscosity (u ∝ η). This simple proportionality holds for the bulk material. Nevertheless, in the thin films the decoupling of the crystal growth rate from the inverse viscosity occurs, and the standard kinetic growth models need to be corrected. Such corrections provide better description of experimental data and more realistic value of the parameter describing the mean interatomic distance in the crystal-liquid interface layer, where the crystal growth takes place.

The isothermal crystal growth kinetics in Se70Te30 thin films was investigated using the microscopy and in situ X-ray diffraction (XRD) measurements. Plate-like crystals grew linearly with time which is the sign of liquid-crystal interface kinetics. In the studied temperature range, from 68 °C to 88 °C, crystal growth rates exhibit simple exponential behavior with an activation energy of crystal growth EG = 168 ± 12 kJ mol−1. The growth data obtained from the microscopy measurements were combined with viscosity data, melting parameters and the appropriate crystal growth model was assessed. The relation between the kinetic coefficient of crystal growth and viscosity (u∝η-ξ) is described in detail, and a correction of the standard growth model is suggested. The crystal growth data obtained from the in situ XRD measurements were described using the Johnson-Mehl-Avrami nucleation-growth model with the Avrami exponent m = 2.2 ± 0.2. The activation energy of the overall crystallization process EA was estimated and its value is 171 ± 11 kJ mol−1.

The crystal nucleation behavior and kinetics in Ge1.8Sb36.8S61.4 thin films were studied using optical microscopy coupled with a computer-controlled heating stage. The single-stage in situ heat treatment method was chosen for the study of nucleation. In-situ experiments were performed in the temperature range of 236–295 °C. The time evolution of the number of nuclei at various temperatures revealed transient behavior at low nucleation times. The transient nucleation data were described using the Shneidman equation to get values of steady-state nucleation rate, induction period, and time-lag of nucleation for the studied temperatures. On the basis of nucleation experiments, the temperature dependence of crystal–liquid surface energy and decoupling of nucleation rate and viscosity was assessed. The nucleation rate data obtained from microscopy measurements were discussed in terms of classical nucleation theory. It was found that the nucleation curve with the maximum at 288 °C and previously published growth curve with maximum at 309 °C overlap significantly.

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