Strukturelle und kinetische Aspekte der kombinatorischen Materialsynthese am Beispiel der Phasenwechselmedien
Nowadays different competing techniques for reversible storage of huge amounts of data are used. One approach that has found extensive application is phase change recording, whereby data are encoded by using amorphous or crystalline states. Consequently the reading, writing and erasure of information is based on an optical approach whereby laser light is used to induce the corresponding reflectivity levels. In order to satisfy the rising demand for the design of new and superior phase change materials, an absolute experimentally Trial-and-Error-Method has been employed in the past. Efforts have been undertaken in this work to steer away from the trial and error approach by pursuing two systematic approaches to the design of phase change materials: On the one hand, the density functional theory has been utilized to predict the energetically favored crystal structure of an alloy. The influence of the crystal structure on the kinetics of the phase transformation and of the coordination number and the band gap on the optical properties was discussed. To this end, AgInTe and AgSbTe alloys were theoretically investigated for this purpose. The predicted structure for the Indium-based alloy is the chalcopyrite structure and it was also confirmed experimentally. The low coordination number of 4 leads to a weak overlap of the energy bands resulting in an energy gap of 1 eV. Compared to other phase change materials, the low change of density and therefore of the local atomic configuration leads to a weak optical contrast, which disqualifies this material for phase change recording applications. The exchange of the Indium fraction by an Antimony component led to a preference of the rocksalt structure, which was theoretically predicted and experimentally verified. The coordination number of 6 leads to a pronounced change of the optical properties, which accompanies the transformation from the amorphous to the crystalline state. These suitable optical properties of AgSbTe rendered the investigation of the transformation kinetics possible, which showed a minimal duration of 10 ns for a successful recrystallization. The mechanism of recrystallization was identified as volume crystallization. On the other hand the combinatorial material synthesis was employed to analyze the scaling of the physical properties with changes of the chemical composition in the %-range. For this purpose, the AgInSbTe alloy system has been prepared and characterized for optical properties, transformation kinetics, density change and the crystal structure. The optical band gap of the amorphous phase was determined to be in the range of 0.26-0.47 eV depending on the exact chemical composition. This variation is attributed to the change of the imaginary part of the dielectric function at 833 nm using a simple model. The optical properties were correlated with the results of the crystallization kinetics, where minimum durations of 200-1100 ns for a successful crystallization were found. The recrystallization of an amorphous volume could be obtained in 25-120 ns, which scaled in the same way with the chemical composition like the required duration for crystallization. Both processes were connected with the stoichiometry using a thermodynamical model, which describes the correlation of the glass temperature divided by the melting temperature with the kinetics. In summary it is noted that by the employment of the density functional theory, empirical relations and thermodynamic models a systematic design of phase change materials was demonstrated.
info:eu-repo/classification/ddc/530, Optischer Speicher, Strukturelle Phasenumwandlung, Antimontelluride, Dotierung, Sekundärneutralteilchen-Massenspektrometrie, Physik, Massendatenspeicher, kombinatorische Materialsynthese, Phasenwechselmedien, Dichtefunktionaltheorie
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