Chalkogenidlegierungen für optische und elektronische Speichermedien

摘要

Phasechange alloys are commonly used in optical data storage. Within the next few years an additional application of these alloys is expected to be in solid state memories. In contrast to their widespread use, an understanding of the underlying mechnisms is still incomplete. In this work the properties of these alloys have been investigated systematically to improve the knowledge on phase change alloys. Most of these investigations were performed on GeSb2Te4, because of the DFT calculations available for this compound. On this example it is demonstrated how the different properties rely on each other. The Basis for the use of phasechange alloys in next generation solid state memories is built on the large difference in resistivity of the amorphous and crystalline phase. Additionally the crystallization temperature of around 130°C is important, since it enables a long data retention times. In combination with a fast crystallisation speed of 5 ns at elevated temperatures a fast writing at moderate energy consumption is possible. In this work a number of different techniques were employed to reveal the relationship of optical and electrical properties with the structure of these alloys. The GeSbTe alloys under investigation showed a thermally activated conductivity in the amorphous phase. The corresponding activation barrier was around 0.4 eV while the resistivity was about a few Wmat room temperature. Due to structural relaxation the resistivity and the activation barrier for transport increase with time. After crystallization at around 130°C the resistivity drops by three orders of magnitude while the optical bandgap changes from 0.6 eV to 0.4 eV. UV photon spectroscopy measurements revealed that these features are caused by a signifikant shift in the electron density in the valence-band. Additional x-ray photon spectroscopy showed the differences between in the underlying amorphous and crystalline structures. Here two main effects were observed. First a larger amount of free charge carriers was found in the crystalline phase. Secondly a charge transfer from one atomic species to another was observed. This charge transfer explains the large property contrast between the two phases. While the atoms in the crystalline phase are coordinated in the rocksalt structure the amorphous phase appears to be dominated by a spinell like arrangement of atoms. But these measurements also indicated that in the amorphous phase a second local arrangement of atoms can be found.