Phase-change materials : structure, vibrational states and thermodynamics of crystallization

Phase-change materials (PCM) offer the potential to be employed as active switching material in novel, non-volatile memory devices. These devices allow for faster readand write operations as compared to common Flash memory and are superior to state-of-the-art DRAM due to their ability to retain information even when the external power supply is removed. The application of such phase-change memory in various fields (e.g. automotive and home use) requires the adaptation to specific external conditions and hence can be realized only if systematic design rules are available to tune the material’s properties. These design rules can be derived from a microscopic theory for their properties and transitions. Data storage in these devices is generally based on the distinct properties of their amorphous and crystalline phases, from which the present state can be read. Information is written by heating the active material either above the crystallization temperature or above the melting temperature for vitrification. Therefore, research on these materials can be classified into three areas: The static properties of the amorphous phase, those of the crystalline phase and the nature of the quasi-reversible transition. Progress in the systematic understanding of phase-change materials has to be based on an atomistic picture of both, transitions and properties, in particular since only compounds containing some few elements – mostly chalcogenides – show the necessary properties. The atomic short range order has a crucial impact on the property contrast and was investigated here by means of x-ray absorption spectroscopy, total and inelastic neutron scattering. The resulting atomic pair distribution functions revealed that all investigated materials (i.e. GeSb2Te4, Ge2Sb2Te5, In3SbTe2, Sb2Te, AIST, GeTe und Ge15Sb85) show 5% to 15% longer average interatomic distances in the crystalline phase. Also their coordination number was found to increase. This increase is particularly pronounced in the case of Ge2Sb2Te5, where the average bond length increases from 2.72(1) Å by 11% upon crystallization, while at the same time the coordination number increases from 2.8(1) by 88(5)%. This modification of the local atomic structure indicates a change in the bonding mechanism and increases the contrast e.g. in optical reflectivity. In GeSb2Te4 and Ge2Sb2Te5, however, also the variance of interatomic distances increases upon crystallization. This observation is surprising, since a larger variance of bond lengths in the crystalline phase as compared to that of the amorphous phase at equal temperatures indicates a softening of the interatomic potentials upon crystallization. Measurements of the mechanical stress, however, show that the crystalline phase is mechanically harder than the amorphous phase. Measurements of the density of vibrational states were performed to resolve this discrepancy. They confirmed that the crystalline phase consists of softer bonds. The changes of the interaction potential are accompanied by a change in the bonding mechanism: While the coordination numbers of the amorphous phase of all investigated phase-change materials is well described by the 8-N rule, the coordination numbers of the respective crystalline phases is generally higher and in good agreement with resonance bonding, which is accompanied by a particularly large optical dielectric constant. This transition from stiffer bonds in the amorphous and softer bonds in the crystalline phase is favorable for the switching properties of memory devices. Recently, a metal-insulator transition was reported for various compounds from the pseudobinary line GeTe-Sb2Te3 (GST), whose order parameter is the atomic disorder. This transition can be controlled by changing the annealing temperature of amorphous phase-change materials above the crystallization temperature. At low annealing conditions, the material behaves semiconducting and becomes metallic after heating to higher temperatures. Motivated by these results, the local