International audienc

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Several chalcogenide alloys exhibit a pronounced contrast between the optical absorption in the metastable rocksalt and in the amorphous phase. This phenomenon is the basis for their application in optical data storage. Here we present ab initio calculations of the optical properties of GeTe and Ge1Sb2Te4 in the two phases. The analysis of our computations and experimental data reveal the correlation between local structural changes and optical properties as well as the origin of the optical contrast in these materials. We find that the change in optical properties cannot be attributed to a smearing of transition energies as commonly assumed for amorphous semiconductors: the optical contrast between the two phases can only be explained by significant changes in the transition matrix elements.

International audiencePhase-change materials are successfully employed in optical data storage and are becoming a promising candidate for future electronic storage applications. Despite the increasing technological interest, many fundamental properties of these materials remain poorly understood. However, in the last few years the understanding of the material properties of phase-change materials has increased significantly. At the same time, great advances have been achieved in technological applications in electronic as well as optical data storage. We review the latest scientific and technological developments in the field, focusing on the electronic, optical, and kinetic properties of phase-change materials

The structure of molten elemental (Si, Ge) or binary (III-V, II-VI) semiconductors is well known, and has been shown to depend on the average number of valence s-and p-electrons (N sp ) [1] However, studies on binary systems have been limited to stoichiometric compounds, corresponding to a discrete set of N sp values. Studying ternary compounds allows us to investigate homogeneous liquids with varying average valenceelectron numbers. These materials have the additional advantage of often having lower melting temperatures, which renders them experimentally feasible for studies in an accessible temperature range. Among these ternary systems, a subset of Sb-and Te-based alloys shows a unique combination of properties. On the one hand, their electrical resistivity and optical reflectivity change dramatically with the transition between amorphous and crystalline states, indicating significant structural differences between these two phases. On the other hand, re-crystallization of the amorphous phase using laser or current pulses at temperatures between the glass transition (T g ) and melting (T m ) temperatures is fast, and proceeds in less than 10 ns. These properties are used in phase-change memories, [2,3] with a number of suitable materials for optical and electronic phase-change storage having been identified by trial and error.

Phase change materials possess a unique combination of properties, which includes a pronounced property contrast between the amorphous and crystalline state, i.e., high electrical and optical contrast. In particular, the latter observation is indicative of a considerable structural difference between the amorphous and crystalline state, which furthermore is characterized by a very high vacancy concentration unknown from common semiconductors. Through the use of ab initio calculations, this work shows how the electric and optical contrast is correlated with structural differences between the crystalline and the amorphous state and how the vacancy concentration controls the optical properties. Furthermore, crystal nucleation rates and crystal growth velocities of various phase change materials have been determined by atomic force microscopy and differential thermal analysis. In particular, the observation of different recrystallization mechanisms upon laser heating of amorphous marks is explained by the relative difference of just three basic parameters among these alloys, namely, the melt-crystalline interfacial energy, the entropy of fusion, and the glass transition temperature.

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First-principles molecular dynamics simulations are used to study the structural properties of liquid and crystalline SnSe 2. We reproduce the experimental structure factor with confidence and fully describe the pair-correlation functions and the local structure of the liquid. It is shown that, unlike other group IV chalcogenides such as GeSe 2 , SnSe 2 does not display tetrahedral ordering in the liquid and contains a large amount of fivefold tin atoms with selenium atoms lying in an equatorial plane and at the edges of the polyhedra. A certain number of homopolar defects are found whose rate is substantially lower however than in GeSe 2. Compared to the crystalline system the density in the liquid decreases by 8.5%, which is accompanied by a decrease in the atomic coordination. Local distortions as found in typical phase-change materials are present in SnSe 2 .