Phase-change materials are of tremendous technological importance ranging from optical data storage to electronic memories. Despite this interest, many fundamental properties of phase-change materials, such as the role of vacancies, remain poorly understood. 'GeSbTe'-based phase-change materials contain vacancy concentrations around 10% in their metastable crystalline structure. By using density-functional theory, the origin of these vacancies has been clarified and we show that the most stable crystalline phases with rocksalt-like structures are characterized by large vacancy concentrations and local distortions. The ease by which vacancies are formed is explained by the need to annihilate energetically unfavourable antibonding Ge-Te and Sb-Te interactions in the highest occupied bands. Understanding how the interplay between vacancies and local distortions lowers the total energy helps to design novel phase-change materials as evidenced by new experimental data.

SnSe, SnSe2, and Sn2Se3 alloys have been studied to explore their suitability as new phase change alloys for electronic memory applications. The temperature dependence of the structural and electrical properties of these alloys has been determined and compared with that of GeTe. A large electrical resistance contrast of more than five orders of magnitude is achieved for SnSe2 and Sn2Se3 alloys upon crystallization. X-ray diffraction measurements show that the transition in sheet resistance is accompanied by crystallization. The activation energy for crystallization of SnSe, SnSe2, and Sn2Se3 has been determined. The microstructure of these alloys has been investigated by atomic force microscopy measurements. X-ray reflection measurements reveal density increases of 5.0%, 17.0%, and 9.1% upon crystallization for the different alloys.

Crystallization processes in different Te alloys, employed in phase change materials for optical data storage, have been investigated by in situ mechanical stress measurements. Upon crystallization a considerable stress buildup is observed, which scales with the volume change upon crystallization. Nevertheless the observed stress change only corresponds to approximately 9% of the stress estimated for a purely elastic transformation. Further evidence of stress relief phenomena comes from the temperature dependence of the stress in the crystalline and amorphous states. Ultrathin dielectric layers have a profound influence on the crystallization process as evidenced by simultaneous optical reflectance and mechanical stress measurements. This observation can be explained by heterogeneous nucleation of crystallites at the interface between the dielectric layer and the phase change film.

The influence of Bi doping upon the phase change characteristics of Ge2Sb2Te5 alloys has been investigated using four-point-probe electrical resistance measurements, grazing incidence x-ray diffraction (XRD), x-ray reflectometry (XRR) and variable incident angle spectroscopic ellipsometry, a static tester and atomic force microscopy. For a Ge2Sb2Te5 alloy doped with 3% Bi, two transition temperatures are observed in the temperature dependent sheet resistance measurements at 136°C and 236°C, respectively. The evolution of structures upon annealing, investigated by XRD, reveals that the first transition is caused by the crystallization of the amorphous film to a NaCl-type structure, while the second transition is related to the transition to a hexagonal structure. The density values of 5.87±0.05gcm−3, 6.33±0.05gcm−3, and 6.41±0.05gcm−3 are measured by XRR for the film in the amorphous, NaCl-type, and hexagonal structure, respectively. Ultrafast crystallization, which is correlated with a single NaCl-structure phase and the reduced activation barrier, is demonstrated. Sufficient optical contrast is exhibited and can be correlated with the density change upon crystallization.

At present, the discovery and development of phase change materials is mainly based upon empirical strategies and trial and error approaches. Here, we present a structural criterion that needs to be met to enable the mandatory fast recrystallization with sufficient optical contrast that characterizes suitable phase change materials. Comparing the behavior of AgInTe 2 and AgSbTe 2 films it is demonstrated that only the AgSbTe 2 films, which show a cubic coordination, have sufficient density contrast, and hence, also optical contrast to allow phase change recording.