The identification of materials suitable for non-volatile phase-change memory applications is driven by the need to find materials with tailored properties for different technological applications and the desire to understand the scientific basis for their unique properties. Here, we report the observation of a distinctive and characteristic feature of phase-change materials. Measurements of the dielectric function in the energy range from 0.025 to 3 eV reveal that the optical dielectric constant is 70-200% larger for the crystalline than the amorphous phases. This difference is attributed to a significant change in bonding between the two phases. The optical dielectric constant of the amorphous phases is that expected of a covalent semiconductor, whereas that of the crystalline phases is strongly enhanced by resonant bonding effects. The quantification of these is enabled by measurements of the electronic polarizability. As this bonding in the crystalline state is a unique fingerprint for phase-change materials, a simple scheme to identify and characterize potential phase-change materials emerges.

Localization of charge carriers in crystalline solids has been the subject of numerous investigations over more than half a century. Materials that show a metal-insulator transition without a structural change are therefore of interest. Mechanisms leading to metal-insulator transition include electron correlation (Mott transition) or disorder (Anderson localization), but a clear distinction is difficult. Here we report on a metal-insulator transition on increasing annealing temperature for a group of crystalline phase-change materials, where the metal-insulator transition is due to strong disorder usually associated only with amorphous solids. With pronounced disorder but weak electron correlation, these phase-change materials form an unparalleled quantum state of matter. Their universal electronic behaviour seems to be at the origin of the remarkable reproducibility of the resistance switching that is crucial to their applications in non-volatile-memory devices. Controlling the degree of disorder in crystalline phase-change materials might enable multilevel resistance states in upcoming storage devices.

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.

Defective Bi2Te3 structures have been studied with the aim of lowering the thermal conductivity in order to improve the thermoelectric figure of merit. The cross-plane thermal conductivities of structures containing point defects have been computed by means of molecular dynamics techniques, finding a maximum decrease of 70% for a 4% concentration of tellurium atom vacancies. Superlattices with modified stoichiometries have also been considered in order to find the configuration having the lowest thermal conductivity. In this case, a maximum decrease of 70% was also found. These predictions open the way to the design of efficient bulk thermoelectric materials having optimised thermal properties similar to those of superlattices.

Scanning tunneling microscopy and spectroscopy have been employed to reveal the evolution of the band gap and the Fermi level as a function of the annealing temperature for Ge1Sb2Te4, a promising material for phase change memory applications. The band gap decreases continuously from 0.65 eV in the amorphous phase via 0.3 eV in the metastable crystalline phase to zero gap in the stable crystalline phase. The Fermi level moves from the center of the gap in the amorphous phase close to the valence band within the crystalline phases. Moreover, the metastable phase has been imaged with atomic resolution, presumably showing the Te lattice at negative sample bias and the Ge/Sb/vacancy lattice at positive bias.

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