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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.

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.