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Controlling a state of material between its crystalline and glassy phase has fostered many real-world applications. Nevertheless, design rules for crystallization and vitrification kinetics still lack predictive power. Here, we identify stoichiometry trends for these processes in phase change materials, i.e. along the GeTe-GeSe, GeTe-SnTe, and GeTe-Sb2Te3 pseudo-binary lines employing a pump-probe laser setup and calorimetry. We discover a clear stoichiometry dependence of crystallization speed along a line connecting regions characterized by two fundamental bonding types, metallic and covalent bonding. Increasing covalency slows down crystallization by six orders of magnitude and promotes vitrification. The stoichiometry dependence is correlated with material properties, such as the optical properties of the crystalline phase and a bond indicator, the number of electrons shared between adjacent atoms. A quantum-chemical map explains these trends and provides a blueprint to design crystallization kinetics., Tailoring the crystallization kinetics of materials is important for targeting applications. Here the authors observe a remarkable dependence of crystallization and vitrification kinetics and attribute it to systematic bonding changes for a class of materials between metallic and covalent bonding.

Phase change memory (PCM) is the most mature among the novel memory concepts. Embedded PCM technology can be a real breakthrough for process cost saving and performances. Nevertheless, for specific applications some improvement in high temperature data retention characteristics is needed. In this paper, we present an optimized GexSbyTez phase change material, able to guarantee code integrity after soldering thermal profile and data retention in extended temperature range. In particular, extrapolation of data retention at 10 years for temperatures higher than 150°C cell level has been demonstrated, thus enabling automotive applications. Despite the tradeoff between the SET speed and RESET data retention, competitive performances with respect to present floating gate memories have been confirmed. Finally, solid data collection based on a 4-Mb test chip integrated in a standard 90-nm CMOS technology platform has been performed. Functionality and performances are well in line with today industrial targets.

A long-standing question for avant-garde data storage technology concerns the nature of the ultrafast photoinduced phase transformations in the wide class of chalcogenide phase-change materials (PCMs). Overall, a comprehensive understanding of the microstructural evolution and the relevant kinetics mechanisms accompanying the out-of-equilibrium phases is still missing. Here, after overheating a phase-change chalcogenide superlattice by an ultrafast laser pulse, we indirectly track the lattice relaxation by time resolved x-ray absorption spectroscopy (tr-XAS) with a sub-ns time resolution. The approach to the tr-XAS experimental results reported in this work provides an atomistic insight of the mechanism that takes place during the cooling process; meanwhile a first-principles model mimicking the microscopic distortions accounts for a straightforward representation of the observed dynamics. Finally, we envisage that our approach can be applied in future studies addressing the role of dynamical structural strain in PCMs.

Phase change materials based on chalcogenides are key enabling technologies for optical storage, such as rewritable CD and DVD and recently also electrical nonvolatile memory, named phase change memory (PCM). In a PCM, the amorphous or crystalline phase affects the material band structure, hence the device resistance. Although phase transformation is extremely fast and repeatable, the amorphous phase suffers structural relaxation and crystallization at relatively low temperatures, which may affect the temperature stability of PCM state. To improve the time/temperature stability of the PCM, novel operation modes of the device should be identified. Here, we present bipolar switching operation of PCM, which is interpreted by ion migration in the solid state induced by elevated temperature and electric field similar to the bipolar switching in metal oxides. The temperature stability of the high resistance state is demonstrated and explained based on the local depletion of chemical species from the electrode region.

A systematic design of practicable media suitable for rewritable, ultrahigh density (>;1Tbit/sq.in.), high data rate (>;1Mbit/s/tip) scanning-probe phase-change memories is presented. The basic design requirements were met by a Si/TiN/Ge2Sb2Te5 (GST)/diamond-like carbon structure, with properly tailored electrical and thermal conductivities. Various alternatives for providing rewritability were investigated. In the first case, amorphous marks were written into a crystalline starting phase and subsequently erased by recrystallization, as in other already established phase-change memory technologies. Results imply that this approach is also appropriate for probe-based memories. However, experimentally, the successful writing of amorphous bits using scanning electrical probes has not been widely reported. In light of this, a second approach has been studied, that of writing crystalline bits in an amorphous starting matrix, with subsequent erasure by reamorphization. With conventional phase-change materials, such as continuous films of GST, this approach invariably leads to the formation of a crystalline “halo” surrounding the erased (reamorphized) region, with severe adverse consequences on the achievable density. Suppression of the “halo” was achieved using patterned media or slow-growth phase-change media, with the latter seemingly more viable.

The microstructure of Ni-5at%W (Ni-W) and Ni-11at%V (Ni-V) biaxially textured substrates has been investigated using X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). The correlation between the substrate microstructure and superconducting transport properties of YBa/sub 2/Cu/sub 3/O/sub 7-y/ (YBCO) film grown on it has been studied on the YBCO/CeO/sub 2//Ni-W and YBCO/CeO/sub 2//NiO/Ni-V architectures. Our study has ascertained that the in-plane texture of the substrates is one of the most important factors, limiting the critical current density. The Ni-V substrate has a lower percolation area due to the larger number of twinned grains and a broader in-plane angular distribution and, as a consequence, the YBa/sub 2/Cu/sub 3/O/sub 7-y/ (YBCO) film grown on it has a critical current density of 0.6 /spl times/ 10/sup 6/ A/cm/sup 2/, depressed by factor 2 with respect to YBCO grown on the Ni-W substrate. For the Ni-V substrate, another limiting factor is its low oxidation resistance. In contrast to Ni-V, the Ni-W substrate has a larger percolation area, mainly due to the absence of twinned grains, and a high oxidation resistance.

The thermal conductivity of Ge2Sb2Te5 (GST) layers, as well as the thermal boundary resistance at the interface between the GST and amorphous SiO2, were measured using a PhotoThermal Radiometry experiment. The two phase-changes of the Ge2Sb2Te5 were retrieved, starting from the amorphous and sweeping to the fcc crystalline state at 130 °C and then to the hcp crystalline state at 310 °C. The thermal conductivity resulted to be constant in the amorphous phase, whereas it evolved between the two crystalline states. The thermal boundary resistance at the GST-SiO2 interface was estimated to be higher for the hcp phase than for the amorphous and fcc ones.

International audience; The thermal conductivity of Ge2Sb2Te5 (GST) layers, as well as the thermal boundary resistance at the interface between the GST and amorphous SiO2, was measured using a photothermal radiometry experiment. The two phase changes in the Ge2Sb2Te5 were retrieved, starting from the amorphous and sweeping to the face centered cubic (fcc) crystalline state at 130 °C and then to the hexagonal crystalline phase (hcp) at 310 °C. The thermal conductivity resulted to be constant in the amorphous phase, whereas it evolved between the two crystalline states. The thermal boundary resistance at the GST-SiO2 interface was estimated to be higher for the hcp phase than for the amorphous and fcc ones.

International audience; In this article we present the first results of a quite simple phenomenological model we have developed to simulate the cationic substitutions in Nd1+xBa2-xCu3O7+x/2-d (Nd123). Although elementary concepts from Statistical Mechanics had been used in it, significant results have been obtained, as the reconstruction of the substitution region limits and their dependence on temperature. Particularly interesting is the prediction of strong temperature dependence for the minimum of the substitution parameter x.