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Author: Bragaglia, V. × Type: Text × Publisher: London : Nature Publishing Group × WGL Institute: PDI ×

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    Role of interfaces on the stability and electrical properties of Ge2Sb2Te5 crystalline structures
    (London : Nature Publishing Group, 2017) Mio, A.M.; Privitera, S.M.S.; Bragaglia, V.; Arciprete, F.; Cecchi, S.; Litrico, G.; Persch, C.; Calarco, R.; Rimini, E.
    GeSbTe-based materials exhibit multiple crystalline phases, from disordered rocksalt, to rocksalt with ordered vacancy layers, and to the stable trigonal phase. In this paper we investigate the role of the interfaces on the structural and electrical properties of Ge2Sb2Te5. We find that the site of nucleation of the metastable rocksalt phase is crucial in determining the evolution towards vacancy ordering and the stable phase. By properly choosing the substrate and the capping layers, nucleation sites engineering can be obtained, thus promoting or preventing the vacancy ordering in the rocksalt structure or the conversion into the trigonal phase. The vacancy ordering occurs at lower annealing temperatures (170 °C) for films deposited in the amorphous phase on silicon (111), compared to the case of SiO2 substrate (200 °C), or in presence of a capping layer (330 °C). The mechanisms governing the nucleation have been explained in terms of interfacial energies. Resistance variations of about one order of magnitude have been measured upon transition from the disordered to the ordered rocksalt structure and then to the trigonal phase. The possibility to control the formation of the crystalline phases characterized by marked resistivity contrast is of fundamental relevance for the development of multilevel phase change data storage.
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    Mapping the band structure of GeSbTe phase change alloys around the Fermi level
    (London : Nature Publishing Group, 2018) Kellner, J.; Bihlmayer, G.; Liebmann, M.; Otto, S.; Pauly, C.; Boschker, J.E.; Bragaglia, V.; Cecchi, S.; Wang, R.N.; Deringer, V.L.; Küppers, P.; Bhaskar, P.; Golias, E.; Sánchez-Barriga, J.; Dronskowski, R.; Fauster, T.; Rader, O.; Calarco, R.; Morgenstern, M.
    Phase change alloys are used for non-volatile random-access memories exploiting the conductivity contrast between amorphous and metastable, crystalline phase. However, this contrast has never been directly related to the electronic band structure. Here we employ photoelectron spectroscopy to map the relevant bands for metastable, epitaxial GeSbTe films. The constant energy surfaces of the valence band close to the Fermi level are hexagonal tubes with little dispersion perpendicular to the (111) surface. The electron density responsible for transport belongs to the tails of this bulk valence band, which is broadened by disorder, i.e., the Fermi level is 100 meV above the valence band maximum. This result is consistent with transport data of such films in terms of charge carrier density and scattering time. In addition, we find a state in the bulk band gap with linear dispersion, which might be of topological origin.
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    Far-infrared and Raman spectroscopy investigation of phonon modes in amorphous and crystalline epitaxial GeTe-Sb2Te3 alloys
    (London : Nature Publishing Group, 2016) Bragaglia, V.; Holldack, K.; Boschker, J.E.; Arciprete, F.; Zallo, E.; Flissikowski, T.; Calarco, R.
    A combination of far-infrared and Raman spectroscopy is employed to investigate vibrational modes and the carrier behavior in amorphous and crystalline ordered GeTe-Sb2Te3 alloys (GST) epitaxially grown on Si(111). The infrared active GST mode is not observed in the Raman spectra and vice versa, indication of the fact that inversion symmetry is preserved in the metastable cubic phase in accordance with the Fm3 space group. For the trigonal phase, instead, a partial symmetry break due to Ge/Sb mixed anion layers is observed. By studying the crystallization process upon annealing with both the techniques, we identify temperature regions corresponding to the occurrence of different phases as well as the transition from one phase to the next. Activation energies of 0.43 eV and 0.08 eV for the electron conduction are obtained for both cubic and trigonal phases, respectively. In addition a metal-insulator transition is clearly identified to occur at the onset of the transition between the disordered and the ordered cubic phase.