The role of vacancies in the pressure amorphisation phenomenon observed in Ge-Sb-Te phase change alloys
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The role of vacancies in the pressure amorphisation phenomenon observed in Ge-Sb-Te phase change alloys M. Krbal1&2 , A. V. Kolobov1 , P. Fons1 , J. Tominaga1 , J. Haines2 , A. Pradel2 , M. Ribes2 , C. Levelut3 , R. Le Parc3 and M. Hanfland4 1 Nanodevice Innovation Research Centre, Advanced Industrial Science and Technology 1-1-1 Higashi, Tsukuba, Japan 2 Institute Charles Gerhardt, UMR 5253 CNRS-UM2-ENSCM-UM1, PMDP/PMOF, Universit Montpellier II, Place Eugne Bataillon, Montpellier Cedex 5, France 3 Laboratoire des Colloides, Verres et Nanomatriaux, Universit Montpellier II, Place Eugne Bataillon, Montpellier Cedex 5, France 4 European Synchrotron Radiation Facility, Grenoble, France ABSTRACT We demonstrate, both experimentally and by computer simulation, that while the metastable face-centered cubic (fcc) phase of Ge-Sb-Te becomes amorphous under hydrostatic compression at about 15 GPa, the stable trigonal phase remains crystalline. We present evidences that the pressure-induced amorphisation phenomenon strongly depends on the concentration of vacancies included in the Ge/Sb sublattice, but is thermally insensitive. Upon higher compression, a body-centered cubic phase is obtained in both cases at around 30 GPa. Upon decompression, the amorphous phase is retained when starting with the fcc phase while the initial structure is recovered when starting with the trigonal phase. We argue that the presence of vacancies and the associated subsequent large atomic displacements lead to nanoscale phase separation and the loss of the initial structure memory in the fcc staring phase of GeSb-Te. We futher compare the amorphous phase obtained via the pressure route with the melt quenched amorphous phase. INTRODUCTION The Te-based multicomponent alloys along the GeTe-Sb2 Te3 quasibinary tie-line are widely studied for both electronic and optical memories applications [1]. The high speed, high stability and excellent scalability of phase-change electronic memory has led to the suggestion that it may replace the currently widely used flash memory [2]. In both cases, the encoding of the information is based on the reversible phase transformations of chalcogenide alloys between amorphous and crystalline states. The importance of phase-change materials for both present and future memory applications clearly requires better knowledge of their fundamental properties and the physics behind the utilized phase transition. The density of the amorphous structure is about 5 % smaller than that of the crystalline phase. In optical discs, the recoded bits are amorphous marks that want to expand their volume when they are formed. At the same time, they are confined within the crystalline background and cladding layers. These two opposing aspects can lead to the generation of significant transient stress in recorded bits on order of a few GPa. The incurred pressure can
affect reversibly or irreversibly the structure of the crystalline surroundings of the amorphous bit and as well as the stability of recorded information, as the str
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