In-situ transmission electron microscopy studies of the crystallization of N-doped Ge-rich GeSbTe materials
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Research Letter
In-situ transmission electron microscopy studies of the crystallization of N-doped Ge-rich GeSbTe materials Marta Agati, and François Renaud, CEMES-CNRS, 29 Rue Jeanne Marvig, 31055 Toulouse, France Daniel Benoit, STMicroelectronic, 850 Rue Jean Monnet, 38920 Crolles, France Alain Claverie, CEMES-CNRS, 29 Rue Jeanne Marvig, 31055 Toulouse, France Address all correspondence to Alain Claverie at [email protected] (Received 1 June 2018; accepted 6 August 2018)
Abstract We have studied by electron microscopy and x-ray diffraction techniques the amorphous-to-crystalline phase transition which occurs during annealing of a highly Ge-rich and N-doped amorphous GeSbTe material. The crystallization onset occurs at 380 °C with the diffusion and segregation of Ge followed by the formation of Ge nanocrystals. The GeSbTe face-centered cubic (FCC) crystalline phase only appears at 400 °C. Phase separation occurs because the Ge concentration is well above what can be accommodated by the Ge2Sb2Te5 lattice. The possible formation of a two-phase material should be considered in order to simulate device characteristics and optimize material composition for electronic memory applications.
Introduction Phase change materials (PCMs) have been extensively studied, particularly in the perspective of data storage applications.[1,2] Their fundamental property relies in the reversible transition between two different microstructural states, i.e., the amorphous-to-crystalline phases, which show two distinct sets of physical properties that can be macroscopically measured. Indeed, variations of the optical reflectivity upon amorphous to crystalline phase transition may attain up to 30%,[2] while the electrical resistivity can change by more than three orders of magnitude,[3] producing a metal–insulator transition.[4] Beyond the pronounced contrast of the physical properties between the two phases, the fast switching between these states renders these materials suitable for data storage applications.[1] Currently, PCMs are exploited as optical data storage media in rewritable compact discs (CDs), digital versatile discs (DVDs), and Blu-ray discs. The information bit is encoded in the contrast of the refractive index relative to the two phases.[2] Later on, PCMs have also been recognized as effective building blocks for future electronic memory devices [phase change-random access memories (PC-RAMs)], where the bit of information is encoded in two distinct resistive states corresponding to the (high-resistive) amorphous state and the (lowresistive) crystalline state. PC-RAMs have demonstrated sub-10 ns switching speed,[5,6] extremely good cyclability,[7] data retention ability[8] as well as possible integration in largearrays[9] and, more importantly, scalability to sub-10 nm dimensions,[7,10] thus representing potential candidates in the search of future alternatives to flash memory technology.[11] The fast crystallization process (t ∼ 10–100 ns) occurring in
pseudo-binary chalcogenide-based alloys, in particular Ge2Sb2Te5
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