Electronic and Atomic Structure of Ge2Sb2Te5 phase change memory material
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0918-H01-02
Electronic and Atomic Structure of Ge2Sb2Te5 phase change memory material John Robertson, Ka Xiong, and Paul Peacock Engineering, Cambridge University, Cambridge, CB2 1PZ, United Kingdom Abstract Electronic structure calculations are presented for various model structures of the crystalline and amorphous phases of Ge2Sb2Te5 (GST). The structures are all found to possess a band gap of order 0.5 eV, indicating closed shell behaviour. It is pointed out that structural vacancies in A7-like GST are not electronically active. In addition, A7-like structures do not support valence alternation pair (VAP) defects, which are one model of the conduction processes in the glassy phase in non-volatile memories. Introduction Chalcogenide-based phase change memory (PCM) materials are used in re-writable optical memories [1]. They use the difference in optical band gap and reflectivity between the crystalline (c-) and amorphous (glassy) phases. They will also be used in electronic non-volatile memory devices because of their possible superior scaling possibilities compared to Flash [2]. This has resulted in an increased number of studies of their structural and thermal properties [3-6]. However, the local atomic structure is still the matter of debate [7,8], particularly for the glassy phase, as are their electronic properties. The prototypical phase change material is GST, Ge2Sb2Te5. The crystalline phase is a distorted cubic rocksalt structure, according to X-ray diffraction, and this transforms into a related hexagonal crystal above ~200C [6]. There have been two studies of the local structure of the crystal and glassy phases by extended x-ray fine structure (EXAFS)[7,8]. The crystal phase is consistent with the structure found by X-ray diffraction. However, there is debate about the glassy structure, as either one in which the Te atoms form a lattice like in the cubic phase as in the model of Kolobov et al [7], or whether they form a structure as in amorphous GeSe, with lower coordinations and Ge-Ge bonds, as suggested by Baker et al [8]. This brings into focus the point that sulfides and selenides are good glass-forming chalcogenides, whereas tellurides are different. They have a lower glassforming ability. This is what makes the phase change between crystal and amorphous phases rapid and useful for storage. A second question concerns their electronic structure. The rock-salt lattice of GST is not fully occupied, it has vacancies on the Te sublattice. Are these electronically active? Finally, based on the historical view of chalcogenides, it was proposed that the electronic defects of the glassy phase are valence alternation pairs (VAPs)[9] as in a-As2Se3. However, this may not be so if Te is sufficiently different to Se. Calculations The electronic structure of various structural models of the crystalline and amorphous phases have been calculated by the ab-initio total energy, plane wave pseudopotential
method, using the CASTEP code [10]. We use ultra-soft pseudopotentials [11], a plane wave cutoff energy of 200
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