Ab initio determination of the elastic properties of cubic Ge 1 b 2 Te 4
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Ab initio determination of the elastic properties of cubic Ge1Sb2Te4 K. Kohary, A. S. H. Marmier, and C. D. Wright College of Engineering, Mathematics, and Physical Sciences, University of Exeter, Harrison Building, North Park Road, Exeter, EX4 4QF, United Kingdom ABSTRACT The elastic properties of chalcogenide materials used for phase change applications in rewritable optical media (such as CD-RW, DVD-RW, etc) are still poorly characterized and the previously published experimental and theoretical values show large discrepancies. In this manuscript, we review these results and carry out a careful analysis of the elastic properties of a model system, crystalline Ge1Sb 2Te4, using density functional theory and elastic anisotropy considerations. We show that Ge1Sb 2Te4 exhibits significant anisotropy in its elastic properties. INTRODUCTION Chalcogenide glasses based on phase-change materials play an important role in rewritable optical disks, such as CD-RW, DVD-RW, and are considered for the emerging memory application PCRAM (phase-change read access memory) [1]. The dominant material for practical applications is based on the ternary alloy Ge-Sb-Te (GST). Two of the important compositions are Ge1Sb2Te4 (GST124) and Ge2Sb2Te5 (GST225), which both have a quasi-cubic symmetry [2] due to the rocksalt atomic structure, where one fcc sublattice is occupied by Te, whereas the other sublattice is occupied by Ge, Sb atoms and vacancies. While the optical and electrical properties have been extensively investigated, the elastic properties of GST alloys are still poorly characterized. The published information on the elastic properties of GST is limited and there is a considerable disagreement concerning the values for bulk and Young’s modulus in the literature. In addition, the investigations so far assumed crystalline isotropy and therefore a full set of elastic constants has not been derived yet. In this paper we carry out a theoretical analysis of the elastic properties for a model system of GST124 using density functional theory (DFT) and we calculate the full set of elastic constants. ELASTIC PROPERTIES With the exception of [3], previous works on the elastic properties of GST focused on individual properties and did not attempt to fully characterize the elastic tensors (stiffness and compliance). Macroscopic methods such as curvature measurement (CM) [4-6] and nanoindentation (NI) [6,7] have been used to access Young’s and biaxial moduli of GST124 and GST225, in addition to X-rays diffraction technique (XRD) to measure the bulk modulus [8,9]. Brillouin light scattering (BLS) has been the choice to elucidate the elastic constants for GST124 by Blachowitz et al. [10], but with the important limitation of assuming isotropic elastic properties for GST. Atomistic simulations using the density functional theory (DFT) have also been used to calculate the bulk modulus of GST124 [3] and GST225 [3,11].
Table I Summary of elastic properties of GST124 published in literature.
GST124
Method
K (GPa)
E (GPa)
Other (GPa)
Referen
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