Global and local structures of the Ge-Sb-Te ternary alloy system for a phase-change memory device

A detailed theoretical investigation on the global and local structures of the Ge-Sb-Te(GST) ternary alloy system for the phase-change memory is presented. We examine the cohesive energy of the ${(\mathrm{Ge}\mathrm{Te})}_{n}{({\mathrm{Sb}}_{2}{\mathrm{Te}}_{3})}_{m}$ homologous series as well as the dependence of the energy on the atomic distribution. We show that the cohesive energy decreases with increasing vacancy concentration and the vacancies repel each other to minimize the number of dangling bonds. In ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$, Sb and Ge atoms favor two-dimensional (layered) and three-dimensional (agglomerated) arrangements, respectively. In ${\mathrm{Ge}}_{1}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$, on the other hand, Ge atoms tend to form a two-dimensional layered structure. Possible structural building blocks of the GST system are proposed based on the density-functional theory total energy calculations.