Abstract
Interfacial Phase Change Memories (iPCMs) based on (GeTe)2/Sb2Te3 superlattices have been proposed as an alternative candidate to conventional PCMs for the realization of memory devices with superior switching properties. The switching mechanism was proposed to involve a crystalline-to-crystalline structural transition associated with a rearrangement of the stacking sequence of the GeTe bilayers. Density functional theory (DFT) calculations showed that such rearrangement could be achieved by means of a two-step process with an activation barrier for the flipping of Ge and Te atoms which is sensitive to the biaxial strain acting on GeTe bilayers. Within this picture, strain-engineering of GeTe bilayers in the GeTe–chalcogenide superlattice can be exploited to further improve the iPCM switching performance. In this work, we study GeTe–InSbTe superlattices with different compositions by means of DFT, aiming at exploiting the large mismatch (3.8%) in the in-plane lattice parameter between GeTe and In3SbTe2 to reduce the activation barrier for the switching with respect to the (GeTe)2–Sb2Te3 superlattice.
Highlights
Chalcogenide alloys are used as active materials in non-volatile phase change memories (PCMs) which rely on the reversible and fast transition between the amorphous and crystalline phases of the alloy induced by Joule heating.[1,2,3,4,5,6]
In these devices, referred to as interfacial phase change memories,[7] it was suggested that the transformation involves small displacements of a subset of atoms without melting and amorphization, the material remaining in a crystalline phase in both SET and RESET states.[7]
The substitution of the Sb2Te3 block with the In3SbTe2 one might prevent the incorporation of GeTe.8 The (GeTe) bilayers into the Sb-rich blocks as occurs in (GeTe)n/(Sb2Te3)m superlattices grown by molecular beam epitaxy (MBE) and by other means.[14,17,19,20]
Summary
Chalcogenide alloys are used as active materials in non-volatile phase change memories (PCMs) which rely on the reversible and fast transition between the amorphous and crystalline phases of the alloy induced by Joule heating.[1,2,3,4,5,6] The two states of the memory can be discriminated thanks to a large difference in electrical resistivity between the two phases. It has been shown that (GeTe)n(Sb2Te3)m pseudobinary compounds arranged in superlattice (SL) geometries require a switching power in SET/RESET operations much lower than that needed in conventional GST alloys.[7] In these devices, referred to as interfacial phase change memories (iPCMs),[7] it was suggested that the transformation involves small displacements of a subset of atoms without melting and amorphization, the material remaining in a crystalline phase in both SET (low resistivity) and RESET (high resistivity) states.[7] The SET state was proposed to correspond to a ferroelectric. In spite of all the different alternative models proposed so far, we might still conceive that the switching mechanism based on crystal-to-crystal transition between the Ferro-GeTe and Inverted-Petrov phases could occur in other SLs in which the replacement of the Sb2Te3 sub-blocks by other materials could prevent the incorporation of the GeTe blocks In this respect, we studied the geometry, the switching mechanism and the electronic structure of GeTe/In3SbTe2 SLs by means of DFT calculations. We found that the large lattice mismatch between the In3SbTe2 and GeTe blocks can be exploited as a means to further reduce the activation barrier for SET/RESET transitions as proposed in ref. 12 for GeTe/Sb2Te3 SLs
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