Origin of resistivity contrast in interfacial phase-change memory: The crucial role of Ge/Sb intermixing

Abstract

Phase-change memories based on reversible amorphous-crystal transformations in pseudobinary GeTe-Sb2Te3 alloys are one of the most promising non-volatile memory technologies. The recently proposed superlattice-based memory, or interfacial phase change memory (iPCM), is characterized by significantly faster switching, lower energy consumption and better endurance. The switching mechanism in iPCM, where both the SET and RESET states are crystalline, is still contentious. Here, using the ab initio density functional theory simulations, a conceptually new switching mechanism for iPCM is derived, which is based on the change in potential landscape of the band-gap, associated with local deviations from the pseudo-binary stoichiometry across the van der Waals gaps, and the associated shift of the Fermi level. The crucial role in this process belongs to Ge/Sb intermixing on the cation planes of iPCM. These findings offer a comprehensive understanding of the switching mechanisms in iPCM and are an essential step forward to the insightful development of phase-change memory technology. Phase change random access memory (PCRAM) is one of the most promising candidates for the next generation non-volatile memory with phase change materials forming a key component of the recently commercialized 3D XPoint memory technology where sophisticated three-dimensional integration of chalcogenide-based memory and selector components enables much faster writing speeds than Flash and denser capacity than dynamic random access memory (DRAM). 1,2 The development of phase-change materials dates back over three decades, with the most studied materials being Ge-Sb-Te (GST) ternary alloys on the GeTe-Sb2Te3 pseudobinary tie-line, and which found great success in optical disc devices. 3 On the other hand, as optical disc and non-volatile memories clearly have very different requirements, the required material properties also differ. Using the same material, GST alloys, for PCRAM memories does not in itself solve the various difficulties in developing non-volatile memories. A major breakthrough in PCRAM materials was the development of interfacial phase change memory (iPCM) inspired by the Ge-atom flip-flop model. 4-8 In iPCM, the two binary end compounds of the GeTe-Sb2Te3 pseudobinary tie line, GeTe and Sb2Te3, are alternately stacked to form an atomically aligned superlattice structure with van der Waals (vdW) gaps separating covalently bonded blocks. Devices based upon iPCM showed excellent performance such as ultra-low power consumption, faster switching speeds, and longer endurance than conventional alloy-type PCRAM. 4,9 After the development of chalcogenide superlattices and their devices based on GeTe/Sb2Te3 superlattices fabricated from sputter-deposited films, 4,10-13 several specific switching models were proposed based on the idea of bi-layer switching within GeTe blocks. 14-20 In particular switching between inverted Petrov and Ferro structures (Figure 1) was proposed based on the arguments that the relative stability of these two pha...

Phase change thin films for non-volatile memory applications

“…14(a) and (b)) and simulations are also of high importance to understand the switching mechanisms in layered GeTe-Sb 2 Te 3 based heterostructures. 74,[138][139][140][141]…”

High‐Speed Bipolar Switching of Sputtered Ge–Te/Sb–Te Superlattice iPCM with Enhanced Cyclability

“…An important note regarding the structural basis of the bipolar switching process described above is that it requires the existence of two equivalent low‐resistance states. Namely, while a direct field‐driven transition occurs from the high‐resistance Te–Ge–Ge–Te stacking phase to the low‐resistance Ge–Te–Ge–Te stacking phase, a similar transition (from Te–Ge–Ge–Te to Te–Ge–Te–Ge stacking) should occur upon application of an electric field with the opposite polarity (the detailed switching mechanism will be discussed elsewhere) . But such a transition was not observed experimentally.…”

“…. Thus, a Ge‐rich bilayer may be more easily switchable . Also, considering an alternative concept, it is possible that it is the Sb‐Te bilayer that participates in the switching process has an excess of valence electrons comparing to the Ge‐Te layer.…”

A first-principles study of the switching mechanism in GeTe/InSbTe superlattices

“…On the basis of DFT calculations other mechanisms were then proposed involving the motion of the stacking fault 21 or the inversion of SbTe bilayers leading to a reconguration of the van der Waals gaps which breaks the local chemical stoichiometry and results in a reversible metal-insulator transition. 22,23 Experimental evidence for the inversion of SbTe bilayers has been reported in ref. 24.…”