Reconfiguration of van der Waals Gaps as the Key to Switching in GeTe/Sb 2 Te 3 Superlattices
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.444
Reconfiguration of van der Waals Gaps as the Key to Switching in GeTe/Sb2Te3 Superlattices A.V. Kolobov, P. Fons, Y. Saito, J. Tominaga Nanoelectronics Research Institute, National Institute of Advanced Industrial Science & Technology, Tsukuba Central 5, 1-1-1 Higashi, Ibaraki 305-8565, JAPAN
Abstract
GeTe/Sb2Te3 superlattices, also known as interfacial phase-change memory (iPCM), exhibit significantly faster switching and are characterized by much lower power consumption and longer data retention compared to devices based on alloyed materials. In early work, the superior performance of iPCM was linked to a crystal-crystal transition between the SET and RESET states. As the primary mechanism, a change in the stacking order of Ge and Te planes within a GeTe block was suggested. Subsequent STEM studies on epitaxial GeTe/Sb2Te3 superlattices demonstrated that the GeTe blocks were not located between Sb2Te3 quintuple layers but, were incorporated inside the latter, providing a serious challenge to the early explanation. In this work, we demonstrate that changes associated with the reconstruction of the SbTe terminating layers nearest to van der Waals gap leads to a pronounced change in the density of states and can serve as an alternative explanation for a large property contrast between the SET and RESET states in GeTe/Sb2Te3 superlattices.
INTRODUCTION GeTe-Sb2Te3 quasibinary alloys are characterized by an unusually large property contrast between the crystalline and amorphous phases, which is the basis of their use in optical and electronic memory devices. The original idea belongs to S.R. Ovshinsky [1] and has been successfully implemented in digital versatile discs and bluray discs [2] as well as non-volatile phase-change random-access memory (PCRAM) [3]. A significant step forward in device performance was the proposal to spatially separate GeTe and Sb2Te3 in the form of a superlattice (SL) [4]. Such superlattices, also called interfacial phase-change memory (iPCM), are characterized by significantly lower power consumption, fast switching rates and larger cyclability than otherwise identical devices fabricated using alloys of the same average composition.
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Figure 1: The umbrella-flip of Ge atoms in an alloy [panel (a): left vs. right] and in a layered structure [panel (b): top vs. bottom]. Te atoms are shown in tan and Ge atoms are red in panel (a) and green in panel (b). Panel (a) has been reproduced from ref. [5] with permission from the publisher.
Of special interest is the fact that iPCMs do not melt during the transformation to the high-resistance state and the transition between the SET and RESET states is of a crystal-crystal type. The atomistic mechanism of the aforementioned crysta
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