Abstract
We study the switching process in chalcogenide superlattice (CSL) phase-change memory materials by describing the motion of an atomic layer between the low and high resistance states. Two models have been proposed by different groups based on high-resolution electron microscope images. Model 1 proposes a transition from Ferro to Inverted Petrov state. Model 2 proposes a switch between Petrov and Inverted Petrov states. For each case, we note that the main transition is actually a vertical displacement of a Ge layer through a Te layer, followed by a lateral motion of GeTe sublayer to the final, low energy structure. Through calculating energy barriers, the rate-determining step is the displacive transition.
Highlights
The GeSbTe chalcogenide superlattice’ (CSL) have the basic formulae (GeTe)n(Sb2Te3)m
The van der Waals gap is between the two Sb2Te3 blocks
In the Ferro structure, the (GeTe)[2] block has a Ge-Te-Ge-Te sequence and the van der Waals (vdW) gap is between the (GeTe)[2] and Sb2Te3 block
Summary
For a one at a time case with the situation where two GeTe bilayers move together. Interestingly, the energy barrier is similar. We calculated the situation where the lateral motion occurred by the migration of Ge vacancies This is considered because Takaura[11] noted that a Ge deficit in the CSLs favored the observation of switching. The switching transition in CSL materials occurs as a 2-step process, a vertical flip of Ge sublayers through Te sublayers driven by the applied field, followed by a lower energy lateral motion of the GeTe sublayers back to the more stable configurations. The energy barrier for the vertical atomic flip of Ge layers is a high energy process with a barrier of 2.56 to 3.1 eV, and is followed by a lateral motion of GeTe sublayers with barriers of 0.44 eV, whether by vacancy diffusion, or by a more concerted bulk motion
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