Morphology and Electric Conductance Change Induced by Voltage Pulse Excitation in (GeTe)2/Sb2Te3 Superlattices.

Leonid Bolotov,Yuta Saito,Junji Tominaga,Tetsuya Tada

doi:10.1038/srep33223

Abstract

Chalcogenide superlattice (SL) phase-change memory materials are leading candidates for non-volatile, energy-efficient electric memory where the electric conductance switching is caused by the atom repositioning in the constituent layers. Here, we study the time evolution of the electric conductance in [(GeTe)2/(Sb2Te3)1]4 SLs upon the application of an external pulsed electric field by analysing the structural and electrical responses of the SL films with scanning probe microscopy (SPM) and scanning probe lithography (SPL). At a low pulse voltage (1.6–2.3 V), a conductance switching delay of a few seconds was observed in some SL areas, where the switch to the high conductance state (HCS) is accompanied with an SL expansion under the strong electric field of the SPM probe. At a high pulse voltage (2.5–3.0 V), the HCS current was unstable and decayed in a few seconds; this is ascribed to the degradation of the HCS crystal phase under excessive heating. The reversible conductance change under a pulse voltage of opposite polarity emphasised the role of the electric field in the phase-transition mechanism.

Highlights

Chalcogenide superlattice (SL) phase-change memory materials are leading candidates for non-volatile, energy-efficient electric memory where the electric conductance switching is caused by the atom repositioning in the constituent layers
The switching in SL films requires an order of magnitude of less energy than the traditional process, and it occurs in a much shorter time[7], indicating that another mechanism of the crystal-to-crystal phase transition involving the repositioning of atoms in the crystal layers is responsible[8]
There was an unclear correlation among the high conductance state (HCS) current values and Voltage pulses with an amplitude (VIN) and Δt, indicating that other factors play a role in the variations observed in the switching voltage

Summary

Introduction

Chalcogenide superlattice (SL) phase-change memory materials are leading candidates for non-volatile, energy-efficient electric memory where the electric conductance switching is caused by the atom repositioning in the constituent layers. We study the time evolution of the electric conductance in [(GeTe)2/(Sb2Te3)1]4 SLs upon the application of an external pulsed electric field by analysing the structural and electrical responses of the SL films with scanning probe microscopy (SPM) and scanning probe lithography (SPL). This structure is predicted to be the most stable configuration above 125K15 Another model assumes that the switching involves a Te-Ge-Te bond flip in two adjacent atomic layers in opposite directions, called a Petrov model[9]. A detailed study of the interface formation in [(GeTe)(1 nm)/(Sb2Te3)(3 nm)]15 SLs using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) has shown SL grains with mixed Ge/Sb atomic layers prepared by molecular beam epitaxial growth on the Sb-terminated. Correspondence and requests for materials should be addressed to L.B. (email: bolotov.leonid@ aist.go.jp) www.nature.com/scientificreports/

Methods

Results

Conclusion