Abstract
Multicomponent chalcogenides, such as quasi-binary GeTe–Sb2Te3 alloys, are widely used in optical data storage media in the form of rewritable optical discs. Ge2Sb2Te5 (GST) in particular has proven to be one of the best-performing materials, whose reliability allows more than 106 write–erase cycles. Despite these industrial applications, the fundamental kinetics of rapid phase change in GST remain controversial, and active debate continues over the ultimate speed limit. Here we explore ultrafast structural transformation in a photoexcited GST superlattice, where GeTe and Sb2Te3 are spatially separated, using coherent phonon spectroscopy with pump–pump–probe sequences. By analysing the coherent phonon spectra in different time regions, complex structural dynamics upon excitation are observed in the GST superlattice (but not in GST alloys), which can be described as the mixing of Ge sites from two different coordination environments. Our results suggest the possible applicability of GST superlattices for ultrafast switching devices.
Highlights
Multicomponent chalcogenides, such as quasi-binary GeTe–Sb2Te3 alloys, are widely used in optical data storage media in the form of rewritable optical discs
Interfacial phase-change memory consists of a superlattice (SL) structure formed from alternating layers of GeTe and Sb2Te3. iPCM was designed to utilize a solid–solid phase transformation between the covalently bonded (RESET) and resonantly bonded (SET) phases, induced predominantly by the displacement of Ge atoms at the interface, to achieve both faster and lower power threshold switching than in conventional GST alloys7. iPCM structures with certain thicknesses and atomic order in the individual blocks were argued to be topological insulators, and recently, it was reported that iPCM could be switched between the Dirac-semimetal and gapped phases[15], an effect that has the potential to lead to novel spin memory devices and as a platform to study the topological properties of SLs
We report on systematic studies of the structural dynamics in a prototypical iPCM structure, [Ge2Te2/Sb2Te3]20, using coherent phonon spectroscopy (CPS) under strong photoexcitation employing both single- and doublepump-pulse excitation
Summary
Multicomponent chalcogenides, such as quasi-binary GeTe–Sb2Te3 alloys, are widely used in optical data storage media in the form of rewritable optical discs. As a further step, using weak femtosecond laser pulses with pump fluences below 100 mJ cm À 2, Makino et al.[22] demonstrated that for a prototypical iPCM structure, [Ge2Te2/Sb2Te3]20, a phase change from the RESET into the SET phase could be induced by selectively exciting a phonon mode that involves Ge atoms using a double-pulse excitation. Exploration of the non-thermal pre-phase transformations in GST materials will enable a deeper understanding of the local lattice structure far from equilibrium, and will potentially make it possible to increase the speed limit of switching in the phase-change process beyond the current limit of nanoseconds down to sub-picosecond timescales
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