Kirill V. Mitrofanov

Selective detection of tetrahedral units in amorphous GeTe-based phase change alloys using Ge L3-edge x-ray absorption near-edge structure spectroscopy

Using Ge L3-edge x-ray absorption near-edge structure (XANES) studies, we demonstrate a noticeable difference in local structure between amorphous and thermally crystallized GeTe-based phase change alloys. The pronounced change appears as a step-like feature at the absorption edge corresponding to a 2p → 5s (4d) electron transition. Comparison with ab initio XANES simulations suggest that the step-like feature is due to the presence of tetrahedrally coordinated Ge atoms in the as-deposited samples. The obtained results demonstrate that Ge L3-edge XANES can be used as a structural probe for the existence of tetrahedral Ge sites in GeTe-based phase change alloys.

Femtosecond structural transformation of phase-change materials far from equilibrium monitored by coherent phonons

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.

Ge L3-edge x-ray absorption near-edge structure study of structural changes accompanying conductivity drift in the amorphous phase of Ge2Sb2Te5

A gradual uncontrollable increase in the resistivity of the amorphous phase of phase-change alloys, such as Ge2Sb2Te5, known as drift, is a serious technological issue for application of phase-change memory. While it has been proposed that drift is related to structural relaxation, no direct structural results have been reported so far. Here, we report the results of Ge L3-edge x-ray absorption measurements that suggest that the drift in electrical conductivity is associated with the gradual conversion of tetrahedrally coordinated Ge sites into pyramidal sites, while the system still remains in the amorphous phase. Based on electronic configuration arguments, we propose that during this process, which is governed by the existence of lone-pair electrons, the concentration of free carriers in the system decreases resulting in an increase in resistance despite the structural relaxation towards the crystalline phase.

Picosecond strain dynamics in Ge2Sb2Te5 monitored by time-resolved x-ray diffraction

Coherent phonons (CP) generated by laser pulses on the femtosecond scale have been proposed as a means to achieve ultrafast, non-thermal switching in phase-change materials such as Ge

Sub-nanometre resolution of atomic motion during electronic excitation in phase-change materials

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Study of band inversion in the PbxSn1−xTe class of topological crystalline insulators using x-ray absorption spectroscopy

Sb

Doping of ZnO nanowires using phosphorus diffusion from a spin-on doped glass source

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Anomalous phase change process in [(GeTe)2/(Sb2Te3)]20 superlattice observed by coherent phonon spectroscopy

Te

Ultrafast Lattice Dynamics of Phase-Change Materials Monitored by a Pump-Pump-Probe Technique

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Local structure of the SnTe topological crystalline insulator: Rhombohedral distortions emerging from the rocksalt phase

(GST). Here we use ultrafast optical pump pulses to induce coherent acoustic phonons and stroboscopically measure the corresponding lattice distortions in GST using 100 ps x-ray pulses from the ESRF storage ring. A linear-chain model provides a good description of the observed changes in the diffraction signal, however, the magnitudes of the measured shifts are too large to be explained by thermal effects alone implying the presence of transient non-equilibrium electron heating in addition to temperature driven expansion. The information on the movement of atoms during the excitation process can lead to greater insight into the possibilities of using CP-induced phase-transitions in GST.