Ge–Sb–Te (GST) phase change materials exhibit a metal-to-insulator transition and therefore are expected to be useful for a variety of terahertz wave applications in addition to their primary application in optical and electrical memory devices.

With rapid advances occurring in terahertz (THz) radiation generation techniques, the interaction between matter and intense THz fields has become an important research topic. Among different types of THz radiation sources, the free electron laser (FEL) is a promising experimental tool that is expected to pave the way for new forms of material processing, control of phase transitions, and serve as a test bench for extreme operating conditions in high-speed small-size electrical and magnetic devices through the exploitation of strong THz electrical and magnetic fields without the presence of interband electronic excitation. In the current work, we irradiated Ge₂Sb₂Te₅ phase change memory material with intense THz pulse trains from an FEL and observed THz-induced surface changes due to damage as a precursor to ablation and the formation of fine surface undulations whose spatial period is comparable to or slightly smaller than the wavelength of the excitation THz pulses in the material. The formation of undulations as well as the fact that no significant thermal effect was observed below the volume expansion threshold suggests that THz-induced effects mainly contributed to the observed changes. To the best of our knowledge, this is the first experimental observation of THz-induced undulations (so-called “LIPSS”), which are of potential importance for laser material processing.

Phase-change memories based on reversible amorphous-crystal transformations in pseudobinary GeTe-Sb₂Te₃ alloys are one of the most promising nonvolatile memory technologies. The recently proposed superlattice-based memory, or interfacial phase-change memory (iPCM), is characterized by significantly faster switching, lower energy consumption, and better endurance. The switching mechanism in iPCM, where both the SET and RESET states are crystalline, is still contentious. Here, using the ab initio density functional theory simulations, a conceptually new switching mechanism for iPCM is derived, which is based on the change in the potential landscape of the bandgap, associated with local deviations from the pseudobinary stoichiometry across the van der Waals gaps and the associated shift of the Fermi level. The crucial role in this process belongs to Ge/Sb intermixing on the cation planes of iPCM. These findings offer a comprehensive understanding of the switching mechanisms in iPCM and are an essential step forward to the insightful development of phase-change memory technology.

We report on the dynamics of coherent phonons in semimetal 1T′-${\text{MoTe}}_2$ using femtosecond pump-probe spectroscopy. On an ultrafast sub-picosecond timescale at room temperature, a low frequency and long-lifetime shear phonon mode was observed at 0.39 THz, which was previously reported in the form of a characteristic phonon only in the low temperature ${\mathrm{T}}_{\mathrm{d}}{-\text{MoTe}}_2$ phase. Unlike the other optical phonon modes, the shear phonon mode was found to strongly couple with photoexcited carriers. Moreover, the amplitude of the shear mode surprisingly decreased with increasing excitation density, a phenomenon that can be attributed to be a consequence of the lattice temperature increasing after excitation. These results provide useful physical information on ultrafast lattice symmetry switching between the normal semimetal 1T′ and the lattice inversion symmetry breaking Type-II Weyl semimetal ${\mathrm{T}}_{\mathrm{d}}$ phases.

This paper summarizes recent progress on thin film growth of chalcogenides by sputtering. The materials discussed include Sb–Te, Bi–Te, Ge–Te, and their superlattices, materials that are technologically important particularly for non-volatile phase change memory. In this work, the sputter-growth behavior of high-quality layered chalcogenide films is discussed. Sputtering is one of the most commonly used thin-film growth techniques in the semiconductor industry, however, the complex interrelationship between growth parameters can lead to difficulty in fabricating high-quality films although the deposition method itself is relatively simple. Here, we successfully demonstrate the fabrication of highly-oriented layered chalcogenide materials by sputtering. The selection of the appropriate sputtering target is important. In particular, it was found that a Te-rich Sb–Te alloy target such as Sb ₃₃ Te ₆₇ is necessary in order to obtain a stoichiometric Sb ₂ Te ₃ film. Moreover, the growth temperature is also a key factor in obtaining a highly-oriented film, namely the ideal growth temperature for an Sb ₂ Te ₃ film is between 230 °C and 250 °C after the growth of an amorphous seed layer at room temperature. Furthermore, it was found that this technique is also useful to grow films epitaxially on Al ₂ O ₃ or Si(111) substrates even though there are some misoriented grains as well as twins present. Finally, we demonstrate the growth of highly-oriented Sb ₂ Te ₃ films on a flexible substrate. The versatility of sputtering will become technologically more and more important for the various applications represented by phase change memory.