Yongjin Chen

Element-resolved atomic structure imaging of rocksalt Ge2Sb2Te5 phase-change material

Disorder-induced electron localization and metal-insulator transitions (MITs) have been a very active research field starting from the seminal paper by Anderson half a century ago. However, pure Anderson insulators are very difficult to identify due to ubiquitous electron-correlation effects. Recently, an MIT has been observed in electrical transport measurements on the crystalline state of phase-change GeSbTe compounds, which appears to be exclusively disorder driven. Subsequent density functional theory simulations have identified vacancy disorder to localize electrons at the Fermi level. Here, we report a direct atomic scale chemical identification experiment on the rocksalt structure obtained upon crystallization of amorphousGe2Sb2Te5. Our results confirm the two-sublattice structure resolving the distribution of chemical species and demonstrate the existence of atomic disorder on the Ge/Sb/vacancy sublattice. Moreover, we identify a gradual vacancy ordering process upon further annealing. These findings not only provide a structural underpinning of the observed Anderson localization but also have implications for the development of novel multi-level data storage within the crystalline phases.

Microstructural fingerprints of phase transitions in shock-loaded iron

The complex structural transformation in crystals under static pressure or shock loading has been a subject of long-standing interest to materials scientists and physicists. The polymorphic transformation is of particular importance for iron (Fe), due to its technological and sociological significance in the development of human civilization, as well as its prominent presence in the earths core. The martensitic transformation α→ε (bcc→hcp) in iron under shock-loading, due to its reversible and transient nature, requires non-trivial detective work to uncover its occurrence. Here we reveal refined microstructural fingerprints, needle-like colonies and three sets of {112}<111> twins with a threefold symmetry, with tell-tale features that are indicative of two sequential martensitic transformations in the reversible α→ε phase transition, even though no ε is retained in the post-shock samples. The signature orientation relationships are consistent with previously-proposed transformation mechanisms, and the unique microstructural fingerprints enable a quantitative assessment of the volume fraction transformed.

Computation of Lyapunov values for two planar polynomial differential systems

The complex formulas of computing the Lyapunov values are investigated for two planar polynomial systems with a pair of pure imaginary eigenvalues. The relations between the two systems and their standard forms are discussed and the transformation formulas are given. The corresponding complex formulas of computing the Lyapunov values are obtained. The results obtained here can be applied to judging the type of singular points and the number of limit cycles in planar polynomial systems.

Direct observation of structural transitions in the phase change material Ge2Sb2Te5

Phase change memory, which is based on the reversible switching of phase change materials between amorphous and crystalline states, is one of the most promising bases of nonvolatile memory devices. However, the transition mechanism remains poorly understood. In this study, via in situ transmission electron microscopy with an externally applied DC voltage and nanosecond electrical pulses, for the first time we revealed a reversible structural evolution of Ge2Sb2Te5 thin films from an amorphous state to a single-crystal state via polycrystals as an intermediate state. This transition is different from the traditional understanding of structural changes in Ge2Sb2Te5, i.e., from an amorphous structure to a hexagonal close-packed structure via face-centered cubic as an intermediate structure. In situ observations indicate that this poly-to-single crystal structural transition is caused by coalescence of neighbouring grains induced by an electric field, in which a fast heating/cooling rate is found to be essential. Our study opens a new avenue for the realization of the multi-level operation of phase change materials.

Electrical properties and structural transition of Ge2Sb2Te5 adjusted by rare-earth element Gd for nonvolatile phase-change memory

Phase change memory has been considered as the next generation in non-volatile electronic data storage. The property modulation of such materials by the doping of rare-earth elements has drawn a lot of attention, which motivates us to search for the optimal dopants and reveal the underlying mechanisms. Here, we investigate the role of Gd as a dopant in Ge2Sb2Te5, which exhibits higher crystalline resistance and better thermal stability and antioxidant capacity than the undoped counterpart. Moreover, Gd dopants suppress both the processes of phase transition and grain growth. The crystalline structure remains unchanged with Gd dopants and vacancies are randomly distributed. Furthermore, the bonding mechanism was theoretically investigated. In the amorphous state, Gd atoms modify the local structures around Ge, Sb, and Te atoms. The large coordination number of Gd and the “Gd–Te distorted pentagonal bipyramidal-like” structure can be attributed to the good thermal stability. These microscopic findings figure out some of the key issues about the bonding mechanism, electrical properties, and crystallization behaviors of Gd doped phase change memory materials, which could be useful for storage devices.Phase change memory has been considered as the next generation in non-volatile electronic data storage. The property modulation of such materials by the doping of rare-earth elements has drawn a lot of attention, which motivates us to search for the optimal dopants and reveal the underlying mechanisms. Here, we investigate the role of Gd as a dopant in Ge2Sb2Te5, which exhibits higher crystalline resistance and better thermal stability and antioxidant capacity than the undoped counterpart. Moreover, Gd dopants suppress both the processes of phase transition and grain growth. The crystalline structure remains unchanged with Gd dopants and vacancies are randomly distributed. Furthermore, the bonding mechanism was theoretically investigated. In the amorphous state, Gd atoms modify the local structures around Ge, Sb, and Te atoms. The large coordination number of Gd and the “Gd–Te distorted pentagonal bipyramidal-like” structure can be attributed to the good thermal stability. These microscopic findings figur...

Microstructure evolution of the phase change material TiSbTe

The crystallization process and crystal structure of the phase change material TiSbTe alloy have been successfully established, which is essential for applying this alloy in phase change memory. Specifically, transmission electron microscopy (TEM) analyses of the film annealed in situ were used in combination with selected-area electron diffraction (SAED) and radial distribution function (RDF) analyses to investigate the structural evolution from the amorphous phase to the polycrystalline phase. Moreover, the presence of structures with medium-range order in amorphous TST, which is beneficial to high-speed crystallization, was indicated by the structure factors S(Q)s. The crystallization temperature was determined to be approximately 170°C, and the grain size varied from several to dozens of nanometers. As the temperature increased, particularly above 200°C, the first single peak of the rG(r) curves transformed into double shoulder peaks due to the increasing impact of the Ti–Te bonds. In general, the majority of Ti atoms enter the SbTe lattice, whereas the remainder of the Ti atoms aggregate, leading to the appearance of TiTe2 phase separation, as confirmed by the SAED patterns, high-angle annular dark field scanning transmission electron microscopy (HAADFSTEM) images and the corresponding energy-dispersive X-ray (EDX) mappings.