Impact of Bonding on the Stacking Defects in Layered Chalcogenides

Abstract

Phase‐change materials for high‐density data storage traditionally exploit the amorphous‐to‐crystalline phase transition. A number of these compounds are organized in blocks, separated by van der Waals‐like gaps. Such layered chalcogenides are attracting interest due to their unique material properties and the possibility to change their properties upon local rearrangements at the gap, giving rise to novel applications. To better understand the behavior of layered chalcogenides, the connection between structural defects, physical properties, and the bonding situation is highlighted here using electron microscopy, X‐ray diffraction, and density functional theory. In particular, stacking defects in hexagonal Ge4Se3Te, GaSe, and Sb2Te3 are characterized experimentally, followed by an investigation of the influence of observed and hypothetical stacking defects on optical and electronic properties by theoretical means. Then, a connection between observed defects and the bonding situation in these materials is drawn and related to the presence of van der Waals and metavalent bonding in chalcogenides. Finally, additional experiments are performed to validate the conclusions for other metavalently bonded layered chalcogenides. Transmission electron microscopy provides a powerful tool for direct detection of defects and, when combined with diffraction experiments and ab initio theory, it facilitates the precise investigation of the bonding mechanisms in layered chalcogenides.

Interface controlled thermal resistances of ultra-thin chalcogenide-based phase change memory devices

“…Diffuse streaks emanate from the {210} Bragg spots as a result of the faulted grain. Highly faulted structures have been observed in a variety of chalcogenides including Ge-Sb-Te compounds [36][37][38][39][40][41] . The large, faulted grains grew laterally by growth ledges that often nucleate at the W/GST interface.…”

“…By shifting each nine-layer building block by a partial lattice vector, the c-GST becomes h-GST and vice versa. Recent literature suggests that the weak bonding between the sesqui-chalcogenides building blocks, such as Sb 2 Te 3 , significantly exceeds those of van der Waals forces and, therefore, possesses more of a metavalent bond nature 40,44 . As a result of the weak bonding between the blocks, there is a low-energy barrier to passing partial dislocations that transform the lattice and cause faults.…”

Dimensional transformation of chemical bonding during crystallization in a layered chalcogenide material

“…It should be noted that some layered materials, such as Sb 2 Te 3 , have been pointed out not to be rigorous van der Waals solids as the spacing across the vdW gaps is shorter than suggested by the vdW radius of the constituent atoms, suggesting stronger interactions 35 . Based on the DFT simulations of the interlayer binding energy 36 , 37 , the observation of stacking defects 38 , 39 , and the estimation of the Te–Te gap spacing, Cr 2 Ge 2 Te 6 appears more likely to possess the characteristics of vdW solids. In either case, we believe that the present work will pave the way to the understanding of the crystallization mechanism of other layered materials and hence contribute to the fabrication of high-quality functional layered materials for future electronics applications.…”

Novel Polymorph of GaSe

“…The discrepancy between the overlaid atomic model of GaSe and the experimental atomic positions obtained by HAADF‐STEM shows that the vdW film is not well aligned with the underneath GaSe passivation layer on the Si surface. Such a deviation has been attributed to the creation of a translational shear fault (TSF), [ 36 ] wherein the TLs stacking in γ‐GaSe is not preserved.…”