Role of grain boundaries in Ge–Sb–Te based chalcogenide superlattices

Abstract

Interfacial phase change memory devices based on a distinct nanoscale structure called superlattice have been shown to outperform conventional phase-change devices. This improvement has been attributed to the hetero-interfaces, which play an important role for the superior device characteristics. However, the impact of grain boundaries (GBs), usually present in large amounts in a standard sputter-deposited superlattice film, on the device performance has not yet been investigated.Therefore, in the present work, we investigate the structure and composition of superlattice films by high resolution x-ray diffraction (XRD) cross-linked with state-of-the art methods, such as correlative microscopy, i.e. a combination of high-resolution transmission electron microscopy and atom probe tomography to determine the structure and composition of GBs at the nanometer scale. Two types of GBs have been identified: high-angle grain boundaries (HAGBs) present in the upper part of a 340 nm-thick film and low-angle grain boundaries present in the first 40 nm of the bottom part of the film close to the substrate. We demonstrate that the strongest intermixing takes place at HAGBs, where heterogeneous nucleation of Ge 2 Sb 2 Te 5 can be clearly determined. Yet, the Ge 1 Sb 2 Te 4 phase could also be detected in the near vicinity of a low-angle grain boundary. Finally, a more realistic view of the intermixing phenomenon in Ge-Sb-Te based chalcogenide superlattices will be proposed. Moreover, we will discuss the implications of the presence of GBs on the bonding states and device performance.

Chalcogenides by Design: Functionality through Metavalent Bonding and Confinement

“…There are also no major obstacles applying the atom probe to these materials to determine the bond rupture. Recently, APT has even been used already to characterize chalcogenide superlattices [ 187 ] as well as nanoscale phase separation. [ 188 ] Hence, regarding the property portfolio and the bond breaking, it is possible to conclude without doubt if a certain solid employs MVB.…”

Mg Deficiency in Grain Boundaries of n‐Type Mg3Sb2 Identified by Atom Probe Tomography

“…APT has the capability of chemically characterizing materials in 3D on an atom‐by‐atom basis, which helps to understand the formation of nanostructures in metals and semiconductors. The technique has been applied to detect off‐stoichiometry around the grain boundaries in various semiconductors such as silicon, chalcogenides, filled skutterdites, zintls, oxides, and other systems . The results suggest that the resistance of the grain boundaries in Mg 3 Sb 2 ‐based materials can be attributed to a Mg deficiency that greatly reduces the local free charge carrier concentration.…”

Origin of the concentration-dependent effects of N on the stability and electrical resistivity in polycrystalline Ge1Sb2Te4

“…24 These concentration-sensitive doping effects so far lack the corresponding microscopic explanations. On the other hand, the grain boundary plays a vital role in the behavior of GST alloys, 25,26 for instance, the grain boundary provides favorable doping sites for small dopants as observed in the experiments. 27,28 In addition, the structural distortions brought in by the boundary have an influence on the electronic structures.…”

“…43,44 Cojocaru-Mirédin et al reported that over 90% of grain boundaries in the GST alloys are high-angle grain boundaries, in which Σ3 twin boundaries account for a large proportion. 26 Therefore, the Σ3 (111) grain boundary (shown in Fig. 1(c)) containing 84 atoms is first constructed so as to simulate the polycrystalline model.…”