Role of grain boundaries in Ge-Sb-Te based chalcogenide superlattices.

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, 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 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 grain boundaries at the nanometer scale. Two types of grain boundaries have been identified; high-angle grain boundaries 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 high-angle grain boundaries, where heterogeneous nucleation of Ge2Sb2Te5 can be clearly determined. Yet, the Ge1Sb2Te4 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 grain boundaries on the bonding states and device performance.