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New Research on Phase Change Materials as analyzed by APT

Monday, April 8, 2019

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

Oana Cojocaru-Mirédin, Henning Hollermann, Antonio M Mio, Anthony Yu-Tung Wang and Matthias Wuttig

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 flm, on the device performance has not yet been investigated.

Therefore, in the present work, we investigate the structure and composition of superlattice flms 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 identifed: high-angle grain boundaries (HAGBs) present in the upper part of a 340 nm-thick flm and low-angle grain boundaries present in the frst 40nm of the bottom part of the flm close to the substrate. We demonstrate that the strongest intermixing takes place at HAGBs, 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 GBs on the bonding states and device performance.

The article can be found here.


Unique Bond Breaking in Crystalline Phase Change Materials and the Quest for Metavalent Bonding

Min Zhu, Oana Cojocaru-Mirédin, Antonio M. Mio, Jens Keutgen, Michael Küpers, Yuan Yu, Ju-Young Cho, Richard Dronskowski, and Matthias Wuttig

ABSTRACT: Laser-assisted feld evaporation is studied in a large number of compounds, including amorphous and crystalline phase change materials employing atom probe tomography. This study reveals signifcant differences in feld evaporation between amorphous and crystalline phase change materials. High probabilities for multiple events with more than a single ion detected per laser pulse are only found for crystalline phase change materials. The specifcs of this unusual feld evaporation are unlike any other mechanism shown previously to lead to high probabilities of multiple events. On the contrary, amorphous phase change materials as well as other covalently bonded compounds and metals possess much lower probabilities for multiple events.

Hence, laser-assisted feld evaporation in amorphous and crystalline phase change materials reveals striking differences in bond rupture. This is indicative for pronounced differences in bonding. These fndings imply that the bonding mechanism in crystalline phase change materials differs substantially from conventional bonding mechanisms such as metallic, ionic, and covalent bonding. Instead, the data reported here confrm a recently developed conjecture, namely that metavalent bonding is a novel bonding mechanism besides those mentioned previously.

The article can be found here.