Ulrich Ross

Direct imaging of crystal structure and defects in metastable Ge2Sb2Te5 by quantitative aberration-corrected scanning transmission electron microscopy

Knowledge about the atomic structure and vacancy distribution in phase change materials is of foremost importance in order to understand the underlying mechanism of fast reversible phase transformation. In this Letter, by combining state-of-the-art aberration-corrected scanning transmission electron microscopy with image simulations, we are able to map the local atomic structure and composition of a textured metastable Ge2Sb2Te5 thin film deposited by pulsed laser deposition with excellent spatial resolution. The atomic-resolution scanning transmission electron microscopy investigations display the heterogeneous defect structure of the Ge2Sb2Te5 phase. The obtained results are discussed. Highly oriented Ge2Sb2Te5 thin films appear to be a promising approach for further atomic-resolution investigations of the phase change behavior of this material class.

Focused high- and low-energy ion milling for TEM specimen preparation

Abstract For atomic-resolution aberration-corrected (Cs-corrected) scanning transmission electron microscopy (STEM) the quality of prepared TEM specimens is of crucial importance. High-energy focused gallium ion beam milling (FIB) is widely used for the production of TEM lamella. However, the specimens after conventional FIB preparation are often still too thick. In addition, damage and amorphization of the TEM specimen surface during the milling process occur. In order to overcome these disadvantages, low-energy Ar ion milling of FIB lamellae can be applied. In this work, we focus on TEM specimen preparation of different thin films (GaN, Ge 2 Sb 2 Te 5 , TiO 2 ) and interface structures (GaN/6H-SiC, SrTiO 3 /TiO 2 , Ge 2 Sb 2 Te 5 /Si) using a combination of FIB with a focused low-energy Ar ion polishing. The results show that this combination enables the routine preparation of high quality TEM lamellae with a smooth surface and uniform thickness, even at the interface region between two different materials and over a lateral range of several micrometres. The prepared lamellae exhibit less surface damage and are well suited for atomic-resolution Cs-corrected STEM/TEM imaging at medium and low accelerating voltages. These results are in a good agreement with Monte Carlo simulations performed by the Stopping and Range of Ions in Matter (SRIM) software.

Local atomic arrangements and lattice distortions in layered Ge-Sb-Te crystal structures

Insights into the local atomic arrangements of layered Ge-Sb-Te compounds are of particular importance from a fundamental point of view and for data storage applications. In this view, a detailed knowledge of the atomic structure in such alloys is central to understanding the functional properties both in the more commonly utilized amorphous–crystalline transition and in recently proposed interfacial phase change memory based on the transition between two crystalline structures. Aberration-corrected scanning transmission electron microscopy allows direct imaging of local arrangement in the crystalline lattice with atomic resolution. However, due to the non-trivial influence of thermal diffuse scattering on the high-angle scattering signal, a detailed examination of the image contrast requires comparison with theoretical image simulations. This work reveals the local atomic structure of trigonal Ge-Sb-Te thin films by using a combination of direct imaging of the atomic columns and theoretical image simulation approaches. The results show that the thin films are prone to the formation of stacking disorder with individual building blocks of the Ge2Sb2Te5, Ge1Sb2Te4 and Ge3Sb2Te6 crystal structures intercalated within randomly oriented grains. The comparison with image simulations based on various theoretical models reveals intermixed cation layers with pronounced local lattice distortions, exceeding those reported in literature.

Low temperature epitaxy of Ge-Sb-Te films on BaF2 (111) by pulsed laser deposition

Pulsed laser deposition was employed to deposit epitaxial Ge2Sb2Te5-layers on the (111) plane of BaF2 single crystal substrates. X-ray diffraction measurements show a process temperature window for epitaxial growth between 85 °C and 295 °C. No crystalline growth is observed for lower temperatures, whereas higher temperatures lead to strong desorption of the film constituents. The films are of hexagonal structure with lattice parameters consistent with existing models. X-ray pole figure measurements reveal that the films grow with one single out-of-plane crystal orientation, but rotational twin domains are present. The out-of-plane epitaxial relationship is determined to be Ge2Sb2Te5(0001) || BaF2(111), whereas the in-plane relationship is characterized by two directions, i.e., Ge2Sb2Te5 [-12-10] || BaF2[1-10] and Ge2Sb2Te5[1-210] || BaF2[1-10]. Aberration-corrected high-resolution scanning transmission electron microscopy was used to resolve the local atomic structure and confirm the hexagonal structure o...

Van der Waals interfacial bonding and intermixing in GeTe-Sb2Te3-based superlattices

Interfacial phase change memory (iPCM) based on GeTe and Sb2Te3 superlattices (SLs) is an emerging contender for non-volatile data storage applications. A detailed knowledge of the atomic structure of these materials is crucial for further development of SLs and for a better understanding of the resistivity switching characteristics of iPCM devices. In this work, crystalline GeTe-Sb2Te3-based SLs, produced by pulsed laser deposition onto a Si(111) substrate at temperatures lower than in previous studies, are analyzed by advanced scanning transmission electron microscopy. The results reveal the formation of Ge-rich Ge(x+y)Sb(2–y)Tez building blocks with specific numbers of ordered Ge cation layers (between 1 and 5) and disordered cation layers (4) for z = 6–10, as well as intermixed cation layers for z = 5, within the SLs. The G Ge(x+y)Sb(2–y)Tez units are separated from the Sb2Te3 building blocks by van der Waals gaps. In particular, the interlayer bonding is promoted by the formation of outermost cation layers consisting of intermixed GeSb within the building blocks adjacent to the van der Waals gaps. The Ge(x+y)Sb(2–y)Tez units with z > 5 retain metastable crystal structures with two-dimensional bonding within the SLs. The present study shed new light on the possible configurations of the building units that can be formed during the synthesis of GeTe-Sb2Te3-based iPCM materials. In addition, a possible switching mechanism active in iPCM materials is discussed.