Ge-Sb-Te (GST) compounds are of high interest due to their outstanding optical and electronic properties. Thin films of GST alloys are widely used as phase change materials (PCMs) in optical and electronic data storage devices [1]. The operating principle of conventional PCMs is based on ultrafast, reversible transformation between the amorphous and metastable (cubic) crystalline phases.  Recently, a new type of phase change memory device, so called ‘interfacial phase change memory’ (iPCM) was proposed. iPCM consists of GeTe-Sb₂Te₃ superlattices. In the case of iPCMs, the phase transitions occur between two crystalline structures, thus allowing for a drastic reduction in energy consumption in memory devices. However, it has been experimentally demonstrated that the structure of iPCM corresponds to van der Waals bonded layers of Sb₂Te₃ and various layered GST crystal structures [2]. The switching mechanism of iPCM and the electronic properties of these materials are determined by the local atomic arrangement of Ge and Sb atoms. Consequently, knowledge on the proper local atomic arrangement in layered GST crystal structures is of paramount importance. The aim of this work is to study the local atomic arrangements and lattice distortions in GST thin films consisting of layered Ge₂Sb₂Te₅ (GST225), Ge₁Sb₂Te₄ and Ge₃Sb₂Te₆ crystal structures using a combination of atomic-resolution Cs-corrected HAADF-STEM imaging and theoretical image simulation. 

In this work, large crystallites of trigonal GST were prepared by ex-situ heating of amorphous GST225 thin films [3].  Fig. 1 shows the microstructures of GST thin films heated at different temperatures. The thin films consist of various building blocks with 7, 9 and 11 layers, indicating pronounced chemical disorder along the c-axis. The averaged composition of the thin films was verified to be 20 at.% of Ge, 24 at.% of Sb and 56 at.% of Te. Consequently, the disorder is attributed to deviations in local chemical composition of GST thin films which appears to be typical for layered GST compounds.

Fig. 2 gives a HAADF image of a single 9-layer GST225 building block. The block consists of alternating cation (GeSb) and anion (Te) layers. There are four different stacking sequences proposed in the literature, which differ in site occupancy of the distinct cation layers and in thermal displacement parameters (B). Due to the sensitivity of image intensities to B factors, the parameters can be used for the distinguishing between various stacking sequences. Fig. 2(b) and Figs. 2(c)-(f) show the comparisons between experimental and theoretical averaged intensity maxima for specific lattice sites in a GST225 lattice, respectively. The results reveal that the Ge and Sb atomic species tend to form intermixed cation layers with different ratios of Ge to Sb between distinct cation layers. Moreover, the Ge and Sb atoms in the studied structures are off-centre displaced from the centre of (GeSb)Te6 octahedrons. However, the distortions in Ge₁Sb₂Te₄ and Ge₃Sb₂Te₆ lattices are found to be larger than in the literature reported structures and strongly depended on the annealing temperature. Thus, the crystal structure of a single GST building block is conceptually similar to the local structure of the cubic GST [4].

In conclusion,the outcomes of this work shed new insights into the local structure of layered GST compounds, which may assist theoretical modelling of the switching mechanism of iPCM.

[1] S. Raoux, W. Wełnic, D. Ielmini, Chem. Rev. 110, 240 (2010).

[2] J. Momand, R. Wang et al., Nanoscale 7, 19136 (2015).

[3] U. Ross, A. Lotnyk et al., Appl. Phys. Lett. 104, 121904 (2014).

[4] A. Lotnyk, S. Bernütz et al., Acta Mater. 105, 1 (2016).

Figures:

Fig. 1. Cs-corrected HAADF-STEM images of GST samples prepared at (a) 493 K and (b) 523 K. The number of layers in each building block is inserted into (a) and (b). The bright dots in (a) and (b) are Te atomic columns whereas the darker dots are GeSb atomic columns. The dark lines are van der Waals gaps.

Fig. 2. (a) HAADF-STEM image of GST225 lattice in [11-20] zone axis. (b) Experimental intensity maxima for Te and GeSb atomic rows. Intensity maxima taken from simulated images using GST225 crystal structures with (c) Matsunaga sequence (-Te3-Ge0.44Sb0.56(2)-Te2-Ge0.56Sb0.44(1)-Te1-) and (d) Urban sequence (-Te3-Ge0.33Sb0.66(2)-Te2-Ge0.60Sb0.36(1)-Te1-). The intensities of (b) match better the intensities of (d).

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EMC Abstracts - https://emc-proceedings.com/abstract/evaluation-of-local-atomic-arrangements-and-lattice-distortions-in-layered-ge-sb-te-crystal-structures/