Electron beam induced in situ writing and recovering of vacancy layers in
Ge2Sb2Te5
crystal lattice
Authors: Lotnyk Andriy , Bernütz Sabine , Sun Xinxing , Ross Ulrich , Ehrhardt Martin , Rauschenbach Bernd 
Published: December 2016

Phase change materials (PCMs), such as Ge‐Sb‐Te‐based alloys, are of high interest due to their technologically eminent optical and electronic properties. Among the Ge‐Sb‐Te based PCMs, Ge
2
Sb
2
Te
5
(GST225) is a well‐known compound and the most used PCM. GST225 is utilized in optical and electronic data storage devices [1]. Technological relevant phases of GST225 are an amorphous phase, a metastable (cubic) phase containing 20% of intrinsic vacancies and a stable trigonal phase. Despite comprehensive studies on amorphisation‐crystallisation processes of GST225, the phase transformation from the cubic to the trigonal phase has not been experimentally investigated up to now. In this work, the phase transformation is investigated
in‐situ
in a Cs‐corrected STEM by exposing distinct layers of GST225 crystal lattice to repeated line scanning of a focused electron beam [2].

The cubic GST225 phase was prepared by laser irradiation of amorphous GST225 thin films [2]. No planar defects such as vacancy layers (VLs) [3] were observed in the cubic GST225 phase (Fig. 1(a)) [2]. However, VLs can be intentionally produced in the GST225 phase by repeated line scanning of the focused electron beam along individual mixed GeSb/V layers, while no such defects can be created by scanning the electron beam along the Te layers (Fig.1(b)). Most notably, these VLs disappeared after repeated scanning of the focused electron beam over a scanning window covering these defects (Fig. 1(d)). Moreover, the Te‐Te distance in the [001] direction is reduced in these newly formed VLs [2]. The observed ordering of vacancies into layers is due to energy transfer by inelastic interactions of the electron beam with the GeSb layer and takes place by diffusion of Ge and Sb atoms towards vacancies located in the nearest neighbouring GeSb/V layers [2]. Previous DFT calculations indicated that the ordering of vacancies into layers is energetically favourable [4]. It was also shown that the structural transition to the layered trigonal GST225 phase is driven by the ordering of vacancies in the cubic GST225 phase. However, the vacancy ordering necessarily occurs before complete formation of the vacancy planes, the process of which could be observed in this work.

From the above presented results and those reported in Refs. [3,4], a transformation mechanism between the cubic and trigonal GST225 phases can be proposed. The stable GST225 consists of nine layers alternatingly containing Te and GeSb in one unit cell, e.g. ‐Te‐GeSb‐Te‐GeSb‐Te‐VL‐Te‐GeSb‐Te‐GeSb‐, with intrinsic VL between adjacent Te layers (Fig. 2(a)). The stacking sequence in cubic GST225 is ‐Te‐GeSb/V‐Te‐GeSb/V‐Te‐GeSb/V‐Te‐GeSb/V‐Te‐GeSb/V‐ (Fig. 2(b)) and the trigonal GST225 can therefore be derived from the cubic GST225 by removing the vacancies from the sublattice and accumulating them in the VLs. However, movement of Ge and Sb atoms in the cubic GST225 lattice along direction and subsequent shift of newly formed building blocks against each other are required to complete the trigonal lattice. Thus, this phase transition is driven by local short‐distance movements of Ge and Sb atoms towards vacancies without long range diffusion and without change in composition of the parent cubic GST225 phase. Consequently, the phase change between the cubic and trigonal GST225 phases is a diffusionless transformation process similar to martensitic transformation [5].