Real structure of highly oriented
Ge‐Sb‐Te
thin films investigated by
Cs
‐corrected
STEM
Authors: Ross Ulrich , Lotnyk Andriy , Thelander Erik , Rauschenbach Bernd 
Published: December 2016

The phase‐change effect in a wide class of tellurium‐based chalcogenide compounds allows for the fast and reversible transition between crystalline and amorphous states that possess a large contrast in material properties. Application examples include optical storage media, phase‐change RAM. Ge‐Sb‐Te (GST) compounds are often used as a model system for this behavior, and have thus received wide‐spread attention in literature [1‐3]. Recent interest in the material contrast switching behavior of GST has extended to the reversible switching of highly oriented layered superlattices [3]. The exact nature of the crystalline transition is still under debate, and detailed insight into the high‐temperature trigonal phases in an epitaxial environment is required in order to accurately interpret experimental data. While such epitaxial thin films of GST are typically prepared by MOCVD or MBE, the here presented experimental results were obtained by pulsed laser deposition (PLD) from compound targets onto various substrates at elevated temperatures. The aim of the work presented is thus to investigate the microstructure of of GST thin films deposited by PLD, with particular focus on the interface formation, as well as to characterize the stacking sequences and defect structures in the trigonal phases produced [4].

GST thin films were deposited from a stoichiometric Ge
2
Sb
2
Te
5
compound target onto various silicon substrates in a range of temperatures from 110 to 280 °C. The thin films deposited on amorphous surface layer and chemically cleaned substrates possessed closed surfaces and low roughness. While electron and x‐ray diffraction data shows that all films are in the trigonal phase, some were found to be polycrystalline, while others possess are clear uniform epitaxial relation towards the substrate. Furthermore, a comparison of average chemical compositions by STEM‐EDX revealed that the relative concentration of Ge rapidly declines at deposition temperatures above 200 °C.

Two atomic resolution images of polycrystalline columnar grain growth of GST by PLD are shown in Fig. 1. As can be seen in Fig. 1(a), when deposited onto a flat amorphous interface layer, the crystalline grains above can be misoriented towards the substrate, and the onset of systematic layering of the characteristic van der Waals (vdW) layers is delayed. Fig. 1(b) shows the disordered stacking in the bulk of a crystallite deposited onto cleaned Si(111) at 110 °C, with 7, 9 and 11‐layered building blocks and Z‐contrast correlating with Ge
1
Sb
2
Te
4
, Ge
2
Sb
2
Te
5
and Ge
3
Sb
2
Te
6
, respectively. Fig. 2 shows the interface of GST on superficially cleaned Si(111) deposited at 110 °C (Fig. 2(a)) and 280 °C (Fig. 2(b)). All epitaxial thin films possess a surface passivation Te/Sb layer, as well as a vdW gap immediately above [4,5]. We thus find a strong correlation between grain morphology, surface passivation and substrate treatment, as well as characteristic stacking disorder and chemical reordering of the Ge‐rich layers.

We thank Mrs. A. Mill for her assistance in the FIB preparation. The financial support of the European Union and the Free State of Saxony (LenA project; Project No. 100074065) is gratefully acknowledged.