Phase transitions in Cu-Sb₂Te₃ thin film systems induced by focused ion beam milling

Abstract text (incl. figure legends and references)

Specimen preparation for transmission electron microscopy (TEM) using focused ion beam (FIB) is a commonly used method [1]. While it offers many advantages over other methods, like site specific specimen preparation, it also suffers from many disadvantages. Artifacts induced by FIB range from ion implantation to thermal effects [2,3]. In this work, we study the impact of FIB milling on the microstructure of Cu (electrode)-Sb₂Te₃ (thin films) systems used for memory applications [4]. The influence of different factors such as ion beam current, layer stacking sequence, Sb₂Te₃ structure and layer thickness are evaluated.

Sb₂Te₃ thin layers are epitaxially grown on p-type Si (111) substrates, while polycrystalline Sb₂Te₃ thin films are grown on SiO₂ using pulsed laser deposition (PLD) [4]. Cu, Pt/Cu and Cr layers are deposited by magnetron sputtering on top of the Sb₂Te₃ layers. TEM specimens are prepared at varied beam currents using a standard cross-section FIB preparation method [1]. FIB specimens are investigated using advanced methods of aberration-corrected scanning TEM such as atomic-scale HAADF imaging and atomic-scale chemical analysis (EDX and EELS). In situ x-ray diffraction heating of the Cu-Sb₂Te₃ thin film is performed to confirm structural changes.

Dependent on beam current used during FIB lamella preparation and Sb₂Te₃ layer thickness, hole formation in the Cu layer, thickness change and chemical changes of the Sb₂Te₃ layers are observed (Fig. 1). Samples with a 17 nm Sb₂Te₃ layer show uniform intercalation of Cu while a sample with 100 nm thick Sb₂Te₃ layer exhibits differential intercalation behavior. In specimen prepared from the in situ heated samples Sb₂Te₃ and Cu-Te grains are observed. The introduction of a Pt layer between the Cu electrode and Sb₂Te₃ layers hinders structural changes caused by FIB (Fig. 2). Moreover, Cr-Sb₂Te₃ and a Cu-GeTe layer systems show no modifications of Sb₂Te₃ and GeTe thin films during FIB preparation. In polycrystalline Sb₂Te₃ specimen the intercalation of Cu and formation of new phases is also observed.

This work demonstrates that structural changes in Sb₂Te₃ thin film systems can be induced during FIB specimen preparation of Cu (electrode)-Sb₂Te₃ (thin films) systems. The changes are thermally induced transitions caused by FIB process due to local heating, while redeposition of Cu plays a minor role.

The authors thank P. Hertel for magnetron sputtering. We acknowledge the financial support by the German Research Foundation (DFG 448667535).

[1] T. Ishitani, et al., Microscopy 43.5 (1994): 322-326.

[2] J. Mayer, et al., MRS bulletin 32.5 (2007): 400-407.

[3] R. Schmied, et al., Physical Chemistry Chemical Physics (2014), 6153.

[4] H. Bryja, et al., 2D Materials (2021), 045027.

Fig. 1: (a) EDX Map of a lamella prepared with FIB, (b-c) Overview HAADF-STEM images of specimen prepared with normal and reduced FIB beam currents. (d) Atomic-resolution HAADF-STEM image showing new Cu-Te phases.

Fig. 2: Cu/Pt/Sb₂Te₃ layer stack. (a) Overview HAADF-STEM image. (b) Overview EDX elemental map. (c) Atomic-resolution HAADF-STEM image, showing initial Sb₂Te₃ structure.