Preparation and Characterization of Cu-Te Phases Synthesized by Focused Ion Beam Method

Specimen preparation for transmission electron microscopy (TEM) using focused ion beam (FIB) is a commonly used method [1]. An advantage over other methods is site specific specimen preparation that only damages a small area of the sample. However, it also suffers from many disadvantages. Artifacts induced by FIB range from ion implantation and resulting amorphization to thermal effects like phase transitions [2,3]. In this work, we prepared different nanoscale Cu-Te phases from Cu (electrodes) - Sb₂Te₃ (thin films) system [4] using FIB method. Copper chalcogenides have shown promising thermoelectric properties, e.g. high efficiency and tunability, which are key motivations for research on these materials [5].<br/><br/>Sb₂Te₃ thin layers with thicknesses of 17 or 100 nm are epitaxially grown on p-type Si (111) substrates. Samples with a 24 nm thick polycrystalline Sb₂Te₃ thin film are grown on a SiO₂coated waferusing 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]. Ion beam parameters are varied and lowered up to 15 kV, 250 pA for the trenching step before lift out. 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.<br/><br/>Dependent on beam current used during FIB lamella preparation and Sb₂Te₃layer thickness, hole formation in the Cu layer, thickness change and chemical changes of the Sb₂Te₃layers are observed. Layer thickness after FIB specimen prepared at normal milling parameters are 28 nm and 23 nm for reduced parameters in a sample with originally 17 nm Sb₂Te₃. Hole formation is also decreased indicating less material transport. The structural changes are confirmed by in situ x-ray diffraction heating in addition. Samples with a 17 nm Sb₂Te₃layer showed uniform intercalation of Cu while a sample with 100 nm thick Sb₂Te₃layer exhibited differential intercalation behavior. In specimen prepared from the in situ heated samples Sb₂Te₃and Cu-Te grains are observed. The introduction of a Pt layer between the Cu electrode and Sb₂Te₃layers hindered structural changes caused by FIB. The same effect is observed in the heated sample where a Sb layer was formed between the Cu and Sb₂Te₃ layer. Moreover, Cr-Sb₂Te₃and a Cu-GeTe layer systems showed no modifications of Sb₂Te₃ and GeTe thin films during FIB preparation. In polycrystalline Sb₂Te₃specimen the intercalation of Cu and formation of new Cu-Te phases was also observed. Here a formation of a tetragonal Cu₂Te phase is found.<br/><br/>This work shows that structural changes in Sb₂Te₃thin film systems can be induced during FIB specimen preparation of Cu (electrode)-Sb₂Te₃ (thin films) systems. The influence of different factors such as ion beam current, layer stacking sequence, Sb₂Te₃ structure and layer thickness on the formation of Cu-Te phases are evaluated. The changes are thermally induced transitions caused by FIB process, while redeposition of Cu plays a minor role. Similar thermal effects can be observed in heated samples. Other studied systems show no modifications.<br/><br/>Acknowledgements<br/>The authors thank P. Hertel for magnetron sputtering. We acknowledge the financial support by the German Research Foundation (DFG 448667535).<br/><br/>References<br/>[1] T. Ishitani, et al., Microscopy 43 (1994) 322<br/>[2] J. Mayer, et al., MRS Bull. 32 (2007) 400<br/>[3] R. Schmied, et al., Phys. Chem. Chem. Phys. 16 (2014) 6153<br/>[4] H. Bryja, et al., 2D Materials 8 (2021) 045027<br/>[5] T. Wei, et al., Science China Mater. 62 (2019) 8