MS-3-O-1956 Quantitative structural and chemical investigation of amorphous and metastable crystalline phase-change alloy thin films by Cs-corrected STEM

Phase-change composites are of interest as active components in next generation electronics phase-change random access memory (PCRAM) [1]. They display a rapid, reversible transition in resistance and reflectivity states due to their unique crystallization behaviour from amorphous to metastable crystalline phase, as well as a low threshold to re-amorphization. The atomic structure transition is therefore closely linked to those electronic and optical properties making the material class useful for data retention.
We have applied the analytical capabilities of a state-of-the-art probe Cs-corrected FEI Titan³ G2 60-300 TEM to the investigation of the atomic structure and phase transition in various thin film samples deposited by pulsed laser deposition [2]. The chosen material systems of ternary compounds along the (GeTe)_x-(Sb₂Te₃)_1-x pseudobinary line are well established in applications and widely used as test cases for phase change behaviour. The investigations encompassed samples deposited at various temperatures onto a number of single-crystalline substrates, as well as a range of sample treatments applied in order to induce the phase transitions of interest. In particular, we have performed a detailed study of the crystallization behaviour in GeSb₂Te₄ and Ge₂Sb₂Te₅. In addition, as-grown textured and epitaxial metastable thin films were investigated with sub-angstrom resolution by comparison with quantitative STEM image simulations performed with the xHREM/STEM simulation software package [3].
The resulting HRSTEM images as well as STEM-EDX maps from the fourfold super-X EDX detector array (see Fig.1) and EELS spectrum analysis allow us to shed further light onto the functional characteristics of these highly beam-sensitive materials. The investigation of fast epitaxial growth onto oriented substrates in particular reveals the formation of defect networks in the metastable phase (see Fig.2) and may offer the potential for the development of phase change thin film structures with improved switching behaviour.

[1] Raoux, S.; Welnic, W.; Lelmini, D.; Chem. Rev. 2010, 110, 240-267.
[2] Lu, H.; Rauschenbach, B. et al.; Adv. Funct. Mater. 2013, 23, 3621–3627.
[3] Ross, U.; Lotnyk, A.; Thelander, E.; Rauschenbach, B.; Appl. Phys. Lett. submitted 02/2014