New insights on the metastable cubic GexSbyTez phase

Abstract

Ge-rich Ge-Sb-Te (GGST) alloys are of high interest for industrial production of complementary metal-oxide-semiconductor integrated phase change random access memories (PCRAM) of high performance. BEOL-integrated GGST-based PCRAM constitute a relevant solution for low-power on-chip non-volatile memory production and in-memory computing, addressing new challenges of automotive and artificial intelligence applications for example. During cycling, in the low-resistive SET state, the crystallized GGST alloy is mainly made of nano-grains of two phases: diamond Ge and a ternary rock-salt (RS) Ge-Sb-Te (GST) phase. The RS-GST phase is metastable and generally assumed to exhibit the stoichiometry Ge₂Sb₂Te₅ corresponding to the stable hexagonal Ge₂Sb₂Te₅ compound. However, recent works suggest that this metastable RS-GST phase is not stoichiometric and can accept higher Ge contents, meaning that the Ge content of RS-GST could vary depending on the Ge excess level in the GGST alloy compared to the Ge₂Sb₂Te₅ reference stoichiometry. Consequently, the performance of GGST-based PCRAMs could vary with Ge composition of GGST alloy, suggesting the existence of an ideal GGST alloy composition showing a good compromise between crystallization temperature, programming current density, resistance, threshold voltage drift… In the present work, the structure and composition of crystallized GGST films elaborated through Ge, Sb, and Te co-sputtering were studied by X-ray diffraction, high resolution transmission electron microscopy, and atom probe tomography. The metastable RS-GST phase crystallized in GGST films exhibiting a Ge excess between 23% and 42% is found to be able to incorporate a large amount of Ge, of about 50 at% and sometimes beyond.