Ge-Sb-Te ternary alloys are key materials for phase-change random access memory (PCRAM). In PCRAM, data recording relies on reversible switching between amorphous and crystalline phases by electrical pulse induced Joule heating. We have proposed and developed GeTe/Sb₂Te₃ superlattice phase change memory, which is known as interfacial phase change memory (iPCM), and have demonstrated a significant reduction in switching energy and much longer endurance compared to conventional alloy-type phase change memory.

Since this superlattice is composed of atomically aligned GeTe and Sb₂Te₃ multilayers, fabricating a highly-oriented film is crucial. Furthermore, in order to utilize such material for novel devices in an industrial setting, reliable processes that allow large area deposition of uniform films is critical. Sputtering is one of the most useful, technologically friendly, and reliable methods for thin film fabrication. In this work, the growth mechanism of layered chalcogenide films is discussed.

The superlattice films were grown on various substrate materials by radio-frequency magnetron sputtering. Ge-Te and Sb-Te alloy targets were used. Before the growth of the film, Ar sputter cleaning was carried out to remove a native oxide from the substrate surface. An Sb₂Te₃ seed layer was grown initially at room temperature followed by alternate deposition of GeTe and Sb₂Te₃ layers at high temperature.

The crystal structure of the films as well as the degree of orientation were evaluated by x-ray diffraction (XRD) while the microstructure was observed by transmission electron microscopy (TEM). We found there is a strong dependency on substrate materials in terms of out-of-plane orientation of the superlattice film. On the basis of the obtained results, we will discuss the growth mechanisms of layered chalcogenide films and propose optimal growth conditions for future phase change memory applications.

This work was supported by JST-CREST (JPMJCR14F1).