Si 2Sb 2Te 5 alloys have potential applications for future chalcogenide random access memory (C-RAM). The thermally induced crystallization process of Si 2Sb 2Te 5 alloys was characterized by in situ heating experiments with transmission electron microscopy (TEM). The crystallization of a Si 2Sb 2Te 5 amorphous film was initiated at around 160 °C, concomitant with a phase separation process. The crystallized product of the amorphous Si 2Sb 2Te 5 film was a nano-scale (10 nm in local-domain size) composite material consisting of amorphous Si (a-Si), crystalline Sb 2Te 3 (c-Sb 2Te 3) and crystalline Te (c-Te).