Chalcogenide-based Ge15Te85 thin films have recently been explored for ovonic threshold switching (OTS) selector devices for vertically stackable cross-point memory applications. Despite reasonable understanding over its crystallization kinetics and threshold switching properties, the structural stability and morphological acquaintance at elevated temperatures remain key challenges. In this paper, we investigate the thermal stability, surface morphology and local structure of as-deposited amorphous Ge15Te85 thin film starting from room temperature up to 325 °C. Our experimental results reveal that upon heating, the de-vitrification is initiated in the form of localized segregation of Te atoms at 120 C, followed by crystallization of Te at ∼220 C and GeTe at ∼263 C as corroborated by temperature-dependent measurements of electrical resistance, x-ray diffraction and scanning electron microscopic studies. Furthermore, the crystalline areas of these films are characterized by the fine-grained morphology, which clearly distinguishes the segregation of crystallization of Te and GeTe microstructures. These findings elucidate a deeper understanding of the multi-phase crystallization process through morphological evidence, which will be useful towards optimization of materials for OTS selector applications. © 2019 IOP Publishing Ltd.