Temperature dependent structural evolution and crystallization properties of thin Ge₁₅Te₈₅ film revealed by in situ resistance, x-ray diffraction and scanning electron microscopic studies

Abstract

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.

Approaching the Glass Transition Temperature of GeTe by Crystallizing Ge15Te85

“…Hence, the two crystallization events in Figure 2 could be related to the crystallization of these two crystalline phases. Support for this hypothesis is found in the study by Sengottaiyan et al [31] and others, where tellurium was found to crystallize first. This is confirmed by the XRD data shown in Figure 3.…”

Crystallite growth limits in amorphous oxides

“…The issue in XRD determination of the crystallization threshold is that the method requires a sufficient volume of crystalline material for detection. There are examples of transmission electron microscope (TEM) images which show crystallites in otherwise amorphous films below the crystallization threshold determined in the same study by XRD (Sengottaiyan et al 2019). A small population of crystallites may therefore be present in all mirrors (Glover et al 2018).…”

Thermodynamics and kinetics of glassy and liquid phase-change materials

“…The crystallization of Ge 15 Te 85 consists of two separate processes. The first crystallization process can be attributed to the crystallization of the Te constituent [104]. When the Te crystallizes, the remaining amorphous phase changes its composition towards that of GeTe.…”