In situ study of the crystallization in GeTe0.26 Se0.74 thick film by synchrotron X-ray diffraction

Crystallization in unencapsulated GeTe0.26Se0.74 amorphous 3 µm-thick films produced by thermal co-evaporation technique was studied by in situ temperature-dependent synchrotron powder X-ray diffraction from room temperature up to 418 °C and back to 30 °C. The experimental results obtained with a continuous heating rate of 2 °C/min under a protective nitrogen atmosphere showed that the crystallization process took place in several steps i) total crystallization of the amorphous phase to an intermediate crystallized phase, (ii) total transformation of the intermediate phase to a second crystallized phase. The intermediate phase corresponds to a solid solution with a metastable α-GeTe-like rhombohedral polar structure (R3m) presenting a uniaxial negative thermal expansion along the c-axis. The onset of crystallization to the polar structure is around 270 °C. The high-temperature phase has a stable hexagonal structure with a P63mc space group. When cooling, until back to room temperature, the hexagonal structure is conserved. The effect of a changing heating rate from 2° to 0.1°C/ min was also investigated. In this context, the crystallization scheme is modified. •Several-step crystallization process for GeSe0.74Te0.26 amorphous film in the 20–418 °C range.•At low temperatures, crystallization of a metastable R3m structure with no phase segregation.•Existence of a c-axis negative thermal expansion (in the hexagonal setting) as for α-GeTe.•Transformation of the polar R3m phase to the stable P63mc structure with heating.•The crystallization temperature and the existence domain of the R3m phase increase with the heating rate.