Impact of different annealing conditions on the physical and optical properties of GST and GSST thin films

Germanium–antimony–telluride (GST) and selenium-doped GST (GSST) alloys are widely investigated as phase-change materials for photonic and memory applications due to their tunable crystallization and optical properties. While encapsulation is employed to mitigate degradation, the chemical and structural instability of uncapped GST and GSST thin films, particularly at high temperatures, remains poorly quantified. We investigate the thermal stability, crystallization behavior, and optical response of thermally evaporated GST and GSST films with different selenium contents annealed at 200 °C and 350 °C under low-vacuum (LV) and high-vacuum (HV) conditions. Thickness measurements, energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and optical transmittance were used to decouple thermal effects from oxidation assisted degradation mechanisms. LV annealing promotes significant thickness loss, stoichiometric drift—dominated by tellurium depletion, and disrupted crystallization, especially at 350 °C. In contrast, HV annealing strongly suppresses chemical instability, preserves film thickness within densification limits, and restores crystallization behavior consistent with intrinsic phase evolution. Although GSST films exhibit higher optical transmittance than GST after crystallization, selenium incorporation does not prevent degradation under conditions of high-temperature and oxidizing species availability. These results highlight the critical role of the annealing environment in controlling the stability and performance of uncapped GST-based films at high temperatures.