Optical constants acquisition and phase change properties of Ge₂Sb₂Te₅ thin films based on spectroscopy

In this work, phase change chalcogenide Ge₂Sb₂Te₅ (GST) thin films were fabricated by magnetron sputtering. The optical properties, especially the optical constants (refractive index and extinction coefficient), of such alloys were systematically studied by investigating their thermally and photo-thermally induced switching between different phases. The results show that GST films are highly tunable in microstructure and optical constants, either by post-annealing at 160 °C, 200 °C, 250 °C and 350 °C, respectively, or by laser irradiation of 1 mW, 3 mW, 5 mW and 10 mW power with beam diameter of 7 μm at 532 nm, respectively. From the structural analysis, we can clearly observe different crystallinities and chemical bonding in the different post-treated GST films. The optical constants of GST films under various phases were obtained from spectrophotometry, by fitting their transmittance data with the Tauc-Lorentz (TL) dispersion model. The refractive index and extinction coefficient exhibit notable change upon annealing and laser irradiation, specifically at 1550 nm, from 3.85 (amorphous) to 6.5 (crystalline) in refractive index. The optical constants have been proved capable of fine tuning via the laser irradiation method. Hence, the pronounced adjustability in optical properties due to rapid and repeatable phase change render GST suitable for tunable photonic devices.