Intrinsic evolutions of dielectric function and electronic transition in tungsten doping Ge₂Sb₂Te₅ phase change films discovered by ellipsometry at elevated temperatures

Tungsten (W) doping effects on Ge₂Sb₂Te₅ (GSTW) phase change films with different concentrations (3.2, 7.1, and 10.8%) have been investigated by variable-temperature spectroscopic ellipsometry. The dielectric functions from 210 K to 660 K have been evaluated with the aid of Tauc-Lorentz and Drude dispersion models. The analysis of Tauc gap energy (E_g) and partial spectral weight integral reveal the correlation between optical properties and local structural change. The order degree increment and chemical bond change from covalent to resonant should be responsible for band gap narrowing and electronic transition enhancement during the phase change process. It is found that the elevated crystalline temperature for GSTW can be related to improved disorder degree. Furthermore, the shrinkage of E_g for GSTW should be attributed to the enhanced metallicity compared with undoped GST.