Optical properties and phase - change transition in Ge 2 Sb 2 Te 5 flash evaporated thin films studied by temperature dependent spectroscopic ellipsometry

We studied the optical properties of as-prepared (amorphous) and thermally crystallized (fcc) flash evaporated Ge2Sb2Te5 thin films using variable angle spectroscopic ellipsometry in the photon energy range 0.54 4.13 eV. We employed Tauc-Lorentz model (TL) and CodyLorentz model (CL) for amorphous phase and Tauc-Lorentz model with one additional Gaussian oscillator for fcc phase data analysis. The amorphous phase has optical bandgap energy Eg = 0.65 eV (TL) or 0.63 eV (CL) slightly dependent on used model. The Urbach edge of amorphous thin film was found to be ~ 70 meV. Both models behave very similarly and accurately fit to the experimental data at energies above 1 eV. The Cody-Lorentz model is more accurate in describing dielectric function in the absorption onset region. The thickness decreases ~ 7 % toward fcc phase. The bandgap energy of fcc phase is significantly lower than amorphous phase, Eg = 0.53 eV. The temperature dependent ellipsometry revealed crystallization in the range 130 150 °C. The bandgap energy of amorphous phase possesses temperature redshift -0.57 meV/K (30 110 °C). The crystalline phase has more complex bandgap energy shift, firstly +0.62 meV/K (150 180 °C) followed by -0.29 meV/K (190 -