Optical properties and phase change transition in Ge2Sb2Te5 flash evaporated thin films studied by temperature dependent spectroscopic ellipsometry

Abstract

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 (TL) model and Cody–Lorentz (CL) model for amorphous phase and TL model with one additional Gaussian oscillator for fcc phase data analysis. The amorphous phase has optical bandgap energy Egopt=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 CL 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, Egopt=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, first +0.62 meV/K (150–180 °C) followed by −0.29 meV/K (190–220 °C). The optical properties (refractive index, extinction coefficient, and optical bandgap energy) of as-prepared and fcc flash evaporated Ge2Sb2Te5 thin films are very similar to those values previously reported for sputtered thin films.