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.

Band gap widening with time induced by structural relaxation in amorphous Ge2Sb2Te5 films

“…The e 2 peak around 2 eV is ascribed to the Te p to Sb p states transition. 22 At higher energies, e 2 of the aged film decreases more steeply than the as-deposited film resulting in a shape more similar to that of the crystalline state. 21 Furthermore, the absorption edge at lower energies can be seen to evolve according to a blue shift for increasing time.…”

“…These results are comparable with literature data reporting broad peaks for e 1 and for e 2 around 1.2 and 2.2 eV, respectively. 21,22 Optical functions obtained immediately after deposition and 173 h after deposition, referred to as the aged film, are both reported.…”

Pulsed laser deposited GeTe-rich GeTe-Sb2Te3 thin films

“…For the fabrication of GST thin films, magnetron sputtering12122, thermal (flash) evaporation2324, chemical vapor deposition using metal-organic precursors2526 or also atomic layer deposition27 can be used. In line with mentioned techniques, pulsed laser deposition (PLD) is a suitable deposition method for the thin films growth too.…”

In situ analysis of cadmium sulphide chemical bath deposition by an optical fibre monitor

“…Woollam Co. M-2000UI rotating compensator ellipsometer in conjunction with CompleteEASE software (version 3.81), for the generation of optical models. A mean squared error (MSE) function was used to assess the quality of the fit of the optical models to the experimental data [8].…”

“…This model was used as a guide in this study to create a model for polycrystalline material. For an as-deposited CdS layer, three oscillator functions were used to model the optical constants: a Cody Lorentz oscillator [8] and two Lorentzian oscillators.…”