Influence of Crystallization Degreeon the Optical Parameters Dispersion of Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> Thin Films for Phase-Change Memory
Active research of the Ge2Sb2Te5 material (GST225) is connected with the possibility of creating multi-level non-volatile elements for high-speed integrated optical functional circuits. The principle of multi-level recording in these devices is based on the formation of regions in Ge2Sb2Te5 thin films with partial crystallization and significantly different optical properties. To predict the parameters of the effect that initiates phase transformations and reliably ensure a reversible transition between a set of logical states, it is necessary to have reliable information about the optical characteristics of Ge2Sb2Te5 thin films in states with different degrees of crystallization and the conditions for their production. Thereby, the purpose of this research was to determine the influence of phase state Ge2Sb2Te5 films on the extinction coefficient ( k ) and refractive index ( n ) and study changes in the values of optical band gap depending on the annealing temperature have been considered. The obtained samples of Ge2Sb2Te5 thin films were examined using atomic force microscopy, X-ray phase analysis, and energy dispersion X-ray phase analysis, and energy-dispersive microanalysis to determine the film thickness, morphology, phase state, and composition of the films. The ellipsometry method was used to obtain the spectra of ellipsometric angles ψ and Δ (the amplitude and phase components of light wave) and the parameters n and k were determined. The influence of layer and mathematical models on the calculation dispersion of optical parameters of Ge2Sb2Te5 films, have been considered. It was found a significant increase in the values of n and k on the wavelength of 1550 nm when annealing above 200 °C has been found. It has been shown that in the amorphous state, Ge2Sb2Te5 thin films have an optical band gap equal to 0,71 eV, and in the crystalline = 0,47 eV. It was determined that the dependences of the refractive index, extinction coefficient, and optical band gap on the degree of crystallization of Ge2Sb2Te5 thin films are close to linear.