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 Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> 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 Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> 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 Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> 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 Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> 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 Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> 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 Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> 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, Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> 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 Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> thin films are close to linear.