Influence of the Degree of Crystallinity on the Dispersion of the Optical Parameters of Ge2Sb2Te5 Phase-Change Memory Thin Films

Abstract—Extensive studies of Ge2Sb2Te5 material are associated with the possibility of producing multilevel nonvolatile elements for high-speed integrated optical functional circuits. The principle of multilevel recording in such devices is based on the formation of partially crystallized regions with substantially different optical properties in Ge2Sb2Te5 thin films. To predict the parameters of the effect initiating phase transformations and to reliably provide a reversible transition between many logic states, it is necessary to have reliable data on the optical characteristics of Ge2Sb2Te5 thin films in states with different degrees of crystallinity and on the conditions of attainment of such states. In the study, the influence of the phase state of Ge2Sb2Te5 films on the extinction coefficient and refractive index and variations in the optical band gap in relation to the temperature of heat treatment are investigated. Ge2Sb2Te5 thin-film samples are examined by means of atomic-force microscopy, X-ray phase analysis, and energy-dispersive microanalysis to determine the film thickness, morphology, phase state, and composition. By spectroscopic ellipsometry, the spectra of the ellipsometric angles ψ and Δ (the amplitude and phase components of the light wave) are obtained, and the extinction coefficient and refractive index are determined. The influence of the layer models and mathematical models on calculation of the dispersions of the optical parameters of Ge2Sb2Te5 films is considered. A substantial increase in the extinction coefficient and refractive index at the wavelength 1550 nm on heat treatment at temperatures higher than 200°С is established. It is shown that the optical band gaps of Ge2Sb2Te5 thin films in the amorphous and crystalline states are 0.71 and 0.47 eV, respectively. It is found that the dependences of the extinction coefficient, refractive index, and band gap on the degree of crystallinity of Ge2Sb2Te5 thin films are close to linear.