Pb-additive Se-Te-In nano-chalcogenide thin films: preparation, morphological, optical analysis and material perspective for phase-change memory devices

The present research paper elaborates on the synthesis of nano-chalcogenide Se79-xTe15In6Pbx (x = 0, 1, 2, 4, 6, 8 and 10) thin films through thermal evaporation technique. Distinctive peaks in the X-ray spectrum reveal a nano-range of investigation composition further validated by HRTEM and AFM. Several magnifications of TEM micrographs illustrate honeycomb, rod and nearly spherical shapes of nano-particles. HRTEM micrograph shows that the produced particles have excellent crystallinity and nanostructure. STEM-EDX mapping reveals the formation of nano-particles with Se, Te and In only while depleted in Pb. Also the Se, Te and In contents that present in nano-chalcogenide are homogeneously distributed throughout the nano-particles. Surface morphology of primed thin films analysed with atomic force microscopy illustrates that all nano-films are dense, have irregular grain distribution, well crystallized in nano-form and are composed of nearly spherical-shaped isolated nano-particles uniformly distributed over the surface. The optical transmission spectra of these films have been recorded in the spectral range of 500 to 3000 nm at room temperature. Through Swanepoel's method, the maxima and minima of the interference fringes obtained from spectra are used to deduce film thickness and optical constants viz refractive index, absorption coefficient and extinction coefficient. Thereafter, refractive index and extinction coefficient are used to obtain other optical parameters: volume energy loss function, surface energy loss function, dielectric constant, dielectric loss function, optical conductivity and optical electronegativity. Optical bandgap (Egop) for indirect transitions is determined by Tauc relation and is found to be least for Pb = 1 at.wt.% among examined thin films. The variation in different optical parameters with Pb substitution is also reported and discussed.