| Abstract: |
This study follows how controlled isochronal annealing changes the structure and optical response of an antimony-bearing chalcogenide glass, Ge₁₅Sb₂₀Se₆₅, prepared by melt quenching and deposited as thin films by thermal evaporation. Films about 600 nm thick were annealed for two hours in argon at 373, 423, 473 and 523 K and compared with the as-deposited state. Differential scanning calorimetry on the parent glass gave a glass transition near 468 K and a crystallization onset near 553 K at 10 K/min, with activation energies close to 190 kJ/mol for both processes. X-ray diffraction showed an amorphous structure up to 473 K and incipient Sb₂Se₃-type crystallinity (about 12%, crystallite size 18 nm) at 523 K. Raman spectra indicated a steady fall in the proportion of Se–Se homopolar bonding, with the Se–Se to network band ratio dropping from 0.55 to 0.29 at 473 K. Over the same range the refractive index at 1550 nm rose from 2.62 to 2.77, the Tauc gap narrowed from 1.46 to 1.37 eV, and the Urbach energy decreased from 88 to 63 meV. At 523 K the index reached 2.86 and the gap 1.30 eV, but the Urbach energy rose to 82 meV as crystallites introduced disorder and scattering. A single-oscillator analysis showed that the dispersion energy and static index grew together while the oscillator energy fell. The results link network relaxation, bond rearrangement and early crystallization to quantifiable optical tuning, and define a processing window for index adjustment without major loss. |
