Structural ordering and optical band gap evolution in amorphous Sb₂Se₃ thin films under low-temperature annealing

Low-temperature thermal annealing offers an effective strategy for tailoring the local atomic structure of amorphous chalcogenides without inducing crystallization; however, the mechanisms governing the early stages of structural relaxation remain poorly understood. Here, stoichiometric bulk Sb₂Se₃ was synthesized and employed as the evaporation source to deposit ∼40 nm thick amorphous Sb₂Se₃ films, which were subsequently annealed at 140 °C to investigate the relationship between structural relaxation and optical response. X-ray diffraction confirmed that both as-deposited and annealed films remained predominantly amorphous, while annealing enhanced intermediate-range ordering without forming crystalline phases. Raman spectra exhibited characteristic Sb–Se vibrational modes at 190, 209, 253, and 295 cm⁻ 1 , whose narrowing and slight frequency shifts provide evidence of thermally induced local atomic rearrangement and enhanced short-range ordering. SEM and AFM revealed homogeneous morphology with only minor localized surface modifications, while EDX confirmed preserved film stoichiometry. These results support a structural-relaxation mechanism involving redistribution of free volume, increased average inter-chain spacing, probable fragmentation of large nanovoids into numerous smaller pores, and the development of local interfacial mechanical stresses within the amorphous matrix. These structural rearrangements reduced the optical band gap from 1.40 to 1.36 eV without changing the Urbach energy, indicating that the electronic structure evolved through structural relaxation rather than increased structural disorder. This work establishes a direct structure–property relationship between thermally induced atomic-scale relaxation and the optical response of amorphous Sb₂Se₃, demonstrating that low-temperature annealing provides an effective route for engineering functional chalcogenide thin films for optoelectronic and phase-change memory applications.