In Situ Monitoring of Crystallinity of FeSe2 Thin Films During Thermal Annealing and the Annealing Effects on the Structural, Optical and Dielectric Properties

Herein stacked layers of iron selenide (FeSe₂) thin films are deposited by the physical evaporation technique and thermally annealed. An in situ monitoring of the crystallinity during the annealing process has shown that the crystallinity is reached at 100 °C. The crystallinity of the films that preferred the orthorhombic phase is enhanced with increasing annealing temperature. Evidences about the improved crystallinity are presented by the increased crystallite and grain sizes, decreased microstrain values, decreased stacking faults, and decreased defect densities with increasing annealing temperature. Optical investigations have shown impressive effect of the annealing process on the optical reflectance, optical contrast, and light absorbability. Namely, respective improvement percentages exceeding 170%, 64%, and 140% is achieved near E≈2 eV for samples annealed at 200 °C for 20 min. Both direct and indirect optical transitions are dominant in the film. In addition the annealing increased the dielectric constant in the spectral range of 1.17–4.20 eV. Maximum dielectric enhancement by 214% is reached near ≈2.10 eV. Moreover, the annealing process increases the optical conductivity and drift mobility of the FeSe₂ films. The improvement in the crystallinity that resulted in enhanced optical properties makes the thermally annealed FeSe₂ films promising for optoelectronic technology applications.