Abstract
This work reports on the modeling and precise determination of the optical parameters of mechanically exfoliated gallium selenide (ε-GaSe) thin films from both measured transmittance (T meas ) and reflectance (R meas ) at room temperature. Then, the absorption α(λ) and the reflection R(λ) coefficients were accurately calculated, at any wavelength. More general relations taking into account the multiple transmissions and the multiple reflections on both sides of the ε-GaSe film were considered. The fundamental optical constants that are the complex refractive index N(λ) and the complex dielectric constant ε(λ) were then accurately determined. Modeling the imaginary part of the latter parameter permits a precise determination of the positions of the resonances observed in the transmittance and reflectance spectra. The modeling was handled by considering an increasing number from three to five oscillators within the Lorentz model. The assignation of these resonances to the related critical points of the Brillouin zone at and above the band gap was investigated. The splitting on the valence band and the close indirect-direct nature of the configuration in the conduction band have been considered simultaneously. For GaSe material, the refractive index varies from 2.8 and 3.3 with strong peaks above the band gap corresponding to resonances at 3.2 eV and 3.7 eV. The analysis of the oscillator strengths and the lifetimes of the observed transitions allowed to clearly identifying the indirect nature of the fundamental band gap.
Its value of 1.97 eV is largely separated from the direct band gap of about 230 meV. The transition lifetimes were found to vary between 0.6 fs and 24 fs.