Evaluation of spectral dispersion of optical constants of a-Se films from their normal-incidence transmittance spectra using Swanepoel algebraic envelope approach

Spectral dispersions of index of refraction n(λ ) and extinction coefficient κ (λ ) of undoped amorphous selenium ( a -Se) films of three thicknesses ( d ≈ 0.5, 0.75, and 1.0 µm) were evaluated by analyzing experimental room-temperature normal-incidence transmittance-wavelength ( T_exp(λ ) - λ ) data ( λ = 400–1100 µm) of their air-supported a -Se film/thick glass slide-stacks using Swanepoel’s transmission envelope theory of uniform films. Above a wavelength λ _c ≈ 640 nm , as-measured T_exp(λ ) - λ spectra display well-resolved maxima and minima, with minor shrinkage in transparent and weak absorption regions (750–1100 nm). Below λ _c , a smeared sharp decline of T_exp(λ ) with decreasing λ , signifying strong absorption in a -Se films and existence of band-tail localized states. For λ > λ c , the n (λ ) - λ data retrieved from algebraic envelope procedures followed a Sellmeier-like dispersion relation, with the best-fit values of high-frequency dielectric constant ε _∞ ≈ 4.9 , static index of refraction n_0 = n( E → 0) ≈ 2.43 , and resonance wavelength λ _0 ≈ 490 nm , which may be assigned to onset of photogeneration in a -Se. Urbach-like dependency of absorption coefficient α (hν) of a -Se films on photon energy hν was realized with an Urbach-tail breadth of 85 meV. All achieved optical parameters were found to be slightly dependent on film thickness. Findings of present algebraic analysis are consistent with reported literature results obtained on the basis of other optical analytical approaches.