Retrieval of optical constants of undoped amorphous selenium films from an analysis of their normal-incidence transmittance spectra using numeric PUMA method

Abstract

The as-measured room-temperature normal-incidence transmittance–wavelength (T exp(λ) − λ) spectra of undoped amorphous selenium (a-Se) films, which were thermally deposited onto glass slides, exhibit well-resolved interference-fringe maxima and minima λ > λ c (≈630 nm), below which they fall rather sharply to zero transmittance. In the transparency and weak absorption region, the maxima transmittance is close to the substrate transmission, implying good uniformity of the a-Se films. The geometric thicknesses of the films and the spectral dependency of their optical constants n(λ) and κ(λ) were retrieved by analyzing the T exp(λ) − λ spectra by the PUMA method, based on the full T(λ)-formula for air-supported {uniform thin film/thick transparent substrate}-stacks, without the need for dispersion relations in prior and regardless of the number of interference fringes. The n(λ) − λ data of the transparency and weak absorption regions were found to fit the Wemple–DiDomenico and modified Sellmeier dispersion relations. The ω-dependency of the absorption coefficient α(ω) in the absorption-edge region has been analyzed in view of various interband transition models and was found to be nearly described by the linear power-law relation $$\alpha \hbar \omega \propto \hbar \omega - E_{\text{g}}$$ , with E g ≈ 2.2 eV over a broad spectral range.