Determination of Optical Properties of Undoped Amorphous Selenium (a-Se) Films by Dielectric Modeling of Their Normal-Incidence Transmittance Spectra

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Normal-incidence specular transmittance T_exp (?) of undoped amorphous selenium ( a -Se) films of several thicknesses (0.25-1 ?m) thermally evaporated on thick microscopic glass-slide substrates upheld at temperatures near 30 o C (below glass transition temperature of undoped Se) has been measured at room temperature in the spectral wavelength?range 300-1100 nm. Above a threshold wavelength ?_c (~ 600 nm), their as-measured T_exp (?)-? curves display transmittance values near that of glass substrates and exhibit well-resolved interference-fringe maxima and minima, signifying good uniformity of fabricated a -Se films. Below?_c, the T_exp (?)-? curves decline progressively to zero transmittance, preceded by a tailing-like trend of possible structural disorder origin. The spectral dependencies of optical constants n(?) and?(?) of the studied a -Se films on ? were retrieved from iterative curve-fitting of their T_exp (?)-? data to theoretical T_theor (?)-formulations of an air-supported {thin film/thick substrate}-stack using the O’Leary-Johnson-Lim (OJL) interband-transition dielectric model, combined with a dielectric constant , representing the dielectric background at very high photon energies (at spectral wavelengths much smaller than measured). Regardless of the number of observed interference fringes, reliable simulation to the as-measured normal-incidence T_exp (?)-? spectra has been achieved, but were remarkably curve-fitted if we presume that a thin roughness layer (<5 nm) of selenium was formed on top of the fabricated undoped a -Se films. The retrieved n(?)-? spectra display a broad peak around??0.52 ?m, below which n(?) was found to vary with wavelength in accordance with a Sellmeier-like (Wemple-DiDominco) single-oscillator (normal) dispersion formula, with a static index of refraction n(E=0)=?(?_s )?2.42, where ?_s is the static dielectric constant of the material at zero photon energy , and an oscillator “average” bandgap energy of E_o?3.93 eV?2 E_g^opt, the optical (Tauc) bandgap energy. The values of E_g^opt of studied undoped a -Se films deduced from Tauc-like plots were around 1.92 eV (± 0.02 eV) and the retrieved values of OJL band-tailing energy parameter ? (Urbach-tail parameter ?_U) was found to be in the range 40-50 meV, slightly dependent on film thickness.