Extracting the optical parameters of thermally evaporated Se film by modelling of transmittance spectra: Effect of heat treatment

Abstract

Thin Se films with an appropriate thickness were deposited by the thermal evaporation technique on glass substrates that are grasped at a temperature of ∼300 K. The as-deposited amorphous Se thin films were heat-treated in N2 ambient for 1 h at different annealing temperatures that ranged from 323 to 363 K. The transmittance spectra of green and annealed Se films have been measured at vertical incidence angle along the wavelength range of 300–2500 nm. For the first time, the dielectric function (OJL2) model modified by O’Leary, Johnson, Lim was used alone to fit very well the transmission spectra along the whole wavelength range and extract the optical parameters of Se films. For each value of photon energy, the value of the complex dielectric function of all examined Se films was obtained. By simulating, the model parameters (absorption coefficient α, real and imaginary dielectric constants, refractive index n and optical bandgap Eg) and the film thickness have been deduced. We found that the index of refraction increased by increasing the annealing temperatures. Optical bandgap energy of Se films changes slowly from 2.0 eV of virgin a-Se film to 1.6 eV as the annealing temperature was raised to 343 K and then increased again from 1.93 eV to 2.2 eV for films annealed at temperatures that ranged from 348 to 363 K. By heat treatment crystallization of a-Se films occurred at and above the annealing temperature of 348 K that was confirmed by XRD patterns. The amorphous film showed a smooth surface consisting of very small grains of 40 nm size and low roughness of 1.9 nm. The grain size decreases, while roughness increased by increasing the annealing temperatures.