Structural and optical properties of selenium-rich CdSe (SR-CdSe) thin films prepared by thermal evaporation
are studied as a function of annealing temperature. X-ray diffraction (XRD) patterns show that
the as-prepared films were amorphous, whereas the annealed films are polycrystalline. Analyzing XRD
patterns of the annealed films reveal the coexistence of both (hexagonal) Se and (hexagonal) CdSe crystalline
phases. Surface roughness of SR-CdSe films is measured using atomic force microscope (AFM).
Analyses of the absorption spectra in the wavelength range (200–2500 nm) of SR-CdSe thin films indicates
the existence of direct and indirect optical transition mechanisms. The optical band gap (Eg) of
as-prepared film is 1.92 and 2.14 eV for the indirect allowed and direct allowed transitions respectively.
After annealing, the absorption coefficient and optical band gap were found to decrease, while the values
of refractive index (n) and the extinction coefficient (kex) increase. The dispersion of the refractive index is
described using the Wimple–Di Domenico (WDD) single oscillator model and the dispersion parameters
are calculated as a function of annealing temperature. Besides, the high frequency dielectric constant
(ε∞) and the ratios of the free carrier concentration to its effective mass (N/m*) are studied as a function
of annealing temperature. The results are discussed and correlated in terms of amorphous-crystalline
transformations.