Refined analysis of absorption spectra of CdS thin films

We demonstrate that optical absorption spectra of CdS thin films in the visible/near ultraviolet range can be precisely described by four different contributions of electron transitions from Gamma points of three branches of the valence band maxima (Gamma(7), Gamma(9), Gamma(7'),) to the lowest two minima in the conduction band (Gamma(e), Gamma(e')), in accordance with the theoretical results of the work (Lew Yan Voon et al 1996 Phys. Rev. B 53 10703). By using the method of successive extractions of different contributions in the absorption coefficient (alpha), four contributions have been found, attributed to the following electron transitions: (Gamma(7') -> Gamma(e)), (Gamma(9) -> Gamma(e)), (Gamma(7) -> Gamma(e)) and (Gamma(7) -> Gamma(e')). Corresponding bandgaps for films annealed at 400 degrees C in CdCl2 obtained in this work are 2.20 eV, 2.30 eV, 2.43 eV and 3.88 eV, that are close to the theoretical values for bulk CdS 2.24 eV, 2.28 eV, 2.43 eV and 3.8 eV. Bandgaps for as grown films are somehow higher because of the confinement effect in microcrystals of a smaller size. For the films annealed in air due to creation of Cd vacancies, the bandgap decreases down from the bulk values; thus, the values of bandgaps for films annealed at 400 degrees C in air are: 1.82 eV, 2.11 eV, 2.25 eV and 3.87 eV. Strong absorption onset, considered as the fundamental absorption edge, corresponds to direct allowed transitions (Gamma(7) -> Gamma(e')) at photon energies of about 3.8 eV. Absorption due to direct allowed transitions (Gamma(7) -> Gamma(e)) at photon energies of about 2.3 eV are usually reported as the optical bandgap of CdS saturates at relatively low values alpha approximate to 5 x 10(4) cm(-1).