Origin of the optical absorption of In_2O_3 thin films in the visible range

摘要

Optical absorption of thin films of In_2O_3 obtained by spray pyrolysis was studied in the visible part of the spectrum, using a numerical procedure for extraction of different contributions in the absorption coefficient. One contribution from a direct forbidden (3.29 eV) and three indirect allowed transitions (2.09 eV, 3.42 eV and 3.58 eV) were identified. They were attributed to the transitions from the valence band to the two lowest minima in the conduction band separated from the valence band by gaps ≈2 eV and 3.3 eV large, in agreement with the results of ab intiio calculations done in Erhart et al (2007 Phys. Rev. B 75 153205). The fundamental absorption edge that has been observed at ≈4 eV in many earlier experimental works is attributed to the transitions Γ_8-Γ_1, where Γ_1 is the lowest minimum in the conduction band at Γ point and Γ_8 is a maximum substantially lower than the highest maximum in the valence band (Γ_4) (Walsh et al 2008 Phys. Rev. Lett. 100 167402). Therefore, the high conductivity of In_2O_3 is due to the generation of carriers by transitions from the valence band in the two lowest maxima of the conduction band, going along with a high transmission in the visible range, up to the photon energies of ≈4 eV, at which direct allowed transitions occur, leading to strong absorption.