Electronic structures and excitonic transitions in nanocrystalline iron-doped tin dioxide diluted magnetic semiconductor films: An optical spectroscopic study

Nanocrystalline iron-doped tin dioxide (Sn_1-xFe _xO₂) films with x from 0 to 0.2 were prepared on c-sapphire substrates by pulsed laser deposition. X-ray diffraction and Raman scattering analysis show that the films are of the rutile structure at low compositions and an impurity phase related to Fe₂O₃ appears until the x is up to 0.2, suggesting the general change of lattice structure due to the Fe ion substitution. The dielectric functions are successfully determined from 0.0248 to 6.5 eV using the Lorentz multi-oscillator and Tauc-Lorentz dispersion models in the low and high photon energy regions, respectively. With increasing Fe composition, the highest-frequency transverse optical phonons E_u shifts towards a lower energy side and can be well described by (608 - 178x) cm^-1. From the transmittance spectra, the fundamental absorption edge is found to be decreased with the Fe composition due to the joint contributions from SnO₂ and Fe₂O ₃. It can be observed that the doped films exhibit evident excitonic excitation features, which are strongly related to the Fe doping. Among them, the 6A_1g → 4T_2g transition contributes to the onset of optical absorption. Moreover, the remarkable intensity reduction and a red-shift trend with the doping composition, except for the pure film, can be testified by the photoluminescence spectra. It can be concluded that the replacement of Sn with the Fe ion could induce the 2p-3d hybridization and result in the electronic band structure modification of the Sn _1-xFe_xO₂ films.