Electronic band-edge structure and linear optical response of rutile SnO2 and TiO2 are studied and compared, employing a fully relativistic full-potential linearized augmented plane wave (FPLAPW) method [1] within the local density approximation (LDA). A quasi-particle model corrects the LDA band-gap energies to Eg(SnO2) = 3.3 eV and Eg(TiO2) = 3.0 eV, which is then used to fit the on-site Coulomb self-interaction correction (SIC) potential LDA+U. We show that inclusion of this k-dependent SIC-like potential correction, as well as inclusion of polaronic screening are of outermost importance for accurately determining the electronic structure and its band-edge effective electron and hole masses in these transition metal-oxides. Also, spin-orbit interaction affects the energy dispersion of the second and third uppermost valence bands. The calculated complex dielectric function e(w) = e1(w) + ie2(w) and its high-frequency constant e(infinity) show very strong anisotropic optical linear response of SnO2. The electron-optical phonon interaction affects the static dielectric constant e(w~0) especially for TiO2 [2]. The calculated absorption coefficient a(w) reveals very modest optical band-edge absorption in the photon energy region Eg < hw < ~ Eg+D (with D = 0.8 eV for SnO2 and D = 0.5 eV for TiO2) originating from symmetry-forbidden optical transition at the Gamma-point. The strong onset to absorption at Eg+D is due to transitions from energetically lower lying valence bands with flat energy dispersion. The main difference between SnO2 and TiO2 is the presence of the unoccupied low-energy Ti 3d conduction-band states which yield a very strong optical absorption hw > Eg+D in TiO2.
Acknowledgements: Supported by the Swedish Research Council (VR), Swedish Foundation for International Cooperation in Research and Higher Education (STINT), and the Brazilian National Research Council (CNPq).
References:
[1] P. Blaha, K. Schwarz, G.K.H. Madsen, D. Kvasnicka. and J. Luitz, WIEN2k, An APW + local orbitals program for calculating crystal properties (K. Schwarz, Techn. U. Wien, Austria), 2001. ISBN 3-9501031-1-2.
[2] C. Persson and A. Ferreira da Silva, Appl. Phys. Lett. 86, 231912 (2005).
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