Polarization dependence of the optical response in SnO2 and the effects from heavily F doping

The optical properties of intrinsic SnO2 (TO) and fluorine doped (FTO) are characterized in terms of the dielectric function epsilon(h omega) = epsilon(1) (h omega) + i epsilon(2)(h omega) by electronic structure calculations. The intrinsic TO shows intriguing absorption characteristics in the 3.0-8.0 eV region: (i) the low energy region of the fundamental band gap (3.2<h omega<3.9 eV), the optical transitions Gamma(+)(3) -> Gamma(+)(1) (valence-band maximum to conduction-band minimum) is symmetry forbidden, and the band-edge absorption is therefore extremely weak. (ii) In the higher energy region (3.9<h omega<5.1 eV) the Gamma(-)(5) -> Gamma(+)(1), transitions (from the second uppermost valence band) is strongly polarized perpendicular to the main c axis. (iii) Transitions with polarization axis parallel to c axis are generated from Gamma(-)(2) -> Gamma(+)(1) transitions (from the third uppermost valence bands), and dominates at high energies (5.1<h omega eV). Heavily F doped TO (FTO) with doping concentrations n(F) = 4 x 10(20) cm(-3) changes the absorption significantly: (iv) Substitutional F-O generates strong inter-conduction band absorption at 0.8, 2.2, and 3.8 eV which affects also the high frequency dielectric constant epsilon(infinity). (v) Interstitial F-i is inactive as a single dopant, but act as a compensating acceptor in highly n-type FTO. This explains the measured non-linear dependence of the resistivity with respect to F concentration. (C) 2009 Elsevier B.V. All rights reserved.