Strong inter-conduction-band absorption in heavily fluorine doped tin oxide

The optical, electrical and structural properties of thin film tin oxide (TO), F-doped tin oxide (FTO; nF ≈ 6 × 1020 cm−3) and highly F-doped tin oxide (hFTO; nF ≈ 10 × 1020 cm−3), grown by spray pyrolysis technique, are studied by atomic force microscopy, Hall effect, X-ray fluorescence and transmission/reflection measurements. The resistivity (ρ = 32 × 10−4 Ω cm for intrinsic tin oxide) shows intriguing characteristics when F concentration nF is increased (ρ = 6 × 10−4 Ω cm for FTO but 25 × 10−4 Ω cm for hFTO) whereas the carrier concentration is almost constant at high F concentration (nc ≈ 6 × 1020 cm−3 for FTO and hFTO). Thus, F seems to act both as a donor and a compensating acceptor in hFTO. The high carrier concentration has a strong effect on the optical band-edge absorption. Whereas intrinsic TO has room-temperature band-gap energy of Eg ≈ 3.2 eV with an onset to absorption at about 3.8 eV, the highly doped FTO and hFTO samples show relatively strong absorption at 2-3 eV. Theoretical analysis based on density functional calculations of FTO reveals that this is not a defect state within the band-gap region, but instead a consequence of a hybridization of the F donor states with the host conduction band in combination with a band filling of the lowest conduction band by the free carriers. This allows photon-assisted inter-conduction band transitions of the free electrons to energetically higher and empty conduction bands, producing the below-gap absorption peak.