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≈6×1020cm−3) and highly F-doped tin oxide (hFTO; nF≈10×1020cm−3), grown by spray pyrolysis technique, are studied by atomic force microscopy, Hall effect, X-ray fluorescence and transmission/reflection measurements. The resistivity (ρ=32×10−4Ωcm for intrinsic tin oxide) shows intriguing characteristics when F concentration nF is increased (ρ=6×10−4Ωcm for FTO but 25×10−4Ωcm for hFTO) whereas the carrier concentration is almost constant at high F concentration (nc≈6×1020cm−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≈3.2eV with an onset to absorption at about 3.8eV, the highly doped FTO and hFTO samples show relatively strong absorption at 2–3eV. 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.