The Effects of Sb, In, and Sn Doping on the Optical Properties of Tin Dioxide (Part A)

Abstract

<p> This study was focused on the absorption spectra of tin dioxide thin films on optical quartz substrates. The films were doped with antimony from zero to ten percent, indium from zero to ten percent, and intrinsically, by heating undoped samples in vacuum and air. The surface resistances were also measured.</p> <p> The results for antimony doping show that the energy required for electron transitions from valence to conduction bands, the associated phonon energies, and the optical absorption by free carriers all increase while resistance decreases with increased doping. These results are consistant with antimony acting as a donor in SnO2 and elevating the Fermi level, which is in the conduction band for the undoped material, to a higher level, thus increasing the free carrier concentration. The results possibly also indicate a strain on the lattice caused by doping.</p> <p> The results for indium doping show a similar increase in energies along with a decrease in optical absorption by free carriers and an increase in resistance with increased doping. The indium acts as an acceptor and, in so doing, causes the Fermi level to drop into the valence band, at the doping levels used in this study. This is probably due to the formation of an acceptor band above or overlapping the valence band and resulting in a reduction of the free carrier concentration. The increase in phonon energy indicates that the doping imposes a strain on the lattice.</p> </p> Heating undoped films in vacuum appears to drive off oxygen resulting in reduced resistance and therefore higher free carrier concentration due to lattice defect doping, and reduced valence to conduction band transition energy possibly due to the formation of a conduction band in the forbidden band-gap. The changes were reversible by re-heating in air.</p>