Doping and Conductivity Limitations in Sb:SnO2 Nanowires Grown by the Vapor Liquid Solid Mechanism

Abstract

Sb doped SnO2 nanowires have been grown via the vapor liquid solid mechanism on fused SiO2 at 800 °C and 1 mbar under a flow of Ar and O2 by using an excess of metallic Sb in conjunction with Sn. We obtain highly crystalline Sb:SnO2 nanowires with diameters of 100 nm, lengths up to 100 μm, and a tetragonal rutile crystal structure that contains Sb donor impurities as confirmed by Raman spectroscopy. The Sb:SnO2 nanowires have a maximum carrier density of 8 × 1018 cm–3 and conductivity of 2500 S/m at 300 K determined from THz conductivity spectroscopy and the Hall effect, which are significantly higher than 3 × 1016 cm–3 and 11 S/m in undoped SnO2 nanowires. The one-dimensional electron gas charge distribution has a maximum at the core but extends all the way up to the surface as shown via the self-consistent solution of the Poisson-Schrödinger equations in contrast to the case of undoped SnO2 where the surface depletion leads to a confinement of the charge distribution to the core by taking into account the energetic position of the Fermi level with respect to the conduction band at the surface. We show that the Sb impurities are not incorporated into the SnO2 lattice through the Au particles, while higher growth temperatures between 900 to 1000 °C and/or lower growth pressures between 10–2 to 10–1 mbar do not result into the incorporation of Sb impurities in the SnO2 nanowires. This is attributed to the depletion of metallic Sb during the temperature ramp, before the onset of one-dimensional growth and/or the re-evaporation of Sb impurities arriving on the surface of the SnO2 nanowires and implies a fundamental doping limitation that is challenging to overcome especially in view of the fact the one-dimensional growth was suppressed under a significant excess of Sb.