Epitaxially Oriented Sn:In2O3 Nanowires Grown by the Vapor–Liquid–Solid Mechanism on m-, r-, a-Al2O3 as Scaffolds for Nanostructured Solar Cells

Abstract

We have grown highly directional, epitaxial Sn:In2O3 nanowires via the vapor–liquid–solid mechanism on m-, r- and a-Al2O3 between 800 and 900 °C at 1 mbar. The Sn:In2O3 nanowires have the cubic bixbyite crystal structure and are tapered with lengths of up to 80 μm, but they are inclined at ϕ ≈ 60° along one direction on m-Al2O3 while those on r-Al2O3 are inclined at ϕ ≈ 45° and oriented along two mutually orthogonal directions. In contrast, vertical Sn:In2O3 nanowires were obtained on a-Al2O3. We obtain excellent uniformity and reproducible growth of Sn:In2O3 nanowires up to 15 mm × 15 mm on m- and r-Al2O3, which is important for the fabrication of nanowire solar cells. All of the Sn:In2O3 nanowires had a resistivity of 10–4 Ω cm and carrier densities on the order of 1021 cm–3, in which case the charge distribution has a maximum at the surface of the Sn:In2O3 nanowires as a result of the occupancy of sub-bands residing well below the Fermi level, as shown via the self-consistent solution of the Poisson–Schrödinger equations in the effective mass approximation. We also show that the Sn:In2O3 nanowires are capable of light emission and exhibited room-temperature photoluminescence at 3.1 eV as a result of band-to-band radiative transitions but also at 2.25 eV as a result of donor-like states residing energetically in the upper half of the energy band gap. We discuss the advantages of using ordered networks of Sn:In2O3 nanowires in solar cell devices and issues pertaining to their fabrication.