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

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 degrees C at 1 mbar. The Sn:In2O3 nanowires have the cubic bixbyite crystal structure and are tapered with lengths of up to 80 mu m, but they are inclined at phi approximate to 60 degrees along one direction on m-Al2O3 while those on r-Al2O3 are inclined at phi approximate to 45 degrees 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 x 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) Omega cm and carrier densities on the order of 10(21) 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-Schrodinger 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.