Paper EM-ThM5
STM and XPS studies of ITO (001) Surfaces

Thursday, November 12, 2009, 9:20 am, Room B1

Tin-doped Indium Oxide (ITO) is a transparent conducting oxide and extensively used in flat panel displays, solar cells, and organic-light-emitting-diodes. Despite the industrial significance and versatility of this material, little is known about its surface structure.

Epitaxial In₂O₃ (001) thin films with a Sn content between 0 and 30 at% were grown on Yttria stabilized Zirconia YSZ (001) using oxygen-plasma-assisted molecular beam epitaxy (MBE). The growth was monitored with Reflection-high-energy-electron-diffraction (RHEED). Low-energy-electron-diffraction (LEED) showed well ordered surfaces.

Angle-resolved X-ray photoemission (ARXPS) performed in situ shows that Sn⁴⁺ substitutes In³⁺ in the In₂O₃ bixbyite lattice and Sn enrichment in the near-surface region. A shoulder at the Sn 3d_5/2 peak at off-normal-emission angles indicates a Sn²⁺ oxidation state. The O 1s peak shows significant asymmetry in In₂O₃ and a more symmetric peak shape with increasing Sn content. This indicates that the charge imbalance, produced by substituting Sn⁴⁺ for In³⁺ in the In₂O₃ lattice, is compensated by introducing extra oxygen in the bixbyite structure.

Preparing a flat ITO (001) surface is challenging due to its polar character. A Sn concentration of more than 15 at% is necessary to stabilize ITO (001). Empty-states Scanning Tunneling Microscopy (STM) shows terraces 100 to 400nm² in size and an overall surface roughness of 0.5nm. In agreement with previous theoretical work we find that the ITO (001) surface is oxygen terminated. Features in atomically-resolved STM images are interpreted with a model involving dimerization of surface oxygen.