Growth and surface characterization of tin-doped indium oxide thin films.

摘要

The geometrical and electronic surface properties of In2O 3 and Sn-doped In2O3 (ITO) have been investigated. Sn-doped In2O3 is widely used as a transparent conducting oxide in flat panel displays, organic-light-emitting-diodes, solar cells, and electrochromic windows. Despite the fact that surface and interfaces are important in all these applications, a fundamental understanding of the surface properties of this material is lacking. Meaningful surface investigations are best conducted on single-crystalline samples, thus epitaxial thin films of In2O3 and ITO were grown and used as samples for the surface investigations.;This work focuses on two low-index surfaces of ITO, the non-polar (111) orientation and the (100) orientation, which, in its bulk-terminated form, is polar. The epitaxial films were grown with oxygen-plasma assisted molecular beam epitaxy (MBE) on yttria-stabilized zirconia, which exhibits a cube-on-cube epitaxy as well as a small lattice mismatch with respect to In2O 3.;The YSZ(111) substrate was characterized with Re ection-high-electron-energy-diffraction (RHEED) and Low-energy-electron-diffraction (LEED) and its surface was found to be (1x1) terminated. RHEED and LEED measurements on the substrate were possible if the substrate was kept at 300°C in order to avoid charging effects of this insulating material. RHEED exhibited 2-dimensional growth mode for the Sn-doped In2O3 thin films. Using LEED it was found that the surface of In2O3 and Sn-doped In 2O3 poses a (1x1) terminated surface. A de-convolution of X-ray core level photoemission (XPS) of In 3d peaks; into one component that is due to regular photoemission and one that is due to interaction of core holes with electronic plasmons, provided the plasmon energy, E p; From Ep the electron density n of the doped films was obtained. For an ITO film with 6.2 at% of Sn, it was found that 1/3 of the Sn atoms contribute one electron to the conduction band. Scanning-tunneling-microscopy (STM) was measured and atomic resolution was achieved. Bright and dark atomic features were assigned to a surface with bulk (1x1) termination. Density-functional-theory (DFT) calculations performed by collaborators confirmed that indium atoms are being imaged bright and dark in empty-states STM, depending on the configuration of their oxygen neighbors; bright when neighboring three-fold and dark when neighboring four-fold oxygen atoms.;Epitaxial (100)-oriented thin films of In2O3 and Sn-doped In2O3 were also grown. The YSZ substrate was characterized using RHEED and LEED and was found to have a (2x2) reconstructed surface. RHEED showed that the Sn-doped In2O3 grows in a 3-dimensional growth mode. Using LEED it was found that the surface of Sn-doped In2O3 exposes a surface with a c(1x1) termination. Considering symmetry arguments and the systematic absence of LEED spots, it was concluded that the surface can have only an oxygen termination. Due to the polarity of this orientation, it was found that unusually high concentrations of tin, ca. 15.6 at%, are needed to render the surface flat. Comparing STM measurements with DFT calculations published in the literature, it was concluded that surface oxygen atoms are dimerized, with all O dimers present for a high Sn concentration in ITO. At low Sn-concentrations the observed STM images are consistent with a model where only 2/3 of the oxygen atoms are present, again consistent with published DFT predictions. It is thus concluded that both, the Sn concentration and oxygen dimerization, are instrumental in stabilizing the polar ITO (001) surface.