Surface and Bulk Electronic Structure of Bixbyite Transparent Conducting Oxides

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Subsolidus phase relationships within the ZnO-In<sub>2</sub>O<sub>3</sub>-SnO<sub>2</sub> system at 1275 <sup>o</sup>C were established by conventional solid state reaction methods. No new compounds or structures were observed within the ternary diagram. Equilibrium in the ZnO-corner, between the homologous compounds and the spinel phase, was found difficult to achieve. This problem was overcome by utilizing mixtures of the k=11 (ZnO) <sub>11</sub>In<sub>2</sub>O<sub>3</sub> homologous series compound plus spinels with high indium content. The defect chemistry and surface electronic properties of the zinc- and tin- co-doped In<sub>2</sub>O<sub>3</sub> , In<sub>(2-2x)</sub>Sn<sub>x</sub>Zn<sub>x</sub>O<sub>3</sub> (x=0-0.40) or ZITO, were investigated. The ZITO material is an n-type conductor and carriers are generated via an inherent cation off-stoichiometry of tin donors to zinc acceptors, i.e., n=[Sn<sub>In</sub><sup>*</sup>]-[ Zn<sub>In</sub><sup>'</sup>]> 0. pO<sub>2</sub>-dependent conductivity and thermopower measurements completed on a bulk specimen of the terminal ZITO composition showed the presence of the Frank and Kostlin neutral defect cluster, (2Sn<sub>In</sub><sup>*</sup>O<sub>i</sub><sup>' '</sup>)<sup>x</sup>, which is also prevalent in ITO. Photoelectron spectroscopy studies were made of the surfaces of bulk ceramic pellet specimens of ZITO. Small variations were seen in the Fermi level position and core level binding energies upon oxidation and reduction. The surfaces of the bulk specimens were rich in zinc as well as oxygen. The 'surface oxygen enrichment' was observed as a shoulder on the high binding energy side of the O1s core level emission. Sputter depth-profiling showed that the features characteristic of the 'surface oxygen enrichment' are mostly removed after one minute of sputtering. In addition, no variation in the Fermi level position was seen upon going from a significantly zinc-rich to a slightly tin-rich surface. <em>In situ</em> thin film deposition and annealing of ZITO films were carried out, and analyzed by XPS/UPS without breaking vacuum. Deposition under different oxygen contents leads to differences in the Fermi level position and core level binding energies consistent with changes in the concentration of Frank and Kostlin neutral defect clusters, (2Sn<sub>In</sub><sup>*</sup>O<sub>i</sub><sup>' '</sup>)<sup>x</sup>, affecting the carrier content in ZITO (i.e., reducing conditions result in high E<sub>F</sub>, oxidizing conditions result in low E<sub>F</sub>). In addition, significant reversible changes in the Fermi level and core level binding energies of 300-600 meV upon oxidation and reduction annealing were seen. These changes were attributed to changes in the concentration of the Frank and Kostlin neutral defect cluster, which affects the carrier content. <em>Ex situ</em> annealing of a previously <em>in situ</em> deposited film in air for 48 hours at 450 <sup>o</sup>C created the 'surface oxygen enrichment' observed on bulk ceramic specimens. Subsequent \<em>in situ</em> oxidation and reduction annealings no longer resulted in the large variations of the Fermi level or core level binding energies that were seen for the same treatments completed on <em>in situ</em> deposited films. It was suggested that the surface oxygen enrichment inhibits the oxygen exchange necessary for changes in the Fermi level position in ZITO.

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  • 09/20/2018
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