engleski

Structural studies of tin-doped indium oxide (ITO)

Tin doped In_2O_3 is an n-type transparent conducting oxide with extensive commercial applications, including transparent heating elements of aircraft and car windows, antistatic coatings on electronic instrument display panels, transparent electrodes and solar cells [K. L. Chopra, S. Major, D. K. Pandya, Thin Solid Films 102 (1983) 1]. Despite the widespread use of ITO, the understanding of its defect structure is rather incomplete. The crystal structure of In_2O_3 and ITO is cubic, space group Ia3, isostructural with mineral bixbyite (Fe, Mn)_2O_3 [M. Marezio, Acta Cryst. 20 (1966) 723]. The structure contains two octahedral cation sites referred as sites B and D, respectively. Each cation resides at the center of a distorted cube, with six corners occupied by oxygen anions. The remaining two corners are empty and play an important role in the defect chemistry of ITO. A set of nanocrystalline ITO samples containing 0-14 at% Sn was prepared by a sol-gel technique from InCl_3 and SnCl_4 reagent grade chemicals, followed by thermal treatment at 300 ^oC for 2 h. Structural studies of the samples were performed using X-ray diffraction and ^119Sn Mössbauer spectroscopy. Unit-cell parameter of the as-prepared ITO samples increased with Sn-doping level up to 8 at% Sn and decreased above. Such behaviour of the unit-cell parameter indicated that tin substitution for indium on sites B and D is non-uniform and is dependent on tin content. ^119Sn Mössbauer spectra showed that tin is present only in Sn^4+ state in the samples. The best fits of ^119Sn Mössbauer spectra were obtained by assuming the presence of two doublets, corresponding to B and D positions in cation sublattice of the In_2O_3 structure. Isomer shifts (IS) for the described B and D positions are in the range 0.35-0.464 and 0.09-0.17 mm s-1, respectively, [H. Binczycka, M. Uhrmacher, M. L. Elidrissi-Moubtassim, J.-C. Jumas, P. Schaaf, phys, stat. sol. (b), 1-8 (2005) / DOI 10.1002]. The pronounced difference is observed in the relative area of the two doublets: a decreasing area for the one doublet and an increasing area for the other. Only for the sample containing 8 at% Sn they were almost equal. The samples were additionally annealed at 1000 ^oC for 1h and slowly cooled to room temperature. Lattice parameter of the annealed samples increased linearly with Sn-content in the examined concentration range. ^119Sn Mössbauer spectra of the annealed samples contained extremely broad subspectrum in addition to two doublets corresponding to Sn^4+ cations on sites B and D. Decrease of temperature from RT to 10 K caused disappearance of the broad component. The unusual broad Mössbauer subspectrum could be explained by a diffusional motion of Sn^4+ ions.

In_2O_3; Tin doping; X-ray diffraction; Mössbauer spectroscopy

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