PHASE COMPOSITION OF SnO_x THIN FILMS OBTAINED BY REACTIVE DC SPUTTERING

Résumé
Par pulvérisation sur l'étain métallique en atmosphère Ar+O₂ (taux de O₂ variable) un matériau du type SnO_x (1≤x≤2) a été obtenu. L'analyse des phases en présence, réalisée par spectrométrie Mössbauer et rayons X, permet de caractériser la variation du taux de SnO₂ dans l'échantillon.

Abstract
Transparent and conducting thin SnO₂ films have numerous applications as heating elements, antistatic coatings and light transmitting electrodes. The aim of this work was to determine the phase composition of the SnOx (1≤x≤2) thin films obtained on the glass substrates by DC reactive sputtering of metallic tin. The samples were prepared in Ar+O₂ mixture for which oxygen concentration was varied. The Mössbauer spectra for six samples obtained at various oxygen concentration in the chamber during the sputtering were measured in a transmission geometry using the BaSn source. The measurements were done at room and liquid nitrogen temperatures. The isomer shift and quadrupole splitting values of pure and stoichiometric SnO₂, α-SnO and β-Sn were used for the identification of the phases which are observable in the MS. The fitted values of the Mössbauer parameters for the measured spectra are listed in Table I. As expected the fraction for the SnO₂ phase increases with oxygen concentration in the cahmber, on the contrary the fraction of the SnO decreases and dor the higher oxygen concentrations the films are composed of the SnO₂ phase only. Due to the small recoil free fraction for the Sn lattice at room temperature, the MS taken at 293 K are free from the lines which might be attributed to that phase. At liquid nitrogen temperature at which recoil free fractions for Sn, SnO and SnO₂ are comparable the Sn phase was found in the film obtained at the lowest oxygen concentration 4.8%. The percentage contribution of the particular phases present in the films were estimated comparing the areas over the MS lines and taking to the account the different values of the recoil free fractions. The samples were also X-ray analysed. The α-Sn and β-SnO phases are being revealed in the diffraction pattern for the sample obtained at 4.8% oxygen concentration in the chamber. Only the α-SnO phase was recorded in the samples obtained at 6.8% and 8.0% oxygen concentration. In the other samples the SnO₂ phase appears in addition to the disappearing SnO phase. Our conclusions are : 1/ For the low oxygen concentration in the chamber, the SnO₂ phase turns out to be a noncrystalline solid solution in the SnO matrix; 2/ As the oxygen concentration increases the amount of the α-SnO phase diminishes and the SnO₂ crystalline phase emages.