Oxidation of Sn Thin Films to SnO 2 . Micro-Raman Mapping and X-ray Diffraction Studies
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Oxidation of Sn thin films to SnO2 . Micro-Raman mapping and x-ray diffraction studies Luigi Sangaletti, Laura E. Depero, Brigida Allieri, Francesca Pioselli, Elisabetta Comini, Giorgio Sberveglieri, and Marcello Zocchi Istituto Nazionale per la Fisica della Materia and Dipartimento di Fisica e Chimica per i Materiali, Universit`a di Brescia, Via Valotti, 9-25133 Brescia, Italy (Received 1 October 1997; accepted 19 December 1997)
The oxidation of tin layers deposited onto alumina substrates is investigated with the aim to identify the different steps of the process and obtain information on the sample homogeneity, phase segregation, and degree of oxidation. It is shown that at least three phases coexist at 450 ±C, Sn, SnO, and SnO2 , and remarkable inhomogeneities, already visible at an optical inspection, are found in the thin film. A micro-Raman mapping of the layer shows that these inhomogeneities are related to the presence of different Sn oxidation states, as evidenced by the inhomogeneous distribution of SnO and SnOx Raman bands. The thin film becomes homogeneous after annealing treatments above 550 ±C, where only the SnO2 cassiterite phase is detected.
One of the main goals in the production of thin film gas sensors is the structural and chemical stability of the layer and the reduction of drift in the electrical response. Many factors may induce drift in the electrical response upon gas exposure. The most important are structural phase transitions, phase segregations, crystallite size growth,1 oxidation of substoichiometric materials, contamination, and poisoning of the surface layer. The first four factors can be related to the sensor working temperature which, depending on the kinetics of structural and chemical transformations, may change the microstructural properties of the layer and also promote the oxidation of substoichiometric areas. While surface contamination and poisoning can be detected by such surface analysis techniques as photoemission spectroscopies, the microstructural properties can be effectively investigated by x-ray diffraction (XRD) and Raman spectroscopy. The aim of the present work is to investigate the oxidation process of the Sn layer deposited by the rheotaxial growth and thermal oxidation (RGTO) technique. This technique, which is based on the deposition of metallic tin by rf sputtering onto a substrate held at a temperature above Sn melting point, is widely used for depositing SnO2 layers with a high surface area and remarkable response to polluting gases.2 In addition to the phase identification during the oxidation process, the main result is the detection by Raman spectroscopy—in the 250–450 ±C temperature range—of a phase ascribed to substoichiometric SnOx . With the aid of micro-Raman mapping, this phase was found distributed in transparent areas of the film, well separated by gray areas where the SnO phase was detected. This result shows that microRaman mapping can be a very effective tool in the characterization of structura
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