Stoichiometry Control in Molecular Beam Deposited Nanocrystalline SnO 2 and TiO 2 Thin Films
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Tin and titanium dioxides, Sn0 2 and TiO 2 , are prospective materials for gas sensors'-. Both oxides are polycrystalline materials, with similar crystallography (both are tetragonal), inherently non-stoichiometric (with 0-deficit). Electrical conductivity of Sn0 2 and TiO 2 strongly depends on the environmental gases. SnO 2 sensors are sensitive to reducing gases (H2, CO, CH 4 etc.) due to the low-temperature chemisorption of the environmental gases on the grain surface that changes the surface electron state level, governing the grain surface conductivity. The TiO 2 sensors are sensitive to gasesoxydants (02 , NO, etc. ). The bulk diffusion of oxygen from outside into the TiO 2 , compensating an original deficiency of oxygen in the TiO 2-1attice, increases the TiO 2 resistivity by oxygen atoms occupation of 0-vacancies (the donor centers). TiO 2 sensors are stable at high temperatures and used in combustion systems to control CO/0 2 ratio. Both oxides have good resistance to corrosive gases, low cost and are easy to handle. Main disadvantage of SnO2 sensors as compared to Ti0 2 are their poor selectivity and structure instability at temperatures above 4000 C. Similarly to other metal-oxide sensors, Sn0 2 and TiO 2 sensors are produced by sintering and sol-gel processes (in the case of thick films) or by reactive ion sputtering (metal targets) and e-gun deposition (oxide targets) in the case of thin films. Recently, molecular beam deposition (alternative version of the e-gun deposition) demonstrated some advantages for thin film processing: clean substrate surface and simple controlled oxide flux in a high-vacuum molecular beam chamber is useful in preparation the oxides that are uniform on a large-area scale, and oxide mixtures with stable composition. However, in the case of oxides with low oxygen pressure (as Sn02), the film deposition complicates due to oxygen and oxides evaporation in high vacuum from the high-temperature melt. Fig. la shows the calculated oxygen pressure temperature dependence for the Sn02 or TiO 2 melts 5. In this study we have studied the behaviour of Sn02 and TiO 2 films molecular beam deposited and annealed in vacuum or in oxygen to correct an initially oxygen non-stoichiometry. EXPERIMENTAL The Sn02 and TiO 2 films were deposited by means of electron gun molecular beam (MB) deposition from the appropriate commercial targets. During deposition the substrates, Si0 2/(00l)Si and amorphous carbon covered Ni grids (for in-situ TEM experiments), were kept at a temperature of 100 0C. The film thickness ranged from 50 to 500nm. Rutherford
Mat. Res. Soc. Symp. Proc. Vol. 581 © 2000 Materials Research Society
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