X-Ray Diffraction and Modelling Studies of Multilayer SnO 2 Thin Film Gas Sensors
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ABSTRACT Structural studies have been carried out on SnO 2 multilayer thin film grown by the Rheotaxial Growth and Thermal Oxidation method on A120 3 substrates. A preliminary analysis of the X-ray diffraction patterns shows that, in addition to the Sn0 2 cassiterite phase, a strong contribution from an orthorhombic Sn02 phase is present. In the case of the 3-layer film, the orthorhombic phase is structurally and microstructurally stable after an annealing up to 32 h at 400 'C. The cation coordination is similar to that found in cassiterite, but the chains of edge-sharing [Sn0 6]8 - octahedra run in a zig-zag fashion along the [100] direction, each straight unit containing four octahedra. The relationship between the two phases is discussed on the basis of structural simulations including twinning planes in the crystal structure. INTRODUCTION The characterisation of the structural properties of Sn0 2-based thin film gas sensors is essential to understand the mechanisms of thin film growth and stability. In particular, the study of structural stability upon annealing treatments is basic to obtain reliable gas sensors for working at high temperatures. Tin dioxide in the form of thin films is finding increasing attention as a method for producing gas sensors. An original PVD method for preparing thin films with high surface to volume ratio has been developed [1-3]. This technique named Rheotaxial Growth and Thermal Oxidation, is a two step process in which the first step consists of the evaporation of metallic Sn onto substrates heated above the melting temperature, Tm, of Sn (Tsubstrate > Tm = 232'C). Sn metal tends to cluster on the substrate surface into small droplets which do not touch each other. In a second step, the Sn droplets are oxidised by thermal annealing in air at temperatures in the 500 - 700 0C range. The second step is critical in that an incomplete oxidation of the metal droplet can be responsible of an unacceptably large drift in the sensor response. the preparation conditions for obtaining well oxidised RGTO Sn02 thin film were identified in a previous work [3]. The oxidation temperature has a lower limit at 600 'C, which is the lowest temperature required to have the oxidation of the metal droplets, and a higher limit, which is due to damages in the substrate, including heating elements and electrical contact pads, that may occur at high annealing temperatures. One way to cope with these requirements is the growth of thin layers, with an average layer thickness not exceeding 0.2 - 0.3 gtm, which allows a complete oxidation at low temperatures. When thick Sn02 films are required, RGTO-SnO 2 films can be prepared by means of alternating Sn evaporation and oxidation treatments in order to keep the average grain size reasonably small. Following this procedure, a Sn0 2 phase, previously identified at high pressure conditions, has been detected. This phase can be structurally related to cassiterite by introducing a periodic microtwinning in this phase. Starting from the periodic microgemination in the
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