Structure-Property Relationships of Tin Dioxide Thin Films Grown on Sapphire Substrates by Femtosecond Pulsed Laser Depo
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STRUCTURE-PROPERTY RELATIONSHIPS OF TIN DIOXIDE THIN FILMS GROWN ON SAPPHIRE SUBSTRATES BY FEMTOSECOND PULSED LASER DEPOSITION J. E. Dominguez , L. Fu and X. Q. Pan Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, Michigan 49109 ABSTRACT The sensitivity of semiconductive tin dioxide (SnO2) to reducing gases is determined by the electrical conductivity change in the material. This change in conductivity strongly depends on the thickness of the electron depletion layer near the oxide film surface. In this paper we study the effect of crystal defects and interfaces on the electrical properties and the gas sensing performance of SnO2 thin films. SnO2 thin films with the thickness varying from 15 nm to 100 nm were deposited on sapphire substrates with different surface crystallographic orientations by femtosecond pulsed laser deposition. Films grown on the (1012) sapphire (R-cut) are epitaxial, single crystal. High resolution transmission electron microscopy studies showed the existence of a large number of crystal defects including crystallographic shear planes and misfit dislocations at the film/substrate interface. Films grown on the (0001) sapphire substrates (Ccut) are nanocrystalline with (200) texture. The gas sensitivity of the films was measured in a gas reactor at high temperature. It was found that the sensitivity to reducing gases increases with decreasing film thickness. Electrical transport properties of the SnO2 thin films were investigated by Hall effect measurements. Models correlating the microstructures of thin films to electrical properties are proposed. INTRODUCTION Tin dioxide (SnO2) thin films have recently become the subject of many investigations due to their improved properties over bulk materials and to their possible implementation with micromachined devices.1 SnO2 thin films are usually found in transparent electrodes, solar cells and gas sensors. Gas sensors measure the change in electrical conductance or resistance of the material as a function of atmosphere at constant temperature. Synthetic air is usually used as the carrier gas and reducing (e.g. H2) or oxidizing gases (e.g. NOx) are the sensed species. The sensing mechanism is based on the change in electrical properties as the film interacts with gaseous species. At relatively high temperatures, oxygen adsorbs on the oxide surface and traps electrons from the material. When a reducing gas (for example) is introduced, it reacts with the adsorbed oxygen on the surface. Adsorbed oxygen reacts with reducing gases and its surface coverage decreases. As a result electrons are injected back into the material, which increases its electrical conductivity. The electrical properties and gas sensing performance of SnO2 based sensors strongly depend on the microstructure, surfaces, and crystal defects of the film. For example, it is known that porous SnO2 sensors show a high sensitivity due to increased oxygen diffusion and large surface area.2 Grain size can also influence greatly the response to reducing gas
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