STRUCTURE-PROPERTY RELATIONSHIPS OF TIN DIOXIDE THIN FILMS GROWN ON SAPPHIRE SUBSTRATES BY FEMTOSECOND PULSED LASER DEPOSITION

摘要

The sensitivity of semiconductive tin dioxide (SnO_2) 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 SnO_2 thin films. SnO_2 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 (C-cut) 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 SnO_2 thin films were investigated by Hall effect measurements. Models correlating the microstructures of thin films to electrical properties are proposed.