Epitaxial ͑101͒ tin dioxide thin films with thickness ranging from 6 and 100 nm were deposited on the (1012) ␣-Al 2 O 3 substrate by femtosecond pulsed laser ablation. Due to the lattice and thermal expansion mismatch with the substrate, the SnO 2 film shows interfacial misfit dislocations, antiphase boundaries ͑APBs͒, and partial dislocations. The APBs lie along the (101) planes with a displacement of 1/2͓101͔. The densities of APBs and partial dislocations vary with film thickness, whereas the average spacing of misfit dislocations remains constant. Hall effect measurements showed that both electron concentration and mobility decrease with a reduction in the film thickness, which is ascribed to the scattering of electrons by crystal defects and interfaces and the effect of a native space charge region at the near-surface region of the films. The response of the films to reducing gases was found to depend on the electron concentration of the film and the relative fraction, with respect to film thickness, of material that is depleted of electrons.

An ultrafast (100 fs) Ti sapphire laser (780 nm) was used for the deposition of SnO2 thin films. The laser-induced plasma generated from the SnO2 target was characterized by optical emission spectroscopy and electrostatic energy analysis. It was found that the ionic versus excited-neutral component ratio in the plasma plume depends strongly on the amount of background oxygen introduced to the deposition chamber. Epitaxial SnO2 films with high quality and a very smooth surface were deposited on the (1̄012) sapphire substrate fabricated at 700 °C with an oxygen background pressure of ∼0.1 mTorr. The films are single crystalline with the rutile structure, resulting from the high similarity in oxygen octahedral configurations between the sapphire (1̄012) surface and the SnO2 (101) surface. Hall effect measurements showed that the electron mobility of the SnO2 film is lower than that of bulk single crystal SnO2, which is caused by the scattering of conduction electrons at the film surface, substrate/film interface, and crystal defects.

Nanocrystalline tin dioxide ͑SnO 2 ͒ thin films of different thicknesses were fabricated on the ͑0001͒ surface of ␣-Al 2 O 3 ͑sapphire͒ using femtosecond pulsed laser deposition. X-ray diffraction and transmission electron microscopy ͑TEM͒ analysis revealed that the microstructure of the films strongly depends on the film thickness. The films with a small thickness ͑Ͻ30 nm͒ are composed of nanosized columnar ͑100͒ oriented grains ͑3-5 nm in diameter͒ which grow epitaxially on the substrate with three different in-plane grain orientations. The ͑101͒ oriented grains ͑25 nm in diameter͒ appear when the film thickness becomes larger than a critical value ͑about 60 nm͒. The volume fraction of the ͑101͒ grains increases with film thickness. Cross-section TEM studies indicated that the ͑101͒ oriented grains nucleate on the top of the ͑100͒ oriented nanosized grains and show abnormal grain growth driven by surface energy minimization. As a result, the electrical transport properties are strongly dependent on the film thickness.

This work demonstrates the correlation between the microstructure of nanocrystalline SnO 2 thin films and their electrical transport properties and sensitivities to reducing gases. SnO 2 thin films were deposited on the (1 012) surface of ␣-Al 2 O 3 ͑sapphire͒ using electron beam evaporation of a pure SnO 2 ceramic source, followed by postdeposition annealing in synthetic air. SnO 2 thin films with randomly oriented nanosized grains were obtained by annealing an amorphous SnO film deposited at room temperature. Films with nanosized SnO 2 laminates were obtained by annealing epitaxial ␣-SnO films deposited at 600°C. The laminates are oriented with their ͑101͒ planes parallel to the substrate surface and have a high density of coherent twin boundaries. Hall measurements indicate that the electron concentration of the film with laminate grains is much lower than for the film with random grains. It is proposed that the high density twin boundaries inside the laminates trap conducting electrons and significantly reduce the electron concentration. As a result, the sensitivity to reducing gases of the laminar film is higher than that of the corresponding film with randomly oriented SnO 2 grains. It was also found that the grain size has strong effects on the sensitivity of SnO 2 films.

Tin dioxide (SnO2) has been extensively studied and used as gas sensors to detect toxic gases such as CO, NOxand flammable gases like H2.[l] Recently, considerable researches have focused on thin film sensors due to their high performance as well as their integration compatibility with semiconductor technology for making microsensors and sensor arrays. [2] The performance of thin film sensors is remarkably influenced by the way they were fabricated.[3] Among various deposition techniques, pulsed laser deposition (PLD) has shown great prominence in the deposition of a wide variety of oxide thin film materials such as high Tc superconductors, semiconductors and dielectrics. in this work we present our experimental results on tin dioxide films deposited using pulsed laser ablation on sapphire (α -Al2O3) substrates with different surface orientations.Tin oxide films with a thickness of 100 nm were deposited on the (1012) and (0001) sapphire by pulsed laser ablation of ceramic SnO2 targets.