Omcvd Synthesis, Characterization, and STM Nanometer Lithography Studies of Tin Oxide Films

In order to better understand the relationship between basic film properties and performance in practical applications, fabrication techniques are needed for growing tin oxide films having reproducible properties. For this work, an OMCVD process has been optimized for fabricating tin oxide films having well controlled morphology and preferred orientation. The dependence of the gas sensitivity of these films on film properties and gas pressure was also investigated. Tin oxide films were produced by organometallic chemical vapor deposition using tetramethyltin and oxygen as reactants. Highly oriented single-crystal-like films having submicron-sized crystallites were grown on TiO _2 (110) and rm Al_2O _3 (1102) crystals. The preferred orientation of films deposited on fused quartz strongly depends on growth rate and temperature. All films have typical resistivities on the order of l Omega-cm and average transmittance of 80% through the visible spectra. Gas sensitivities of palladium-dosed SnO _2 films were characterized by measuring changes in conductance relative to initial film conductances during alternating exposure and evacuation cycles of H _2, O_2, and CO. Gas sensitivities up to 600% were recorded during H_2 and O_2 exposures, while smaller sensitivities were generally observed during CO exposures. The sensitivities of films to all three tested gases were found to be inversely proportional to approximately the square root of the initial film conductances. In addition to the above studies, a unique lithography technique using a scanning tunneling microscope (STM) was utilized to modify gold, graphite, copper, and tin oxide surfaces. Nanometer-sized surface features were written by pulsing the bias voltage between the sample and a solid gold STM tip. In all cases, the writing probability was characterized by a threshold bias voltage of 3 to 4V. The apparent STM tip radius of curvature was indirectly measured by scanning the tip across copper slip bands having known geometry. It was found that the so-measured tip radius varied with time while scanning and that the size of clusters deposited using the same tip increased proportionally with the tip radius.