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The dependence of the electrical conductivity Pd-doped SnO2 thick films in an oxygen atmosphere is studied. The conduction mechanism when the atmosphere is changed from vacuum to oxygen can be explained considering the modification of the intergranular potencial barriers. The films exposure at temperatures higher than 300°C favors the oxygen indiffusion that causes the annihilation of oxygen vacancies. The difference between thick films doped with Pd and films without additives is analyzed.En este trabajo se estudia la dependencia de la conductividad eléctrica en oxígeno de películas de SnO2 dopadas con Pd. Los cambios en la conductividad, al modificar la atmósfera de vacío a aire, pueden ser explicados si consideramos que la adsorción de oxígeno produce un incremento en el ancho y en el alto de las barreras de potencial intergranulares. La exposición de las películas a estos gases a temperaturas mayores que 300°C favorece la difusión interna del oxígeno lo que provoca la aniquilación de vacantes de oxígeno y por ende la alteración de las barreras intergranulares. Se analiza en particular la diferencia en el comportamiento eléctrico entre las películas de SnO2 dopadas y sin dopar.

In this work the dependence of the electrical conductivity in air and in carbon monoxide of SnO2 films is studied. The conduction mechanism when the atmosphere is changed from vacuum to air and vacuum to carbon monoxide can be explained considering the modification of the intergranular potential barriers. The films exposure at temperatures higher than 300°C favors the oxygen indiffusion that causes the annihilation of oxygen vacancies.En este trabajo se estudia la dependencia de la conductividad eléctrica en aire y en monóxido de carbono de películas de SnO2. El proceso de conducción, al cambiar la atmósfera de vacío a aire y de vacío a monóxido de carbono, puede ser explicado si consideramos cómo la adsorción de estos gases altera las barreras de potencial formadas en los bordes de grano. La exposición de las películas a estos gases a temperaturas mayores que 300°C favorece la difusión interna del oxígeno lo que provoca la aniquilación de vacantes de oxígeno.

ZnO-Varistors have been studied by scanning tunnelling microscopy and spectroscopy. Tunnelling current-voltage characteristics indicate that grains present a rectifying behaviour. Since the prebreakdown zone is controlled by Schottky barriers formed by intergranular states, current density vs. temperature curves were fitted considering the tunnelling current through these barriers. Then, the Schottky barrier heights determined using the model are compared with the barrier heights obtained from the scanning tunnelling spectroscopy analysis in the grain boundary.Varistores de ZnO fueron estudiados mediante el empleo de microscopía y espectroscopía de efecto túnel. Las curvas de corriente túnel vs. tensión indican que las muestras presentan una conducta rectificante. Dado que la conducción en la zona de pre-ruptura es controlada por la formación de barreras de Schottky en los bordes de grano debidas a la presencia de estados intergranulares, las curvas de densidad de corriente vs. temperatura fueron ajustadas considerando la corriente túnel que atraviesa dichas barreras. La altura de las barreras de Schottky determinada con el modelo es comparada con las alturas obtenidas a través del análisis de barrido por espectroscopía túnel (STS) en la región del borde de grano.

Changes in the concentration of oxygen vacancies within the grains of polycrystalline SnO2, due to different atmosphere exposures, were detected using impedance and photoemission spectroscopies. From measured capacitance values, variations of the potential barrier widths could be determined. It is shown that under the presence of an oxygen rich atmosphere, at relatively low temperature, the width of intergranular potential barriers increase to the point that grains become completely depleted of carriers. With subsequent exposure to vacuum, capacitance adopts a higher value, indicative of intergranular barriers and quasi-neutral regions at the center of the grains. X-ray and ultraviolet photoemission spectroscopy measurements showed that SnO2 samples treated in oxidizing or reducing environments have similar barrier heights and different work functions. Results are especially relevant in the study of mechanisms responsible for metal oxide gas sensing. [ABSTRACT FROM AUTHOR]

Resistivity transients of tin oxide films at step isothermal changes in oxygen pressure are investigated. It is expected that, after exposing the samples to oxygen, the resistivity would increase monotonically as barriers become higher to finally reach a plateau at steady state. Here we present experimental results showing a nonmonotonic resistivity transient response that cannot be explained by only considering changes in the Schottky barrier heights. We provide an explanation based on the effects of intragrain oxygen diffusion that accounts for the observed main features of conduction in this polycrystalline material. Oxygen diffuses into the grains annihilating vacancies; the donor concentration is then reduced affecting the sample conductivity. [ABSTRACT FROM AUTHOR]

The ac electrical resistance and capacitance of SnO2 thick films under vacuum and air atmosphere were analyzed using the Cole–Cole plot. To fit the experimental results, a simple circuit model that considers a capacitance and a resistance in parallel was employed. An explanation for the resistance variation considering spherical grains with different characteristics is proposed. A careful analysis of the resulting depletion layers and doping levels gives evidence for gas diffusion into the grains. [ABSTRACT FROM AUTHOR]

Presents a study which examined the surface reactions that occur when adatoms of lanthanum are deposited onto the high-temperature superconductor YBa[sub2]Cu[sub3]O[sub6.9]. Experimental details; Results and discussion; Conclusion.

The study of electronic and chemical properties of semiconductor oxides is motivated by their several applications. In particular, tin oxide is widely used as a solid state gas sensor material. In this regard, the defect structure has been proposed to be crucial in determining the resulting film conductivity and then its sensitivity. Here, the characteristics of vacancy-like defects in nanocrystalline commercial high-purity tin oxide powders and the influence of the annealing treatment under different atmospheres are presented. Specifically, SnO2nanopowders were annealed at 330 °C under three different types of atmospheres: inert (vacuum), oxidative (oxygen) and reductive (hydrogen). The obtained experimental results are discussed in terms of the vacancy-like defects detected, shedding light to the basic conduction mechanisms, which are responsible for gas detection. [ABSTRACT FROM AUTHOR]

Gas sensors based on Titanium dioxide films have been a subject of interest due to its high sensitivity and low cost. The sensing mechanism of this type of sensors is based on the conduction mechanism, which is governed by the potential barriers formed at the inter-grains of the polycrystalline structure. The shape of these potential barriers strongly depends on the characteristics of the material, and then it is expected that a doping aggregation will affect significantly the conductivity and thus the sensitivity of the sensor. In this work, we study the effect in the oxygen sensitivity of titanium dioxide films due to N-doping. We developed a model in order to explain our experimental results based on the fact that, for the particle size of our titanium dioxide samples, grains are completely depleted of carriers. [ABSTRACT FROM AUTHOR]

The punctual character and random nature of the impurity positions in depletion regions lead to inhomogeneities that can significantly affect the potential intergranular barriers at polycrystalline semiconductors and, in general, of potential barriers at any semiconductor interface. This would reflect in Arrhenius plots for the electrical conductivity that become curved due to fluctuations of the intergranular barrier heights. Experimental results for polycrystalline tin oxide can be fitted assuming thermionic emission conduction at grain boundaries with a Gaussian distribution of barrier height fluctuations. However, resorting to a computational numerical model, we found that spatial fluctuations in barrier heights due to the discreteness of the donors and their statistical distribution at the depletion region differ from a Gaussian distribution. The type of obtained fluctuations, considering thermionic emission conduction, cannot explain the Arrhenius plots for the electrical conductivity found experimentally, especially at low temperature. Conversely, the tunneling contribution to conduction, without resorting to fluctuations, presents the observed trends. [ABSTRACT FROM AUTHOR]