Suchergebnisse

[eng] The air and environment quality is, nowadays, one of the main political concerns of the governmental institutions. Suitable gas detection is nowadays an important requirement, which is provided by gas sensors. Metal oxide semiconductors are the most common materials used as semiconducting gas sensors. They can be used with different type of operational mechanisms, like resistive, capacitive or optical based sensors, among others. The main objective of this dissertation is to contribute towards the improvement of gas sensors based on semiconducting nanowires. The easy integration of nanowires in low consumption devices is the fundamental idea that guides this dissertation, and the subsequent characterization of the sensors. Furthermore, the low power consumption of the device is a constant condition of the presented procedures, implemented by means of MEMS substrate that has been used as platform for all the fabricated sensors. The easy integration of nanowire-based devices by using localized growth on top of sensing platforms has been developed, by a site-specific growth of SnO2 and Ge nanowires on top of micromembranes and microhotplates, on the sensing area of the gas sensors. The fabrication procedure allows in single process the growth of NW networks on top of the electronic platforms that will be used for the gas sensing, an important step forward towards the integration of nanowires on electronic devices. On the other hand, the fabricated devices can be used as gas sensors readily after the growth. The gas sensing behaviour of SnO2 networks have been characterized towards different gases; specifically, the kinetics of ammonia response in dry and humid air has been analysed in detail. In addition, the influence of water vapour is analysed, and thus, the chemical paths of the interaction with ammonia have been related to the operating temperature. Furthermore, the synthesized meshes of Ge NWs have been for the first time, at the best of our knowledge, studied as a gas sensor. The chemical interactions towards oxidizing and reducing gases are analysed, paying important attention on the structural characterization, which results primordial for the analysis of sensing behaviour. A procedure based on electron beam lithography is explored in order to fabricate individual nanowire-based devices on top of microhotplates and micromembranes. The experimental procedure for that is detailed in the different steps. The individual nanowires have also studied as a gas sensor, whose results are discussed and compared to their mesh counterpart. ; [spa] La tesis titulada New Fabrication Methodologies for the Development of Low Power Gas Sensors Based on Semiconducting Nanowires, se enmarca dentro de los sensores de gas para la monitorización ambiental de la calidad del aire, con el objetivo de detectar la presencia de gases nocivos para la salud humana. El trabajo desarrollado se basa en el uso de sensores de gas resistivos, es decir, que la adsorción de un gas en la superficie del sensor da lugar a un cambio en la conductividad del sensor, el cual repercute en un cambio de su resistencia medible experimentalmente. Los materiales utilizados son los óxidos metálicos, materiales semiconductores de banda prohibida ancha (entre 2 y 4 eV). Generalmente, los óxidos metálicos necesitan ser calentados a una temperatura por encima de 150 ºC para promover la interacción con los gases adsorbidos, que se lleva a cabo mediante la denominada quimisorción, una interacción localizada que implica una transferencia de carga entre el semiconductor y la especie de gas. Dado que el gas se adsorbe en la superficie del material, el mecanismo es considerado como superficial, cuya sensibilidad depende en gran medida, entre otros efectos, del ratio superficie/volumen. Así pues, las nanoestructuras aparecen como candidatas óptimas para ser utilizadas como sensor de gas. En la tesis mencionada, se utilizan estructuras unidimensionales, concretamente nanohilos semiconductores como parte activa. El principal desafío para la comercialización de sensores basados en nanohilos es, actualmente, la integración de estos en un dispositivo de forma eficiente y escalable. Un nuevo método de fabricación que mejor la integración de los nanohilos en sus plataformas electrónicas ha sido desarrollado, basado en el crecimiento localizado de nanohilos monocristalinos sobre microplacas calefactoras, es decir, membranas de dimensiones micrométricas que contienen un microcalefactor enterrado. El calefactor es utilizado para proporcionar la temperatura necesaria para la termólisis del precursor durante el crecimiento, y para calentar y promover la reacción durante el sensado de gas. Mediante este proceso han sido crecidos nanohilos de SnO2 y de Ge, en forma de redes con múltiples nanohilos conectados entre sí. Los dispositivos fabricados han sido caracterizados como sensores de gas. Los nanohilos de dióxido de estaño presentan la máxima respuesta ante amoníaco. Los mecanismos químicos que se dan lugar durante la interacción entre el amoníaco y el semiconductor han sido analizados a partir de la respuesta cinética y los distintos fenómenos observados, en aire seco y húmedo. Los nanohilos de Ge han sido estudiados por primera vez como sensores de gas, manteniendo una temperatura de 100 ºC. El comportamiento de tipo p de los nanohilos ha sido determinado a partir de medidas de efecto campo. Los nanohilos presentan una capa de óxido de Ge en la superficie, de alrededor de 1 nm, que posee un papel clave en la interacción, ya que contiene sitos de adsorción para el sensado de gas. El mecanismo de sensado de Ge se concluye como consistente con un óxido metálico de tipo p. Por otro lado, han sido fabricados sensores de gas basados en nanohilos individuales de SnO2 mediante un proceso basado en litografía por haz de electrones, sobre micromembranas calefactores suspendidas. El proceso ha sido integrado y adaptado para el tipo de micromembranas, que presentan rugosidades en la superficie. Los dispositivos fabricados han sido caracterizados como sensor de gas y comparados con las redes de nanohilos del mismo material.

The air and environment quality is, nowadays, one of the main political concerns of the governmental institutions. Suitable gas detection is nowadays an important requirement, which is provided by gas sensors. Metal oxide semiconductors are the most common materials used as semiconducting gas sensors. They can be used with different type of operational mechanisms, like resistive, capacitive or optical based sensors, among others. The main objective of this dissertation is to contribute towards the improvement of gas sensors based on semiconducting nanowires. The easy integration of nanowires in low consumption devices is the fundamental idea that guides this dissertation, and the subsequent characterization of the sensors. Furthermore, the low power consumption of the device is a constant condition of the presented procedures, implemented by means of MEMS substrate that has been used as platform for all the fabricated sensors. The easy integration of nanowire-based devices by using localized growth on top of sensing platforms has been developed, by a site-specific growth of SnO2 and Ge nanowires on top of micromembranes and microhotplates, on the sensing area of the gas sensors. The fabrication procedure allows in single process the growth of NW networks on top of the electronic platforms that will be used for the gas sensing, an important step forward towards the integration of nanowires on electronic devices. On the other hand, the fabricated devices can be used as gas sensors readily after the growth. The gas sensing behaviour of SnO2 networks have been characterized towards different gases; specifically, the kinetics of ammonia response in dry and humid air has been analysed in detail. In addition, the influence of water vapour is analysed, and thus, the chemical paths of the interaction with ammonia have been related to the operating temperature. Furthermore, the synthesized meshes of Ge NWs have been for the first time, at the best of our knowledge, studied as a gas sensor. The chemical interactions ...