Advances in semiconducting nanowires for gas sensing: synthesis, device testing, integration and electronic nose fabrication

The work presented in this dissertation is focused on the fabrication, integration and test of chemoresistive gas sensor devices and systems based on semiconducting nanowires (NWs). The first objective of this dissertation was to grow monocrystalline In2O3 and Ga2O3 NWs via the vapor-liquid-solid mechanism using a chemical vapor deposition furnace and solid precursors. Subsequently devices based on individual NWs were fabricated, contacted on top of microhotplates using Focused Electron Beam Induced Deposition (FEBID), and their gas sensing properties were characterized. The gas sensors based on individual In2O3 NWs present considerable response towards ethanol at 300 ºC, with a response time of about 4 minutes. On the other hand, the gas sensors based on individual Ga2O3 NWs showed high selectivity towards relative humidity at room temperature, with resistance variations up to 90% in times as short as 2 minutes, and with minimal response to other gases (NO2, CO, ethanol and H2). This behavior is completely different from that reported on this material and is a direct consequence of the NW growth method, which gives rise to a carbon shell around the NWs. Furthermore, the sensing behavior ressembles that of carbon-containing materials.. A second objective was to deepen into methodologies to integrate the sensing material in the substrates where the gas sensing devices are fabricated, with the aim of simplifying the integration procedures and increasing the throughput. With this in mind, dielectrophoretic alignment of NWs was the first methodology proposed to fabricate chemoresistors based on arrays of individual WO3 NWs. The maximum gas response of the fabricated arrays of individual NWs was towards 5 ppm of NO2 for the pristine and towards 100 ppm of EtOH for the Pt-functionalized WO3 NWs, respectively. This higher response of the Pt-functionalized WO3 NWs-based gas sensors is related to the surface decoration of these NWs, which increases the amount of oxygen adsorbed species at their surface, allowing EtOH molecules to be more easily adsorbed than on pristine NWs. The second approach proposed for contacting individual SnO2 NWs on top of suspended microhotplates was Electron Beam Lithography (EBL) in combination with lift-off. The method allows fabricating several devices sequentially but without breaking the vacuum of the EBL system and required optimization of the holder for spinning and of the electron dose used to modify the photoresist properties. The gas characterization of these devices showed higher resistance variations than that obtained for the reference fabrication technique, FEBID. This superior behavior can be the result of the better electrical characteristics of the Ti/Pt contacts in front of the FEBID Pt-deposition. This demonstrates the potentiality of this techniques for contacting individual NWs on top of micromembranes. The third objective was, to a certain point, a natural extension of the device integration activity when considering one of the major drawbacks of chemoresistors: their lack of selectivity. For this, SnO2, WO3 and Ge NWs, have been grown on well-defined and pre-specified regions of one single chip, allowing their simultaneous operation. Here, NO2, CO and relative humidity (RH), diluted in dry synthetic air, have been tested. The calibration of each individual sensor has been carried out exposing the whole chip to the individual gases but with only this particular sensor heated and biased, while the others were unheated and unbiased. This has allowed determining the optimal operation conditions for each sensor. Next, at these optimal temperatures, all the sensors have been tested, simultaneously, towards each gas specie alone. Finally, tests of the three sensors, operating simultaneously, towards mixtures of the three gases were performed. The data from all the mentioned measurements have been treated according to the Principal Component Analysis (PCA) methodology and the results demonstrate that the fabricated sensor system can discriminate and quantify the concentration of the three different studied analytes. The three sensors, made of three different materials and operating simultaneously, constitute an electronic nose, which we here call nano electronic nose.Aquesta tesi doctoral està enfocada al desenvolupament de dispositius i sistemes sensors de gas basats en nanofils semiconductors monocristal·lins. El primer objectiu aconseguit és el creixement de nanofils d’In2O3 i de Ga2O3 utilitzant un forn pel dipòsit químic en fase vapor. Els nanofils fabricats s’han transferit a xips amb micromembranes suspeses amb calefactor i nanofils individuals s’han contactat amb els seus elèctrodes, emprant el dipòsit assistit per feixos d’electrons. Els nanofils d’In2O3 mostren una resposta considerablement selectiva enfront d’etanol a partir dels 200 ºC i els de Ga2O3 són molt selectius a la humitat relativa a temperatura ambient, fruit del mètode de creixement. Un segon objectiu d’aquesta tesi ha estat explorar dos mètodes d’integració de nanofils individuals: la dielectroforesi, per alinear nanofils de WO3, que han mostrat respostes enfront d’etanol i NO2, a 250 ºC, i la litografia per feixos d’electrons, per contactar nanofils de SnO2 sobre microplataformes calefactores suspeses, que han mostrat respostes satisfactòries enfront d’amoníac a 200 ºC. En ambdós casos s’ha demostrat la viabilitat d’aquestes tècniques. L’últim objectiu ha estat la fabricació i caracterització d’un nas electrònic basat en xarxes de nanofils de SnO2, WO3 i Ge integrades en cadascuna en una micromembrana d’un mateix xip. Aquests nanofils s’han fet créixer directament en regions localitzades i predefinides del xip, les micromembranes, integrats directament en ell, evitant la necessitat de transferir-los després de fer-los créixer. Primer s’ha realitzat el calibratge de cada sensor individual enfront de tres gasos individualment, després enfront de tres gasos simultàniament i, finalment, els tres sensors s’han fet funcionar alhora enfront dels tres gasos. Les dades de totes les mesures s’han tractat segons la metodologia d’anàlisi de components principals i els resultats demostren que aquest sistema és capaç de discriminar entre diverses mescles de NO2 i CO diluïts en aire sintètic sota diferents nivells d’humitat relativa. Els tres sensors, formats per els tres materials diferents i funcionant simultàniament, constitueixen un nano nas electrònic