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Molecular modulation of surface electrical conduction in silicon.

Dubey, Girjesh (2011). Molecular modulation of surface electrical conduction in silicon. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 229 p.

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Adsorption of charged or polar species on semiconductor surfaces can modulate the electrical properties through long-range field effects. Hydrogen-terminated silicon-on- insulator (SOI-H) is an interesting model system for investigating this sensitivity to surface processes. Sheet resistance, Hall effect, and accumulation mode pseudo- MOSFET measurements have been used to probe molecular adsorption and reaction events on SOI-H. The sheet resistance of SOI-H increases significantly with time in ambient air, due to depletion of majority carriers caused by oxidation. Physisorbed water further modulates the conductivity, inducing downward band bending and accumulation of majority carriers on n-doped films, without changing the carrier mobility significantly. Adsorption of water in a controlled vacuum system is found to strongly and reversibly increase the conductivity of both n-type and p-type SOI-H substrates. These conductivity changes can be attributed to water induced field effects that lead to accumulation of majority carriers on n-type and the formation of a minority carrier channel (inversion) on p-type substrates. The surface charge densities required to account for these effects are on the order of ~1011 q•cm-². Pyridine adsorption in the torr range gives rise to similar yet stronger reversible conductivity modulation effects as compared with water (several fold) on both n-type and p-type substrates, inducing positive surface charges of the order 1011-1012 q•cm-2. The ability of pyridine, water and other nitrogen containing molecules such as ammonia and triethylamine to reversibly bias p-type surfaces into inversion demonstrates a new type of molecular triggered electronic switch where adsorption is used to reversibly gate transport through the silicon substrate. In addition to conductivity/Hall Effect observations, the current-voltage characteristics of SOI-H pseudo-MOSFET n-channel devices are also found to be sensitive to the environment. The accumulation threshold voltage, or flat-band voltage, exhibits large reversible changes upon cycling between ambient atmosphere, high vacuum and exposure to water and pyridine vapour at pressures in the torr range. Both these adsorbates shift the flat band potential to more negative values, consistent with their ability to act as effective electron donors. The field-effect mobility is found to be comparatively less affected through these transitions. Adsorption of the well known electron acceptor tetracyanoethylene (TCNE) is shown to cause depletion, with ppm levels of TCNE vapour in ambient atmosphere found to rapidly decrease the saturation current by over two orders of magnitude. The effect is only partially reversible on the hydrogen terminated surface, due to the accumulation of strongly bound TCNE molecules on the surface. In addition, oxidation of the H-terminated surface is seen to result in irreversible shifts in both the flat-band voltage and field-effect mobility. In order to passivate the surface from these irreversible processes, a photochemical gas phase reaction with decene was used to form a decyl monolayer on the SOI(100)-H surface. Formation of this monolayer is found to result in a relatively small shift of threshold voltage and only a slight degradation of the field effect mobility. Decyl passivation only slightly decreases the response of the FET to TCNE adsorption while significantly improving the reversibility of the response. These results suggest that alkyl monolayer dielectrics formed by the gas phase photochemical method can function as good passivating dielectrics in field effect sensing applications.

Type de document: Thèse
Directeur de mémoire/thèse: Rosei, Federico
Co-directeurs de mémoire/thèse: Lopinski, Gregory
Informations complémentaires: Résumé avec symboles
Mots-clés libres: silicone; semi-conducteurs
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 06 août 2014 20:51
Dernière modification: 17 nov. 2015 19:57
URI: http://espace.inrs.ca/id/eprint/2177

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