In this study, we describe an original method to build up a superficial and nanostructured porous polysilicon (POpSi) thin film on a Silicon substrate for thermoelectric (TE) applications. The POpSi layer is electrically isolated from the conductive Silicon substrate by an SiO2 interlayer. In thermoelectricity, nanostructuration is commonly used to engineer the thermal conductivity of the material. Here, we show that the nanostructuration brought by porosification breaks the thermal conductivity of polysilicon layers (polySi) and has no detrimental impact on their Seebeck coefficient: a POpSi with up to 62% porosity has a Seebeck value maintained at about 255µV/°C when the standard polysilicon, it is built from, is n type with a carrier density of 3.4×10 19 /cm 3 . On the other hand, the benefit of the thermal conductivity reduction (by a factor of about 30) clearly over-compensates the degradation of the electrical conductivity measured in the POpSi layers, in terms of impact on the factor of merit ZT. Compared to the standard polySi layer, the ZT value is increased 25-fold in POpSi with 44% porosity (it increases from 0.004 to 0.1). Such a novel material is a good candidate for planar thermoelectric devices using Silicon technologies. As a demonstration, a modeling shows that if the optimum POpSi thin film is integrated into a planar TE microgenerator (µTEG) with suspended membranes, it would permit an improvement of conversion efficiency of up to 28% compared to a µTEG integrating the standard polySi layer counterpart.
Dans cet article, nous présentons des microgénérateurs thermoélectriques (µTEG) planaires réalisés en technologie Silicium compatible CMOS, et destinés à la récupération de toute forme d'énergie thermique. Ceux-ci utilisent des matériaux faible coût, abondants et respectueux de l'environnement. La captation de la chaleur est effectuée à l'aide d'un concentrateur, en Silicium. La configuration « semi 3D » développée utilise des membranes suspendues et nécessite, pour éviter de les casser, la mise en oeuvre de structures de maintien, appelées bossages. L'objectif de ce travail est d'évaluer l'amélioration des performances de conversion des µTEG résultant de la réduction des pertes thermiques latérales suite à l'anodisation sélective de ces bossages. La simulation thermique de µTEGs intégrant de tels bossages en Si poreux montre que les puissances générées pourront être augmentées jusqu'à 65%. ABSTRACT. In this paper, we present a planar thermoelectric microgenerator (µTEG) based on CMOS-compatible silicon technology to harvest all forms of thermal energy. These have made by using low cost, abundant and ecofriendly materials. The heat harvesting has performed with a silicon concentrator. The developed semi-3D configuration needs safety support called 'boss' to avoid the breaking up of the membranes. The objective of this work is to evaluate the improvement of these μTEGs performance resulting from the reduction of lateral heat losses by using selective anodizing of these bosses. The thermal modeling of µTEGs embedding porous silicon bosses shows an increase in output power up to 65%. MOTS-CLÉS. énergie, microgénérateur, thermoélectrique, planaire, silicium poreux, modélisation.
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