Elaboration and thermoelectric characterization of nanostructured polysilicon thin films by anodization

Over the last decade, the Internet of Things (IoT) market has grown almost exponentially. The energy issue has become one of the major challenges for IoTs but also, more broadly, for all microsystems. All innovative and environmentally friendly solutions to recover energy now play a strategic role in the development of these new technologies and are even the subject of increasingly important research and publications. The abundance of heat which represents energy in its most degraded form – all energy tends to become heat – gives a particular interest to thermoelectricity. For miniature applications, the thermoelectric effect (TE) has so far been exploited mainly in thermal sensors or in infrared detectors, where it favors high sensitivities despite low conversion efficiency.

The conversion efficiency of TE materials, used in conventional thermogenerators, is directly related to their dimensionless figure of merit (where is the electrical conductivity, a the thermoelectric power and the thermal conductivity of the TE material). These physical properties are oppositely dependent on carrier density and temperature, which complicates their performance optimization. The main avenues studied for the improvement of these materials use nanostructuring, in particular to reduce the phononic component of the thermal conductivity. The use of nanostructures, low dimensional systems or nano-composite structuring allows to confine the thermal transport of phonons in 2D or even quasi-1D.

The work carried out at IEMN, in the field of thermoelectricity, has allowed the development of a new method to fabricate thin layers of nanostructured polysilicon, electrically isolated from their silicon substrate. Characterizations have shown that the porosification of polysilicon (POpSi) layers drastically reduces their thermal conductivity without impacting their thermoelectric power. This improvement of the thermal component overcompensates the degradation of the electrical conductivity and allows a 25 times increase of the figure of merit (for a POpSi layer with a porosity of 44%, ZT ~ 0.1). Simulations under COMSOL 3D® show that the integration of such layers in planar thermogenerators realized on suspended membranes (IEMN technologies) would allow a 30% increase of their conversion efficiency.

Flow chart of the porous polysilicon layer (a to d) and plated thermocouple strip (f)

Katir Ziouche, Ibrahim Bel-Hadj and Zahia Bougrioua
Nano Energy, 80 (2021) 105553, https://doi.org/10.1016/j.nanoen.2020.105553