Jury :

Masahiro NOMURA, Professor, Tokyo-University, Japan, Rapporteur

Nicolas STEIN, Senior Lecturer, Institut Jean Lamour, Nancy, Rapporteur

Olivier BOURGEOIS, CNRS Research Director, Institut Néel, Grenoble, Examiner

Sylvie HEBERT, CNRS Research Director, CRISMAT, Caen, Examiner

Katir ZIOUCHE, Professor, IUT Université de Lille, Lille, Examiner

Nolwenn FLEURENCE, R&D Engineer, LNE, Trappes, Guest

Jean-François ROBILLARD, Lecturer-Researcher, Junia, Lille, Thesis supervisor

Emmanuel DUBOIS, CNRS Research Director, Lille, Thesis Director

Summary:

In thermoelectricity, silicon nanostructures represent an attractive alternative to conventional thermoelectric materials due to their abundance, non-toxic nature and compatibility with CMOS technology. Researchers have investigated various methods for improving the zT figure of merit of silicon by increasing the σ/κ ratio; decreasing the thermal conductivity κ using (i) nanometric structures such as thin membranes or nanowires, (ii) using surface roughness, (iii) oxidising the surface, which results in a low value of κ. However, few experimental measurements of zT on crystalline silicon nano-objects have been presented, and these data show significant scatter. Usually, the figure of merit is obtained by an independent measurement of the transport properties of the materials (κ, σ and S). The uncertainty of zT can easily reach 50 %, given that each parameter has an uncertainty of 5 % to 20 %. The Harman technique is a simple and fast method for directly measuring zT in bulk materials. This thesis proposes an adaptation of the transient Harman technique for suspended crystalline nano-membranes. A correction factor is introduced to take into account the effects of radiation, contact resistances and Joule heating. In addition, an implementation of the device is presented, allowing direct access to zT through purely electrical measurements, thus eliminating the need for separate measurements of transport parameters. In addition, elementary devices are examined for determining the thermal properties of silicon, including thermal conductivity, Seebeck coefficient and electrical conductivity. The measured Seebeck coefficient, showing a similarity with bulk Si near room temperature, raises a fundamental question about the relative contributions of electron diffusion and phonon transport.

Abstract:

In thermoelectricity, silicon nanostructures represent an interesting alternative to conventional thermoelectric materials due to Si abundance, non-toxic nature, and its compatibility with CMOS technology. Researchers have investigated methods to enhance the silicon figure of merit zT by increasing the σ/κ ratio; decreasing the thermal conductivity κ by (i) using nanometric structure such as thin membranes or nanowires, (ii) roughening, (iii) oxidation of the surface achieved relatively low value of κ. Yet, few experimental measurements of zT in crystalline silicon nano-objects were presented with important data dispersion. Usually, the thermoelectric figure of merit is obtained through independent measurement of materials transport properties (κ, σ and S). The uncertainty of zT can easily reach 50% considering that each parameter has an uncertainty of 5% to 20%. Harman's technique is a simple and rapid method to measure zT directly in bulk materials. This thesis proposes an adaptation of the transient Harman technique for suspended crystalline nano-membranes. A correction factor is introduced to account for radiation, contact resistances, and Joule heating effects. Furthermore, a device implementation is presented, enabling direct access to zT through purely electrical measurements, eliminating the need for separate measurements of transport parameters. Additionally, elementary devices are examined to determine the transport properties of silicon, including thermal conductivity, Seebeck coefficient, and electrical conductivity. The measured Seebeck coefficient, showing similarity to Bulk Si near room temperature, raises a fundamental question concerning the relative contributions of electron diffusion and phonon transport.