Thesis presented November 16, 2015

Abstract:
This PhD project is an exhaustive study on the characterization of the thermal properties of a new type semiconducting materials based on germanium. It is a germanium matrix containing nano-inclusions with the objective of creating a perfect "electron crystal - phonon glass" material. The materials are thin films of an epitaxial germanium matrix embedded with Ge:Mn nano-inclusions, grown on a Germanium-on-insulator (GOI) substrate in CEA-INAC in Grenoble. From TEM images of the thin films it has been demonstrated that both the matrix and inclusions are monocrystalline, and the nano-inclusions have generally a spherical form with a diameter distribution ranging from 5 to 50 nm. Depending on the growth parameters in molecular beam epitaxy, i.e. the Mn concentration and the annealing temperature, the geometries, mean diameters and diameter distributions of nano-inclusions in Ge:Mn can be varied. With these unique structural features, these Ge:Mn thin films are one of the most interesting models for the study of the influence of nano-inclusions on thermal transport in a crystalline matrix. The characterization of the thermal properties of the material have been done using two advanced techniques: the 3-omega method in Institut Néel, and the Scanning Thermal Microscopy (SThM) in CETHIL (Centre d'Energétique et de Thermique de Lyon) in Lyon. A highly sensitive differential 3-omega measurement setup has been developed in the work, which permits precise (error~12%) measurements of electrical conductive thin films having low thermal conductivities. Dramatically reduced thermal conductivities have been revealed for Ge:Mn thin films containing different Mn% and having different inclusion geometries at room temperature, compared to crystalline bulk Ge. A minimum value of 3.3 Wm^-1K-¹ was found for Ge:Mn thin film containing 10% Mn, beating the “alloy limit” of thermal conductivity set by SiGe alloys at room temperature (6-12 Wm^-1K^-1). The measurement results of SThM confirmed the low thermal conductivities for all Ge:Mn/GOI samples at room temperature. Numerical simulations using different models have been performed to try to interpret the experimental results and to understand the mechanisms of the influence of the nano-inclusions on the phonon transport in semiconductor materials.

Keywords:
Thermoelectricity, 3-Omega method, Thin film, Nano-Inclusion, Germanium, Phonon transport

On-line thesis.