Nowadays, a major challenge towards improving the performance of microchips and processors is the management of power consumption and heat dissipation within dense arrays of devices. This issue could potentially be addressed by the development of integrated thermoelectric generators (TEGs), which convert on-chip waste heat into electricity. However, most common thermoelectric materials used today are incompatible with Complementary Metal-Oxide-Semiconductor (CMOS) processing. An alternative material platform for CMOS-integrated active on-chip coolers are alloys composed of carbon (C), tin (Sn), and silicon (Si) combined with germanium (Ge). Due to their direct bandgap and high carrier mobility, the electronic and optical properties of these group IV alloys have been the object of intensive research. Meanwhile, their thermal properties have received comparatively little attention. The significant alloy scattering due to mass fluctuations in (C:)(Si)Ge1-x-ySnx alloys affects phonon transport and may play a crucial role in shaping their thermoelectric performance. This thesis helps to close this knowledge gap by investigation of the thermal characteristics of (C:)(Si)Ge1-xSnx alloys, focussing on several factors that impact their thermoelectric efficiency. The major objective is to gain deeper insight into the mechanisms that lead to reduction in lattice thermal conductivity within C:(Si)Ge1-xSnx, thus contributing to a fundamental understanding of the dynamics of phonons in complex semiconductor alloys. For this purpose, a systematic study is carried out by varying the composition, growth conditions, and dislocation density of the samples. Firstly, an in-depth analysis of critical factors such as short-range ordering and phonon interactions in (C:)(Si)Ge1-xSnx is provided through polarisation-dependent Raman spectroscopy. This method allows to reveal a trend of repulsion between Sn-Sn and Si-Sn atomic pairs hasbeen revealed. Secondly, to investigate the multi-phonons interactions, the impact of temperature on the Raman shift in Ge1-xSnx layers was explored. This study did not reveal a significant effect of the Sn content on the phononic interactions of the alloy. Lastly, the thermal conductivity of the Ge1-xSnx lattice was examined using Raman thermometry and the 3ω method. The most effective strategy to minimise thermal conductivity is identified as targeting an Sn content of approximately 14at.% or by enhancing the defect density in alloys with lower Sn concentrations. Together, the results presented in this thesis shed light on how stoichiometry, structural disorder, and phonon behaviour are interconnected in group IV semiconductor alloys. In addition to advancing the fundamental understanding of (C:)(Si)Ge1-x-ySnx, they offer a guidance for engineering materials with tailored thermoelectric properties.
Agnieszka Anna Corley-Wiciak (Wed,) studied this question.