Energy harvesting for Internet of Things applications requires efficient thermoelectric (TE) materials, which should ideally be semiconductors compatible with Si technology. While pure Si or Ge are not good candidates for these applications, mostly because of their large lattice thermal conductivity, the GeSn and SiGeSn alloys are much more promising options, both because of the small lattice thermal conductivity and because of large electron mobility in cases where the band gap is direct. This thesis will perform the theoretical / modelling investigation of the potential of GeSn and SiGeSn alloys as TE materials for low-grade heat to electricity conversion. The previously calculated expected TE figures of merit of ZT ∼ 0.4 at 300 K and an impressive 1.04 at 600 K. These values are extremely promising in view of the use of GeSn/Ge layers operating in the typical on-chip temperature range.
The modelling will be based on k.p and effective mass method for the band structure parameters in the conduction and valence band, followed by calculations of carriers mobility, Seebeck coefficients, and thermal conductivity due to various scattering processes (acoustic, optical, and intervalley phonon scattering, alloy disorder, ionised impurity, and lattice defects induced scattering). Based on these calculations the promising alloy compositions will be identified, to deliver a large power factor and / or large ZT parameter, which describe the power output or the conversion efficiency of bulk material based thermoelectric generators.
Further work will then be directed towards more complex structures which can improve the TE generators performance. Instead of homogeneous-composition bulk alloys, the modulated-composition structures will be explored, where the composition modulation leads to the modulation of the conduction and valence band edges, which enables energy filtering in carrier transport, eventually leading to improved heat-to-electricity conversion. Optimal design of devices with energy filtering effect will then be investigated.