Details

Autor(en) / Beteiligte

• Upadhyaya, Meenakshi
• Aksamija, Zlatan
• Andreoni, Wanda
• Yip, Sidney

Titel

Thermal Conductivity of Nanostructured Semiconductor Alloys

Ist Teil von

• Handbook of Materials Modeling, p.917-951

Ort / Verlag

Cham: Springer International Publishing

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• This chapter provides a comprehensive description of phonon transport in nanostructures of silicon-based Group-IV alloys. We employ the phonon Boltzmann transport equation (pBTE) formalism with full phonon dispersion and a partially diffuse momentum-dependent specularity model for boundary roughness scattering. The results show thermal conductivity in Si-Ge nanostructures including thin films, superlattices (SLs), nanowires (NWs), and nanocomposites (NCs) to be well below their bulk counterparts, reaching almost to the amorphous limit. Further minimization of lattice thermal conductivity is shown in binary (SiSn and GeSn) and ternary (SiGeSn) alloys and their thin films. Si-Sn alloys have the lowest conductivity (3 W/mK) of all the bulk alloys, which reduces further to around 1 W/mK in 20-nm-thick Si-Sn films. Thermal transport in nanostructures is tunable by extrinsic boundary effects such as sample size in thin films, period thickness in SLs, length/diameter in NWs, and grain size in NCs. Additionally, boundary/interface properties, such as roughness, orientation, and composition, also play a significant role in thermal transport and offer additional degrees of freedom to control the thermal conductivity in nanostructured semiconductor alloys. Thermal conductivity can be minimized in SLs by growing short-period Si∕Si1−xGex SLs with the Si-Ge alloy layer thicker than the Si one. We describe a Monte Carlo method of sampling the phonon mean free paths (MFPs), capable of capturing both partially diffuse boundaries and ballistic effects in the calculation of thermal conductivity of Si-Ge NWs. The computed phonon flights are comprised of a mix of long free flights over several μm interrupted by bursts of short flights, resulting in a heavy-tailed distribution of flight lengths, indicating superdiffusive phonon transport, which results in L1∕3 scaling across a wide range of NW lengths up to nearly 10 μm. Lastly, our pBTE model for nanocomposites, based on Voronoi tessellation to capture the grain structure in NCs, is described. The size scaling of thermal conductivity observed in NWs persists in NCs as well and is found to be insensitive to the variance in grain sizes.