Semiconductor Thermal and Electrical Properties Decoupled by Localized Phonon Resonances

• Published in: Advanced materials (Weinheim) Vol. 35; no. 26; pp. e2209779 - n/a
• Main Authors: Spann, Bryan T., Weber, Joel C., Brubaker, Matt D., Harvey, Todd E., Yang, Lina, Honarvar, Hossein, Tsai, Chia‐Nien, Treglia, Andrew C., Lee, Minhyea, Hussein, Mahmoud I., Bertness, Kris A.
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 01.06.2023; Wiley Blackwell (John Wiley & Sons)
• Subjects: Charge transport; Cooling; Current carriers; Decoupling; Electric conductors; Electrical properties; Energy recovery; gallium nitride; Heat conductivity; Heat transfer; Materials science; Membranes; nanophononic metamaterials; nanopillars; Phonons; Power factor; Reduction; Thermal conductivity; thermal transport; Thermoelectric materials; thermoelectrics; gallium nitride; nanopillars; thermoelectrics; thermal transport; nanophononic metamaterials
• ISSN: 0935-9648; 1521-4095
• DOI: 10.1002/adma.202209779

Thermoelectric materials convert heat into electricity through thermally driven charge transport in solids or vice versa for cooling. To compete with conventional energy‐conversion technologies, a thermoelectric material must possess the properties of both an electrical conductor and a thermal insulator. However, these properties are normally mutually exclusive because of the interconnection between scattering mechanisms for charge carriers and phonons. Recent theoretical investigations on sub‐device scales have revealed that nanopillars attached to a membrane exhibit a multitude of local phonon resonances, spanning the full spectrum, that couple with the heat‐carrying phonons in the membrane and cause a reduction in the in‐plane thermal conductivity, with no expected change in the electrical properties because the nanopillars are outside the pathway of voltage generation and charge transport. Here this effect is demonstrated experimentally for the first time by investigating device‐scale suspended silicon membranes with GaN nanopillars grown on the surface. The nanopillars cause up to 21% reduction in the thermal conductivity while the power factor remains unaffected, thus demonstrating an unprecedented decoupling in the semiconductor's thermoelectric properties. The measured thermal conductivity behavior for coalesced nanopillars and corresponding lattice‐dynamics calculations provide evidence that the reductions are mechanistically tied to the phonon resonances. This finding paves the way for high‐efficiency solid‐state energy recovery and cooling. The in‐plane thermal conductivity of semiconductor membranes is reduced by coupling heat‐carrying phonons with localized phonon resonances in deposited nanopillars. This full‐spectrum resonance‐hybridization mechanism decouples the electrical properties of the membrane from its thermal properties, paving the way for novel thermoelectric materials and device designs. The experimental results are a manifestation of nanophononic metamaterial effects previously shown in molecular‐dynamics simulations.