The thermal conductance between electrons and phonons in a solid state system becomes comparatively weak at sub-Kelvin temperatures. In this work five batches of thin heavily doped silicon-on-insulator samples with the electron concentration in the range of 2.0-16 -1019 cm-3 were studied. Below 1 K all the samples were in the dirty limit of the thermal electron-phonon coupling, where the thermal phonon wavelength exceeds the electron mean free path. The heat flow between electrons and phonons is proportional to (T6e-T6ph), where Te (Tph) is the electron (phonon) temperature. The constant of proportionality of the heat flow strongly depends on the electron concentration and its magnitude is roughly two orders of magnitude smaller than in normal metals like Cu. These properties of degenerate Si make it promising material for many low temperature device applications.
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