We report the reduction of thermal conductivity and phonon mean-free-path below the Casimir limit originated by phonon backscattering in silicon nanowires with scallop shaped surface modulation. The scallop surface modulation with a period of 55–110 nm on 200 nm diameter nanowires was created using top-down deep-reactive-ion-etching. The measured thermal conductivity was 15.13 W m−1 K−1, 34% lower than those of nanowires fabricated by metal-assisted-chemical-etching with a similar diameter, and 60% lower than the Casimir limit. An analysis using the phonon Boltzmann transport equation and radiation analogy of ballistic phonons shows that this drastic reduction stems from the phonon backscattering at the scallop surface. The thermal conductivity measured at 300–500 K showed a nearly temperature-independent trend suggesting that the structural control can overcome a material limit. Our results provide a systematic nanostructure control approach to improve the thermoelectric efficiency more than the limit imposed by a material.

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