Electron and phonon transport in silicon nanostructures
Understanding electron and phonon transport in silicon nanostructures is essential for developing advanced electronic and thermoelectric systems. As opposed to electrons that contribute to conduction only near the Fermi surface, phonons show broadband spectrum when they transport heat in silicon. By controlling the dimension and morphology and accounting for characteristic length scale of electrons and phonons, we can engineer their transport properties. In this work, the transport of electrons and phonons in silicon nanostructures was investigated to improve future designs of thermoelectrics as well as nanoelectronics and phononics.
Phonon scattering at boundaries in solid is considered diffusive or specular depending on the wavelength of phonons and the length scales of surface roughness. Thermal conductivity will be at minimum when the phonon scattering is purely diffusive, which is known as the Casimir limit. However, in rough silicon nanowires, the thermal conductivity was found to break the limit. Hence, the role of the roughness parameters (rms, correlation length and p, high frequency roughness factor) of etched VLS nanowires was systematically addressed for the suppression of thermal conductivity. The surface roughness, characterized by amplitude (rms) and lateral length scale of roughness (correlation length), is demonstrated to dictate phonon transport in such nanowires down to 5W/mK. More interestingly, high frequency spectrum window of surface roughness close to dominant phonon wavelength (1 - 10 nm) at room temperature was chosen to correlate to thermal conductivity and it showed better the trend with the high frequency roughness factor. Electroless etched (EE) nanowires regardless of their unique irregular morphologies also further have been shown to follow the trend, which indicates sub-diffusive regime of phonon is mainly governed by high frequency surface roughness.
The measurement platforms where both electrical and thermal properties can be characterized for the same holy silicon (HS) ribbon were first developed. It was unclear how the morphology, the doping concentration, and the dimensions affect the transport properties of nanostructures mainly for thermoelectric applications. Hence we developed this platform for HS ribbons known for enhancing thermoelectric performance. Electron and phonon transport was independently controlled by tuning the neck size, which is the limiting distance between adjacent holes. Owing to accurate thermal conductivity characterization by the superior device design, the necking effect was quantified with a fixed pitch distance. The thermal conductivity in this structure was found below the incoherent phonon scattering regime due to the periodicity in nano-hole array, which possibly opens the new realm of coherent phonon scattering.
Finally, the first demonstration of ballistic phonon transport in the cross-plane direction of silicon has been shown with HS structures. We showed that long wavelength phonons can propagate ballistically up to a few hundred nanometers despite the lateral dimension of 20nm. The fundamental understanding in thermal conduction has direct relevance for heat management in silicon based electronics and thermoelectrics. Furthermore, phononic devices could potentially be realized via ballistic phonon components in silicon nanostructures.