Electrifying hot silicon nanowires

New simulations predict that doping silicon nanowires with heavier silicon isotopes can dramatically lower their thermal conductivity-an important step towards the realization of high efficiency thermoelectric nanowire devices.

Heat is normally thought of as a wasteful by-product, but thermoelectric devices could be used to convert thermal energy into electrical power. When one end of thermoelectric material is hotter than the other, a voltage is generated between the two ends that produces an electric current in an external circuit.

Materials used for fabricating high efficiency thermoelectric devices must easily conduct electricity, but not heat. In this respect, nanostructures are promising as they have a high surface to volume ratio, which causes phonons—waves that transport heat in a material—to scatter frequently and consequently carry heat less efficiently.

Nanomaterials can also be doped and synthesized with great precision to reduce phonon transport, concepts that theoreticians in Singapore are proposing to produce high performance thermoelectric devices from silicon nanowires. Gang Zhang at the Institute of Microelectronics in collaboration with Nuo Yang and Baowen Li of National University of Singapore predict that doping silicon (Si) nanowires with heavier isotopes of Si can dramatically reduce their thermal conductivity.¹

The efficiency with which phonons transport heat in a solid depends on the purity of the crystal structure. Substituting isotopes of Si that have heavier masses than ²⁸Si—the most abundant isotope of silicon—makes it harder for phonons to carry heat through the material.

Zhang and co-workers used molecular dynamics simulations to calculate the thermal conductivity of Si nanowires in which ²⁸Si was substituted randomly with ²⁹Si and ⁴²Si. Notably, the largest decrease in thermal conductivity was found to occur when the doping level was varied up to 20%, with the thermal conductivity dropping to 77% and 27% of the pure ²⁸Si value for ²⁹Si and ⁴²Si dopants, respectively.

Fig. 1: Different isotopes of silicon are periodically arranged in a model silicon nanowire that is placed between ‘hot’ (red) and ‘cold’ (blue) heat baths. The large spheres are ⁴²Si and the small spheres are ²⁸Si, the most abundant isotope of Si. The periodicity can be tuned to increase or decrease the thermal conductivity of the nanowire.

The group has also considered the effects of non-random doping of the nanowires, such as an arrangement in which the wire contains five rows of ²⁹Si and then five rows of ²⁸Si (Fig. 1).

Although only 5% of silicon contains ²⁹Si and, the ⁴²Si isotope is rare and unstable, these findings give a valuable insight into the effects of mass on the thermal conductivity of silicon nanowires.

Zhang says, “Given the ideal interface compatibility between silicon nanowires and conventional silicon-based IC chips, this work could result in huge economic benefits for the electronics industry.”