Use of Field-Effect Density Modulation to Increase ZT for Si Nanowires: A Simulation Study

Abstract

Modulation doping is a promising means of increasing the electrical conductivity of thermoelectric (TE) materials and achieving a high figure of merit (ZT). We compared, qualitatively and quantitatively, the TE performance of a field-effect density modulated Si nanowire channel of diameter D = 12 nm with that of its doped counterpart, by use of self-consistent atomistic tight-binding simulations coupled to the Boltzmann transport equation. We describe the simulation model, and show that as a result of a large improvement in electrical conductivity, gating, rather than doping, can result in greater than three-fold improvement of the TE power factor. Despite the large increase in the electronic part of the thermal conductivity, the total thermal conductivity is still dominated by phonons. Thus, a ZT more than three-fold higher can also be achieved in the gated channel compared with the doped channel. Finally, we show that the power factor peak is obtained when the Fermi level resides ∼k B T below the band edge, as is observed for doped channels.