Low-temperature enhancement of the thermoelectric Seebeck coefficient in gated 2D semiconductor nanomembranes

An increasing need for effective thermal sensors, together with dwindling energy resources, have created renewed interests in thermoelectric (TE), or solid-state, energy conversion and refrigeration using semiconductor based nanostructures. Effective control of electron and phonon transport due to confinement, interface, and quantum effects has made nanostructures a good way to achieve more efficient thermoelectric energy conversion. Theoretically, a narrow delta-function shaped transport distribution function (TDF) is believed to provide the highest Seebeck coefficient, but has proven difficult to achieve in practice. We propose a novel approach to achieving a narrow window-shaped TDF through a combination of a step-like 2-dimensional density-of-states (DOS) and inelastic optical phonon scattering. A shift in the onset of scattering with respect to the step-like DOS creates a TDF which peaks over a narrow band of energies. We perform a numerical simulation of carrier transport in silicon nanoribbons based on numerically solving the coupled Schrodinger-Poisson equations together with transport in the semi-classical Boltzmann formalism. Our calculations confirm that inelastic scattering of electrons, combined with the step-like DOS in 2-dimensional nanostructures leads to the formation of a narrow window-function shaped TDF and results in enhancement of Seebeck coefficient beyond what was already achieved through confinement alone. A further analysis on maximizing this enhancement by tuning the material properties is also presented.