Quantum Modeling of Thermoelectric Properties of Si/Ge/Si Superlattices

Using a nonequilibrium Green´s function approach, quantum simulations are performed to assess the device characteristics for cross-plane transport in Si/Ge/Si-superlattice thin films. The effect of quantum confinement on the Seebeck coefficient and electrical transport and its impact on the power factor of superlattices are studied. In this case, decreasing well width leads to an increased subband spacing causing the Seebeck coefficient of the superlattice to decrease. Electron confinement also causes a drastic reduction in the overall available density of states. Results show that confinement effects in the silicon barrier are responsible for a 40% decrease in the electrical conductivity of superlattices where barrier films are thinner by a factor of 1.5. In the same two devices, there is a negligible change in the Seebeck coefficient, which results in a decrease in the power factor corresponding to the decrease in conductivity. This decrease in the electrical performance for superlattices with thinner layers may offset the previously hypothesized gains of highly scaled superlattice structures resulting from reduced thermal conductivity. Simulations of the present superlattice structure at varying doping levels show a decrease in power factor with a decrease in device size parameters.