Enhancement of the Thermoelectric Figure of Merit in a Si/Si_1-xGex Quantum Well:

In bulk form, Si_1-xGe_x is a promising thermoelectric material for high temperature applications. It has been shown recently that the power factor (and presumably also the thermoelectric figure of merit, Z) of n-type and p-type PbTe quantum wells in a PbTe/Pb_1-xEu_xTe superlattice can be increased relative to bulk PbTe. We show here that a similar increase in the power factor S^2σ in a Si/Si_1-xGe_x quantum well can be achieved over the bulk value through quantum confinement effects. We report the results from theoretical modeling as well as experimental investigations of quantum confinement effects in the presence of δ-doping within the barrier layers. The δ-doping layers are introduced by growing very thin layers of a wide band gap material within the barrier layers in order to increase the effective barrier height and thereby reduce the barrier width necessary for the quantum confinement of carriers within a single quantum well. The overall thermoelectric figure of merit is thus increased due to the reduced barrier width. The δ-doping would also serve to reduce the thermal conductivity in the barriers, by introducing phonon scattering centers at interfaces within the barrier region. The optimum placement of the δ-doping layers within the barrier region is discussed.