Directional thermal conductivity of a thin Si suspended membrane with stretched Ge quantum dots

We model a nanomaterial showing a hybrid thermal behavior between dissipative and insulating regimes. The nanomaterial is made up of a thin Si suspended membrane covered by self-assembled Ge quantum dots (QDs) with facets. A membrane plane is constituted from the orthogonal [100] and [001] directions (x and z, respectively). The QDs are stretched in [001] forming nanoscale phonon waveguides. When hot and cold junctions are connected to the membrane following [001], the throughput thermal conductivity alpha shows a significant exaltation with respect to the in-plane orthogonal direction [001] where QD constriction is defined. This property can be used for the design of nanoscale dissipaters to remove heat in only one main direction. Indeed, low leakage heat currents are obtained in other directions so that they cannot affect thermal budget in other parts of a device to cool as a silicon chip. In our theoretical model, a deflection angle beta is taken in a membrane plane from the axis x. The anisotropic thermal conductivity is analyzed as a function of beta. In an example molecular-scale device, alpha can be exalted by 4 to 5 folds, or from 0.7 to 2.9 W/m/K, when beta is increased from 0deg (x) to 90deg (z), respectively. Therefore, the QD-waveguide nanomaterial presents a different thermal insulating behavior in the direction [100] and can as well be used for the design of both dissipative and thermoelectric devices. The transition between both contra effects is obtained for the in-plane close-packed directions <110>.