ICT2014 Abstracts

The manipulation of phonons similarly to what is currently achieved with electrons and photons is the ultimate goal to realize a new generation of devices where heat flow can be controlled. Some causes for thermal rectification theoretically predicted, like non-linearity of the structures that produce a strong dependence of the vibrational density of states with temperature or asymmetric electron-phonon interaction occurring at the interface between two dissimilar materials, have been evidenced in few experimental realizations. It has been recently proposed a possible preferential heat flow sense in graded materials.

In this work we demonstrate the thermal rectification of heat flow across a compositionally-modulated Si_1-xGe_x superlattice (SLs).  We overcome the limitation for the bidirectional measurement of thermal conductivity in epitaxial samples by growing a perfect isomorphic structure in direct and reverse order. The set of samples were grown on Si(001)  substrates by MBE, at relatively low temperatures (400^oC) to prevent plastic relaxation, with a linear compositional gradient increasing or decreasing Ge content (x from 0 to 0.55 or 0.70). The final SLs consist of 4 periods of 40 nm of graded Si_1-xGe_x alloy separated by 5 nm of Si. High resolution X-ray diffraction tools are used to map the SL strain and certify structure symmetry.

We use the 3ω method to obtain κ in the T range 50-400 K. The compositionally-graded Si_1-xGe_x SLs show a significant reduction of phonon transport compared to traditional SLs superlattices, in which transport is dominated by the high density of interfaces, with conductivity values below 2.7 W/mK at room temperature. Furthermore, the asymmetry of the structure has a remarkable effect on heat flow propagation, as the thermal conductivity varies by an outstanding difference as large as 40%, if the heat flow is parallel or anti-parallel to the concentration gradient.