Hexagonal Si

A great deal of progress has been made in the past years in controlling the growth processes of Si nanowires (NWs). Among other achievements, experimental groups have been able to fabricate hexagonal Si NWs, in which the crystal structure is different from that of Si under normal conditions. In this article we investigate the thermal conductivity of hexagonal Si (both in bulk and NW systems) and compare it to that of cubic ("normal") Si. We find that the thermal conductivity in the hexagonal phase is reduced by as much as 40% with respect to the cubic phase. This reduction is found to be caused by the symmetry reduction of the crystal and the appearance of new three-phonon scattering channels for optical vibrational modes in the 10 THz region.