  Several hitherto unexplained features of thermal conductivity in group IV
materials, such as the change in the slope as a function of sample size for
pure vs. alloyed samples and the fast decay in thermal conductivity for low
impurity concentration, are described in terms of a transition from a
collective to kinetic regime in phonon transport. We show that thermal
transport in pure bulk silicon samples is mainly collective, and that
impurity/alloy and boundary scattering are responsible for the destruction of
this regime with an associated strong reduction in thermal conductivity,
leaving kinetic transport as the only one allowed when those resistive
scattering mechanisms are dominant.
