Phonon conductivity of insulators and semiconductors

Phonon conductivity calculations for materials from groups IV, III–V, II–VI and I–VII are presented. The method uses an effective relaxation time for a phonon taking into account departure from equilibrium of all other phonons which participate in both three-phonon N- and U-processes. It is shown that an important role is played by the off-diagonal part of the three-phonon collision operator above the boundary regime. In particular it is found that the off-diagonal U-processes operator gives a negative contribution to the conductivity at high temperatures. The role of optical phonons in heat transport and in acoustic-optical phonon scattering is also studied. For materials with the mass ratio m₁/m₂ ? 1 we have used the Debye dispersion law in the ‘reduced zone’ scheme, while for materials with m₁/m₂ > 1 but <4 we have used the Debye dispersion law for the acoustic phonons and the Einstein dispersion law or the optical phonons. Inclusion of the optical phonons does not modify the temperature dependence of the conductivity but does change the magnitude significantly.