Phonon dispersion effects and the thermal conductivity reduction in GaAs/AlAs superlattices

The experimentally observed order-of-magnitude reduction in the thermal con ductivity along the growth axis of (GaAs)(n)/(AlAs)(n) (or nxn) superlattic es is investigated theoretically for (2x2), (3x3) and (6x6) structures usin g an accurate model of the lattice dynamics. The modification of the phonon dispersion relation due to the superlattice geometry leads to flattening o f the phonon branches and hence to lower phonon velocities. This effect is shown to account for a factor-of-three reduction in the thermal conductivit y with respect to bulk GaAs along the growth direction; the remainder is at tributable to a reduction in the phonon lifetime. The dispersion-related re duction is relatively insensitive to temperature (100 < T < 300 K) and n. T he phonon lifetime reduction is largest for the 2x2 structures and consiste nt with greater interface scattering. The thermal conductivity reduction is shown to be appreciably more sensitive to GaAs/AlAs force constant differe nces than to those associated with molecular masses. (C) 2000 American Inst itute of Physics. [S0021- 8979(00)06415-X].