Hot phonons in a single heterostructure

The lifetime of long-wavelength optical phonons in bulk semiconductors is known to be a function of electron density, decreasing with increasing density. An explanation of this has been given in terms of the effect of plasmon coupling on the anharmonic decay route. This explanation is limited to small wave-vectors and is not applicable to the case of phonons in general, since their interaction with electrons involves larger wave-vectors for which the plasma coupling is Landau damped. Nevertheless, a similar decrease of lifetime with density is observed for hot phonons in the channel of a GaN HFET. We offer an explanation of this in terms of a lifetime of interface modes significantly shorter than that of bulk modes. Our explanation also applies qualitatively to the observation of the non-monotonic behaviour of phonon lifetime with electron density.