First-order optical and intervalley scattering in semiconductors

The scattering rate and momentum relaxation time for nonpolar optical and intervalley scattering is determined for the case in which the interaction matrix element is of first order in the wave vector of the phonon. This process is expected to be important in many cases in which the zero-order interaction is forbidden by symmetry selection rules. The results are applied to silicon where, for reasonable values of the coupling constants, scattering via first-order coupling is comparable to the weakly coupled value that until now has been improperly assumed for the zero-order low-energy phonon interaction, which is actually forbidden.