EFFECTS OF INTERBAND EXCITATIONS ON RAMAN PHONONS IN HEAVILY DOPED N-SI

Raman-active inter-conduction-band transitions from the 1 to 2 bands in heavily doped n-Si (n 1.5×1020 cm-3) interfere with the zone-center optical phonon to produce Fano-type asymmetric phonon line shapes typical of a discrete-continuum interaction. We have studied the line shapes as a function of exciting frequency and uniaxial stress along the [001] and [111] directions. The asymmetry is removed under [001] uniaxial stress for the doublet component of the phonon that couples to the stress-depleted doublet valley, and is enhanced for the singlet component that couples to the carrier-enhanced singlet valley. We have calculated from microscopic theory the parameters that describe the Fano interaction the asymmetry parameter, the broadening, and the frequency shift due to the self-energy of the phonon deformation-potential interaction with the free electrons in the conduction-band valley. These parameters have been calculated for zero stress and a high stress and a high stress along the [001] direction, and are found to be in excellent agreement with experiment. We have also investigated the second-order acoustical-phonon scattering [2TA(X)]; it shows no change in line shape for heavily doped n-Si. Under uniaxial stress along the [001] and [111] directions, it exhibits only the hydrostatic shift, as does pure Si, which was measured for comparison. © 1978 The American Physical Society.