Effect of Free Carriers on Zone-Center Vibrational Modes in Heavily Doped<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>p</mml:mi></mml:math>-type Si. II. Optical Modes

Abstract

The effect of free holes on the optical phonons of Si at the zone center is investigated with Raman scattering. The presence of holes produces an inter-valence-band, electronic, broad, scattering background which overlaps with the one-phonon Raman line. This overlap results in a Fano-type continuous-discrete interference. The parameters $\ensuremath{\Gamma}$ and $q$ of the resulting Fano scattering line can be varied by changing (a) the laser wavelength and (b) the free-carrier concentration. A theoretical interpretation of these results based on the detailed valence-band structure of Si and on the resonant characteristics of the vibronic and electronic Raman scattering is presented. The effect of uniaxial stress on this composite Raman scattering of $p$-type Si is also investigated. At large stresses the electronic background disappears and the phonon lines recover the linewidths of the intrinsic material. The behavior in the intermediate-stress range is very similar to that reported for the acoustical phonons in the previous paper.