Effect of free carrier accumulation or depletion on zone-center vibrational mode in Ge

Heavy impurity doping is known to soften the lattice system in Si and Ge, experimentally [1, 2]. Although it is theoretically explained by the Fano-type electron-phonon interaction [3], it is practically important as well whether this effect may come from heavy impurity doping or high carrier density in semiconductors. Moreover, it may become more important in nano-devices, because the whole channel will be either in volume inversion or volume accumulation. It is, however, difficult to experimentally distinguish free carrier effects from dopant atom ones in heavily doped semiconductors. Then, the objective of this work is to clarify which plays a more significant role, by studying electron-phonon interactions in lightly doped (carrier accumulation) as well as in heavily doped Ge (carrier depletion), experimentally.