Observation of Dipole-Forbidden Transitions through Fano Antiresonance in Boron-Doped Silicon

Infrared absorption in boron-doped silicon has been investigated in the photon energy range from 450 to 1000 cm -1 . Below the continuous background absorption by hole transition, a series of profound dips are observed for dipole-forbidden transitions of phonon and electronic states. These transitions become permitted through configuration interaction with the electronic continuum of the hole state, leading to a sharp antiresonance structure in the spectrum. This interpretation is confirmed by our measurement of spectral dependences on acceptor concentration, donor compensation and sample temperature.