Infrared vibrational spectra of amorphous Si and Ge

We report the observation of the room-temperature infrared absorption spectrum of sputtered films of amorphous Si and Ge between 35 and 700 cm^-1. The spectra show a broad frequency range of absorption corresponding to the frequency range of the vibrational density of states in crystalline Si and Ge. We interpret the results in terms of a disorder-induced breakdown of the selection rules for vibronic infrared absorption in much the same way as occurs in Raman scattering. Discussion is given of related phenomena in alloys and neutron-irradiated crystals.