Vibrational spectroscopy of acceptor-hydrogen complexes in silicon: Evidence for low-frequency excitations

The infrared spectra of acceptor-H centers in passivated Si for B, Al, and Ga provide evidence for an unexpected low-frequency excitation of the complexes. The broad vibrational bands observed near 2000 ${\mathrm{cm}}^{\mathrm{-}1}$ at room temperature shift to higher frequency and narrow dramatically upon cooling to He temperature. While the B-H–related band remains structureless at intermediate temperatures, the vibrational bands observed for Al-H and Ga-H complexes show thermally populated sidebands to the low-energy side of the main vibrational bands. The sidebands indicate the presence of a low-frequency excitation of the complex. We have determined Boltzmann energies of 78 and 56 ${\mathrm{cm}}^{\mathrm{-}1}$ from the intensity of the sidebands as a function of temperature for Al-H and Al-D complexes, respectively. The absence of an anomalously large isotope shift is consistent with this excitation being due to an ordinary vibration rather than a tunneling splitting as is sometimes observed for hydrogen-containing complexes.