H-2 molecules in crystalline silicon

Infrared spectra from low-doped, hydrogenated silicon have revealed very we ak absorption from hydrogen molecules. In Czochralski silicon, vibrational modes from molecules paired with interstitial oxygen atoms (O-i-H-2) have b een identified, together with a vibrational mode (v(3HH)) from molecules tr apped at a second site. A low temperature annealing study (T < 200 degrees C) has now led to the proposal that this second site is an interstitial lat tice site with the axes of the isolated molecules aligned along either [111 ] or [110] to account for their IR activity. The v(3HH) mode is also detect ed in hydrogenated, boron-doped float zone (FZ) Si, together with the stret ch mode of H-B pairs. An estimate of the molecular concentration indicates that this is the source of so-called 'hidden hydrogen' that is observed for boron-doped Si. This identification is confirmed in the present sample by a second low temperature annealing study. Further work is in progress to es tablish the dissociation mechanism of H-2 molecules and the subsequent form ation of H-B pairs. (C) 1999 Elsevier Science S.A. All rights reserved.