H2 molecules in crystalline silicon

Infrared spectra from low-doped, hydrogenated silicon have revealed very weak absorption from hydrogen molecules. In Czochralski silicon, vibrational modes from molecules paired with interstitial oxygen atoms (OiH2) have been identified, together with a vibrational mode (ν3HH) from molecules trapped at a second site. A low temperature annealing study (T<200°C) has now led to the proposal that this second site is an interstitial lattice site with the axes of the isolated molecules aligned along either 〈111〉 or 〈110〉 to account for their IR activity. The ν3HH mode is also detected in hydrogenated, boron-doped float zone (FZ) Si, together with the stretch 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 establish the dissociation mechanism of H2 molecules and the subsequent formation of HB pairs.