Correlation of dynamical, vibrational and optical properties in c-Si with bond-centered-hydrogen and other hydrogen complexes

Hydrogen, introduced into crystalline (c-) or amorphous (a-) silicon (Si), plays an important role in improving Si properties for photovoltaics application. Low temperature proton implantation in c-Si and a-Si or H-doping of Si films introduces metastable hydrogen in the bond-centered-position (BCH), which dissociates with increasing temperatures into new metastable complexes. Using ab-initio molecular dynamics we report on the stability of BCH in crystalline Si and its temperature-induced dissociation products at close to solar cell operating temperatures, mainly the newly experimentally discovered H
2
** dimer, and the H
2
* dimer, as well as isolated interstitial hydrogen and monohydrides. Each complex leaves a characteristic signature in the frequency spectrum and in the imaginary part of the dielectric constant that agrees well with experiments. All complexes modify the vicinity of the energy gap of clean c-Si. BCH introduces characteristic donor levels just below the conduction band that are occupied, hence raising the Fermi energy considerably, and causing a strong peak in the imaginary part of the dielectric constant in the infrared. The H
2
** and H
2
* dimers introduce a low energy tail in the imaginary part of the dielectric constant. The results are important for experimental in-situ optical characterization of Si film growth.