Hydrogen-induced platelets in silicon: Infrared absorption and Raman scattering

The local vibrational modes of hydrogen-induced platelets in crystalline silicon have been studied using infrared absorption and Raman-scattering spectroscopy. Four overlapping but distinct bands are observed in the region 2000--2200 cm^-1. Hydrogen-isotope substitution confirms that these are hydrogen-related stretching vibrations. Hydrogen concentrations obtained from the integrated infrared band intensities are comparable to total hydrogen concentrations obtained from secondary-ion mass spectroscopy. Relative intensities of the bands depend on sample history and can be varied by annealing. This indicates that the four bands arise from at least three distinct structures. Polarization-sensitive Raman measurements indicate that the Raman-active local modes transform according to the fully symmetric representation of a trigonal point group. One structure possesses distinct ir and Raman-active local modes. We conclude that the platelets are highly ordered structures with fundamental building blocks consisting of silicon atoms with one hydrogen-saturated bond, and that at least one inversion-symmetric structure exists. We compare our results with previously proposed structures.