Hydrogenation of semiconductor surfaces: Si and Ge (111)

The relaxations of hydrogenated Si and Ge (111) surfaces are determined using ab initio self-consistent calculations in a slab configuration. The Si-H and Ge-H bonds are found to be considerably larger than the sum of covalent radii. The substrate relaxations are small and their physical origin can be explained in terms of electronic charge transfer which eliminates the surface dipole moment. The calculated frequencies of the hydrogen vibrational modes are in excellent agreement with experiment. A surface-atom vibrational mode is compared to similar modes in the amorphous hydrogenated materials. The comparison predicts that internal surfaces (microvoids) in the amorphous network are locally much softer than the corresponding crystalline surface configuration.