Disorder in tetrahedrally bonded amorphous solids

We have recently found that the density of the low-energy excitations in amorphous silicon films and, in particular, in hydrogenated amorphous silicon (a-Si:H) films can decrease by over three orders of magnitude, to a level of complete disappearance in certain cases from that observed in all other amorphous solids. This observation breaks down the universality of these excitations in amorphous solids. Extending this study, we have measured the low-temperature internal friction of a variety of a-Si: H films, hydrogenated amorphous germanium films and amorphous carbon films, under different deposition conditions or post-annealing treatments. We find that the density of the low-energy excitations in these systems is smaller than in all other amorphous solids, and that they vary considerably between different films. We attribute this property to be a unique feature of their tetrahedral bonding, where the overconstrained structure restricts the tunnelling motion of atoms. We conclude that the low-energy excitations in tetrahedrally bonded amorphous solids are sensitive to the existence of local floppy modes caused by structural inhomogeneity.