LOW-ENERGY EXCITATIONS IN AMORPHOUS FILMS OF SILICON AND GERMANIUM

We present measurements of internal friction and shear modulus of amor phous Si (a-Si) and amorphous Ge (a-Ge) films on double-paddle oscilla tors at 5500 Hz from 0.5 K up to room temperature. The temperature-ind ependent plateau in internal friction below 10 K, which is common to a ll amorphous solids, also exists in these films. However, its magnitud e is smaller than found for all other amorphous solids studied to date . Furthermore, it depends critically on the deposition methods. For a- Si films, it decreases in the sequence of electron-beam evaporation, s puttering, self-ion implantation, and hot-wire chemical-vapor depositi on (HWCVD). Annealing can also reduce the internal friction of the amo rphous films considerably. Hydrogenated a-Si with 1 at.% H prepared by HWCVD leads to an internal friction more than two orders of magnitude smaller than observed for all other amorphous solids. The internal fr iction increases after the hydrogen is removed by effusion. Our result s are compared with earlier measurements on a-Si and a-Ge films, none of which had the sensitivity achieved here. The variability of the low -energy tunneling states in the a-Si and a-Ge films may be a consequen ce of the tetrahedrally bonded covalent continuous random network. The perfection of this network, however, depends critically on the prepar ation conditions, with hydrogen incorporation playing a particularly i mportant role. [S0163-1829(98)07838-2].