Elastic Properties of Amorphous Thin Films

In an attempt to gain a better understanding of the universal low temperature properties of amorphous solids, a technique has been developed for measuring the elastic properties of amorphous thin films. The technique has been applied to a study of electron beam evaporated amorphous SiO_2 films with thicknesses ranging from 0.75 nm to 1000 nm. We find that the anomalous elastic properties associated with tunneling states in bulk amorphous solids are still present in films with thicknesses as small as 0.75 nm. This indicates that the universal thermal and elastic properties found in amorphous solids are not due to interactions which occur between tunneling states, at least on length scales on the order of the minimum film thickness studied. Additionally, the technique has also been applied to a study of thin films of amorphous silicon and germanium produced by a variety of techniques. In this work, for the first time, we have found two amorphous solids whose low temperature lattice vibrations can be varied by over an order of magnitude depending on the method of preparation. In particular, we report on the elastic properties of a-Si prepared by electron beam evaporation and chemical vapor deposition. We find that the elastic properties of a-Si, which are similar to those found in all amorphous solids, can be significantly altered by the addition of hydrogen which passivates the dangling bonds of the a-Si structure. The presence of hydrogen reduces the density of tunneling states by approximately one order of magnitude below the lower limit observed in all other amorphous solids. The implications of these findings on our understanding of the lattice vibrations of amorphous solids will be discussed.