Elastic properties of amorphous and nanocrystalline silicon

Abstract

We review the present status of the elastic properties of amorphous silicon (a-Si) and germanium (a-Ge) thin films produced by either plasma-assisted chemical-vapor-deposition (PECVD) or hot-wire (cat) CVD (HWCVD). Using an extremely sensitive c-Si mechanical oscillator, we measure the low temperature oscillator damping for a variety of amorphous and nanocrystalline Si thin films deposited on the oscillator and thus determine their internal friction. This internal friction is a direct measure of the phonon loss mechanisms. We find that HWCVD a-Si with H content below approximately 3 at.% is the first example that the usual two-level tunneling states (TLS) found in all amorphous solids can be reduced by nearly three orders of magnitude. However, the TLS are present in a-Si prepared by other methods. This low H material also shows an enhanced resistance to the Staebler–Wronski effect. We will present results of light soaking on the elastic properties of both HWCVD and PECVD a-Si. Although the low temperature elastic properties of PECVD and HWCVD are significantly different, their room temperature thermal conductivity is virtually the same.