Acoustic Anomalies in Amorphous Thin Films of Si and SiO2

Abstract

The ultrasonic absorption and variation of the sound velocity have been measured in sputtered films of amorphous silicon and amorphous silicon oxide from 0.5 to 300 K at a frequency of 300 MHz. A Rayleigh surface wave propagating along the surface of a piezoelectric crystal, which is covered by a thin film of amorphous Si${\mathrm{O}}_2$, allows a measurement of the well-known acoustic anomalies originating from two-level tunneling systems. In clear contrast, films of $a$-Si do not exhibit these typical anomalies.