Internal friction measurements of low energy excitations in amorphous germanium thin films

Abstract Motivated to create a germanium analog to nearly two-level-tunneling-system (TLS)-free amorphous silicon, six germanium films, all about 350 nm thick, were deposited by molecular beam epitaxy onto substrates held at temperatures between room temperature and 280∘C. The internal friction and speed of sound of the films was measured between 375 mK and 300 K. Although the intent was to study amorphous thin films, those grown at 200∘C and higher were shown to be at least partially crystalline. The tunneling strength C, a measure of the interaction between phonons and two-level tunneling systems, decreased monotonically with increasing substrate growth temperature, and for films grown on substrates at temperatures above room temperature, C was below the glassy range. The lowest value for an amorphous film, for the 160∘C-grown film, was C = 1.9 × 10 − 5 .