Donor States in Hydrogenated Amorphous Silicon and Germanium

We report the first direct observation of phosphorus and arsenic donor states in hydrogenated amorphous silicon and germanium by electron-spin resonance. The states are identified via their characteristic hyperfine structure, and are attributed to neutral donors. A comparison with crystalline silicon and germanium shows that the disorder potential of the amorphous phase leads to a stronger localization and a wide distribution of donor states.