We propose that Si-H-Si three-center bonds exist in hydrogenated amorphous silicon. These centers give rise to states in the energy gap which have a negative effective electronic correlation energy, $U$. Our model can explain many of the known properties of this material. We make suggestions about how to obtain materials which may prove useful in electronic device applications.

• Received 18 May 1978

DOI:https://doi.org/10.1103/PhysRevLett.41.889