Negative-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>U</mml:mi></mml:math>States in the Gap in Hydrogenated Amorphous Silicon

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.