Hydrogen Flip Model for Light-Induced Changes of Amorphous Silicon

We propose a new metastable defect associated with hydrogen atoms in amorphous silicon. A higher energy metastable state is formed when H is flipped to the backside of the Si-H bond at monohydride sites. The defect is described by a double-well potential energy. The dipole moment of this ``H-flip'' defect is larger and increases the infrared absorption. This defect accounts for large structural changes observed on light soaking including larger infrared absorption and volume dilation.