Metastable Defects and Recombination in Hydrogenated Amorphous Silicon

Metastable defects were created by light exposure in thin films of a-Si:H. Effects on the electronic properties are investigated and the interpretation is based on a proposed recombination model. Changes in the defect density were measured by Photothermal Deflection Spectroscopy (PDS) and Electron Spin Resonance (ESR) for different exposure times. The samples were also characterized by dark conductivity and photoconductivity as a function of light flux. A number of parameters were monitored along the photodegradation process: Fermi level position, dangling bond density, Urbach slope, density of paramagnetic centers, photoconductivity, and the exponent of its light intensity power dependence. The experimental results are consistent with numerical calculations with a recombination model involving localized band tail states and one class of correlated dangling-bond states. The calculations make it possible to explain light-soaking effects on the transport and recombination properties of the a-Si:H films as a consequence of changes in the electronic occupation of the gap states produced by light-induced defects. (Author)