Metastable defects in hydrogenated amorphous silicon

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Abstract
Measurements of the properties of metastable defects in hydrogenated amorphous silicon are presented. The emission energy and time are found with use of deep-level transient spectroscopy. In some cases the metastable states can be associated with specific impurities. The use of transient capacitance and photovoltaic measurements to study light- and current-induced defects in hydrogenated amorphous silicon, a-Si:H, is described in detail for p-i-n and n-i-p solar-cell structures. The sign and annealing energy of defects in the i layer are found. The sign of the defect correlates with the injected charge. Both positive and negative charged defects are produced by uniformly absorbed illumination or double injection. The positively charged defect can be identified with the Staebler-Wronski effect. Both defects cause solar-cell degradation. The activation energy for annealing the negatively charged defect is about 1 eV for most samples. Some samples have a distribution of activation energies. The hole-trap defect has an activation energy higher than 1.4 eV. The defect-production rate is thermally activated with an activation energy slightly higher than the annealing energy. It is argued that the defects are positively and negatively charged dangling bonds, thermally produced when a charge is injected at a band edge.