Memory effects in highly resistive p–i–n heterojunctions for optical applications

Abstract

Large area p–i–n diode structures based on amorphous hydrogenated silicon can be used as single element image sensors where the information is read out by a scanning laser beam. A high sensitivity is reached with silicon–carbon alloy contact layers. The higher defect density in the large band gap material is usually a problem for efficient carrier collection in solar cell applications. When used as an image sensor, however, the charge stored in deep defects represents an easy way to realize short-term image storage. In the case of a p-(Si:H)/i-(Si:H)/n-(Si x C 1− x :H) sensor structure we have measured a memory effect of approximately 1% after several minutes of image projection. Metastable sensor degradation is observed in accordance with the Staebler–Wronski effect. Fast degradation of sensor performance — corresponding to 90% erasable image storage capability — was studied in an unalloyed structure using a Nd:YAG laser system. The response can be modeled by a stretched exponential decay with parameters depending on the laser pulse energy.