Photoconductivity and light-induced change in<i>a</i>-Si:H

Abstract

Capture-rate constants for the trapping of free carriers into different gap-state species are determined by calculating dc photoconductivity at intermediate generation rates versus the inverse temperature and fitting to data measured on undoped a-Si:H in the as-grown and light-soaked states. In the as-grown state, recombination is dominated by high-capture-rate centers near the Fermi level (dangling bonds) at and above 300 K. To explain the second peak and rapid decrease in photoconductivity at lower temperatures, an energy-dependent capture rate must be invoked on the smaller-cross-section band-tail states, indicating a multiphonon energy-loss process for the trapping of free charge. After 90 h of exposure to 100 mW/${\mathrm{cm}}^{2}$ of light, the decrease found in photoconductivity is explained only by increases in both dangling-bond recombination at and above room temperature and tail-state recombination at lower temperatures. The increased recombination at lower temperatures after light soaking is attributed to additional donor states located 0.23 eV above the valence-band edge.