LIGHT-INDUCED METASTABILITY IN A-GE-H

Light-induced metastable changes in the conductivity of hydrogenated amorphous germanium (a-Ge:H) thin films deposited by rf-sputtering are reported. The films are photoconductive with room temperature photo-to-dark conductivity ratio sigma-ph/sigma-d = 1.0-1.5 under AM1 illumination (100 mW/cm2). The dark conductivity and the photoconductivity of the films decrease after exposure to AM1 irradiation, which is attributed to light-induced defect formation. The changes are metastable and the conductivity returns to its original value after some hours in the dark. In order to establish the kinetics of defect formation and annealing, a differential set-up was used to measure conductivity transients under different temperatures and illumination conditions. The time evolution of the defect generation and annealing processes follows a stretched exponential behavior similar to the one observed for a-Si:H. Both processes are temperature activated, with activation energies of 0.40 and 0.45 eV, respectively. This behavior indicates that the kinetics of defect formation involve excitation over a wide distribution of barrier heights separating the annealed and the metastable states. The connection between defects kinetics and dispersive hydrogen diffusion is analyzed.128324325