Temperature and electric field dependences of transient photocurrents in a-Si:H

Temperature and electric field dependences of photocurrents measured at fixed times after pulsed-light excitation in hydrogenated amorphous silicon have been investigated. The photocurrents plotted against inverse absolute temperature for undoped samples exhibit activated behaviours. It is found that the activation energies for all different fixed times after photo-excitation and for all applied electric fields are the same. The results indicate the existence of quasi-equilibrium trapping level during the transit of excess electrons in undoped samples. However, in boron-doped samples the excess holes communicate with the effective trapping levels which move away from E_v (valence band mobility edge) during the transit within the time scale of measurement, which is consistent with the data that the width of valence band tail is wider than that of conduction band tail.