Transient photocharge measurements and electron emission from deep levels in undopeda-Si:H

We report transient photocurrent and photocharge measurements in several undoped hydrogenated amorphous silicon (a-Si:H) specimens in the time domain from 10 ns to 50 s; transients were measured using coplanar electrodes between 260 and 380 K. The photocharge measurements exhibit three features which we identify with electron deep trapping, trap emission, and recombination. We analyzed the typical emission times to obtain the activation energy and frequency prefactor. The activation energy varied between 0.4 and 0.6 eV for different specimens; the frequency prefactor varied exponentially with activation energy (``Meyer-Neldel'' behavior). We discuss the Meyer-Neldel dependence in terms of a temperature shift of D-center levels with respect to the transport edge. The emission time observations also account for the large difference between electron mobility-lifetime products estimated from time-of-flight and steady-state photocurrent measurements.