Electric-field dependence of low-temperature recombination in a-Si:H.

Abstract

Detailed results on the quenching of photoluminescence (PL) by electric fields in hydrogenated amorphous silicon (a:Si:H) are presented. Applying a modulated voltage to films with a sandwich structure results in a typical quenching value of 40% at an electric field of ${10}^{5}$ V/cm and a temperature of 80 K. By analyzing the photocurrent transients we observe a non-exponential dielectric relaxation. We measure the field quenching as a function of excitation intensity and temperature. Below a generation rate of \ensuremath{\sim}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ the field quenching is constant, whereas above this value it gradually decreases. This phenomenon is interpreted in terms of a transition from geminate to nongeminate recombination. Considering the temperature dependence of the PL quenching we observe almost identical dependence on electric field above \ensuremath{\sim}80 K. Below 80 K the quenching is weak. The experimental data are discussed on the basis of various models, including the Poole-Frenkel effect, carrier separation in extended states, and field induced hopping in tail states (the effective temperature approach). Good agreement between experiment and theory is provided by the latter model with a linear dependence of the effective temperature on electric field and temperature, in contrast to the relationship previously derived by Marianer and Shklovskii. We also performed time resolved PL measurements as a function of electric field. We observe a shift of the peak of the lifetime distribution around 10 \ensuremath{\mu}s at 80 K with electric field to shorter times, indicating an enhancement of nonradiative recombination processes. At 10 K the millisecond lifetime, which is attributed to a mostly radiative transition, shows a slight increase with field. This is supposed to be due to separation of electrons and holes by field induced hopping in band tails, thereby increasing the electron-hole distance.