Field-enhanced generation in hydrogenated amorphous silicon

The study of the electric field dependence of carrier thermal emission from deep traps has provided important information on deep defects in III-V semiconductors. In this paper we analyze the effect of the electric field on the electron-hole thermal generation process in hydrogenated amorphous silicon. A detailed model is proposed to explain the experimental field dependence of the thermal generation process. The calculation was carried out by taking into account the trivalent statistics for deep states and two field mechanisms, which enhance the carrier emission, the Poole-Frenkel effect, and thermally assisted tunneling. The Poole-Frenkel effect alone is not strong enough to account for the experimental field dependence of the thermally generated current, and the thermally assisted tunneling mechanism must be included in order to reproduce the experimental results. If a strong electron-phonon coupling is considered within an “electron-lattice interaction” model for the thermally assisted tunneling process, a good agreement with the experimental data is obtained for a carrier effective mass value m*=0.3me. This value is very close to that predicted by effective-mass theory calculations for amorphous silicon. We discuss the possibility of including in the model other nonstandard processes, such as the lowering of the mobility edge induced by the electric field and defect relaxation. Hierarchical defect relaxation processes may explain some of our previous results concerning high electric field forming of reverse biased p-i-n diodes.