Comprehensive numerical simulation of defect density and temperature-dependent transport properties in hydrogenated amorphous silicon.

Abstract

In this work the mobility-lifetime products of electrons and holes, (\ensuremath{\mu}\ensuremath{\tau}${)}_{\mathit{e}}$ and (\ensuremath{\mu}\ensuremath{\tau}${)}_{\mathit{h}}$, the response time of the photocurrent ${\mathrm{\ensuremath{\tau}}}_{\mathit{R}}$ and the electron drift mobility ${\mathrm{\ensuremath{\mu}}}_{\mathit{d}\mathit{e}}$ have been numerically simulated as a function of the dangling bond density ${\mathit{N}}_{\mathit{d}}$ and temperature T in hydrogenated amorphous silicon. We have considered all possible recombination and reemission processes occurring between extended and localized states. The simulated results are in good agreement with experimental data. (\ensuremath{\mu}\ensuremath{\tau}${)}_{\mathit{h}}$ in undoped a-Si:H is insensitive to ${\mathit{N}}_{\mathit{d}}$ in the low-${\mathit{N}}_{\mathit{d}}$ range in contrast to (\ensuremath{\mu}\ensuremath{\tau}${)}_{\mathit{e}}$. This asymmetric ${\mathit{N}}_{\mathit{d}}$ dependence of (\ensuremath{\mu}\ensuremath{\tau}${)}_{\mathit{e}}$ and (\ensuremath{\mu}\ensuremath{\tau}${)}_{\mathit{h}}$ is attributed to the inherent asymmetry of the density-of-states distribution of the conduction- and valence-band tails. The \ensuremath{\mu}\ensuremath{\tau} products and ${\mathrm{\ensuremath{\tau}}}_{\mathit{R}}$ decrease with the generation rate, whereas ${\mathrm{\ensuremath{\mu}}}_{\mathit{d}\mathit{e}}$ increases. The effect of thermal broadening of the band-tail states must be taken into account in the simulation, especially the increase of the characteristic energy of the conduction band-tail states with T. Apart from the defect states, the band-tail states play a very important role in the determination of the photocarrier lifetimes. In the low-${\mathit{N}}_{\mathit{d}}$ range, recombination via the band-tail states dominates over that via dangling bonds, while dangling bonds become the predominant recombination centers in the high-${\mathit{N}}_{\mathit{d}}$ range. The transition from the tail-state-dominated to the defect-state-dominated recombination process depends essentially on the defect density, the temperature, the generation rate, and the Fermi-level position.