Photoconductivity, trapping, and recombination in discharge-produced, hydrogenated amorphous silicon

Abstract

Photoconductivity, trapping, and recombination have been studied in undoped, hydrogenated amorphous silicon ($a\ensuremath{-}\mathrm{Si}{\mathrm{H}}_{x}$) films prepared by the discharge decomposition of silane. In this study the effects of the photoinduced, reversible conductivity changes have been taken into account in the characterization of the different types of electron trapping and recombination kinetics. These kinetics, which over the temperature range of \ensuremath{\sim}350 to 120 K are found to be consistent with free-carrier transport, are correlated with the densities, energies, and free-carrier capture cross sections of the states in the gap. The electron lifetimes, between \ensuremath{\sim}${10}^{\ensuremath{-}6}$ and ${10}^{\ensuremath{-}3}$ s, are shown to be dependent on two types of recombination centers located at or below midgap with one of these centers having an electron capture cross section, ${S}_{n}$, of \ensuremath{\sim}${10}^{\ensuremath{-}19}$ ${\mathrm{cm}}^{2}$. The electron lifetimes are found to be sensitive to these centers even though their densities are \ensuremath{\lesssim}${10}^{\ensuremath{-}4}$ that of the hydrogen present in the films. The electron trapping is determined by the states above midgap, which have densities of \ensuremath{\sim}${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ ${\mathrm{eV}}^{\ensuremath{-}1}$ over the energy range of \ensuremath{\sim}0.6 to 0.35 eV from the free electron band and for energies within \ensuremath{\sim}0.2 eV, densities of \ensuremath{\sim}${10}^{19}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. No evidence is found for a large peak in the densities of states at \ensuremath{\sim}0.4 eV from ${E}_{c}$, a peak which has been extensively reported for $a\ensuremath{-}\mathrm{Si}{\mathrm{H}}_{x}$ films.