Transient photoconductivity and photo-induced optical absorption in amorphous semiconductors

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Abstract
Amorphous semiconductors and insulators display the phenomenon of dispersive transport: the average mobility of the carriers decreases with time after pulsed excitation. A simple model explains how multiple trapping (MT) in a continuous distribution of localized states gives rise to dispersion. This model is then used to show what can be learned about the material from a complete study of its dispersive transport when MT is known to be the origin of the dispersion. Transient photo-induced optical absorption (PA) provides a direct test of the presence of the MT mechanism. Transient photocurrent (PC) then provides the spectrum of the localized states when examined in the time regime before recombination begins. Using this density of states, many of the parameters that characterize transport and recombination, both monomolecular (MR) and bimolecular (BR), can be determined. The assumption that thermal excitation from localized states to higher-energy transport states limits both thermalization and recombination can be tested directly by a comparison between the time dependence of PA and PC in the BR regime. All the predictions of the model have been tested for the case of a-As2Se3. The major conclusions are that the model completely describes transport and recombination in this material, that a-As2Se3 has an exponential distribution of localized states extending at least from 0·25 to 0·6 eV above the valence-band edge with exponential width ∼0·05eV, and that the mobility in the transport states is quite high (∼4 cm2 V−1 s−1) which is typical of extended-state transport. A number of other important parameters are determined: the capture radius of the recombination centres is found to be ∼ 10 Å, the activation energy for thermal generation of recombination centres is found to be ∼0·5 eV and the prefactor of the density of localized states is found to be ≳ 1021 cm−3 eV−1.