An investigation of the conductivity prefactor in a-Si as a function of Fermi level position using the field-effect experiment

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Abstract
This paper describes the results of an investigation of the conductivity prefactor σ*0 in a-Si films as a function of the Fermi level position using the field-effect experiment. The values of σ*0 and the relative statistical shift δ of the Fermi level have been calculated from the temperature dependences of the I D(V G) curves in SiNxa-Si field-effect structures. It was found that, in a given sample, significant changes in σ*0 with (εc — εf)0 were obtained that could be attributed to the statistical shift of εf. However, the changes in σ*0 from one sample to another could be explained only if another effect was taken into consideration. This second mechanism is shown to be consistent with the model of a temperature shift of the mobility edge εc proposed previously by Spear et al. Finally, the shape of the density of states, g(ε), that is consistent with the experimental data was also derived. There is evidence for a peak in g(ε) at 0·70–0·75 eV below εc, which could be identified with a distribution of negatively charged dangling bonds. Also the present results appear to be consistent with a conduction-band tail-state distribution that is linear near the electron mobility edge and varies quasi-exponentially at lower energies.