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

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 SiN_xa-Si field-effect structures. It was found that, in a given sample, significant changes in σ*₀ with (ε_c — ε_f)₀ were obtained that could be attributed to the statistical shift of ε_f. 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 ε_c 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 ε_c, 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.