Some problems encountered during the measurement of the activation energy of dark conductivity of undoped hydrogenated amorphous silicon films

Abstract

The effect of band bending at the interfaces in structures of planar configuration can be overcome by using a sandwich configuration for the transport measurement. However, other difficulties arise owing to the presence of ohmic or injecting contacts. In the case of n +/n/n + or a-Si/a-Si:H/a-Si configurations it was shown that if the film thickness L is smaller than the Debye length L D associated with localized states at the Fermi level the activation energy of conductivity measured in the ohmic region of the J(V) characteristics is W′ = W − 2eV 0( 1 − L 2L D) where W is the true activation energy and V 0 is the surface potential corresponding to the case where L 2L D ⪢ 1 . In the injection regime, easily obtained for low values of applied voltage V (typically 10 -1–10 -2 V), the measured activation energy is given by W′ = W − ( 2αε eNL 2 )V where α ≈ 0.6 and N is the density of states at the Fermi level.