Evaluation of the density of states parameters of a-Si : H by AC photoconductivity measurements and numerical simulation

In this study, we discuss the results of AC-photoconductivity (AC-PC) experiments on intrinsic a-Si:H, and the results of a numerical model developed for AC-PC. High quality samples were illuminated with a monochromatic light in the photon energy region between 1.0 and 1.7 eV. The light was chopped with frequencies ranging from 10 to 600 Hz, and incident photon flux could be varied, between I X 10(13) and 3 x 10(15) ph cm(-2) s(-1). Within these ranges, the magnitude of the photoconductivity and its phase difference were measured as a function of the photon energy, the chopper frequency and the photon flux using a lock-in amplifier. The numerical model has been developed from Simmons-Taylor statistics [Phys. Rev. B4 (1976) 502]. By means of simulation, we studied the influence of the excitation chopper frequency, the photon flux and the photon energy. The choice of the parameters strongly influences the energy and frequency dependence of the phase shift, as well as its absolute value. The most important parameters that affect the absolute value of the phase are charged to neutral capture cross section ratio, c. Application of the simulation model to the considered set of samples allows us to determine both the order of magnitude of the capture cross section and the ratio of the capture cross section of the charged and neutral states. (C) 2003 Elsevier B.V. All rights reserved.