Analysis of the oscillating photocarrier grating technique

Abstract

In this paper we present a complete theoretical analysis of the oscillating photocarrier grating (OPG) method, starting from the generalized equations that describe charge transport and recombination under oscillating grating illumination conditions. The solution of these equations allows us to implement a calculation reproducing the experimental OPG curves. We study both experimentally and from our calculations the dependence of the OPG curves on different external parameters, such as the applied electric field, grating period and illumination intensity. We find that the response of the sample is linked to a characteristic time of the material, which could be the dielectric relaxation time or the small signal lifetime depending on the regime at which the experiment is performed. Therefore, the OPG technique provides a simple method to estimate these parameters. In addition, we demonstrate that the small signal lifetime provides information on the density of states of the material.

Density of states evaluations from oscillating/moving grating techniques

“…[6]. We made measurements using both techniques and we were able to obtain the DOS of the aSi:H sample used for these experiments.…”

“…[6] we have demonstrated that ω max − 1 is related to some characteristic time of the sample, which can be the dielectric relaxation time (τ diel ) or the small signal lifetime for electrons (τ n ′). We have also shown that the small signal lifetime is related to the density of states at the electrons quasi-Fermi level, E Fn , by the approximate equation…”

“…[6] we made a complete analysis of the physics involved in the OPG experiment, explaining the origin of the photocurrent, its dependence on the frequency ω and the typical shape of the OPG/MGT curves.…”

“…Moreover, procedures to extract part of the DOS from measurements involving grating techniques have been proposed [4,5]. Very recently, the oscillating photocarrier grating (OPG) technique emerged as a promising technique to estimate the DOS of hydrogenated amorphous silicon (a-Si:H) [6]. In that publication, the authors argue that the OPG can be considered an ac version of the moving grating technique (MGT) proposed some time ago by Haken et al [2].…”

Obtainment of the density of states in the band tails of hydrogenated amorphous silicon

“…8,9 Some methods based on making interference between two coherent light beams have also been proposed for this part of the DOS of a-Si:H; 10 in particular, we have recently proposed the combined use of Oscillating Photocarrier Grating (OPG) and Moving Grating Technique (MGT) for this purpose. 11 These methods provide information about the small-signal excess electron lifetime, s n 0 , which is the characteristic time with which the total electron concentration adapts to a small change in the illumination intensity. In recent publications, we have proposed a simple and a more sophisticated formula for assessing the DOS in the conduction band tail (CBT) from measurements of OPG/MGT.…”