ROLE OF DANGLING BOND CHARGE IN DETERMINING MU-TAU PRODUCTS FOR A-SI-H

By using a model of recombination via dangling bond states, one shows that the steady-state mu tau products deduced from photoconductivity and ambipolar diffusion length measurements, executed on uniform a-Si:H layers, are products of the form: band mobility x recombination time. The latter are dependent on the occupation functions f(+), f(o), f(-) of dangling bond states. They are not material constants and their values differ considerably according to the specific experimental situation. The mu tau products of the form band mobility x capture time are those that have a direct significance for material quality and device calculations, since f(+), f(o) and f(-) do not enter into their definitions. Particular conditions on f(+), f(o) and f(-) are required so that recombination times are reduced to capture times, in other cases a connection procedure is necessary. The situation for mu tau products derived from time of flight measurements is shown to be similar, except that here one has to consider the occupation functions f(+), f(o) and f(-) in the dark state rather than under illumination. An illustrative example based on experimental data is given.