Determination of the Absorption and the Free Carrier Distribution in Silicon at High Level Photogeneration at 1.06 μm and 294 K

In experiments with high-level photogeneration of carriers in a semi-conductor crystal the concentration and the spatial distribution of the carriers are often not well known. The reason for this is that the influence of the surface and of the free carrier absorption cannot be directly determined but has to be estimated. In this paper we describe an experimental method which allows us to determine the interband absorption coefficient, the free carrier absorption cross section, and the absolute value and spatial variation of the concentration of generated carriers in silicon. A pulsed Nd: YAG laser is used giving pulses of 15-20 ns duration at λ = 1.06 μm. An analytical expression for the nonlinear absorption caused by the generated carriers is presented. We show that it is possible to determine the absorption parameters from the measured attenuation without using a separate probe beam.