Experimental and Theoretical Investigations of Free Exciton Transport in Si

An all optical Time-of-Flight method is used to study transport properties of laser pulse generated ambipolar carrier systems. Exciting carrier pulses at the front of Si wafers and probing the carrier density by a thin layer at the back yields a high temporal and spatial resolution. The evaluation with a simple hydrodynamical and a kinetic model yields the mobility and the distribution function as a function of space and time at different temperatures. The most important feature of surface generated ambipolar carriers is the motion in a nonequilibrium state over relatively large distances, though no electric field is applied. The moving carriers are identified as free excitons showing the classical mobility μ( T ) ∼ T –1.5 by scattering with acoustic phonons via the deformation potential.