Drift and Diffusion of Free Excitons in Si

The drift of free excitons in an applied strain gradient in Si is directly observed for the first time. The exciton luminescence is detected with temporal, spatial, and spectral resolution in order to map the local force field and measure the exciton velocity. An absolute measurement of the momentum relaxation time $\tau$ is made between 1.3 and 20 K and is well explained by acoustic-phonon scattering. Both diffusion and drift regimes are observable, revealing extremely high exciton mobilities.