Free-exciton diffusion and decay in zero-stress Ge

We report the first direct measurement of the free-exciton (FE) diffusivity $D_x$ in germanium. The evolution in time of spatial profiles of FE luminescence were measured. From these FE density profiles we determined the diffusivity $D_x$(4.2 K)≊300 ${\mathrm{cm}}^2$ ${\mathrm{s}}^{\mathrm{-}1}$ which is limited by phonon scattering, and the surface recombination velocity S≊3000 cm? sup -1—, and an FE lifetime ${\tau }_x$≊27 μs for dislocation free Ge and ${\tau }_x$≊12 μs for dislocated Ge with surface recombination effects on these ${\tau }_x$ excluded. Evidence is presented which suggests that the FE recombination velocity S at a crystal surface is not sensitive to the detailed character of that surface. It is concluded that previous measurements could have been dramatically affected by the surface recombination of FE, especially those measurements made on crystals whose dimensions are on the order of the FE diffusion length. An FE-phonon scattering time ${\tau }_p$(4.2 K)≊2×${10}^{\mathrm{-}10}$ s is deduced from the measured diffusion constant.