Temperature dependence of the positron diffusion length in Ge single crystals

The emitted positronium fraction (f) is measured in Ge single crystals for positron implant energies from 25 to 5025 eV and sample temperatures from 300 to 1090 K. A one-dimensional model is fitted to these data to extract the positron diffusion length (L/sub+/). The gradual decrease in L/sub +/ detected between 600 and 850 K cannot be explained by positron-vacancy trapping. An interpretation in terms of a transition from a Bloch-like positron to a positron-polaron is proposed. An abrupt decrease in L/sub +/ is also observed at 1020 K which could be associated with defects in the near-surface region.