The Temperature Dependence of Positron Diffusion in Germanium and Silicon.

Positron interactions with low-index surfaces of single crystals of Ge and Si have been investigated. A "slow-positron beam" source (at Brookhaven National Laboratories) of monoenergetic ((DELTA)E(,+) < 0.5 eV) positrons with controllable implantation energies (E(,+) = 0 (--->) 6 keV) was used in an ultra-high vacuum (UHV) surface chamber. All samples were Ar-sputtered to remove surface contamination and annealed in situ. Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED) were used to determine surface cleanliness and crystallinity respectively. The fraction (f(,Ps)) of positrons annihilating from the positron -electron bound state (positronium, Ps) after leaving the sample was determined as a function of implantation energy (E(,+)) and sample temperature. A one-dimensional diffusion model was fitted to the f(,Ps) (E(,+)) data at each temperature to determine the positronium branching ratio at the surface (f(,0)) and a parameter (E(,0)) related to the positron diffusion length (L(,+)) in the bulk. The Ps branching ratio is well described by a model of thermally-activated desorption from a surface state as has been found for metals. In Ge, E(,0) decreases rapidly above (TURN) 600 K and then starts to rise again above (TURN) 900 K. This behaviour has been interpreted in terms of a two-state model of self -trapping of the positron in a lattice dilation. Above (TURN) 1000 K both the surface parameter f(,0) and the near-surface parameter E(,0) show evidence of an abrupt transition. This may be of the order-disorder type found in Si at (TURN) 1150 K and observed, in the present studies, to change the surface trap (f(,0) decreases). The diffusion -related parameter (E(,0)) shows little temperature dependence in Si until, above (TURN) 950 K, an increase is seen which may be due to changes in the near-surface electric field created by an inversion layer, or activated motion of a self-trapped state. Exposure of the Ge(110) surface to oxygen was observed to increase the slow-positron yield.