Diffusivity and surface transition rate of positrons in crystalline silicon as a function of dopant concentration

Positron lifetime measurements have been performed on silicon as a function of dopant concentration with the pulsed low-energy positron system (PLEPS II). The lifetime spectra were analysed with a modified version of Positronfit. Values have been obtained for the lifetimes and the corresponding intensities as a function of implantation energy for each sample. Using a new approach for the solution of the diffusion-trapping-model, the mean lifetime and the surface lifetime intensity are used to calculate the diffusion coefficient D and surface transition rate v. Both parameters vary only smoothly in n-doped and weakly p-doped material. For p-type silicon in the concentration range from 1015 to 1021 cm−3 both values first decrease by two orders of magnitude, leading to a minimum in D and v at ca = 1018 cm−3. For higher acceptor concentrations the values for D and v increase again by one order of magnitude. We believe that the accumulation of positrons in so-called Debye-Hückel clouds around negatively charged acceptors in the crystal is the most important contribution.