Abstract
Positron beam timing spectroscopy has been used to investigate positron diffusion in hydrogenated amorphous silicon between 85 and 350 K. The diffusivity is determined from both the contribution of the surface, where the positrons have a different characteristic annihilation rate, and its effect on the shape of the bulk annihilation rate distribution. We observe a single positron state with a temperature independent lifetime of 322 ps. From the temperature dependence of the positron diffusion we show that this is a localized state and present the first ever direct observation of hopping diffusion of positrons in solids. The migration enthalpy for positrons in this state is found to be 17.7(3) meV.
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