Hopping transport of positrons in hydrogenated amorphous silicon - art. no. 217401

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