Contrast in positron reemission microscopy due to positron trapping by dislocations

Positron reemission microscope (PRM) images of 150 nm thick, self-supporting, single crystal Ni(100) films were observed to exhibit high contrast band-like features on the few-μm scale, with boundary definitions of ∼ 0.1 μm. Transmission electron microscope (TEM) images of the films revealed no individual features on the μm scale that would give rise to positron trapping or strong scattering by impurities. Also an examination of the surfaces with a scanning electron microscope (SEM) showed no surface composition variations that could be related to the PRM images. At 10 000× magnification the TEM revealed μm-scale domains containing large dislocation densities, ∼109/cm2, as distinct from domains having ∼107/cm2 or less dislocation densities. Although the lateral size of the individual dislocations were observed to be less than 100 nm, the net effect of a high density of dislocations is proposed to be the contrast mechanism responsible for the μm-scale dark regions observed in the PRM images. The significance of correlating μm-scale patterns in PRM images with dislocation densities is discussed in regard to providing focusing targets for future high magnification (∼50 000×) PRM studies of monovacancies.