Resolution in scanning near-field cathodoluminescence microscopy

Abstract

The lateral resolution in scanning near-field cathodoluminescence microscopy, in the case of bulk materials, is theoretically and experimentally studied. Although the theoretical resolution of near-field optical collection systems is determined by the probe size and thus should not be dependent on the energy dissipation volume in the material, it is theoretically shown that the contribution of the far-field signal caused by the radiative centers situated far from the probe can alter the resolution by adding a background noise to the near-field signal. In order to see the role of the energy dissipation volume, the electron beam accelerating voltage is varied and its influence on lateral resolution is studied in the case of a material that does not present a large energy transfer range, a fluorite doped with 0.3% europium. Cathodoluminescence images of this one confirm that the resolution is improved by working at low accelerating voltage to limit the energy dissipation volume, hence the contribution of the far-field radiative centers. On the other hand, for materials having a large energy transfer range (several micrometers) and therefore for which the far-field contribution can be strongly disturbing, the question concerning the efficiency of the near-field collection systems arises. Our experimental results on indented MgO crystal, which presents a large energy transfer range, demonstrates that near-field collection, despite the large contribution of the radiative centers situated far from the tip, gives still a better resolution than far-field collection.