  In a lot of systems, charge transport is governed by local features rather
than being a global property as suggested by extracting a single resistance
value. Consequently, techniques that resolve local structure in the electronic
potential are crucial for a detailed understanding of electronic transport in
realistic devices. Recently, we have introduced a new potentiometry method
based on low-energy electron microscopy (LEEM) that utilizes characteristic
features in the reflectivity spectra of layered materials [1]. Performing
potentiometry experiments in LEEM has the advantage of being fast, offering a
large field of view and the option to zoom in and out easily, and of being
non-invasive compared to scanning-probe methods. However, not all materials
show clear features in their reflectivity spectra. Here we, therefore, focus on
a different version of low-energy electron potentiometry (LEEP) that uses the
mirror mode transition, i.e. the drop in electron reflectivity around zero
electron landing energy when they start to interact with the sample rather than
being reflected in front of it. This transition is universal and sensitive to
the local electrostatic surface potential (either workfunction or applied
potential). It can consequently be used to perform LEEP experiments on a
broader range of material compared to the method described in Ref. [1]. We
provide a detailed description of the experimental setup and demonstrate LEEP
on workfunction-related intrinsic potential variations on the Si(111) surface
and for a metal-semiconductor-metal junction with an external bias applied. In
the latter, we visualize the Schottky effect at the metal-semiconductor
interface. Finally, we compare how robust the two LEEP techniques discussed
above are against image distortions due to sample inhomogeneities or
contamination.
