On the transient potential in insulators

The methods currently used to study the bulk and surface transport properties of insulators usually consist of charging the sample by corona or electrons, and monitoring the natural decay of the surface potential or its buildup after a temporary short circuit (return potential). These measurements have become quite easy since the advent of reliable potential probes, but their interpretation still raises delicate problems concerning, among others, the sample conductivity and its field dependence, the sample polarization and the interfacial injection efficiency. A discussion of these contributions shows that a strict experimental protocol is required if significant results are to be obtained. An alternate technique proposed here uses a scanning electron microscope (SEM), but no potential probe. Electrons from the gun of the SEM are injected below the surface of a thin insulating sample, having its rear face grounded, then a beam of lower energy, acting as probe, is scattered by the trapped charge and forms on the screen a mirror-image of the gun. If enough charge is trapped, the field from the image charge carries the injected charge down across the sample depth. This causes the mirror to contract at a rate which is related to the mobility of the electrons in the sample. Therefore, the mobility is obtained with the resolution of the microscope. Preliminary results indicate that the mobility of electrons injected in LDPE ranges by at least two orders of magnitude, depending on the local field and the sample morphology.