On the transient potential in insulators

The methods currently used to study the bulk and surface transport properti es of insulators usually consist in charging the sample by corona or electr ons, and monitoring the natural decay of the surface potential or its build up after a temporary short circuit (return potential). These measurements h ave become quite easy since the advent of reliable potential probes, but th eir 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 contributio ns shows that a strict experimental protocol is required if significant res ults are to be obtained. An alternate technique proposed here uses a scanni ng electron microscope (SEM), but no potential probe. Electrons from the gu n of the SEM are injected below the surface of a thin insulating sample, ha ving 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 o f the gun. If enough charge is trapped, the field from the image charge car ries the injected charge down across the sample depth. This causes the mirr or 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 t he microscope. Preliminary results indicate that the mobility of electrons injected in LDPE ranges by at least two orders of magnitude, depending on t he local field and the sample morphology.