Scanning electron microscope technique for measuring electrical conductivity: application to tetrathiafulvalene--tetracyanoquinodimethane

A movable current source provided by the electron beam of a scanning electron microscope is used to map out the potential distribution on crystal faces containing the a-b crystallographic axes. Silver paint contacts are used to return the beam current to ground and measure voltage changes as the beam position is moved. The results of the new technique are confirmed and complemented by the conventional movable-contact method and the extension of both methods to low temperature is discussed. The potential distributions for our samples reveal frequently occurring irregularities which are attributable to sample imperfections and inhomogeneities in the silver paint contacts. Methods are presented whereby the conductivities cr, and o-b can be determined despite the presence of certain current flow irregularities; room-temperature values are found to be o.b =490+80 (Ocm) ' and 0'p 1 21+0,15 (Ocm) '. The relationship of 0~ and ab to the elements of the monoclinic conductivity tensor for TTF-TCNQ is clarified. The influence of contact inhomogeneities on four-probe measurements of the temperature dependence of a.b( T) as determined with an electrolytic tank model are presented as is an evaluation of two simple tests which detect measurements influenced by current-flow irregularities. In the tank, contact inhomogeneities are found responsible for erroneously large measurements of o.b(T) which occur with small probability and which resemble published data.