Direct-current method for differentiating contact and bulk low frequency resistance fluctuations

Abstract

Electronic conductance fluctuations, which have often been used as a detailed probe of electronic transport properties, are known to originate from the metal/material contact region as well as from the material itself. In this article, a measurement technique of conductance fluctuations is presented that differentiates between material bulk fluctuations and contact fluctuations. While it was developed for noise studies on high resistance materials (>100 k Ω) where other methods of eliminating or identifying contact noise may not be possible, the method is applicable for all sample resistance ranges. The measurement circuit is a variation of the well-known four-probe technique used to eliminate the influence of the direct-current contact resistance from the sample resistance measurement. A battery supplies the dc current but does not act as a constant current source, while two low-noise preamplifiers measure the current and voltage fluctuations simultaneously. By comparing the real time fluctuations in the current and voltage across pair-wise variations of the four probes, and in particular, determining whether the current fluctuations are symmetric or antisymmetric with the voltage fluctuations, the spatial region of the sample responsible for the noise can be directly identified. This technique does not eliminate contact noise but is a sensitive means of distinguishing contact from bulk fluctuations. Examples of systems for which this method will be useful include high impedance semiconductors, where an insulating or Schottky barrier at the metal/semiconductor interface may generate noise from a separate process that is comparable to or greater in magnitude than the bulk noise.