The correlation of electronic properties with nanoscale morphological variations measured by SPM on semiconductor devices

In this paper we present two examples of how scanning probe microscopy (SPM) techniques can be used to correlate the electronic properties with the surface topography of electronic devices down to the nanometre range. There is an increasing need for such techniques as the size of devices shrinks continually and therefore nanometre-sized features become more important. To illustrate the usefulness of SPM we first focus on scanning tunnelling microscopy/scanning tunnelling spectroscopy (STM-STS) studies of sintered nanocrystalline gas sensors made from 8 nm SnO2 nano-powder. Analysis of simultaneously acquired STS measurements and STM topography show that the apparent surface band gap measured by STS was significantly narrower at nanoparticles' boundaries than at centres. Changes in the conduction band edge position were much more pronounced than for the valence band and are mostly responsible for the variation in band gap. The preferential chemisorption of oxygen species at sintering necks causing an increase in surface state density has been put forward as a possible interpretation. The second example shows how a modified scanning near-field optical microscope (SNOM) can be used to simultaneously acquire topographic and photocurrent maps of Ni-SiC Schottky diodes. The photocurrent was generated by illuminating with laser light emitted through the nanometre sized SNOM aperture, thus locally exciting electrons over the Schottky barrier. By comparing the photocurrent and topography maps it is therefore possible to detect, with submicron resolution, changes in the local electronic properties of the interface. We have shown that annealing the Ni contacts produces clustering on the surface and an increase in the Schottky barrier height. Local measurements of the photocurrent were mapped simultaneously with the topography and the possibility of artefacts discussed. The photocurrent map of annealed contacts indicated a switching behaviour and the generation of voids within the Ni surface/interface has been proposed as a probable mechanism for this effect.