Scanning tunneling potentiometry study of electron reflectivity of a single grain boundary in thin gold films

Abstract

Spatial variations of the local electric field in current-carrying thin gold films were studied with a scanning tunneling microscope on a nanometer scale. With a refined scanning tunneling potentiometry technique, it was possible to determine the local electric fields within single grains. At grain boundaries, we observe potential drops on length scales of less than 1 nm which exceed the potential difference within a grain greatly. We interpret our findings by applying a theory that models grain boundaries as barriers with a reflectivity R for the conduction electrons. With the assumption of isotropic background scattering within each grain, we determine the local current-density j(x,y) that passes a grain boundary. From that, we obtain the reflectivity of individual grain boundaries and find values of R = 0.7 to R = 0.9 which is much higher than expected from macroscopic experiments.

Simulation of grain boundary effects on electronic transport in metals, and detailed causes of scattering

“…We have presented the first simulations of reflectivity for relaxed twin and non‐twin grain boundaries in Cu and Ag. Our results agree with the experimental reported range of reflectivity 3, 16–18 and with individual grain boundary measurements 19, 18. To gain insight into the mechanisms of grain boundary reflectivity and the impact of the non‐planarity of real grain boundaries, we also investigated the effect of vacancies, orientation, and disorder.…”

“…A survey of experimental results 3, 17, 18 indicates $R$ values for Cu and Ag (computed with MS) in the range of 24–46%, in good agreement with our predictions.…”

“…A more direct comparison can be provided by a few experiments measuring grain boundary resistance directly in a metal. Schneider et al 18 measured potential difference across single grains in Au (an fcc noble metal like Cu and Ag), and found reflectivity in the range 70–90%, depending on orientation. Nakamichi 19 measured interface resistivity $\rho _{{\rm gb}} \equiv R_{{\rm gb}} A$ (where $R_{{\rm gb}}$ is interface resistance) in Al for various single grain boundaries.…”

Optical properties of polycrystalline metallic films

“…These barrier reflectances are higher than those previously reported for other materials using the MS model. 1,25 They are, however, well within the range 0.4 -0.9 of reflectance values measured by scanning tunneling microscopy ͑STM͒ potentiometry on single grain boundaries in Au, 26 and are lower than the value of 0.85 reported by Durkan and Wellan 16 obtained modeling a grain barrier as a missing row of atoms.…”

Local potentiometry using a multiprobe scanning tunneling microscope

“…The method is based on scanning tunneling microscopy 3,4 ͑STM͒, providing simultaneously real space images of the potential distribution and the surface topography. [7][8][9][10][11][12][13][14] It is pointed out that STP is still not a straightforward technique because of the difficulties to prepare suitable systems. 5 Recently, a review on STP was published, 6 summarizing the work done in this field so far.…”