Research: Nanoscale Heterostructures
This project is interested in evaluating novel interface behavior that occurs when the contacts are transitioned from the macro scale to the nanoscale.The electrical behavior of semiconductor/metal contacts having spatially inhomogeneous barrier heights has attracted much attention both theoretically and experimentally. Analytical theories and numerical simulations indicate that the current density through small, low-barrier-height regions on the surface of an otherwise high barrier height semiconductor/metal contact should be a strong function of the spatial dimensions of the low barrier height regions. Specifically, provided that the dimensions of the low barrier height regions are sufficiently small, and the band bending in the surrounding high barrier height regions is sufficiently large, the effective barrier height of such spatially inhomogeneous contacts is predicted to be much higher than that produced when the low barrier height regions act independently of (i.e., in a purely area-weighted fashion with respect to) the high barrier height regions.The pinch-off effect could also be important for semiconductor electrodes coated with metal islands, in that it would produce an electrode that could effectively direct minority carriers toward the catalytic metal sites while not incurring the majority carrier recombination effects that would otherwise deleteriously affect the properties of a semiconductor interface having a high fraction of its area covered with low barrier height semiconductor/metal regions.
To address this issue, an array of organized, periodic, nanometer-sized low barrier height Si/Ni contacts has been interspersed among high barrier height Si/liquid contacts, and the electrical properties of this system have been investigated as a function of the scale length of the Si/Ni and Si/liquid regions. To form the contacts, bilayers of close-packed latex spheres were deposited on Si. The spheres formed a physical mask through which Ni was evaporated to form regularly-spaced Si/Ni contacts. By varying the diameter of the latex spheres from 500 nm to 100 nm, geometrically self-similar Si/Ni structures having triangulary shaped Si/Ni regions of nm in edge length, respectively, were produced. The resulting Si surfaces were then used as electrodes in a solid/liquid junction formed with 1.0 M LiClO4-1,1-dimethylferrocene+/0 in methanol. This procedure produced a geometrically-defined array of high barrier height n-Si/liquid contacts interspersed with low barrier height n-Si/Ni contacts. The current-voltage and photoresponse properties of these mixed barrier height contacts were strongly dependent on the size of the low barrier height contact regions even though the fraction of the Si surface covered by Ni remained constant. Electrodes formed from the largest dimension Si/Ni and Si/liquid contacts behaved as expected for two (area-weighted) Schottky diode regions operating independently and in parallel, whereas the small scale length Si/Ni and Si/liquid contacts behaved in accord with effective barrier height theories that predict a pinch-off effect for mixed barrier height systems of sufficiently small physical dimensions.
Current work is attempting to exploit this ability to make nanostructured interfaces to investigate the properties of single domains of materials as well as to prepare solid state structures having novel device properties.
Related Publications:
Robert C. Rossi, Ming X. Tan and Nathan S. Lewis, Size-Dependent Electrical Behavior of Spatially Inhomogeneous Barrier Height Regions on Silicon Surfaces, Appl.Phys. Lett, 2000, 77, 2698-2700.