Selective-Area Atomic Layer Deposition of Copper Nanostructures for Direct Electro-Optical Solar Energy Conversion

Nanoantennas combined with rectifying diodes have been proposed as new devices for light harvesting in the visible and infrared.  Metallic nanostructures act as antennae to concentrate electric fields at nanogaps and enable the conversion of light into an electrical signal through asymmetric tunneling across metal-vacuum-metal junctions.  Electro-optical conversion occurs when surface-plasmon induced charge oscillations are converted into unidirectional electrical current flow.  The proposed devices would accomplish the electro-optical conversion without the need for semiconductor materials.  The nanofabrication of antenna-coupled tunnel diodes is extremely challenging, and requires sub-nm precision for tuning tunnel junction resistance.  Atomic layer deposition is one of the few methods available with the necessary precision to accomplish this.  In this work, we investigate selective-area atomic layer deposition of copper to modify lithographically-defined antenna structures and form arrays of geometrically-asymmetric tunnel diodes.  Finite-difference time-domain simulations and optical characterization measurements are used to guide antenna design, including shape, polarization dependence, and wavelength response.  Simulations show the electric-field enhancement in the gap to increase significantly with decreasing gap distance; moreover, antenna size and morphology allows for tuning of the peak wavelength response over the infrared to visible range.  Nucleation is found to be a critical factor for tuning the electrical and geometric properties of the tunnel junctions.  Whereas conformal, epitaxial growth is desired, actual growth is sensitive to surface preparation and seed-layer properties.  Analysis of copper growth on palladium seed layers using in-situ spectroscopic ellipsometry reveals a surface chemistry mechanism different than previously understood.  Contrary to expectations, palladium-hydrogen complexes are found to be the stable intermediates during growth cycles, and their sensitivity to surface structure may explain non-epitaxial growth.  The discussion underscores the link between fundamental surface chemistry and advanced applications of atomic layer deposition.