Microstructurally engineered, optically transmissive, electrically conductive metal films

Abstract

Using standard multilayer and effective medium models, we determine microstructures that optimize the near-IR-visible normal-incidence optical transmittance of electrically conducting metal films intended for use as semitransparent contacts for semiconductor devices such as photodetectors or photoelectrochemical converters. Various conditions are considered, including unpolarized and linearly polarized light and electrical conduction both parallel and perpendicular to the surface. For linearly polarized light. the optimum microstructure consists of parane! metal lines of nominally square cross section oriented perpendicular to the polarization vector of the incident light, regardless of the direction of electrical conduction. The line separation and cross-sectional dimensions must both be small compared to the wavelength A. For unpolarized radiation. the optimum microstructure depends on the direction of electrical conduction. For conduction parallel to the surface, the optimum microstructure again consists of parallel lines with the lines oriented perpendicular to the residuallinear polarization, if any, of the incident flux. For conduction perpendicular to the surface, the optimum microstructure consists of cylindrical metal posts of dimension small compared to A. Expressions are derived that allow the thicknesses and refractive indices of protective antireflection coatings to be calculated to first order in the thicknesses of the metal. films. The more general case of antireflection coatings for anisotropic structures is briefly discussed.

Template synthesis of infrared-transparent metal microcylinders: comparison of optical properties with the predictions of effective medium theory

“…0.26 gm in diameter with lengths ranging from 0.3 gm to 3 gm depending on deposition time. The composites display a significant transparency in the infrared spectrum between 2000 and 4000 cm- 1 . The Au volume fraction and effective medium theory screening parameter K were estimated from scanning electron microscopic analyses of cylinder dimensions and orientations in the composite membranes.…”

“…Recently, Aspnes, et al described the microstructural limits for metal-insulator composites that are both electronically conductive and optically transparent [1].…”

Optical properties of composite membranes containing arrays of nanoscopic gold cylinders

“…Spherical particles have K = 2, whereas long, needle-like particles aligned with their principal axes parallel to the direction of light incidence have K = 1 [5]; these values of K are of interest to the work described here because the Au particles with aspect ratio = 10 approach the long, needle-like limiting case, and the particles with an aspect ratio of unity can be treated as spheres. .…”

“…While bulk metals are optically opaque, effective medium theory (EMT) [41 predicts that collections of small metal particles can be transparent, provided that the wavelength of light employed is much greater than a characteristic dimension of the particle [4]. Hence, EMT can be used as a guide to prepare metal-containing composites that are both electronically conductive and optically transparent [5][6][7][8]. Such composites are inherently interesting from a purely scientific viewpoint and have a variety of possible technological applications [7].…”

“…where fm is the volume fraction occupied by the metal particles and K is a "screening" parameter that depends on the shape of the particles [4,5].…”