Microstructurally engineered, optically transmissive, electrically conductive metal films

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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 parallel 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 λ. 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 residual linear 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 λ. 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.