Optical properties of Au: Sample effects

Abstract

Reported optical data for Au are investigated to determine the origin of their differences. A single-parameter model representing voids in an otherwise homogeneous medium is shown to account for the major discrepancies in the above-band-gap ($E>\mathrm{}2.5\mathrm{}$ eV) ${\epsilon }_2$ spectra for Au samples prepared in different ways. Ellipsometric measurements on transmission-electron-microscopy (TEM) characterized thin-film samples on an energy range of 1.5-5.8 eV support the void model but show the importance of measuring both ${\epsilon }_1$ and ${\epsilon }_2$ to separate volume from surface film effects. Differences in below-band-gap data arise from at least two mechanisms: grain-size effects in samples with a large volume fraction of imperfections, and increased surface scattering, probably from thermal grooving, in annealed samples. Two mechanisms are required because the lowest values of ${\epsilon }_2$ in the Drude region are shown to occur for unannealed but smooth, moderately thick film samples evaporated on room-temperature substrates. Our best below-band-gap data, taken on electron-beam evaporated samples, show directly the linearly increasing $d$-band to Fermi-level transition threshold near $X$ at 1.8 eV unobstructed by the Drude tail. These data have an ${\epsilon }_2$ value at 1.5 eV equal to within experimental error to that calculated from the scattering lifetime derived from the known resistivity of the bulk metal, indicating a grain size and quality better than anything previously used for optical measurements in Drude region.