Optical properties and electronic structure of dilute Cu-Au, Cu-Zn, Cu-Al, Cu-Ga, Cu-Si, Cu-Ge, Cu-Sn, and Cu-As alloys

Abstract

A systematic study of the optical properties of a large range of $\ensuremath{\alpha}$-phase copper-based alloys was undertaken. The solutes were selected to include elements with one, two, three, four, and five valence electrons. The differential reflectometer, which is capable of determining within a hundredth of an electron volt the energy for interband transitions of electrons, was used for the investigations. Using a line-shape analysis, three different transitions could be identified: (i) A threshold energy for interband transitions around 2.2 eV which involves the upper $d$ bands and the Fermi energy. This transitional energy does not change for solute concentrations up to approximately 1 at.%, confirming the theory by Friedel concerning screening of solute charges at low solute concentrations. Experimental evidence is given which suggests that both the $d$ bands and the Fermi energy are raised by alloying. (ii) Lower $d$-band to Fermi energy transitions around 5 eV behave in many respects similar to the upper $d$-band to Fermi energy transitions, except that the lower $d$ bands appear to be raised much less due to alloying. (iii) A conduction-band to conduction-band transition around 4 eV was observed to decrease in energy with alloying. Copper-gold alloys do not show any changes in the threshold energy due to solute additions. Additional broad structure around 3 eV indicates that transitions originating from gold $d$ bands may be involved.