Infrared optical properties of amorphous nickel-phosphorus alloys

Abstract

We have measured the room-temperature reflectivity in the spectral range 0.02-2.5 eV of electrodeposited Ni-P amorphous alloys containing 15-26 at.% phosphorus. The data are well fit in the low-frequency region by the Drude model and the optical conductivities determined from the fit are in agreement with the published dc conductivities for these materials. Alloys with greater than 20 at.% P appear Drude-like to frequencies as high as 1.5 eV while those alloys with less than 20 at.% P begin to deviate from free-electron behavior at frequencies around 0.7 eV. Crystalline samples examined for comparison show structure at even lower frequencies. Values of the plasma frequency ${\ensuremath{\omega}}_{p}$ and the inverse scattering time $\frac{1}{\ensuremath{\tau}}$ have been extracted for the amorphous alloys and both are found to decrease with increasing phosphorus content. Such a result is consistent with the gradual filling of the Ni $d$ band, causing the density of states to decrease. This would imply that the $d$- and $s$-electron density of states enter additively into a single Drude term---possibly as a result of strong $s\ensuremath{-}d$ scattering.