Spontaneous Emergence of Optically Polarizing Nanoscale Structures by Co-Deposition of Aluminum with Refractory Metals: Implications for High-Temperature Polarizers

The unexpected growth of highly aligned and optically polarizing metallic fins during physical vapor deposition under modestly oblique conditions is investigated. The fins exhibit nanoscale dimensions and are formed when Al is co-sputtered with any of V, Cr, Nb, Mo, Ta, W, Ru, Fe, Ni, Pt, Zr, Mg, and Ti. It is proposed that the phenomenon is caused by anomalously low atomic mobility in the alloys and intermetallic compounds formed by co-depositing with Al. In contrast, when Cu, Ag, and Au (which diffuse more rapidly in Al) are deposited, no fins form. There is a sharp visible transition in optical properties as the ratio of Al to other element is decreased: the color of the sample changes from black to silver-white for compositions containing less than about 55 atom % Al. The region over which the color change occurs is associated with a very strongly polarized reflectance. Cross-sectional elemental mapping and Monte Carlo simulations suggest that growth of the fins may be nucleated by Al hillocks and enhanced by shadowing effects. The diversity of suitable metals makes this a versatile technique for producing nanoscale polarizing surfaces suitable for high-flux and high-temperature applications.