Niobium nitride films of nominal thickness 1 mum and superconducting c
ritical temperature of 14 K were prepared by reactive d.c. magnetron s
puttering. A spectroellipsometer was used to measure the pseudo-dielec
tric constant [epsilon] over the spectral interval 1.5-5.0 eV for film
s prepared at different nitrogen flow rates. Spectrophotometer measure
ments of film reflectivity were made, which confirmed the ellipsometer
results.