Optical constants of sputtered hafnium nitride films. Intra- and interband contributions

Abstract

Transparent and opaque films of hafnium nitride have been prepared by reactive magnetron sputtering in an argon-nitrogen atmosphere. The films were deposited upon heated Coming glass substrates with a deposition rate of about 2.1 nm/s. The optical constants of the films were calculated using Kramers-Kronig integration and ellipsometry for the opaque samples and a combined R T method for transparent samples. Detailed observation of the thickness variation in the optical constants revealed a clear trend: larger n-values and smaller k-values in the VIS-and NIR-range for thinner films. This was analysed within the framework of the classical Drude model and found to be an effect of shorter relaxation time for thinner films. The effect is as large as a factor of three in the film thickness interval 15–380 nm. The optical effective mass of the d-electrons was found to be in the interval 0.82–0.95 of the free electron mass, which is significantly lower than in TiN but similar to ZrN. The interband contribution to ϵ 2( ω) was obtained by subtraction of the Drude part from the experimental dielectric function. It exhibits a sharp increase for λ < 400 nm, indicating the threshold for interband transistion.