Thickness-dependent optical properties of aluminum nitride films for mid-infrared wavelengths

We present a comparative study of optical properties of aluminum nitride (AlN) thin films with different thicknesses. The films with thicknesses ranging from 66 to 303 nm were deposited by reactive sputtering and plasma enhanced atomic layer deposition (PEALD). Permittivity was extracted from an ellipsometry and Fourier-transform infrared (FTIR) spectroscopy reflection measurements. Our particular focus is on the influence of the deposition method and the film thickness on the AlN dielectric function. Here, we show that both PEALD-deposited and sputtered AlN films exhibit a monotonic dependence of negative permittivity on their thickness for vast mid-IR wavelengths including the reststrahlen band around 11.5–15 μm (∼870–667cm−1). By structural analysis, it was shown that PEALD-deposited layers contain a lower amount of oxygen and have smoother surfaces than deposited by the sputtering technique. On the other hand, sputtered films have higher densities and crystallinity, which results in the higher background permittivity (ε∞). In FTIR spectra of the PEALD-deposited layers, two specific features are observed, which can be explained by a higher concentration of nitrogen due to nitrogen-rich plasma in the ALD process. Such peculiarities of the optical properties of AlN films should be taken into account in potential applications of this material for metamaterials and nanostructured systems.