Structure, electrical, and optical properties of reactively sputter-deposited Ta—Al—N thin films

Abstract

We report on the influence of Al content on the structural, electrical, and optical properties of polycrystalline ternary Ta—Al—N thin films. Ta1−xAlxNy thin films with x up to 0.69 and 0.92 ≤ y ≤ 1.22 were deposited on silicon substrates by means of direct current reactive magnetron co-sputtering from elemental Ta and Al targets. The elemental composition, crystal structure, bonding state, and electrical and optical properties of the deposited films were characterized using wavelength-dispersive x-ray spectrometry, x-ray diffraction, x-ray photoelectron spectroscopy, four-point probe electrical resistance, and spectroscopic ellipsometry (SE), respectively. The real part (ɛ1) and imaginary part (ɛ2) of the dielectric constants were derived from the SE data, which were simulated and fitted by using a Drude and Lorentz oscillator models. The Ta1−xAlxNy films are characterized by a compact microstructure, which becomes more columnar with increasing Al content. It is found that the incorporation of Al into TaN lattice results in the stabilization of cubic Ta1−xAlxNy solid solutions up to x = 0.40, while a hexagonal phase is formed for 0.48 ≤ x ≤ 0.69. This structural transition is accompanied by a change in the chemical bonding state, high electrical resistivity (8–10 mΩ cm), and semi-transparent character. Ternary Ta1−xAlxNy alloys with a small amount of Al (x = 0.02 and 0.06) show superior electrical and optical conductivities compared to a binary TaN compound, making them appealing material candidates for UV plasmonic applications.