Preparation, Structure, and Properties of Sputtered, Highly Nitrided Tantalum Films

Abstract

Highly nitrided films of tantalum have been prepared by cathodically sputtering high-purity tantalum in a pure nitrogen atmosphere. The effects of cathode current density and film thickness on the film structure, crystal structure, composition, resistivity, current-voltage characteristics, and optical energy gap were investigated. Transmission electron microscopy showed the films to be apparently discontinuous throughout the range of cathode current densities investigated for 1600 Å thick specimens. The crystal structure determined by electron diffraction analysis was found to be body-centered tetragonal with c=5.01 Å, a=5.77 Å, and c/a=0.87. The presence of small amounts of hexagonal TaN was also observed. Analysis of the gas content showed the amount of nitrogen to be a function of cathode current density and thickness. Electrical resistivity as a function of temperature in the range of 213°–373°K obeyed the relationship ρ=A exp(ΔE/kT), where ΔE decreased with the increasing cathode current density and film thickness. Current-voltage characteristics obtained at 77°K indicated non-Ohmic behavior at large applied voltages. The resistivity could be expressed by log(ρ@V=0/ρ)=AV1/2 above some critical V, where V critical and A are dependent on both film thickness and cathode current density. This behavior could be attributed to conduction by an ``intergrain tunneling'' mechanism. The apparent optical energy gap was found to be in the range of 1.95–2.6 eV depending on sputtering current and thickness.