Optical reflectance, dielectric functions and phonon-vibrational modes of reactively sputtered Nb-substituted TiN thin films

Abstract

The optical reflectance, dielectric functions and phonon vibrational modes of Ti_1−x Nb _x N (0≤x≤0.77) thin films are reported. Films of 500-nm thickness were deposited on 316LN nuclear grade stainless steel substrates by radio-frequency magnetron sputtering. The reflectance spectra of the films, in the energy range 1.5 to 5.5 eV, are fitted using the Drude–Lorentz model as the response of one Drude parameter and four Lorentz oscillators. It is demonstrated that the properties studied are dependent on Nb concentration, x, and exhibit a behavior transition threshold at x=0.5. The optical properties studied are closer to TiN for x<0.5 and resemble NbN for x>0.5. For example, the films showed a minimum in reflectance at ∼2.33 eV for values of x up to 0.5, corresponding to the Ti–N charge transfer band. Increase in Nb concentration beyond 0.5 caused a large shift in this energy to 3.2 eV, corresponding to the Nb–N charge transfer excitation. Similarly, the real part of the dielectric function is characterized by a screened plasma energy of 2.25 eV for values of x<0.5 (∼TiN) and 3.25±0.2 eV for x>0.5 (∼NbN). The energy at which the loss function reaches a peak value increases linearly for values of x from 0 to 0.41 and decreases very drastically for x>0.5. Phonon-vibrational modes of Ti_1−x Nb _x N thin films studied by Raman spectroscopy show that Nb substitution in TiN results in first-order Raman scattering. The single-phonon acoustical peak at 270 cm⁻¹ of TiN shifted to 265 cm⁻¹ for x=0.77, while the two-phonon acoustical peak of TiN at 620 cm⁻¹ shifted to 630 cm⁻¹ for the same value of x. The reasons for the existence of a behavior transition threshold in Nb concentration are discussed.