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

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.