Nitrogen deficiency and metal dopant induced sub-stoichiometry in titanium nitride thin films: A comparative study

Sub-stoichiometric (nitrogen-deficient) and Nb-substituted (Ti _1-y Nb N, 0 ≤ y ≤ 1) titanium nitride thin films were deposited by means of radio frequency magnetron sputtering on SiO2 and Si (311) substrates and compared. Thickness of TiN films varied in the range 116-230 nm, while the value was constant, at 500 nm, for Ti_1-y Nb_y N films. The sub-stoichiometric TiN_x films deposited at room temperature are amorphous, independent of nitrogen partial pressure (in the range of 1.6 to 3.33 Pa). Annealing of the films at 600 °C resulted in amorphous-crystalline transition only in the film deposited at 3.33 Pa, which crystallized into tetragonal ε-Ti₂ N phase. The other films remained amorphous. The hardness and Young's modulus were highest for the film deposited at a nitrogen pressure of 3.33 Pa, viz., 5.9 GPa and 105 GPa respectively. In contrast, all asdeposited Nb substituted TiN (Ti_1-y Nb N) films crystallized into rock salt structure with (111) orientation up to y = 0.77. The hardness and Young's modulus for Ti_1-y Nb_y N films increased with increase in Nb concentration. The highest values of hardness and Young's modulus obtained were 29 GPa and 320 GPa respectively. This study shows that nitrogen deficient titanium nitrides are more difficult to crystallize and exhibit lower hardness than metal substituted nitrides.