Electrical and structural properties of direct current magnetron sputtered amorphous TiAl thin films

Abstract

Structural and electrical properties of amorphous TiAl films with 50 at.% Ti and 50 at.% Al deposited with a compound target by dc magnetron sputtering were characterized by X-ray diffraction (XRD), atomic force microscopy, scanning electron microscopy, and electrical measurements. We found that as-deposited Ti0.5Al0.5 films with a thickness below 1000 nm were amorphous, whereas small hexagonal crystallites could be detected in thicker films. Such layers exhibited a compressive stress which increased with decreasing the thickness of TiAl. The behaviour of stress with the temperature was qualitatively similar in films independently on their thickness. Thermal annealing at 300 °C reduced the initial compressive stress and no changes of the phase of TiAl were observed. We correlated these observations with annihilation of point-like defects created by the deposition in thin Ti0.5Al0.5 films or with appearance of nanocrystallites in the amorphous matrix of TiAl. These effects were also responsible for the changes of the resistivity observed at elevated temperatures. We showed that a heat treatment induced by an electronic current with a density of about 2 MA/cm2 led to the crystallization of TiAl films independent on their thickness. A similar effect could be achieved by annealing the samples at 520–550 °C in air or N atmosphere. A significant drop of the resistivity down to 90 μΩcm was detected in the annealed films at room temperature. XRD confirmed that two different phases of TiAl (γ and α2) with randomly oriented crystallites appeared in the annealed films.