Transport and Magnetic Properties of TiN Thin Films

Abstract

This research reports a fundamental study of transport and magnetic properties of pulsed laser deposited metallic (film thickness ≈ 25-100 nm) and semiconducting (film thickness ≈ 25 – 10 nm) titanium nitride (TiN) thin films. The study is motivated to discern the critical thickness at which the properties of TiN films digress from bulk-like (3D) to 2D thin films. The study showed that while the crystal structure remains rock salt in the entire thickness range, the films undergo a metal-to-insulator (MI) transition around ~15 nm. The thickness (dimensionality) dependent MI transition, which is free from a structural phase transition, is attributed to a the localization of 3d1 electrons in the valence band, which takes place due to the separation in the energy states larger than the gaps between the valence and conduction bands. Analysis of transport resistivity data in different temperature regions indicated that while the Arrhenius law governs the transport properties for all the TiN films in 300 – 350 K range, the transport properties are governed by a thickness-dependent variable range hopping mechanism below 300 K. The magnetic properties of metallic and semiconducting TiN films were also found to be interesting. A room temperature ferromagnetic behavior was observed for both metallic and semiconducting TiN films, there was a marked correlation between coercivity (Hc) and saturation magnetization (Ms) with TiN film thickness. At room temperature, Hc increased from 66 to134 Oe as the thickness decreased from 100 to 9 nm. Similarly, the Ms (2.8 emu/g) at room temperature for the 9 nm sample was the highest. The maximum values of Hc and Ms are among the best values reported for TiN thin films in the literature. Contrary to previous reported causes of ferromagnetism in TiN due to nitrogen vacancies or oxygen doping, we have attributed the observed magnetic behavior of TiN films to the localized 3d1 electrons in TiN which become more localized with the decrease in film thickness due to the size-dependent enhanced separation of energy states. Thickness dependent ferromagnetism study of TiN thin films can add a new dimension to their current applications and tune the magnetic nature of low dimensional TiN films.