Secondary Phase Limited Metal-Insulator Phase Transition in Chromium Nitride Thin Films

TL;DR

This study demonstrates that controlling phase composition in chromium nitride thin films reveals the metal-insulator transition, which is hindered by secondary phases during growth and can be recovered through annealing.

Contribution

It shows that the presence of secondary Cr2N phase inhibits the transition, and that reducing Cr flux and annealing can enable observation of the phase transition in thin films.

Findings

Cr2N phase formation inhibits the metal-insulator transition.

Reducing Cr flux during growth yields epitaxial CrN films with transition.

Annealing converts Cr2N to CrN, restoring the transition at ~277 K.

Abstract

Chromium nitride (CrN) is a well-known hard coating material that has found applications in abrasion and wear-resistant cutting tools, bearings, and tribology applications due to its high hardness, high-temperature stability, and corrosion-resistant properties. In recent years, CrN has also attracted significant interest due to its high thermoelectric power factor, and for its unique and intriguing metal-insulator phase transition. While CrN bulk single-crystals exhibit the characteristic metal-insulator transition accompanied with structural (orthorhombic-to-rocksalt) and magnetic (antiferromagnetic-to-paramagnetic) transition at ~260-280K, observation of such phase transition in thin-film CrN has been scarce and highly debated. In this work, the formation of the secondary metallic Cr2N phase during the growth is demonstrated to inhibit the observation of metal-insulator phase transition in CrN thin films. When the Cr-flux during deposition is reduced below a critical limit, epitaxial and stoichiometric CrN thin film is obtained that reproducibly exhibits the phase transition. Annealing of the mixed-phase film inside reducing NH3 environment converts the Cr2N into CrN, and a discontinuity in the electrical resistivity at ~ 277 K appears which supports the underlying hypothesis. A clear demonstration of the origin behind the controversy of the metal-insulator transition in CrN thin films marks significant progress and would enable its nanoscale device realization.