Growth and characterization of single crystal cubic TaN and hexagonal Ta$_2$N films on c-plane Sapphire

TL;DR

This study successfully grew and characterized single crystal cubic TaN and hexagonal Ta2N films on sapphire, revealing their structural, surface, and superconducting properties, with potential applications in superconducting devices.

Contribution

It reports the first growth and detailed characterization of phase-pure single crystal TaN and Ta2N films on sapphire using molecular beam epitaxy.

Findings

Both films are metallic at room temperature.

Superconductivity observed below 3.6 K and 0.48 K.

Films exhibit smooth surfaces with RMS roughness < 0.3 nm.

Abstract

Two single crystal phases of tantalum nitride were stabilized on c-plane sapphire using molecular beam epitaxy. The phases were identified to be $\delta$-TaN with a rocksalt cubic structure and $\gamma$-Ta$_2$N with a hexagonal structure. Atomic force microscopy scans revealed smooth surfaces for both the films with root mean square roughnesses less than 0.3 nm. Phase-purity of these films was determined by x-ray diffraction. Raman spectrum of the phase-pure $\delta$-TaN and $\gamma$-Ta$_2$N obtained will serve as a future reference to determine phase-purity of tantalum nitride films. Further, the room-temperature and low-temperature electronic transport measurements indicated that both of these phases are metallic at room temperature with resistivities of 586.2 $\mu\Omega$-cm for the 30 nm $\delta$-TaN film and 75.5 $\mu\Omega$-cm for the 38 nm $\gamma$-Ta$_2$N film and become superconducting below 3.6 K and 0.48 K respectively. The superconducting transition temperature reduces with applied magnetic field as expected. Ginzburg-Landau fitting revealed a 0 K critical magnetic field and coherence length of 18 T and 4.2 nm for the 30 nm $\delta$-TaN film and 96 mT and 59 nm for the 38 nm $\gamma$-Ta$_2$N film. These tantalum nitride films are of high interest for superconducting resonators and qubits.