Structural, electronic, and superconducting properties of MBE-grown tantalum nitride films on c-plane sapphire

Abstract

Two single crystal phases of tantalum nitride were stabilized on c-plane sapphire using molecular beam epitaxy. The phases were identified to be δ-TaN with a rock salt cubic structure and γ-Ta2N 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. The Raman spectrum of the phase-pure δ-TaN and γ-Ta2N obtained will serve as a future reference to determine phase purity of tantalum nitride films. Furthermore, 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 μΩ · cm for the 30 nm δ-TaN film and 75.5 μΩ · cm for the 38 nm γ-Ta2N film and become superconducting below 3.6 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 δ-TaN film and 96 mT and 59 nm for the 38 nm γ-Ta2N film. These tantalum nitride films are of high interest for superconducting resonators and qubits.