[Paper Review] Epitaxial titanium nitride microwave resonators: Structural, chemical, electrical, and microwave properties

Ran Gao, Wenlong Yu+7 more|arXiv (Cornell University)|Nov 7, 2021

Semiconductor Quantum Structures and Devices48 references31 citations

TL;DR

This paper demonstrates highly epitaxial titanium nitride (TiN) thin films grown on sapphire substrates via RF-magnetron sputtering at 300 °C, achieving exceptional microwave performance in superconducting coplanar waveguide resonators. The best internal quality factor reached 3.3 × 10⁶ in the single-photon regime and >1.0 × 10⁷ in the high-power regime, establishing epitaxial TiN as a low-loss platform for high-coherence superconducting quantum circuits.

ABSTRACT

Titanium nitride is an attractive material for a range of superconducting quantum-circuit applications owing to its low microwave losses, high surface inductance, and chemical stability. The physical properties and device performance, nevertheless, depend strongly on the quality of the materials. Here we focus on the highly crystalline and epitaxial titanium nitride thin films deposited on sapphire substrates using magnetron sputtering at an intermediate temperature (300$^{\circ}$C). We perform a set of systematic and comprehensive material characterization to thoroughly understand the structural, chemical, and transport properties. Microwave losses at low temperatures are studied using patterned microwave resonators, where the best internal quality factor in the single-photon regime is measured to be $3.3 imes 10^6$, and $> 1.0 imes 10^7$ in the high-power regime. Adjusted with the material filling factor of the resonators, the microwave loss-tangent here compares well with the previously reported best values for superconducting resonators. This work lays the foundation of using epitaxial titanium nitride for low-loss superconducting quantum circuits.

Motivation & Objective

• To develop a low-thermal-budget, scalable method for growing high-quality epitaxial TiN thin films on sapphire substrates.
• To systematically correlate structural, chemical, and electrical properties of epitaxial TiN with its microwave performance.
• To achieve ultra-high quality factors in superconducting microwave resonators using epitaxial TiN, enabling high-coherence quantum circuits.
• To provide a deterministic platform for decoherence studies by eliminating grain boundaries present in polycrystalline films.

Proposed method

• RF-magnetron sputtering at 300 °C was used to deposit 100-nm-thick epitaxial TiN films on c-cut sapphire substrates.
• High-resolution X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed epitaxial growth and crystallinity with a FWHM of 0.04°.
• X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDX) mapping verified stoichiometric TiN composition.
• Microwave resonators were fabricated using both inductively coupled plasma (ICP) dry etching and wet-etching in SC-1 solution.
• Cryogenic measurements were performed in a dilution refrigerator at ~10 mK using a vector network analyzer and cryogenic amplifiers.
• Quality factors were extracted from S21 scattering parameter measurements across single-photon and high-power regimes.

Experimental results

Research questions

• RQ1 Can epitaxial TiN thin films be grown at low thermal budget (300 °C) using standard sputtering, achieving high crystallinity and low microwave loss?
• RQ2 How do structural defects such as twist domains in epitaxial TiN affect microwave performance?
• RQ3 What is the intrinsic microwave quality factor of epitaxial TiN resonators compared to polycrystalline and other superconducting materials?
• RQ4 Can both dry and wet etching processes yield comparable high-quality factors in epitaxial TiN resonators?
• RQ5 How does the epitaxial interface quality influence microwave loss and coherence in superconducting circuits?

Key findings

• The epitaxial TiN/sapphire heterostructure exhibited a rocking curve FWHM of 0.04°, indicating exceptional crystalline quality.
• XRD and TEM analysis confirmed epitaxial growth with [111]TiN aligned to [0001]sapphire, despite an 8% lattice mismatch.
• XPS and EDX confirmed stoichiometric TiN with minimal chemical inhomogeneity.
• The highest internal quality factor reached 3.3 × 10⁶ in the single-photon regime and >1.0 × 10⁷ in the high-power regime.
• Averaged single-photon quality factors >1.4 × 10⁶ were reproducibly achieved using both dry- and wet-etching techniques.
• The microwave loss tangent of the epitaxial TiN resonators compares favorably with the best-reported values for superconducting resonators.