High internal quality factor (~ 10⁶) superconducting coplanar waveguide resonators from MBE-grown titanium nitride films on c-plane sapphire

Coplanar waveguide microwave resonators with frequencies between 2 and 8 GHz, fabricated from titanium nitride (TiN) films showed high internal quality factors (Q_i ~ 10⁶) at the low power limit irrespective of the deposition methods, substrates, and resonator design [1-6]. This higher limit for the internal quality factor may be improved by understanding the material defects better. We deposited a 50 nm TiN on a 2-inch c-plane sapphire at 600^oC growth temperature using plasma-assisted molecular beam epitaxy and an electron beam source for Ti. Using atomic force microscopy, we identified the 3-dimensional growth mode of TiN with 0.6 nm root mean square surface roughness. X-ray diffraction studies indicated a single crystal (1 1 1) surface orientation with 20 arcsecs of full width at half maximum of the rocking curves, one of the smallest reported values for TiN [6]. A superconducting transition temperature of 5.1 K and residual resistivity ratio of 2.1 were measured. Resonators were fabricated using a Cl-based etch. Using the 3-6-3 geometry for resonators standardized by NIST [7], we obtained a Q_i ~ 10⁶ in the single-photon limit. Scanning transmission electron microscope (STEM) images of the film showed columnar growth above a critical thickness of around 8 nm. Surprisingly, we found a screw dislocation in an otherwise defect-free sapphire substrate just 1 nm below the TiN/sapphire interface. This dislocation propagates into the TiN film. An etch-related damage was also found just below the TiN sidewall in sapphire. These defects, along with other defects, could be responsible for the higher limit of Q_i. Further investigation of these defects and an effort to reduce them are under progress.

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[6] Gao et al., Phys. Rev. Materials 6, 036202 (2022)

[7] Kopas et al., arXiv:2204.07202 (2022)