Cryogenic growth of aluminum: structural morphology, optical properties, superconductivity and microwave dielectric loss

TL;DR

This work addresses how structural disorder and grain size in superconducting aluminum films influence optical, superconducting, and microwave loss properties for quantum devices, by comparing cryogenic growth at ~6 K with room-temperature growth on c-plane sapphire. The authors integrate structural (RHEED, AFM, HAADF-STEM, XRD), optical (spectroscopic ellipsometry and pseudo-dielectric function modeling), electrical transport, and microwave resonator measurements to characterize films down to the single-photon regime. Cryogenic growth yields nanoscale polycrystalline grains with fissures that produce a yellow surface and increased kinetic inductance, while superconductivity is enhanced with T_C = 1.57 K and H_C = 685 Oe, and Δ_SC = 240 μeV with ξ = 69 nm; the optical response requires an EMA layer with voids and yields L_K ≈ 0.79 pH/□ and TLS-dominated microwave loss with Q_i up to 1.2×10^7. These results demonstrate a controllable route to tailor superconducting and optical properties via cryogenic deposition and motivate future studies of disorder, Anderson localization, and growth kinetics in ultra-high vacuum thin films for quantum technologies.

Abstract

We explore the molecular beam epitaxy synthesis of superconducting aluminum thin films grown on c-plane sapphire substrates at cryogenic temperatures of 6 K and compare their behavior with films synthesized at room temperature. We demonstrate that cryogenic growth increases structural disorder, producing crystalline grains that modify the optical, electrical, and superconducting properties of aluminum. We observe that cryogenic deposition changes the color of aluminum from fully reflective to yellow and correlate the pseudo-dielectric function and reflectance with structural changes in the film. We find that smaller grain sizes enhance the superconductivity of aluminum, increasing its critical temperature and critical field. We then estimate the superconducting gap and coherence length of Cooper pairs in aluminum in the presence of disorder. Finally, we fabricate superconducting microwave resonators on these films and find that, independently of the growth temperature, the system is dominated by two-level system loss with similar quality factors in the high and low power regimes. We further measure a higher kinetic inductance in the cryogenically grown films.

Figures (4)

• Figure 1: (a) Crystal structure of aluminum and schematic showing the (111) plane of aluminum and the c-plane of sapphire. Reflection high energy electron diffraction pattern of aluminum grown at (b) 293 K and (c) 6 K. The former corresponds to the Al $<0\bar{1}1>$ crystalline orientation while the latter showed rings in all directions. Atomic force microscopy image of the surface of aluminum grown at (d) 293 K and (e) 6 K.
• Figure 2: High angle annular dark field-scanning transmission electron microscopy image of the Al film grown at 293 K (a) and 6 K (c). X-ray diffraction $\mathrm{2\theta - \omega}$ scan of Al grown at 293 K (b) and 6 K (d).
• Figure 3: (a) Pictures of completely reflective aluminum grown at 293 K and yellow aluminum grown at 6 K. (b) Reflectance spectra of reflective and yellow aluminum as a function of incident photon wavelength. (c) Real and imaginary parts of the pseudo-dielectric function of reflective and yellow aluminum.
• Figure 4: (a) Critical field as a function of temperature for Al grown at 293 K and at 6 K. (b) internal quality factor as a function of photon number in the cavity for the resonators measured in this work. (c) Optical microscopy images of the microwave resonators measured in this work. The gap width is $\mathrm{3.2\ \mu m}$. Internal quality factor as a function of frequency measured at high photon number $\mathrm{\bar{n}=1.6\times10^6}$ (d) and low photon number $\mathrm{\bar{n}<1}$ (e).