Cryogenic growth of tantalum thin films for low-loss superconducting circuits-PHYSICAL REVIEW APPLIED 23, 034025 (2025)
2025-03-14 14:11
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PHYSICAL REVIEW APPLIED 23, 034025 (2025)
α-Ta on several different substrates, which include Al2O3(0001), Si(001), Si(111), SiNx, and GaAs(001).
The substrates are actively cooled down to cryogenic temperatures and remain <20 K during the Ta
deposition. X-ray θ-2θ diffraction (XRD) after warming to room temperature indicates the formation of
polycrystalline α-Ta. The 50-nm α-Ta films grown on Al2O3(0001) at a substrate manipulator temperature of 7 K have a room temperature resistivity (ρ300 K) of 13.4 µ cm, a residual resistivity ratio (RRR)
of 17.3 and a superconducting transition temperature (TC) of 4.14 K, which are comparable to bulk values.
In addition, atomic force microscopy (AFM) indicates that the film grown at 7 K with an rms roughness of
0.45 nm was significantly smoother than the one grown at room temperature. Similar properties are found
for films grown on other substrates. Results for films grown at higher substrate manipulator temperatures
show higher ρ300 K, lower RRR and Tc, and increased β-Ta content. Coplanar waveguide resonators with a
gap width of 3 µm fabricated from cryogenically grown Ta on Si(111) and Al2O3(0001) show low-power
Qi of 1.9 million and 0.7 million, respectively, indicating polycrystalline α-Ta films may be promising for
superconducting qubit applications even though they are not fully epitaxial.
abstract: Motivated by recent advancements highlighting Ta as a promising material in low-loss superconducting circuits and showing long coherence times in superconducting qubits, we have explored the effect of cryogenic temperatures on the growth of Ta and its integration in superconducting circuits. Cryogenic growth
of Ta using a low-temperature molecular beam epitaxy (MBE) system is found to stabilize single-phaseα-Ta on several different substrates, which include Al2O3(0001), Si(001), Si(111), SiNx, and GaAs(001).
The substrates are actively cooled down to cryogenic temperatures and remain <20 K during the Ta
deposition. X-ray θ-2θ diffraction (XRD) after warming to room temperature indicates the formation of
polycrystalline α-Ta. The 50-nm α-Ta films grown on Al2O3(0001) at a substrate manipulator temperature of 7 K have a room temperature resistivity (ρ300 K) of 13.4 µ cm, a residual resistivity ratio (RRR)
of 17.3 and a superconducting transition temperature (TC) of 4.14 K, which are comparable to bulk values.
In addition, atomic force microscopy (AFM) indicates that the film grown at 7 K with an rms roughness of
0.45 nm was significantly smoother than the one grown at room temperature. Similar properties are found
for films grown on other substrates. Results for films grown at higher substrate manipulator temperatures
show higher ρ300 K, lower RRR and Tc, and increased β-Ta content. Coplanar waveguide resonators with a
gap width of 3 µm fabricated from cryogenically grown Ta on Si(111) and Al2O3(0001) show low-power
Qi of 1.9 million and 0.7 million, respectively, indicating polycrystalline α-Ta films may be promising for
superconducting qubit applications even though they are not fully epitaxial.
