Fabrication and Microwave Characterization of Epitaxial Vanadium-Based Superconducting Resonators on Silicon Wafers

With the aim of realizing large-scale quantum processors, the coherence of superconducting qubits has been improved by developing low-loss material systems [1]. Loss mitigation has been demonstrated by tailoring the high-quality surfaces of superconducting metals and interfaces between substrate and superconducting metal interfaces [1]. To further mitigate these losses, it is imperative to focus on the crystallinity of the superconducting films. Although the effect of the structural ordering of superconducting films on losses has not yet been fully elucidated, epitaxial films can offer lower losses compared to disordered ones [1,2]. Here, we focus on vanadium (V) for use as highly ordered superconducting films. We fabricate and measure the epitaxial V-based superconducting resonators on Si wafers. We find that the loss factors other than two-level systems (TLSs) are dominant not only on the V surface but also in other parts of the V-based resonators.

A Nb buffer (5 nm)/V (200 nm)/Ta cap (5 nm) structure was sputtered on a buffered HF (BHF)-treated 4-inch Si(100) wafer (approximately 15 kΩcm). The sputtering pressure was less than 0.05 Pa. Using X-ray diffraction, we confirmed the epitaxial growth of the multilayer with an orientation relationship of (110)V || (110)Nb || (001)Si. We also prepared a sample without a Ta capping layer to examine the loss of the V surface. V-based coplanar waveguide λ/4 resonators were fabricated through microfabrication processes. They were designed to possess a resonant frequency of 10–11 GHz [3]. The resonators were immersed in a BHF solution to remove the oxidation layer on the exposed Si surface and then mounted in a dilution refrigerator within several hours. The complex transmission coefficient (S₂₁) spectra of the resonators were measured at 10 mK using a vector network analyzer at various averaged photon numbers (<n_ph>) ranging from 10^-1 to 10⁶. By analyzing the observed S₂₁ spectra, we derived the internal quality factor (Q_int) of the resonators [1,4]. From the Q_int as a function of <n_ph>, we extracted the intrinsic TLS quality factor (Q_TLS,0) and derived the constant loss originating from residual resistance, radiation, and other sources (δ_other) [1,3].

We successfully observed S₂₁ spectra and obtained Q_int values for the resonators with and without the Ta capping layer. Q_int at a single photon level (<n_ph> = 1), Q_TLS,0, and δ_other for the resonator with (without) the Ta capping layer were derived to be 4.8 × 10⁵ (3.6 × 10⁵), 3.9 × 10⁶ (4.6 × 10⁶), and 1.8 × 10^-6 (2.7 × 10^-6), respectively. The resonator with the Ta capping layer exhibits a larger Q_int at <n_ph> = 1 and a smaller Q_TLS,0 and δ_other compared to the resonator without Ta capping. This suggests that the presence of the Ta capping layer enhances Q_int by mitigating losses from the V surface that originate from non-TLS sources, whereas surface oxidation of the Ta capping layer increases TLS loss. Furthermore, even for the resonator with the Ta capping layer, Q_TLS,0 and δ_other were considerably larger than those of the α-Ta resonators with the same circuit design [3]. Thus, it was concluded that TLSs were not the dominant loss factors, not only on the V surface but also in other parts of the V-based resonators. Further research on the chemical and electrical properties is required to fully understand the loss mechanism of V-based resonators.

This paper was based on results obtained from a project, JPNP16007, commissioned by the New Energy and Industrial Technology Development Organization (NEDO), Japan.

References
[1] C. R. H. McRae et al., Rev. Sci. Instrum. 91, 091101 (2020).
[2] R. Gao et al., Phys. Rev. Materials 6, 036202 (2022).
[3] Y. Urade et al., APL Mater. 12, 021132 (2024).
[4] K. S. Khalil et al., J. Appl. Phys. 111, 054510 (2012).