Epitaxial Tantalum Films for Superconducting Qubits

With the advancement of quantum processors, high-quality superconducting materials have played a critical role in achieving longer relaxation times (T₁ ≈ 0.3–0.5 ms). Conventional in-plane polycrystalline tantalum (Ta) films deposited on C-plane sapphire exhibit a reduced residual resistivity ratio (RRR) and might introduce additional losses through grain-boundary oxides. In this work, we demonstrate the growth of epitaxial Ta(110) and Ta(111) films on a-plane and c-plane sapphire, respectively. Atomic force microscopy (AFM) shows that the Ta(111) surface exhibits a smoother morphology (R_q ≈ 0.3 nm). X-ray diffraction (XRD) θ–2θ and φ scans show that both Ta(110) (in-plane alignment > 99.8%) and Ta(111) films are single crystalline in both out-of-plane and in-plane directions. Electron backscatter diffraction (EBSD) further confirms that both Ta(110) and Ta(111) films exhibit large-area in-plane single-crystalline domains. To evaluate the superconducting performance, the RRR were measured, showing that both Ta(110) and Ta(111) films exhibit more than two times higher RRR than that of polycrystalline Ta. Coplanar waveguide resonators and transmon qubits based on the Ta films were fabricated for evaluating their intrinsic quality factors (Q_i) and relaxation times T₁. Among them, the single-crystalline Ta(110) showed a 50% higher T₁ than that of polycrystalline Ta. The epitaxial Ta films pave the way for improving the T₁ time of superconducting qubits toward large-scale integration of quantum processors.