Heteroepitaxial oxide/α-Ta(110) films on sapphire(11-20) – growth, structure, transport, and microwave properties

For building high-performance superconducting qubits and supportive circuits like a high-Q microwave resonator, we need to precisely control material properties in film crystallinity, hetero-interfaces, and capped oxide layer. Ta superconducting films are usually exposed to air, inevitably forming a native oxide layer, which may enhance energy relaxation channels such as two-level systems (TLS) for superconducting circuits.¹ Here, growth of heteroepitaxial deposited-oxide/α-Ta films on a-plane sapphire substrates was achieved in a multi-chamber UHV system.^2,3 The research aims to investigate the microwave properties of the in-situ deposited heterostructures to understand dielectric losses in superconducting quantum circuits.

α-Ta films 35 nm in thickness grew epitaxially in (110) orientation on (11-20) sapphire substrates, followed with in-situ growth of epitaxial and amorphous Al₂O₃ films. The presence of Pendellösung fringes indicates a high degree of structural coherence over the entire film thickness. The root mean square roughness of the Ta films is 0.8 nm. After the in-situ oxide deposition, the surface roughness decreases to 0.47 nm. The Ta film has a critical temperature of 4.29 K and a high residual resistance ratio of 16.



1. Crowley et al., PRX 13, 041005 (2023).

2. Kwo et al., Appl. Phys. Lett. 49, 319 (1986).

3. Lin et al., J. Cryst. Growth 512, 223 (2019).