Probing Oxidation-Driven Amorphized Surfaces in a Ta(110) Film for Superconducting Qubit

Abstract

Recent advances in superconducting qubit technology have led to significant progress in quantum computing, but the challenge of achieving a long coherence time remains. Despite the excellent lifetime performance that tantalum (Ta) based qubits have demonstrated to date, the majority of superconducting qubit systems, including Ta-based qubits, are generally believed to have uncontrolled surface oxidation as the primary source of the two-level system loss in two-dimensional transmon qubits. Therefore, atomic-scale insight into the surface oxidation process is needed to make progress toward a practical quantum processor. In this study, the surface oxidation mechanism of native Ta films and its potential impact on the lifetime of superconducting qubits were investigated using advanced scanning transmission electron microscopy (STEM) techniques combined with density functional theory calculations. The results suggest an atomistic model of the oxidized Ta(110) surface, showing that oxygen atoms tend to penetrate the Ta surface and accumulate between the two outermost Ta atomic planes; oxygen accumulation at the level exceeding a 1:1 O/Ta ratio drives disordering and, eventually, the formation of an amorphous Ta 2 O 5 phase. In addition, we discuss how the formation of a noninsulating ordered TaO 1−δ (δ < 0.1) suboxide layer could further contribute to the losses of superconducting qubits. Subsurface oxidation leads to charge redistribution and electric polarization, potentially causing quasiparticle loss and decreased current-carrying capacity, thus affecting superconducting qubit coherence. The findings enhance the comprehension of the realistic factors that might influence the performance of superconducting qubits, thus providing valuable guidance for the development of future quantum computing hardware.

Structure and Formation Mechanisms in Tantalum and Niobium Oxides in Superconducting Quantum Circuits

“…Since suboxide species may act as magnetic impurities and lead to quasiparticle loss, ,,, understanding the underlying origin of enhanced performance in Ta-based qubits is often solely attributed to suboxide species. However, given that the predominant oxide species for both Nb and Ta exist in the form of their pentoxides, and extensive research indicates that Nb 2 O 5 is primarily accountable for two-level system (TLS) loss, ,, it suggests there might be another mechanism rooted in the intrinsic material properties of amorphous Nb 2 O 5 and Ta 2 O 5 .…”

“…The observed disparity in F – counts may explain why, in this case, we observe that Ta oxide is roughly 5 nm, whereas previous reports have indicated values closer to 3 nm. 23 , 26 , 27 , 31 , 36 − 38 While it has been reported that the tunneling of H – ions has been proposed as a loss mechanism in the GHz range, 39 , 40 it is worth noting that the F – ions are significantly larger than H – ions, and their impact on loss requires further investigation.…”

“…This demonstrates that Ta oxide is less lossy than Nb oxide and explains why other groups have previously been able to achieve max T 1 values of 360 μs (Place et al, reported Ta-based qubits) and 503 μs (Wang et al, demonstrated a further improvement through the implementation of a dry-etch process) with Ta. This improvement has been attributed to factors such as a thinner total surface oxide thickness, a reduced number or absence of stable suboxides on the Ta surface, ,− or the abrupt transition from higher oxidation states to lower ones. , However, Bal et al suggest no clear correlation between oxide thickness and overall loss. Instead, they propose that this loss may arise from variations in the chemical stoichiometry (i.e., the presence of stable suboxides in the Nb oxide).…”