Mitigation of interfacial dielectric loss in aluminum-on-silicon superconducting qubits

TL;DR

This paper reports on aluminum-on-silicon superconducting qubits with significantly improved energy relaxation times, achieved by reducing oxide-related dielectric loss at the substrate-metal interface through thicker aluminum films.

Contribution

It introduces a strategy of increasing aluminum film thickness to mitigate dielectric loss caused by oxide defects at the substrate-metal interface in superconducting qubits.

Findings

Achieved T1 times up to 270 microseconds and a maximum of 501 microseconds.

Thicker aluminum films (>300 nm) reduce oxide presence and grain boundary defects.

Correlated film thickness with improved qubit performance through materials analysis and simulations.

Abstract

We demonstrate aluminum-on-silicon planar transmon qubits with time-averaged ${T_1}$ energy relaxation times of up to ${270\,\mu s}$, corresponding to Q = 5 million, and a highest observed value of ${501\,\mu s}$. We use materials analysis techniques and numerical simulations to investigate the dominant sources of energy loss, and devise and demonstrate a strategy towards mitigating them. The mitigation of loss is achieved by reducing the presence of oxide, a known host of defects, near the substrate-metal interface, by growing aluminum films thicker than 300 nm. A loss analysis of coplanar-waveguide resonators shows that the improvement is owing to a reduction of dielectric loss due to two-level system defects. We perform time-of-flight secondary ion mass spectrometry and observe a reduced presence of oxygen at the substrate-metal interface for the thicker films. The correlation between the enhanced performance and the film thickness is due to the tendency of aluminum to grow in columnar structures of parallel grain boundaries, where the size of the grain depends on the film thickness: transmission electron microscopy imaging shows that the thicker film has larger grains and consequently fewer grain boundaries containing oxide near this interface. These conclusions are supported by numerical simulations of the different loss contributions in the device.