Observation of Interface Piezoelectricity in Superconducting Devices on Silicon

Abstract: The evolution of superconducting quantum processors is driven by the need to reduce errors and scale for fault-tolerant computation. Reducing physical qubit error rates requires further advances in the microscopic modeling and control of decoherence mechanisms in superconducting qubits. Piezoelectric interactions contribute to decoherence by mediating energy exchange between microwave photons and acoustic phonons. Centrosymmetric materials like silicon and sapphire do not display piezoelectricity and are the preferred substrates for superconducting qubits. However, the broken centrosymmetry at material interfaces may lead to piezoelectric losses in qubits. While this loss mechanism was predicted two decades ago, interface piezoelectricity has not been experimentally observed in superconducting devices. Here, we report the observation of interface piezoelectricity at an aluminum-silicon junction and show that it constitutes an important loss channel for superconducting devices. We fabricate aluminum interdigital surface acoustic wave transducers on silicon and demonstrate piezoelectric transduction from room temperature to millikelvin temperatures. We find an effective electromechanical coupling factor of $K^2\approx 2 \times 10^{-5}\%$ comparable to weakly piezoelectric substrates. We model the impact of the measured interface piezoelectric response on superconducting qubits and find that the piezoelectric surface loss channel limits qubit quality factors to $Q\sim10^4-10^8$ for designs with different surface participation ratios and electromechanical mode matching. These results identify electromechanical surface losses as a significant dissipation channel for superconducting qubits, and show the need for heterostructure and phononic engineering to minimize errors in next-generation superconducting qubits.

• The paper demonstrates interface piezoelectricity at the aluminum-silicon junction, identifying it as a key dissipation channel in superconducting devices.
• It uses aluminum interdigital transducers to measure an effective electromechanical coupling factor of K² ≈ 2×10⁻⁵%, providing a quantitative benchmark for weak piezoelectric effects.
• The findings imply that optimizing heterostructure and phonon engineering may mitigate losses and boost superconducting qubit quality factors from 10⁴ to 10⁸.

Observation of Interface Piezoelectricity in Superconducting Devices on Silicon

The research paper examines the phenomenon of interface piezoelectricity in superconducting devices fabricated on silicon substrates, with a focus on its implications for superconducting qubit technologies. The authors rigorously demonstrate the existence of piezoelectric effects at the aluminum-silicon interface and evaluate their potential impact on superconducting qubit performance.

Experimental Observation and Significance

The paper reports the experimental observation of piezoelectricity at the interface of aluminum and silicon, a non-piezoelectric material. Using aluminum interdigital transducers (IDTs) on silicon, the study demonstrates piezoelectric transduction from room to cryogenic temperatures. The measured effective electromechanical coupling factor is $K^2 \approx 2 \times 10^{-5}\%$, comparable to substrates known for weak piezoelectricity. These findings reveal an interface-induced piezoelectric mechanism as a previously unconsidered loss channel in superconducting devices.

Impact on Superconducting Qubits

Through detailed modeling, the paper shows that interface piezoelectricity constitutes a significant dissipation channel, potentially limiting qubit quality factors to $Q \sim 10^4-10^8$ depending on the qubit design's surface participation ratio and mode matching. This highlights the importance of understanding and mitigating piezoelectric losses to improve qubit fidelity. The findings suggest the necessity for optimized heterostructure and phonon engineering in future generations of superconducting qubits.

Mechanisms and Theoretical Insights

The study considers two primary mechanisms for interface piezoelectricity: surface lattice relaxation and charge transfer-induced dipole formation. The latter is identified as the likely dominant contributor to the observed phenomenon, primarily due to metal-induced gap states and the work function mismatch between aluminum and silicon. The research provides a crucial insight into the material science of qubit fabrication, indicating that both high-quality material interfaces and certain substrate-metal combinations can inherently exhibit piezoelectric characteristics.

Implications and Future Directions

These findings underscore the critical need to account for interface-specific interactions in the material design and fabrication of superconducting circuits. The weak temperature dependence of the transduction efficiency observed across a broad temperature range suggests that modifications to substrate treatments or substrate architecture could mitigate losses. Furthermore, the research explores the potential for exploiting interface piezoelectricity to facilitate electromechanical transduction in quantum applications, possibly enabling simpler fabrication processes compared to current methods requiring additional active materials or complex biasing schemes.

In conclusion, the paper makes a significant contribution to the understanding of interface piezoelectricity in nanofabricated superconducting devices. It identifies a crucial loss mechanism and opens up new pathways for both fundamental research and practical improvements in the field of quantum computing. Future research endeavors might focus on more precise microscopic mechanisms and the development of innovative designs to mitigate or harness interface-induced piezoelectric effects in quantum technologies.