High-quality and field resilient microwave resonators on Ge/SiGe quantum well heterostructures

TL;DR

This paper demonstrates high-quality microwave resonators on Ge/SiGe quantum well heterostructures with significantly improved quality factors, magnetic field resilience, and detailed characterization of their superconducting properties, advancing hybrid quantum device platforms.

Contribution

The authors report the fabrication of microwave resonators with unprecedented quality factors on Ge/SiGe heterostructures, surpassing previous results and analyzing their magnetic field behavior and superconducting properties.

Findings

Achieved internal quality factors >1000, reaching ~49000 at single-photon levels.

Resonators remain well-defined up to 850 mT in-plane magnetic field.

Observed hysteresis in out-of-plane magnetic field indicating vortex- and quasiparticle effects.

Abstract

Superconducting resonators integrated with Ge quantum wells (QWs) offer a promising platform for hybrid quantum devices. Yet, in the most common heterostructure architectures, they have so far been limited by sizable photon losses. Here, we report the fabrication and characterization of microwave resonators patterned in the Al thin film of an in-situ grown superconductor/semiconductor hybrid heterostructure (HS). The semiconductor part of this hybrid HS is grown on a commercial Ge substrate. We consistently achieve internal quality factors $Q_i>1000$, surpassing previous results on Ge QW heterostructures grown using the concept of a virtual Ge substrate on Si substrates. We reach $Q_i \approx 49000$ at single-photon occupation and a plateau of $Q_i \approx 20000$ at sub-one photon, an order of magnitude larger than any previously reported value of resonators on Ge QW structures at low power. We further characterize the thin Al film forming the resonator, extracting its kinetic inductance and superconducting gap, and studying its magnetic field dependence. Notably, the resonance remains well-defined up to in-plane magnetic fields of 850 mT. A hysteresis emerges in the out-of-plane magnetic field dependence, for both the resonance frequency and the quality factor, indicating an interesting interplay between vortex- and quasiparticle loss mechanisms.