Enhancing charge stability of Ge quantum well heterostructures via SiGe layer composition engineering

Composition modulation is a powerful technique for designing materials with tailored properties, fueling the development of advanced semiconductor devices. In this work, we have implemented this technique into Ge quantum well heterostructures, offering a promising avenue to address the critical challenge of charge stability in spin qubit devices. Harnessing the atomic-scale precision of molecular beam epitaxy, we have engineered the band structure of the SiGe top barrier via graded composition modulation, thereby reducing charge accumulation states at the SiGe-dielectric interface and strengthening the effective confinement to the hole gases in the Ge quantum wells. The enhanced charge stability of composition-modulated SiGe/Ge quantum well heterostructures is confirmed in Hall devices, featuring an enlarged stable gate voltage range. We have further fabricated quantum dot devices from the composition-modulated SiGe/Ge quantum well heterostructures and observed remarkably low charge noise with an averaged amplitude of $0.46\,\mathrm{μeV}/\mathrm{\sqrt{Hz}}$ at $1\,\mathrm{Hz}$---the lowest reported value for Ge quantum wells grown on silicon. This exceptional charge stability of the quantum dots persists in the few-hole regime, with no observable voltage drift over $\sim$hours. With reduced charge noise and enhanced energy stability, composition-modulated SiGe/Ge heterostructures exhibit significant potential for applications in building high-performance quantum devices, including spin qubits with a long coherence time.