Nuclear Spin-Depleted, Isotopically Enriched ⁷⁰Ge/²⁸Si⁷⁰Ge Quantum Wells

The p-symmetry of the hole wavefunction is associated with a weaker hyperfine interaction, which makes hole spin qubits attractive candidates to implement quantum processors. However, recent studies demonstrate that hole qubits are still very sensitive to nuclear spin bath, thus highlighting the need for nuclear spin-free germanium (Ge) qubits to suppress this decoherence channel. Herein, this work demonstrates the epitaxial growth of Ge-73- and Si-29-depleted, isotopically enriched Ge-70/silicon-germanium (SiGe) quantum wells. The growth is achieved by reduced pressure chemical vapor deposition using isotopically purified monogermane (GeH4)-Ge-70 and monosilane (SiH4)-Si-28 with an isotopic purity higher than 99.9% and 99.99%, respectively. The quantum wells consist of a series of Ge-70/SiGe heterostructures grown on Si wafers. The isotopic purity is investigated using atom probe tomography (APT) following an analytical procedure addressing the discrepancies caused by the overlap of isotope peaks in mass spectra. The nuclear spin background is found to be sensitive to the growth conditions with the lowest concentration of Ge-73 and Si-29 is below 0.01% in the Ge well and SiGe barriers. The measured average distance between nuclear spins reaches 3-4 nm in Ge-70/(SiGe)-Si-28-Ge-70, which is an order of magnitude larger than in natural Ge/SiGe heterostructures. The spread of the hole wavefunction and the residual nuclear spin background in APT voluminals comparable to the size of realistic quantum dots are also discussed.