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

Abstract

The p-symmetry of the hole wavefunction is associated with a weaker hyperfine interaction as compared to electrons, thus making hole spin qubits attractive candidates to implement long coherence quantum processors. However, recent studies demonstrated that hole qubits in planar germanium (Ge) heterostructures are still very sensitive to nuclear spin bath. These observations highlight the need to develop nuclear spin-free Ge qubits to suppress this decoherence channel and evaluate its impact. With this perspective, this work demonstrates the epitaxial growth of ${}^{\text{73}}$Ge-depleted isotopically enriched ${}^{\text{70}}$Ge/SiGe quantum wells. The growth was achieved by reduced pressure chemical vapor deposition using isotopically purified monogermane ${}^{\text{70}}$GeH${}_{\text{4}}$ and monosilane ${}^{\text{28}}$SiH${}_{\text{4}}$ with an isotopic purity higher than 99.9 $\%$ and 99.99 $\%$, respectively. The quantum wells consist of a series of ${}^{\text{70}}$Ge/SiGe heterostructures grown on Si wafers using a Ge virtual substrate and a graded SiGe buffer layer. The isotopic purity is investigated using atom probe tomography following an analytical procedure addressing the discrepancies in the isotopic content caused by the overlap of isotope peaks in mass spectra. The nuclear spin background in the quantum wells was found to be sensitive to the growth conditions. The lowest concentration of nuclear spin-full isotopes ${}^{\text{73}}$Ge and ${}^{\text{29}}$Si in the heterostructure was established at 0.01 $\%$ in the Ge quantum well and SiGe barriers. The measured average distance between nuclear spins reaches 3-4 nm in ${}^{\text{70}}$Ge/${}^{\text{28}}$Si${}^{\text{70}}$Ge, which is an order of magnitude larger than in natural Ge/SiGe heterostructures.