Nuclear Spin-Free ⁷⁰Ge/²⁸Si⁷⁰Ge Quantum Well Heterostructures Grown on Industrial SiGe-Buffered Wafers

The coherence of hole spin qubits in germanium planar heterostructures is limited by the hyperfine coupling to the nuclear spin bath due to ${}^{29}\mathrm{Si}$ and ${}^{73}\mathrm{Ge}$ isotopes. Thus, removing these nuclear spin-full isotopes is essential to extend the hyperfine-limited coherence times needed to implement robust quantum processors. This work demonstrates the epitaxial growth of device-grade nuclear spin-free ${}^{70}\mathrm{Ge}$ / ${}^{28}{\mathrm{Si}}^{70}\mathrm{Ge}$ heterostructures on industrial SiGe buffers while minimizing the amounts of highly purified ${}^{70}{\mathrm{GeH}}_4$ and ${}^{28}{\mathrm{SiH}}_4$ used. The obtained ${}^{70}\mathrm{Ge}$ / ${}^{28}{\mathrm{Si}}^{70}\mathrm{Ge}$ heterostructures exhibit a dislocation density of $5.3\times {10}^6{\mathrm{cm}}^{-2}$ and an isotopic purity exceeding $99.99\%$ , with carbon and oxygen impurities below the detection sensitivity, as revealed by atom probe tomography. Magneto-transport measurements on gated Hall bars demonstrate effective gate control of hole density in nuclear spin-free quantum wells. Negative threshold gate voltages confirm the absence of intentional doping in the wells, while Hall and Shubnikov-de Haas analyses yield consistent carrier densities ( $\sim 1.4\times {10}^{11}{\mathrm{cm}}^{-2}$ ) and high mobilities ( $\sim 2.4\times {10}^5{\mathrm{cm}}^2/\mathrm{Vs}$ ). Mobility trends reveal interface-trap-limited scattering and percolation concentration below $7\times {10}^{10}{\mathrm{cm}}^{-2}$ . These analyses, along with atomic-level studies, confirm the high quality of epitaxial ${}^{70}\mathrm{Ge}$ / ${}^{28}{\mathrm{Si}}^{70}\mathrm{Ge}$ heterostructures and their relevance as a platform for long-coherence spin qubits.