Buried Unstrained Germanium Channels: A Lattice-Matched Platform for Quantum Technology

Strained germanium ( $\epsilon$ -Ge) and strained silicon ( $\epsilon$ -Si) buried quantum wells have enabled advanced spin-qubit quantum processors. However, in the absence of suitable lattice-matched substrates, $\epsilon$ -Ge and $\epsilon$ -Si are deposited on defective, metamorphic SiGe buffers, which may impact device performance and scaling. Here an alternative platform is introduced based on the heterojunction between bulk unstrained Ge and a lattice-matched strained silicon-germanium ( $\epsilon$ -SiGe) barrier, eliminating the need for metamorphic buffers altogether. In a structure with a 52-nm-thick $\epsilon$ -SiGe barrier, a low-disorder two-dimensional hole gas is demonstrated with a high-mobility of $1.33\times {10}^5{\mathrm{cm}}^2/\mathrm{Vs}$ and a low percolation density of $1.4\left(1\right)\times {10}^{10}{\mathrm{cm}}^{-2}$ . Quantum transport shows that holes confined in the buried unstrained Ge channel have a strong density-dependent in-plane effective mass and out-of-plane $g$ -factor, pointing to a significant heavy-hole-light-hole mixing in agreement with theory. Measurements of Zeeman-split levels in quantum point contacts further highlight this character, showing a two-fold larger in-plane $g$ -factor in Ge than in $\epsilon$ -Ge. The prospects of strong spin-orbit interaction, isotopic purification, and of hosting superconducting pairing correlations make this platform appealing for fast quantum hardware and hybrid quantum systems.