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

Strained germanium ( ε $\varepsilon$ -Ge) and strained silicon ( ε $\varepsilon$ -Si) buried quantum wells have enabled advanced spin-qubit quantum processors. However, in the absence of suitable lattice-matched substrates, ε $\varepsilon$ -Ge and ε $\varepsilon$ -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 ( ε $\varepsilon$ -SiGe) barrier, eliminating the need for metamorphic buffers altogether. In a structure with a 52-nm-thick ε $\varepsilon$ -SiGe barrier, a low-disorder two-dimensional hole gas is demonstrated with a high-mobility of 1.33 × 10 5 cm 2 / Vs $1.33 \times 10^{5} \nobreakspace \mathrm{cm^2/Vs}$ and a low percolation density of 1.4 ( 1 ) × 10 10 cm - 2 $1.4(1)\times 10^{10} \nobreakspace \mathrm{cm^{-2}}$ . 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 $g$ -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 $g$ -factor in Ge than in ε $\varepsilon$ -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.