High-mobility holes at germanane/Ge(111) epitaxial 2D/3D allotropic heterointerface

Germanane (GeH) is essentially a hydrogen-terminated Ge analog of graphene with a direct gap (≈1.6 eV). Record hole mobility μh≈ 67 000 cm2 V−1 s−1 is found at 15 K for a single allotropic heterointerface. This is enabled by making topotactically transformed 2D GeH layers meet the 3D bulk Ge(111). Temperature dependence of μh implies metallic conduction without ionized impurity scattering between 20 and 250 K. Sheet hole density for a Fermi sphere nS=2.8×1011 cm−2 agrees well with 3.0×1011cm−2 of Hall measurements. A 6500% magnetoresistance at 7 T accompanies Shubnikov–de Haas oscillations visible even at 15 K. These imply single-band conduction of holes with small effective mass in the in-plane directions, invoking a 2D hole gas (2DHG) picture that epitaxial allotropic heterointerface between 2D GeH and 3D Ge harbors 2D-confined high-mobility holes. Even without elaborate heteroepitaxy and modulation doping, epitaxial 2D/3D allotropic heterostructures pave the way toward facile 2DHG creation.