High mobility holes at germanane/Ge(111) allotropic cross-dimensional heterointerface

Germanane (GeH) is essentially a hydrogen-terminated Ge analog of graphene with a direct gap (~1.6 eV). Record hole mobility mu_h~67,000 cm2/Vs is found at 15 K for a single allotropic cross-dimensional(D) heterointerface. This is enabled by making topotactically-transformed 2D GeH layers meet the 3D bulk Ge(111). Temperature dependence of mu_h implies metallic conduction without ionized impurity scattering between 20 K and 250 K. Sheet hole density for a Fermi sphere n_S=2.8x10^11 /cm2 agrees well with 3.0x10^11 /cm2 of Hall measurements. A 6,500% 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 allotropic cross-D heterointerface between 2D GeH and 3D Ge harbors 2D-confined high-mobility holes. Even without elaborate heteroepitaxy and modulation doping, allotropic cross-D heterostructures pave the way toward facile 2DHG creation.