Metal–insulator transition in H x Eu y Ge2 thin films promoted by topotactic reaction

Two-dimensional germanene derivatives are an interesting platform exhibiting an intriguing temperature-linear metallic conduction. EuGe2 film on a Ge substrate is a typical example composed of intercalated Eu between two-layers of germanene, being a good metal with a kink at around 50 K corresponding to the Néel temperature. After the fabrication of EuGe2 film by molecular-beam epitaxy, a topotactic reaction with HCl solution at −40 °C was applied to transform EuGe2 to HxEuyGe2 by Eu dissolution and H-termination. The fully reacted material corresponds to H-terminated germanene as germanane, GeH. The Eu content in the film can be well regulated by the duration of the topotactic reaction, which induces a dramatic variation of electrical conduction. In contrast to the single crystalline layered structure of EuGe2, the structure of HxEuyGe2 film is composed of an aggregation of nanocrystals and/or amorphous-like grains. As consistent with the detection of the EuGe2 diffraction peak in the structural characterization, the Eu-rich region in y ≥ 0.89 of HxEuyGe2 shows metallic conduction. Less than the critical value of about y ∼ 0.8, the semiconducting transport feature appears in those films, exhibiting a clear metal–insulator transition in HxEuyGe2. The highest carrier mobility at 100 K evaluated by Hall effect measurement is obtained in semiconducting y ∼ 0.72, the value of which is higher than that of both metallic EuGe2 and semiconducting GeH. This composition controllable HxEuyGe2 film is a useful channel material for a two-dimensional semiconductor applicable to a field-effect transistor.