High Hole Mobility in Two-Dimensional Germanane Thin Films

Germanane (GeH) is a hydrogen-terminated layered crystal of germanium and is expected to be a direct bandgap semiconductor with high electron mobility [1]. Our group reported an electric double-layer transistor on a GeH thin film with the ambipolar transport characteristics and the Hall mobilities of electrons and holes at 120 K, 6500 cm²V^-1s^-1, and 570 cm²V^-1s^-1, respectively [2]. However, due to the high contact resistance between electrode and GeH, the transport properties below 120 K remained unexplored. In this study, we report unexpectedly high hole mobilities of GeH thin films exceeding 67 000 cm²V^-1s^-1 at 15 K, which was achieved by optimizing the electrode materials and crystal growth conditions.<br/>The GeH thin films were prepared by the topotactic transformation of epitaxial CaGe₂ films on a Ge (111) grown by solid-source molecular beam epitaxy (MBE) with a base pressure better than 2x10^-8 Pa. A Ge buffer layer and a Zintl-phase CaGe₂ film were grown on the nominally undoped Ge (111) substrate. The GeH films were obtained by immersing the CaGe₂ films in concentrated aqueous HCl for 1~4 days at -18 °C, and then by rinsing in isopropanol at room temperature. Pt electrodes, in a Hall-bar configuration, were formed by sputtering through a stencil mask. The transport properties were measured on a Quantum Design physical properties measurement system.<br/>The sheet resistance, <i>R</i>s, of the GeH film showed a peak approximately at 250 K. Below 250 K, <i>R</i>s decreased with decreasing temperature from ~1.5 kΩ at 250 K to ~50 Ω at 30 K, which indicates metallic conduction, whereas it showed semiconducting behaviors above 250 K. From the Hall measurement, the majority carriers were confirmed to be holes at low temperature while they were electrons at high temperatures above 250 K. <i>R</i>s above 250 K was likely to be dominated by conduction through the Ge substrate and the buffer layer. At 200 K, the Hall resistance showed nonlinear dependence on the magnetic field, which can be understood in terms of two-carrier transport including the holes from the GeH film and the electrons from the Ge layer. Because the magnetic-field dependence of the Hall resistance becomes linear at low temperature, the contribution of the Ge layers can be ignored below 200 K. That the carriers are holes at low temperatures was also confirmed by the Seebeck coefficients. The hole concentration decreased with decreasing temperature from 2×10¹⁷ cm^-3 at 200 K to 3×10¹⁶ cm^-3 at 15 K. Meanwhile, the Hall mobility increased with decreasing temperature from 2×10³ cm²V^-1s^-1 at 200 K to 6.7×10⁴ cm²V^-1s^-1at 15 K. At 15 K, the magnetoresistance ratio reached 6,500 % at 7 T and Shubnikov-de Haas oscillation was observed. The sheet carrier density by assuming a spherical Fermi surface was estimated to 2.8×10¹¹ cm^-2, which is in good agreement with the Hall carrier density of 3.0×10¹¹ cm^-2 at 15 K. These results indicate the possibility of single-band conduction originating from the light-hole band in GeH.<br/>Our results showed that two-dimensional hydrogen-terminated layered germanium prepared by topotactic reaction exhibits unprecedented high hole mobility and well-resolved Landau levels at 15 K. Further attempts at even lower temperatures would allow the first observation of the quantum Hall effect in this novel layered system.<br/><br/>[1] E. Bianco, S. Butler, S. Jiang, O. D. Restrepo, W. Windl, and J. E. Goldberger, ACS Nano <b>7</b>, 4414 (2013).<br/>[2] Y. Katayama, R. Yamauchi, Y. Yasutake, S. Fukatsu, K. Ueno, Appl. Phys. Lett. <b>115</b>, 122101 (2019).