Transport Properties of Methyl-Terminated Germanane Microcrystallites

Abstract

Germanane is a two-dimensional material consisting of stacks of atomically thin germanium sheets. It's easy and low-cost synthesis holds promise for the development of atomic-scale devices. However, to become an electronic-grade material, high-quality layered crystals with good chemical purity and stability are needed. To this end, we studied the electrical transport of annealed methyl-terminated germanane microcrystallites in both high vacuum and ultrahigh vacuum. Scanning electron microscopy of crystallites revealed two types of behavior which arise from the difference in the crystallite chemistry. While some crystallites are hydrated and oxidized, preventing the formation of good electrical contact, the four-point resistance of oxygen-free crystallites was measured with multiple tips scanning tunneling microscopy, yielding a bulk transport with resistivity smaller than 1 Ω·cm. When normalized by the crystallite thickness, the resistance compares well with the resistance of hydrogen-passivated germanane flakes found in the literature. Along with the high purity of the crystallites, a thermal stability of the resistance at 280 °C makes methyl-terminated germanane suitable for complementary metal oxide semiconductor back-end-of-line processes.

Keywords: germanane; hydration; methylation; resistivity; thermal robustness.

Figure 1

Scanning electron microscopy (SEM) images of typical methyl-terminated germanane microcrystallites drop-casted from an isopropanol solution on a Si surface and annealed at 180 °C in ultrahigh vacuum. (a1–a3) Examples of microcrystallites which are well resolved with SEM. (b1–b3) Examples of microcrystallites which become charged under the electron beam. (c1,c2) Comparison of the SEM image quality upon the microcrystallite manipulation with a STM tip. The crystallite is colorized to be better differentiated from the STM tip. (d) Statistical analysis of the occurrence of charging effects as a function of the lateral sizes of the microcrystallite basal plane.

Figure 2

(a) SEM image of methyl-terminated germanane and corresponding energy-dispersive X-ray spectroscopy elemental mappings for (b) oxygen and (c) germanium. Blue arrows delineate microcrystallites that exhibit charging under electron irradiation. (d) Raman spectroscopy measured in the SEM setup of oxidized (bottom), hydrated (middle), and dehydrated (top) methyl-terminated germanane microcrystallites. The vertical arrows delineate the positions of vibrational modes of Ge (TO: transverse optical phonon mode), GeO₂, and H₂O. (e) Cathodoluminescence spectroscopy of a hydrated methyl-terminated germanane microcrystallite.

Figure 3

(a) SEM of a charged methyl-terminated germanane microcrystallite which is manipulated with two STM tips. (b) The right top corner of the microcrystallite was broken, producing two smaller and stable microcrystallites under electron irradiation.

Figure 4

(a) SEM image of a methyl-terminated germanane microcrystallite contacted with four STM tips. The STM tips have been colorized to be better differentiated from the crystallite. (b) Schematic of the current flow with a compressed current distribution due to the limited thickness of the crystallite. (c) V(I) characteristics measured for six different equidistant tip separations. (d) Linear variation of the four-point resistance as a function of the probe separation. The dashed line corresponds to the best fit. (e) SEM image of a methyl-terminated germanane microcrystallite contacted with four STM tips and (f) related four-point resistance. The dashed line corresponds to the best fit, which is inversely proportional to the equidistant probe spacing. Inset: Schematic illustration of the current flow pattern for a thick microcrystallite. (g) SEM image of a methyl-terminated germanane microcrystallite contacted with STM tips from different facets and (h) corresponding four-point resistances as a function of the position of the source tip and the grounded tip.

Figure 5

(a) Comparison of the product of the four-point resistances to the crystallite thickness for GeCH₃ (C) microcrystallites and H-passivated (H) flakes, measured as a function of the annealing temperature. The spacing between the electrodes or the tips varies in a range extending from 0.5 to 3.0 μm. The data for the H-passivated flakes were deduced from the experimental results published in Refs. [9,18]. (b–d) TEM images and SAED patterns of the thin edge of a microcrystallite collected at room temperature (RT), 185 °C and 280 °C, respectively.