Geometry of the Ge(111)-Au( sqrt 3 x sqrt 3 )R30 degrees reconstruction

Growth, phase transition, and island motion of Au on Ge(111)

Using low energy electron microscopy, Au on Ge(111) is determined to follow a Stranski-Krastanov growth mode consisting of a single layer up to one monolayer (ML), followed by three-dimensional Au-Ge alloy droplets. Near 600 °C, we report the first observation of a reversible first-order phase transition that occurs from the (3 × 3)R30° phase to a (1 × 1) phase, which has a coverage of 0.367 ML. The transition gradually occurs through a coexistence region with a temperature range of about 2 °C and weakly depends on coverage, varying from 640 °C at 1 ML down to 580 °C at 0.8 ML. The phase transition is accompanied by phase fluctuations of small domains or the fluctuations of phase boundaries of large domains. At coverage >1 ML and above 250 °C, the 3D droplets move with stick-slip hopping behavior that has previously been explained by dissolution of Ge at step edges into the alloy droplet, which then comes to concentration and thermal equilibrium via the island motion.

The Synergic Effect of Atomic Hydrogen Adsorption and Catalyst Spreading on Ge Nanowire Growth Orientation and Kinking

Hydride precursors are commonly used for semiconductor nanowire growth from the vapor phase and hydrogen is quite often used as a carrier gas. Here, we used in situ scanning electron microscopy and spatially resolved Auger spectroscopy to reveal the essential role of atomic hydrogen in determining the growth direction of Ge nanowires with an Au catalyst. With hydrogen passivating nanowire sidewalls the formation of inclined facets is suppressed, which stabilizes the growth in the ⟨111⟩ direction. By contrast, without hydrogen gold diffuses out of the catalyst and decorates the nanowire sidewalls, which strongly affects the surface free energy of the system and results in the ⟨110⟩ oriented growth. The experiments with intentional nanowire kinking reveal the existence of an energetic barrier, which originates from the kinetic force needed to drive the droplet out of its optimum configuration on top of a nanowire. Our results stress the role of the catalyst material and surface chemistry in determining the nanowire growth direction and provide additional insights into a kinking mechanism, thus allowing to inhibit or to intentionally initiate spontaneous kinking.

Few layer epitaxial germanene: a novel two-dimensional Dirac material

Monolayer germanene, a novel graphene-like germanium allotrope akin to silicene has been recently grown on metallic substrates. Lying directly on the metal surfaces the reconstructed atom-thin sheets are prone to lose the massless Dirac fermion character and unique associated physical properties of free standing germanene. Here, we show that few layer germanene, which we create by dry epitaxy on a gold template, possesses Dirac cones thanks to a reduced interaction. This finding established on synchrotron-radiation-based photoemission, scanning tunneling microscopy imaging and surface electron diffraction places few layer germanene among the rare two-dimensional Dirac materials. Since germanium is currently used in the mainstream Si-based electronics, perspectives of using germanene for scaling down beyond the 5 nm node appear very promising. Other fascinating properties seem at hand, typically the robust quantum spin Hall effect for applications in spintronics and the engineering of Floquet Majorana fermions by light for quantum computing.

Selective doping in a surface band and atomic structures of the Ge(111) (√3 × √3)R30°-Au surface

Atomic and electronic structures of the Ge(111) (√3 × √3)R30°-Au surface with two metallic bands are studied by scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES). The bias-voltage-dependent periodic structure observed by STM is consistent with the electronic structure calculated for an optimized conjugate honeycomb chained trimer (CHCT) model. Electrons are selectively doped to the electron-like surface metallic band by excess Au atoms, which form triangle structures with the Au trimers of the CHCT model. The discrepancy for the bottom energy of the electron-like band between the ARPES results and those of the calculation is attributed to the doping. The triangle structure is mobile at room temperature, but stable at 80 K. Both Au and Ge atoms deposited at room temperature on the Ge(111) (√3 × √3)R30°-Au surface dope electrons to the electron-like surface metallic band. Moreover, the Au atoms increase the spin-orbit interaction at the surface, and thus make the splitting of the spin-polarized band due to the interaction larger than that before the deposition.