Ge Dimer Rows Research Articles

Electronic transport in planar atomic-scale structures measured by two-probe scanning tunneling spectroscopy

Miniaturization of electronic circuits into the single-atom level requires novel approaches to characterize transport properties. Due to its unrivaled precision, scanning probe microscopy is regarded as the method of choice for local characterization of atoms and single molecules supported on surfaces. Here we investigate electronic transport along the anisotropic germanium (001) surface with the use of two-probe scanning tunneling spectroscopy and first-principles transport calculations. We introduce a method for the determination of the transconductance in our two-probe experimental setup and demonstrate how it captures energy-resolved information about electronic transport through the unoccupied surface states. The sequential opening of two transport channels within the quasi-one-dimensional Ge dimer rows in the surface gives rise to two distinct resonances in the transconductance spectroscopic signal, consistent with phase-coherence lengths of up to 50 nm and anisotropic electron propagation. Our work paves the way for the electronic transport characterization of quantum circuits engineered on surfaces.

From the Very First Stages of Mn Deposition on Ge(001) to Phase Segregation

In this work, we have combined scanning tunneling microscopy with high-resolution transmission electron microscopy (HR-TEM) to investigate the initial stages of Mn deposition on Ge(001) surfaces. The growth temperature was chosen to be (353 ± 5) K, which is typical for the synthesis of Ge1–xMnx thin films. At the early stage of the Mn deposition, two distinct kinds of islands are observed even for Mn coverage much smaller than a monolayer with an average size of 1–2 nm and 4–5 nm, respectively. Small islands were found to nucleate in the hollow between the Ge dimer rows, and they were formed by consuming Ge from two adjacent rows. This indicates that Mn–Ge alloying has been taken place even at the early stage of the Mn deposition. When the Mn coverage increases, coarsening between small islands with newly deposited ad-atoms occurs, giving rise to the formation of monosized islands. Interestingly, these nanostructures have an average size of 4–5 nm and separated by a spacing of 7–8 nm that are similar to the spatial ordering of nanocolumns resulting from spinodal decomposition in (Ge,Mn) thin films. HR-TEM analyses indicate that those nanoislands are epitaxial, defect-free, and perfectly coherent with the Ge substrate. A subsequent anneal will result in the formation of Mn5Ge3 islands.

Preventing carbon contamination of Ge (001) during atomic layer deposition with a barium-based Zintl layer

Carbon contamination of the clean Ge (001) 2 × 1 surface occurs during atomic layer deposition (ALD) of perovskite oxides and introduces impurities that can affect device performance. Zintl layers are formed by exposing a bare Ge (001) 2 × 1-reconstructed surface to 0.5-monolayers of elemental Ba, with the Ba atoms occupying the fourfold sites between Ge-dimer rows. The bare Ge (001) and Ba-Zintl layer surfaces feature a 2 × 1-reconstructed structure. Exposure of bare 2 × 1-reconstructed Ge (001) to 0.5 monolayers of elemental Sr leads to an 8 × 1- or 9 × 1-reconstructed Ge (001) surface. Bare Ge (001) and Ba-Zintl layer Ge (001), and 0.5-monolayer Sr-dosed Ge (001) surfaces were exposed to background molecules in the ALD chamber at 225 °C. Bare Ge (001) and Ba-Zintl layer Ge (001) were also exposed to barium bis(triisopropylcyclopentadienyl) at 225 °C and changes in the surface composition were monitored with x-ray photoelectron spectroscopy. The 2 × 1-reconstructed Ba-Zintl surface protects Ge (001) from carbon accumulation associated with ALD background molecules and cyclopentadienyl-based ligands originating from the ALD precursor. Surface structure is important in protecting Ge (001); the 8 × 1- or 9 × 1-reconstructed Ge (001) surface formed from 0.5-monolayers of Sr does not prevent carbon accumulation.

Initial growth of Ba onGe(001): An STM and DFT study

An ordered alkaline-earth submonolayer on a clean $\mathrm{Si}(001)$ surface provides a template for growth of the atomically sharp, crystalline Si-oxide interface that is ubiquitous in the semiconductor device industry. It has been suggested that submonolayers of Sr or Ba on $\mathrm{Ge}(001)$ could play a similar role as on structurally identical $\mathrm{Si}(001)$, overcoming known limitations of the $\mathrm{Ge}(001)$ substrate such as amorphization of its oxidation layers. In this paper the initial stage of the Ba oxidation process, i.e., adsorption and organization of Ba atoms on the $\mathrm{Ge}(001)$ surface as a function of temperature $(270\ensuremath{-}770\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ for coverage 1.0 monolayer (ML) and $0.15\ensuremath{-}0.4\phantom{\rule{0.16em}{0ex}}\mathrm{ML}$, is studied using scanning tunneling microscopy (STM) and density functional theory (DFT). Three types of features have been identified on the Ba-covered $\mathrm{Ge}(001)$ surface. They originate from isolated Ba adatoms, isolated Ba ad-dimers, and the Ba ad-dimers assembled into short-range, randomly distributed chains that run across the Ge dimer rows. We find from both STM measurements and DFT calculations that the latter is the dominant structure on $\mathrm{Ge}(001)$ with increasing coverage.

Atomic imaging of nucleation of trimethylaluminum on clean and H2O functionalized Ge(100) surfaces

The direct reaction of trimethylaluminum (TMA) on a Ge(100) surface and the effects of monolayer H(2)O pre-dosing were investigated using ultrahigh vacuum techniques, such as scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). At room temperature (RT), a saturation TMA dose produced 0.8 monolayers (ML) of semi-ordered species on a Ge(100) surface due to the dissociative chemisorption of TMA. STS confirmed the chemisorption of TMA passivated the bandgap states due to dangling bonds. By annealing the TMA-dosed Ge surface, the STM observed coverage of TMA sites decreased to 0.4 ML at 250 °C, and to 0.15 ML at 450 °C. XPS analysis showed that only carbon content was reduced during annealing, while the Al coverage was maintained at 0.15 ML, consistent with the desorption of methyl (-CH(3)) groups from the TMA adsorbates. Conversely, saturation TMA dosing at RT on the monolayer H(2)O pre-dosed Ge(100) surface followed by annealing at 200 °C formed a layer of Ge-O-Al bonds with an Al coverage a factor of two greater than the TMA only dosed Ge(100), consistent with Ge-OH activation of TMA chemisorption and Ge-H blocking of CH(3) chemisorption. The DFT shows that the reaction of TMA has lower activation energy and is more exothermic on Ge-OH than Ge-H sites. It is proposed that the H(2)O pre-dosing enhances the concentration of adsorbed Al and forms thermally stable Ge-O-Al bonds along the Ge dimer row which could serve as a nearly ideal atomic layer deposition nucleation layer on Ge(100) surface.

Creation and annihilation of single atom vacancy during subsurface diffusion

Initial reaction of Co with Ge(100) substrate was investigated using scanning tunneling microscopy (STM) in conjunction with density-functional theory (DFT) calculations. We observed that the subsurface Co atoms diffuse after the migration of a Ge atom to the metastable M site. The STM imaging in real time revealed that Ge atom vacancy was created and annihilated during the subsurface diffusion of Co atom along the Ge dimer row. Theoretical calculations confirmed that the migration of Ge atom facilitated the fast diffusion of subsurface Co atom.

Adsorption ofSb4on Ge(001) and Si(001) surfaces: Scanning tunneling microscopy and first-principles calculations

We have performed first-principles total energy calculations and scanning tunneling microscopy experiments to study the energetics and the atomic structures resulting from the adsorption of ${\mathrm{Sb}}_{4}$ molecules on the Ge and $\mathrm{Si}(001)\text{\ensuremath{-}}c(2\ifmmode\times\else\texttimes\fi{}4)$ surfaces. Several metastable arrangements are found: ${\mathrm{Sb}}_{4}$ molecules can adsorb as three-dimensional clusters or as flat tetramers. The latter are more stable and can reside either centered on top of the Ge dimer row or half on-top and half over the trench between dimer rows, and in the trenches between two dimer rows. It is found that it is energetically more favorable for ${\mathrm{Sb}}_{4}$ to split into two ${\mathrm{Sb}}_{2}$ dimers adsorbed parallel and on top of the Ge (Si) dimer rows. Simulated scanning tunneling microscopy images are in excellent agreement with the experimental data. The energetics of the diffusion of the dimers into the first Ge (Si) layer is also discussed.

Atomic-scale structural evolution of Ge(100) surfaces etched by H and D

The atomic-scale structural evolution of Ge(100) surfaces etched by H(g) and D(g) at Ts=400 K is studied using scanning tunneling microcopy (STM) and field emission-scanning electron microscopy (FE-SEM). The STM investigation reveals that etching of the Ge(100) by H(g) and D(g) proceeds initially via the production of single atom vacancies (SV), dimer vacancies (DV), and subsequently, line defects along the Ge dimer rows. It is also observed that D(g) etches the Ge(100) surface eight times faster than H(g) does. After extensive exposures of the surface to H(g), the FE-SEM images show square etch pits with V-groove shapes, indicating that H(g) etching of the Ge(100) surface proceeds anisotropically.

Formation of metastable two-dimensional structures during Ag growth on Ge(100)

Silver growth on Ge(100) between 330 and 570 K was studied using scanning tunneling microscopy. At 330 K, three-dimensional (3D) clusters along with two types of two-dimensional (2D) islands were observed: one elongated perpendicular to the substrate dimer rows, the other parallel to the dimer rows. The spacing and registry with the substrate of the former could be explained by Ag adatoms attaching across the substrate dimer bond, while the latter could be attributed to Ag ad-dimers. Annealing caused the larger 3D clusters to grow at the expense of the smaller 3D clusters and 2D islands; at 470 K the 2D islands were completely eliminated. Low-energy electron diffraction indicated that the 3D clusters were oriented with their (110) planes parallel to the surface. After annealing, the 3D clusters became elongated parallel to the Ge dimer rows indicating that the Ag[110] direction aligns parallel to the Ge 1× direction. Although this suggested that the 3D clusters were lower in energy, Ag growth at 470 K led to two types of 2D islands along with pitting of the Ge surface; the pitting was taken as an indication that the islands were intermixed. Images of the islands revealed 2× 3 and 3×2 periodicities. Images of the 3X2 structure showed a single maximum per unit cell, while images of domain boundaries showed that the 3 X 2 structure changed the outermost Ge layer. A model based on Ag-Ge mixed ad-dimers and Ag substitution into the Ge surface can explain these features. The 2X3 and 3×2 structures could only be seen after Ag deposition at ∼470 K; both annealing films deposited at lower temperatures and higher growth temperatures created only 3D clusters. Thus formation of these intermixed structures was attributed to growth of metastable Ag-Ge nuclei at 470 K that were kinetically inhibited from forming at lower temperatures and whose lifetimes were too short to grow at higher temperatures.

Formation process of very thin Ag structures on Ge( [formula omitted]) surface below RT

Submonolayer Ag deposited Ge(0 0 1) surfaces at 90 K were observed by scanning tunneling microscopy at various temperatures between 65 and 240 K. At 78 K, there are seven configurations of bright dots made of a Ag atom or their aggregates and adsorbed on clean Ge(0 0 1) surfaces. These dots thermally diffuse on the Ge surface. By annealing above 100 K, they form various dimers, trimers and clusters elongating along both directions of the Ge dimer and the Ge dimer row. When the surface is annealed at 240 K, parts of the elongated clusters change into the different structures that have been observed at room temperature. With increasing temperature from 90 K, diffusion, clustering and intermixing lead to the formations of the surface alloys or compounds made of Ag and Ge.

Study of Surface Structure and STM Simulation on Ag Adsorption on Ge(001) Surface Using First-Principle Calculations.

The adsorption of Ag on Ge(001) surface has been studied with first-principle calculations for a coverage range from an isolate Ag adatom up to second Ag atom on the surface. Some stable sites of Ag adatom on Ge(001)-2×1 reconstructed surface were found. The scanning tunneling microscopy (STM) images of each stable or metastable site for the adsorption of a low-coverage of Ag adatom are calculated. Two-dimensional Ag islands observed by STM experiments consist of Ag ad-dimers which lie on the trenches between two Ge dimer rows. The ad-dimers are parallel to the under-lying Ge dimer direction.

Surface structure of low-coverage Ag on Ge(001) surface using first-principles calculations

The adsorption of Ag on the Ge(001) surface has been studied with first-principles calculations for a range of coverages from an isolated Ag adatom up to a second Ag atom on the surface. Two-dimensional islands of Ag, observed by scanning tunneling microscopy (STM) experiments, consist of the Ag ad-dimers which lie on the trenches between two Ge dimer rows. The Ag ad-dimer is parallel to the underlying Ge dimer direction. We found that the Ag ad-dimers reside stably at the position between Ge dimer rows.

Micromorphology of Ge and Si1−xGex layers grown on Si(001) by solution epitaxy and the negative-differential conductivity on the Ge layer

The micromorphology and the electronic structure of heteroepitaxial Ge and Si1−xGex layers grown pseudomorphically by solution epitaxy on Si(001) has been investigated by Auger electron spectroscopy as well as scanning tunneling microscopy (STM) and scanning tunneling spectroscopy. A pure Ge film of 4–5 monolayer thickness typically repeats the structure of the Si substrate surface. A significant decrease of tunneling current at a sample voltage of 1.5 V is observed in areas of 0.5 nm diameter between Ge dimer rows. This decrease is due to a negative-differential conductivity at a tunnel diode configuration consisting of a surface defect structure of the Ge layer and the STM tungsten tip. On Si1−xGex films a strong Ge enrichment was found due to segregation during growth. The high Ge surface concentration on Si1−xGex results in additional surface strain fields and trench formation in the uppermost monolayer.