Ge Islands Research Articles

Carbon dimers as the dominant feeding species in epitaxial growth and morphological phase transition of graphene on different Cu substrates.

Cu substrates are highly preferred for the potential mass production of high-quality graphene, yet many of the important aspects of the atomistic growth mechanisms involved remain to be explored. Using multiscale modeling, we identify C-C dimers as the dominant feeding species in the epitaxial growth of graphene on both Cu(111) and Cu(100) substrates. By contrasting the different activation energies involved in C-C dimer diffusion on terraces and its attachment at graphene island edges, we further reveal why graphene growth is diffusion limited on Cu(111), but attachment limited on Cu(100). We also find even higher potential energy barriers against dimer diffusion along the island edges; consequently, a dendritic-to-compact transition is predicted to take place during graphene enlargement on either substrate, but at different growth temperatures. These findings serve as new insights for better control of epitaxial graphene growth.

Growth of NaCl on thin epitaxial KCl films on Ag(100) studied by SPA-LEED

We investigated the growth of NaCl on thin (100)-oriented films of KCl by spot profile analysis of low energy electron diffraction (SPA-LEED). The underlying question of this investigation was how the system accommodates to the misfit of −10% between the NaCl and KCl lattices. The KCl films (3 atomic layers thick) were epitaxially grown on a Ag(100) single crystal. We studied the heteroepitaxial growth of NaCl on KCl at 300K and at 500K, respectively. At 300K, the first NaCl monolayer (ML) grows pseudomorphically on the KCl film. From the second layer onward, the NaCl lattice relaxes. The NaCl multilayers roughen, and a small rotational disorder (±4°) of the NaCl domains is observed. The roughening results from the formation of multilayer islands of limited lateral size due to the misfit to the pseudomorphic first NaCl layer. At a growth temperature of 500K, no pseudomorphic NaCl layer forms, instead relaxed multilayer island growth of NaCl is observed from the first layer onward. Similarly to the growth at 300K, we find NaCl multilayer islands of limited lateral size. For both temperatures, we explain this growth behavior by the misfit that makes the adsorption sites at the island edges of the first relaxed NaCl layer less favorable for larger islands, promoting nucleation of multilayer islands.

The dynamics of TiNx (x = 1–3) admolecule interlayer and intralayer transport on TiN/TiN(001) islands

It has been shown both experimentally and by density functional theory calculations that the primary diffusing species during the epitaxial growth of TiN/TiN(001) are Ti and N adatoms together with TiNx complexes (x=1, 2, 3), in which the dominant N-containing admolecule species depends upon the incident N/Ti flux ratio. Here, we employ classical molecular dynamics (CMD) simulations to probe the dynamics of TiNx (x=1–3) admolecules on 8×8 atom square, single-atom-high TiN islands on TiN(001), as well as pathways for descent over island edges. The simulations are carried out at 1000K, a reasonable epitaxial growth temperature. We find that despite their lower mobility on infinite TiN(001) terraces, both TiN and TiN2 admolecules funnel toward descending steps and are incorporated into island edges more rapidly than Ti adatoms. On islands, TiN diffuses primarily via concerted translations, but rotation is the preferred diffusion mechanism on infinite terraces. TiN2 migration is initiated primarily by rotation about one of the N admolecule atoms anchored at an epitaxial site. TiN admolecules descend from islands by direct hopping over edges and by edge exchange reactions, while TiN2 trimers descend exclusively by hopping. In contrast, TiN3 admolecules are essentially stationary and serve as initiators for local island growth. Ti adatoms are the fastest diffusing species on infinite TiN(001) terraces, but on small TiN/TiN(001) islands, TiN dimers provide more efficient mass transport. The overall results reveal the effect of the N/Ti precursor flux ratio on TiN(001) surface morphological evolution and growth modes.

Lichen Monitoring Delineates Biodiversity on a Great Barrier Reef Coral Cay

Coral islands around the world are threatened by changing climates. Rising seas, drought, and increased tropical storms are already impacting island ecosystems. We aim to better understand lichen community ecology of coral island forests. We used an epiphytic lichen community survey to gauge Pisonia (Pisonia grandis R.BR.), which dominates forest conditions on Heron Island, Australia. Nine survey plots were sampled for lichen species presence and abundance, all tree diameters and species, GPS location, distance to forest-beach edge, and dominant forest type. Results found only six unique lichens and two lichen associates. A Multi-Response Permutation Procedures (MRPP) test found statistically distinct lichen communities among forest types. The greatest group differences were between interior Pisonia and perimeter forest types. Ordinations were performed to further understand causes for distinctions in lichen communities. Significant explanatory gradients were distance to forest edge, tree density (shading), and Pisonia basal area. Each of these variables was negatively correlated with lichen diversity and abundance, suggesting that interior, successionally advanced, Pisonia forests support fewer lichens. Island edge and presumably younger forests—often those with greater tree diversity and sunlight penetration—supported the highest lichen diversity. Heron Island’s Pisonia-dominated forests support low lichen diversity which mirrors overall biodiversity patterns. Lichen biomonitoring may provide a valuable indicator for assessing island ecosystems for conservation purposes regionally.

Impurity-induced island pinning during electromigration

We study the electromigration-induced drift of monolayer Ag islands on Ag(111) which contain one Cu atom. For this purpose a three-dimensional self-learning kinetic Monte Carlo model was extended, and a realistic many-body potential was used. The only free parameters of the model are the effective valences of the Ag and Cu atoms. Due to the impurity, the island drift is significantly reduced, especially for small islands. This is traced back to sequential pinning and depinning events, which are analyzed in detail. Surprisingly, this phenomenon is qualitatively independent of the impurity's effective valence, as long as the impurity does not detach from the island edge. How strongly the drift velocity is reduced depends on the effective valence.

Influence of lattice orientation on growth and structure of graphene on Cu(0 0 1)

We have used low-energy electron microscopy (LEEM) and diffraction (LEED) to examine the significance of lattice orientation in graphene growth on Cu(001). Individual graphene domains undergo anisotropic growth on the Cu surface, and develop into lens shapes with their long axes roughly aligned with Cu〈100〉 in-plane directions. The long axis of a lens-shaped domain is only rarely oriented along a C〈11〉 direction, suggesting that carbon attachment at “zigzag” graphene island edges is unfavorable. A kink-mediated adatom attachment process is consistent with the behavior observed here and reported in the literature. The details of the ridged moiré pattern formed by the superposition of the graphene lattice on the (001) Cu surface also evolve with the graphene lattice orientation, and are predicted well by a simple geometric model. Managing the kink-mediated growth mode of graphene on Cu(001) will be necessary for the continued improvement of this graphene synthesis technique.

KINETICS OF DOPANT ACCUMULATION IN THE ADSORPTION LAYER DURING MOLECULAR-BEAM EPITAXY

A kinetic model of the doping processes in molecular beam epitaxy is developed. The dopant incorporation into the growing crystal is assumed to occur via both blocking dopant atoms at the kink positions by the host atoms and atomic exchange between dopant adatoms and atoms of the topmost crystalline layer. The dopant surface segregation is treated as accumulation of dopant atoms in energetically favorable adsorption positions on the surface. Two segregation pathways are considered: climbing of dopant adatoms over moving steps and jumping of the dopants out of the topmost crystalline layer. It is shown that increasing supersaturation in the adlayer, e.g. with decreasing temperature or increasing growth rate, leads to more efficient blocking of the dopant atoms and, as a consequence, to the decreasing surface segregation. The supersaturation is partially reduced with appearance of 2D islands on the terraces and creation of kinks at the 2D island edges. This results in weaker growth rate and temperature dependences of the surface segregation ratio. Very strong (superexponential) dependence of the surface segregation ratio on the growth temperature is possible under the condition of the intensive jumping of the dopants out of the topmost crystalline layer. The reason is that the probability of «immurement» of the dopant atoms by the moving step depends exponentially on the jumping-out rate constant. The model reproduces experimentally observed segregation behavior of Sb on Si(100).

Simulating the nucleation and growth of Ge quantum dots on Si using high-efficiency algorithms

Original approaches are presented for elucidating a microscopic mechanism of surface atomic diffusion and simulating the heteroepitaxial nanostructure growth on substrates with a complex relief. Using the molecular dynamics method, the energy surface of a pit-patterned Si substrate is mapped out and the conditions for the nucleation of more than one Ge island in a pit are established. A Monte Carlo model is developed that reproduces the Ge growth on Si with regard to elastic effects in a heterosystem. By the example of this Monte Carlo model, it is demonstrated how to reduce the calculation time by optimizing the description of the crystal lattice state in computer cache memory and by using parallel algorithms for working on a computer cluster.

Improved Surface Quality of the Metal-Induced Crystallized Ge Seed Layer and Its Influence on Subsequent Epitaxy

Al-induced crystallized Ge (AIC-Ge) seeded epitaxy is a promising way to fabricate functional materials on amorphous substrates such as glass or polymer sheets. However, the AIC-Ge had difficulty in producing a continuous thin film without forming fine-grain Ge islands stacked on the surface. In this paper, we solved this problem by initially preparing thicker Ge (100 nm) than Al (50 nm) and then removing excess Ge islands using a unique etching technique. The resulting AIC-Ge allowed for a high (111) orientation (96%) and large grains (>100 μm). Transmission electron microscopy demonstrated that an AIC-Ge-seeded Ge layer, grown by molecular-beam epitaxy at 500 °C, was of high quality reflecting the annihilation of Ge islands. This method opens up the possibility for synthesizing Ge, GaAs, and other functional materials on an amorphous substrate.

Structure and morphology of Ge nanowires on Si (001): Importance of the Ge islands on the growth direction and twin formation

Understanding and controlling the structural properties of Ge nanowires are important for their current and future use in technological applications. In this study, the initial stages of the heteroepitaxial growth of Ge nanowires on Si(001) via the Au catalyzed vapor-liquid-solid (VLS) method are investigated. We observe a Ge island located at the base of each nanowire. We propose that these islands are formed by the VLS mechanism and initiate the nanowire growth. Analysis of the islands morphology helps to explain the 〈011〉 growth direction commonly observed for Ge nanowires. Moreover, our observations provide an insight into the formation of twins that propagate along the growth direction.

Unraveling the edge structures of platinum(111)-supported ultrathin FeO islands: the influence of oxidation state.

We used high-resolution scanning tunneling microscopy to study the structure of ultrathin FeO islands grown on Pt(111). Our focus is particularly on the edges of the FeO islands that are important in heterogeneous catalysis, as they host the active sites on inversed catalysts. To imitate various reaction environments we studied pristine, oxidized, and reduced FeO islands. Oxidation of the FeO islands by O2 exposure led to the formation of two types of O adatom dislocations and to a restructuring of the FeO islands, creating long O-rich edges and few short Fe-terminated edges. In contrast, reducing the FeO islands led to a dominance of Fe-rich edges and the occurrence of few and short O-rich edges. In addition, for reducing conditions we observed the formation of O vacancy dislocations on the FeO islands. Through the identification of O adatom and O vacancy dislocations known from closed ultrathin FeO films and geometrical considerations we unraveled the atomic structure of the predominant FeO boundaries of pristine, oxidized, and reduced FeO islands. The results indicate an astonishing flexibility of the FeO islands on Pt(111), since the predominant edge termination and the island shape depend strongly on the preparation conditions.

Three-Dimensional Fourier Monolayer Stress Microscopy

Many important biological processes, such as endothelial mechanotransduction of hemodynamic forces and neutrophil extravasation, involve the transmission of stresses across a cell monolayer. During these processes, the monolayer undergoes both lateral distortion due to the in-plane traction forces generated by the cells, and bending due to the out-of-plane component of the traction forces. However, the contribution of this bending to the monolayer stresses has been neglected in the literature. Here, we present a novel technique to determine monolayer stresses that considers both lateral distortion and bending. To illustrate the method, and to quantify the relative importance of the lateral and bending stresses, we measure the monolayer stresses in micropatterned endothelial cell islands of varying sizes and shapes. The cell islands are cultured on flexible polyacrylamide gels embedded with fluorescent beads, which deform due to traction forces exerted by the cells. We measure the three-dimensional gel deformation using previously established 3D Traction Force Microscopy methods, and recover the monolayer stresses from the measured deformation using Kirchoff-Love thin plate theory. The equations are solved numerically in an efficient manner using a Fourier pseudo-spectral method. The boundary conditions corresponding to the geometry of the cell islands are enforced within the Fourier framework using a relaxation iterative method. Our results indicate that, regardless of island shape, the three-dimensional bending stresses are dominant at the center of the island while the lateral stresses are more important near the island edge. Also, comparing the results from islands of different sizes shows that the relative importance of the bending stresses decreases with island size. These results suggest that it is necessary to resolve bending stresses to accurately determine the monolayer stresses, and reveal that the transmission of forces across cell junctions is three-dimensional and more complex than previously believed.

Static friction scaling of physisorbed islands: the key is in the edge.

The static friction preventing the free sliding of nanosized rare gas solid islands physisorbed on incommensurate crystalline surfaces is not completely understood. Simulations modeled on Kr/Pb(111) highlight the importance and the scaling behavior of the island's edge contribution to static friction.

Removal of Ge Islands in al-induced layer-exchanged Ge thin films on glass substrates by selective etching technique

Removal of Ge Islands in al-induced layer-exchanged Ge thin films on glass substrates by selective etching technique

Bringing the Benefits of Tree Islands Back to the Everglades

Bringing the Benefits of Tree Islands Back to the Everglades

Formation of GeSn alloy on Si(100) by low-temperature molecular beam epitaxy

GeSn alloys grown on Si(100) by the low-temperature (100 °C) molecular beam epitaxy are studied using scanning tunneling microscopy and Raman spectroscopy. It is found that the effect of Sn as a surfactant modifies substantially the low-temperature growth mechanism of Ge on Si. Instead of the formation of small Ge islands surrounded by amorphous Ge, in the presence of Sn, the growth of pure Ge islands appears via the Stranski-Krastanov growth mode, and a partially relaxed Ge1−xSnx alloy layer with the high Sn-fraction up to 40 at. % is formed in the area between them. It is shown that the observed growth mode induced by high surface mobility of Sn and the large strain of the pseudomorphic state of Ge to Si ensures the minimum elastic-strain energy of the structure.

Si/Ge intermixing during Ge Stranski-Krastanov growth.

The Stranski–Krastanov growth of Ge islands on Si(001) has been widely studied. The morphology changes of Ge islands during growth, from nucleation to hut/island formation and growth, followed by hut-to-dome island transformation and dislocation nucleation of domes, have been well described, even at the atomic scale, using techniques such as scanning tunneling microscopy and transmission electron microscopy. Although it is known that these islands do not consist of pure Ge (due to Si/Ge intermixing), the composition of the Ge islands is not precisely known. In the present work, atom probe tomography was used to study the composition of buried dome islands at the atomic scale, in the three-dimensional space. The core of the island was shown to contain about 55 atom % Ge, while the Ge composition surrounding this core decreases rapidly in all directions in the islands to reach a Ge concentration of about 15 atom %. The Ge distribution in the islands follows a cylindrical symmetry and Ge segregation is observed only in the {113} facets of the islands. The Ge composition of the wetting layer is not homogeneous, varying from 5 to 30 atom %.

STM tip-mediated mass transport on Cu surfaces

Atomic-scale simulations are performed to study atomic motion on Cu surfaces to illustrate the effect of the scanning tunneling microscopy tip on mass transport (MT) in the surfaces and on top of the Co island in heteroepitaxial Co/Cu(001) and Co/Cu(111) systems. First we investigate tip-induced atomic motion of Co atoms embedded in the Cu(001) surface at zero bias voltage. With the help of the tip, the Co atom in the surface can freely diffuse toward its nearby vacancy site. So-called vacancy mechanism is used to interpret this phenomenon. Then tip-mediated atomic motion of Co adatoms on the Co islands supported by a Cu(111) surface is studied. It is revealed that the tip has a significant effect on the diffusion of adatoms on the islands and interlayer mass transport at the island edge. Interlayer mass transport at the island edge is found to depend strongly on the tip height and the lateral distance from the tip. By calculating the diffusion barriers, it is found that the jumping diffusion barrier on the island can be zero by the tip vertical manipulation while the Ehrlich–Schwoebel diffusion barrier at the island edge can be reduced by the tip lateral manipulation. Thus, the quality of thin films can be improved by controlling MT in and/or on the surface.

Orientation-dependent growth mechanisms of graphene islands on Ir(111).

Using low-energy electron microscopy, we find that the mechanisms of graphene growth on Ir(111) depend sensitively on island orientation with respect to Ir. In the temperature range of 750-900 °C, we observe that growing rotated islands are more faceted than islands aligned with the substrate. Further, the growth velocity of rotated islands depends not only on the C adatom supersaturation but also on the geometry of the island edge. We deduce that the growth of rotated islands is kink-nucleation-limited, whereas aligned islands are kink-advancement-limited. These different growth mechanisms are attributed to differences in the graphene edge binding strength to the substrate.

Impurity accumulation in an adsorption layer during MBE doping

A theoretical model of capture and surface segregation of an impurity during MBE doping is proposed. It is assumed that an impurity is incorporated in the surface layer of a crystal doped due to both the blocking of the impurity atom by host atoms in kinks on steps and the exchange between the impurity atoms adsorbed and the host atoms in the surface layer. The surface segregation is considered as an accumulation of an impurity in the adsorption layer due to the jumps of impurity atoms adsorbed over the steps and the migration of impurity atoms from the surface layer into the adsorption layer. It is shown that an increase in the supersaturation near a step with a drop in the temperature and an increase in the growth rate suppresses the surface segregation, due to the more effective impurity blocking in the kinks. The formation of 2D islands on the terraces and nonequilibrium kinks at the island edges results in a partial drop of the supersaturation and weakening of the temperature and growth rate effects on the surface segregation. When impurity atoms intensively migrate from the surface layer to the adsorption one, the blurring of the doping profile sharply (superexponentially) increases with the temperature. This is connected with the exponential dependence of the probability of an impurity immuring in the surface layer by a moving step on the rate constant of the impurity migration from the surface layer into the adsorption one. The model reproduces typical dependences of the width of the doping transition’s concentration region on the temperature and growth rate obtained in experiments on Sb-doping of Si.