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The growth of Ge three-dimensional coherent clusters on Si(001) during gas source molecular-beam epitaxy and post-deposition anneals has been investigated using in situ elevated-temperature scanning tunneling microscopy. By monitoring the growth of individual so-called ``hut'' clusters, this technique allowed us to separate various factors that may affect the final size distribution of entire cluster ensembles. It has been found that during the course of epitaxy the hut clusters grow by nucleation and growth of deposited material on the cluster facets; however, the low growth rate ($r\ensuremath{\propto}{t}^{1/n},$ where $4lnl5$), and the large scatter in absolute rate constants indicate diffusion-limited, rather than interface-limited mass transport, although both the facet-nucleation step and the wetting-layer defects inhibit the cluster growth. The strain-induced energy barriers at the cluster bases prevent material addition, and thus growth of large clusters, facilitating the domination of growth mechanisms other than Ostwald ripening at temperatures below 700 K, leading to symmetric or positively skewed cluster-size distribution functions. The tendency towards negatively skewed and bimodal distributions at higher temperatures signaled the contribution of the ripening in accord with the Lifshitz-Slyozov-Wagner theory. Raising the temperature above 770 K leads to a gradual replacement of the huts by the 〈110〉-based macroscopic clusters.

We have investigated self-assembled island formation, molecular detail and interesting contrast reversal effects for Er3N@C80 and Sc3N@C80 on Au(111) and Ag/Si(111) surfaces using variable temperature scanning tunnelling microscopy (STM) and spectroscopy (STS). The trinitride containing fullerenes have been evaporated onto Ag-passivated Si(111) at room temperature and self-assembled into close-packed 2-D islands. Gentle annealing at 200 - 300 °C is required for the formation of close-packed islands with 20 - 50 nm diameter on the Au(111) surface. Variable-voltage STM reveals bias-dependent contrast anomalies within the islands, and at low temperatures (< 90K), intra-molecular resolution of the fullerenes has been achieved. STS measurements indicate that the bright/dark anomalies may be caused by different electron densities of states for the "normal" and "anomalous" fullerenes. ©2008 IOP Publishing Ltd.

We have observed the development of the surfaces during gas-source growth of silicon and germanium in an elevated temperature ultrahigh vacuum scanning tunneling microscopy (STM), with near-atomic resolution under a range of temperature and flux, which are the two dominant parameters, and applied atomistic modeling to the structures seen by STM to enable us to give confident interpretation of the results. A key role in the growth of silicon and germanium on Si(001) from disilane and germane, respectively, is played by the surface hydrogen. The growth of germanium follows a similar path to that of silicon for the first few monolayers, after which the strain becomes relieved by periodic trenches, and eventually by a combination of faceted pits and clusters, both of which nucleate heterogeneously at surface defects. Understanding these processes is crucial to controlling the self-assembled Ge/Si quantum structures. (C) 1998 American Vacuum Society.

The initial stages of gas-source growth of Si(001) using disilane have been investigated using a combination of elevated-temperature STM and atomistic modelling. The reaction pathway from the initial adsorption of disilane fragments up to the nucleation of short strings of epitaxial dimers is discussed. By the use of our STM to study disilane at the temperatures of interest, and atomistic modelling to calculate structural stability and significant activation barriers, we are able to propose a complete description of the mechanisms which underlie gas-source growth.

Trimetallic nitride template fullerenes have been deposited onto a variety of substrates in order to elucidate the substrate-fullerene interactions. We have investigated self-assembled island formation and molecular detail of Er3N@C80 and Sc3N@C80 on Ag/Si(1 1 1), Au(1 1 1)/mica, Si(1 1 1), and Si(0 0 1) using variable temperature scanning tunnelling microscopy (STM). At room temperature, the fullerenes self-assemble into monolayer-high hexagonal close-packed islands on Ag-passivated Si(1 1 1) whereas annealing at elevated temperatures (250-300 °C) is necessary for the self-assembly of close-packed islands on Au(1 1 1). Intra-molecular resolution of the fullerenes has been achieved at liquid nitrogen temperature on Ag/Si(1 1 1) and already at room temperature on Si(0 0 1), when the rotation of the fullerenes is frozen. Whereas the bonding between the fullerenes and Si surfaces is mainly covalent, it appears to be mainly van-der-Waals on the other surfaces. © 2006 Elsevier B.V. All rights reserved.

By means of in situ high-temperature scanning tunnelling microscopy (STM), the thermal stability of highly ordered artificial Al nanocluster arrays, so-called two-dimensional nanocluster crystals, on vicinal Si (111)-7 × 7 surfaces has been investigated. It was found that Al nanocluster crystals are stable below 500 °C. Above 500 °C, the Al nanocluster crystal transforms into a surface phase with singular triangle shapes on down-terraces (the down-step side of a terrace). However, on up-terraces (the up-step side of a terrace), Al nanoclusters can move on Si (111)-7 × 7 surfaces over a large distance and the Al nanocluster crystal retracts toward the phase until the bare Si (111)-7 × 7 surface is completely recovered on up-terraces. The thermally activated motion of Al nanoclusters was not observed apparently below 400 °C, though Al nanocluster crystals can be formed even at 200 °C. This indicates that it is atom diffusion, not cluster diffusion, which is responsible for the formation process of metal cluster crystals on Si (111)-7 × 7 surfaces.

1D indium islands have been formed by epitaxial growth on a self-assembled template. The templates are formed from Bi nanolines, which are 1.5 nm wide, and can be over 500 nm in length. The islands conform to the template width and are over 100 nm long. The first layer of the island forms a novel chain structure of In and Bi atoms, which accounts for the epitaxial growth of the nanocrystals on the template. The second layer shows a hexagonal structure 19 A or 2.5 dimer rows wide. Epitaxial growth saturates after two layers of In have been deposited. With further deposition, a transition to the growth of 3D In droplets is observed.

The practical realisation of nanoscale devices requires the development of practical nanofabrication techniques. The Bi nanolines which self-assemble on Si(001) are promising templates for atomic scale wires, and also have a fascinating subsurface reconstruction. Elastic interactions are often responsible for the limited dimensionality of epitaxial nanoscale structures. The present work examines the elastic strain field around the Bi nanoline in terms of its interaction with other surface features. A short range tensile strain around the Bi nanoline may be identified by the effect on the electronic structure of the neighbouring dimers. A longer range elastic interaction is exhibited in a repulsive interaction between the nanoline and defects and steps. Atomic resolution variable bias scanning tunnelling microscopy (STM) and first principles electronic structure calculations have been used to elucidate these effects, and excellent agreement has been found between experimental observations and theoretical results.

The structure of InAs(0 0 1)-(2×4) surfaces equilibrated under typical MBE conditions is studied by scanning tunneling microscopy (STM). Depending on the magnitude of the As flux, typical surfaces are found to contain a mixture of α2(2×4) and β2(2×4) reconstructions. The relative populations of the α2 and β2 reconstructions are found to depend on substrate temperature and the magnitude of the As flux. The atomic-scale details of the reconstructed units on these mixed-phase surfaces are definitively determined by comparing atomic-resolution dual-bias STM images to first-principles calculations. The imaging mechanism for revealing atomic-scale details, particularly the trench dimer, is found to be qualitatively similar to that for GaAs, although the effect is less pronounced. Additionally, a significant population of ad-atom related structures are observed on quenched surfaces, apparently unrelated to any equilibrium ad-atom population.

The stability of different surface reconstructions on InAs~001! is investigated theoretically and experimentally. Density-functional theory calculations predict four different surface reconstructions to be stable at different chemical potentials. The two dominant reconstructions are the b2 ~234! for high As, and the a2 ~234! for low As overpressure. This trend is confirmed by scanning tunneling microscopy of carefully annealed InAs~001! surfaces. A similar behavior is predicted for GaAs~001!.

Atomically perfect bismuth lines form on Si(001) by a selective desorption process around the temperature at which most of the bismuth desorbs from bismuth epitaxial layers. The lines are perpendicular to the silicon dimer rows; they are 1 nm wide and can be hundreds of nm long in a flat Si(001) surface. They are utterly free of kinks or other defects. In our proposed model three silicon dimers in the surface are replaced with two Bi dimers, with a rebonded missing dimer defect between them. It is suggested that the lines are straight because of kinetic reasons rather than thermodynamic ones -- they form straight and are unable subsequently to kink.

Scanning tunneling microscopy (STM) has been used to investigate the adsorption of benzene on nominally flat Si(100)-(2×1) substrates. STM images show that benzene adsorbs on top of the dimer rows bonding to the two Si–Si dimer dangling bonds. Bias-dependent imaging indicates that the highest occupied molecular orbital of adsorbed benzene lies approximately 1.2 eV below the top of the valence band and that the lowest unoccupied molecular orbital is at least 3.5 eV above the highest occupied molecular orbital. At higher coverages, the benzene molecules are adsorbed on every other dimer along the dimer row and on every other dimer across the dimer rows resulting in a local c(4×2) periodicity, in agreement with the saturation coverage of ∼0.25 monolayer.

An understanding of the diffusion of hydrogen on Si(001) is important in the gas-source growth of silicon and silicon-germanium alloys. We have imaged the saturated monohydride surface between 600 and 700 K using an elevated-temperature scanning tunneling microscope (STM), and have observed the concerted diffusion of pairs of hydrogen atoms along the dimer rows. The barrier for this process has been estimated from the STM data to be $1.95\ifmmode\pm\else\textpm\fi{}0.2\mathrm{eV}.$ Using our experimental images, we have narrowed down the mechanism to two possible routes, which we have modeled with tight binding. Only one of these routes has a barrier (1.98 eV) which is consistent with the experiment, leading to a conclusive identification of the diffusion mechanism.

The crucial difference between gas-source molecular beam epitaxy (MBE) and conventional MBE is the presence of hydrogen on the growth surface. The amount and behaviour of the hydrogen are controlled by a combination of temperature and disilane flux. In situ observations under growth conditions are essential for an accurate understanding of non-equilibrium growth phenomena such as nucleation and coarsening, because once the flux has been cut off the surface material will redistribute itself. We have found that not only does surface hydrogen block silicon diffusion, but also hydrogen saturation of the substrate step edges blocks step-flow growth so that island growth predominates below 700 K, even at low fluxes. The denuded zones seen in MBE are not observed. Above 700 K, the adsorption barrier at step edges is overcome, and a transition from island growth to step-flow growth is observed as the flux is varied.

A novel structure has been observed by scanning tunneling microscopy (STM) on the Si(001) surface after exposure to disilane between 400 and 600 K. The feature is a bright square, with dark lines running across it forming a cross. The proposed structure, a ring of four silicon atoms bonded together and connected by one back-bond per atom to the underlying silicon dimers, has been modelled using tight-binding and density functional theory (DFT) calculations. This ring has been found to be energetically stable with respect to isolated ad-dimers. As it is the first feature to form from disilane fragments with increasing temperature, and its local bonding configuration is very similar to the rebonded B-type step edge which is known to be the favoured adsorption site for epitaxial growth, it may play a crucial role as the nucleus of the new epitaxial layer during gas-source growth of silicon.

A JEOL scanning tunneling microscope (STM) has been used to image the clean silicon (001) surface at low sample bias voltages (around −0.4 V). At this bias, many dimer vacancies are highlighted by a bright feature on the neighbouring dimers. On other defects, this situation is reversed; the area around the defect becomes dark at low voltages. In both cases, at higher bias voltages (around −0.8 V), this contrast disappears. For a number of proposed structures of the single dimer vacancy, ab initio calculations of charge density as a function of energy have been used to simulate STM images. These images show significant bias voltage dependence, and the low bias voltage images differ markedly between the structures modelled. On this basis, we identify the rebonded structure with the bright defect, and the non-rebonded structure with the dark defect.

We have investigated the decay of reflection high-energy electron diffraction oscillations during growth of InAs (001), as a function of growth parameters, such as the V/III ratio. We have shown that the decay constants are sensitive to changes in growth morphology, as measured by scanning tunneling microscopy. Our experiments show that the values of these decay constants decrease at high V/III ratios, in agreement with previous work. Additionally, we have found that the values of the decay constants diverge as the transition between the (2×4) and (4×2) reconstructions is approached. We propose that the decay constants of the growth oscillations may be used as inputs for control of interface morphology.

By means of variable-temperature scanning tunneling microscopy, we have investigated systematically the formation process of Al magic cluster arrays on the $\mathrm{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ surface at high temperature in situ. It was found that the magic clusters form only when the Al coverage is over a critical value, $\ensuremath{\sim}0.08\ifmmode\pm\else\textpm\fi{}0.015$ ML. These clusters occupy preferentially on the faulted-half-unit cells of the $\mathrm{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ surface, but this preference is coverage dependent. By analyzing systematically the spatial distribution of magic clusters below the saturation coverage, an attractive interaction between clusters was found, which prevents a triangular ordering of the nanocluster array.

Ammonia adsorbs dissociatively on the Si(001) surface above $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The majority dissociation products are -H and -$\mathrm{N}{\mathrm{H}}_{2}$ groups, adsorbed on the dangling bonds of the Si dimers. At low coverages, the ammonia fragments are observed to form clumps, which extend both along and across dimer rows, suggesting interactions between molecules: either direct interactions, or those mediated by the buckled dimers on the Si(001) substrate. The detailed reactions in the adsorption and dissociation of $\mathrm{N}{\mathrm{H}}_{3}$ on Si(001) have been investigated by density functional theory calculations, and compared to data from scanning tunneling microscope images. The buckling of the Si dimers provides a mechanism for preferential adsorption next to existing adsorbed species, along a single dimer row. We find that direct H-bonding interactions between chemisorbed $\mathrm{N}{\mathrm{H}}_{2}$ groups and incoming $\mathrm{N}{\mathrm{H}}_{3}\phantom{\rule{0.3em}{0ex}}$ molecules are also significant, including an interaction across dimer rows, which explains the observed clumping behavior.

We have studied the structure of MBE-grown InAs(001)-(2×4) surfaces exposed to low Sb2 fluxes by scanning tunneling microscopy (STM) and ab initio density functional theory (DFT). Experimentally, we observe an Sb-terminated α2(2×4) phase over a wide range of temperatures (400–510 °C) for low Sb2 flux (2 fluxes used during heterostructure growth.

We have examined Bi-induced surface structures of Si(001), formed in the vicinity of its desorption temperature, by means of scanning tunnelling microscopy and reflection high-energy electron diffraction. Below the desorption temperature, adsorption of bismuth results in a (2× n ) Bi–Si(001) reconstruction with second-layer Bi dimers either parallel or perpendicular to the underlying dimers. Even after the desorption of the bismuth epitaxial layer, some bismuth remains on the surface with characteristic structures which depend upon the formation pathway. Perfectly straight Bi lines 1 nm wide and hundreds of nanometres long can be formed either by annealing of the saturated Bi epitaxial layer, or by exposure to a large flux of bismuth around the desorption temperature. Exposure to a smaller flux of Bi at the desorption temperature generates a c(4×4) reconstruction. The stability of the c(4×4) reconstruction is thought to be due to a strain effect resulting from incorporated Bi. Both the Bi line and the c(4×4) structures disappear after further annealing and the remaining Bi concentration was below the detection limit of secondary ion mass spectroscopy.

A recent article (Miwa et al 2005 Nanotechnology 16 2427) casts doubt on the four-dimer-wide Haiku model for the Bi nanoline on Si(001), suggesting instead that the three-dimer-wide Miki model (which had been ruled out) is a better fit in particular to x-ray data. The reasons why the Haiku model provides the best fit to all published data among currently proposed structures are discussed, concentrating on the width and registry of the Bi nanoline, and mentioning new data which shows that under appropriate conditions the two structures coexist in the same surface.

Heteroepitaxial Ge/Si(001) growth has been investigated using in situ scanning tunneling microscopy. While at 620 K the epitaxial strain is relieved by formation of three-dimensional islands (so-called ``hut'' clusters), at 690 K the strain is first relieved by hut pits, having the cluster shapes but with their apex pointing down. Although predicted theoretically to have lower energy than clusters, hut pits have never been observed individually before. Details of cluster and pit nucleation are also presented for the first time.

In this study we have investigated the mechanism of Si and Ge growth from gaseous precursors, at intermediate temperatures, in elevated-temperature scanning tunnelling microscope (STM). Based on our observations, we suggest a plausible mechanism for the growth of Ge and Si from GeH4 and Si2H6.

A method for measuring the V/III ratio for epitaxial growth using threshold photoemission is demonstrated. The variation in photoyield observed during reconstruction changes of InAs (001), attributed to the variation of As dimer concentration on the surface, can be used to determine the As incorporation rate, and hence the V/III ratio in situ, in a manner analogous to As-limited reflection high energy electron diffraction oscillation measurements. Although As-limited photoyield oscillations are observed and reported here, the photoyield method for determining the V/III ratio is not dependent on the measurement of oscillations. The photoyield measurement approach is still applicable at high temperatures where step flow growth competes with island nucleation and oscillations are difficult to resolve.

We have studied the chemistry of [(t-butyl)GaS]4 on Si(100)-(2×1) in ultrahigh vacuum. The characterization techniques used in this study were Auger electron spectroscopy (AES), temperature programmed desorption (TPD), and low-energy electron diffraction (LEED). Exposing [(t-butyl)GaS]4 to Si(100)-(2×1) at temperatures below 700 K gives evidence of gallium, sulfur, and carbon on the surface, as judged by AES, while exposing at an elevated temperature (>700 K) results in nearly carbon free adsorption. TPD spectra measured after exposure at 200 K show that the hydrocarbon ligands of the [(t-butyl)GaS]4 undergo a β-hydride elimination reaction and desorb predominantly as isobutene. For low exposures, the GaS core of the [(t-butyl)GaS]4 molecule dissociates upon annealing, as judged by the SiS and gallium TPD peaks. TPD experiments conducted after adsorption of more than 20 A of [(t-butyl)GaS]4 on the Si(100)-(2×1) surface show evidence of Ga2S desorption. For deposition at 700 K, LEED shows that the (2×1) re...