Search

Extreme ultraviolet (EUV) lithography (13.5 nm) is the newest technology that allows high-throughput fabrication of electronic circuitry in the sub-20 nm scale. It is commonly assumed that low-energy electrons (LEEs) generated in the resist materials by EUV photons are mostly responsible for the solubility switch that leads to nanopattern formation. Yet, reliable quantitative information on this electron-induced process is scarce. In this work, we combine LEE microscopy (LEEM), electron energy loss spectroscopy (EELS), and atomic force microscopy (AFM) to study changes induced by electrons in the 0–40 eV range in thin films of a state-of-the-art molecular organometallic EUV resist known as tin-oxo cage. LEEM–EELS uniquely allows to correct for surface charging and thus to accurately determine the electron landing energy. AFM postexposure analyses revealed that irradiation of the resist with LEEs leads to the densification of the resist layer because of carbon loss. Remarkably, electrons with energies as low as 1.2 eV can induce chemical reactions in the Sn-based resist. Electrons with higher energies are expected to cause electronic excitation or ionization, opening up more pathways to enhanced conversion. However, we do not observe a substantial increase of chemical conversion (densification) with the electron energy increase in the 2–40 eV range. Based on the dose-dependent thickness profiles, a simplified reaction model is proposed where the resist undergoes sequential chemical reactions, first yielding a sparsely cross-linked network and then a more densely cross-linked network. This model allows us to estimate a maximum reaction volume on the initial material of 0.15 nm3 per incident electron in the energy range studied, which means that about 10 LEEs per molecule on average are needed to turn the material insoluble and thus render a pattern. Our observations are consistent with the observed EUV sensitivity of tin-oxo cages.

The imaging of dynamical processes at interfaces and on the nanoscale is of great importance throughout science and technology. While light-optical imaging techniques often cannot provide the necessary spatial resolution, electron-optical techniques damage the specimen and cause dose-induced artifacts. Here, optical near-field electron microscopy (ONEM) is proposed, an imaging technique that combines noninvasive probing with light, with a high-spatial-resolution readout via electron optics. Close to the specimen, the optical near fields are converted into a spatially varying electron flux using a planar photocathode. The electron flux is imaged using low-energy electron microscopy, enabling label-free nanometric resolution without the need to scan a probe across the sample. The specimen is never exposed to damaging electrons.

Crystalline films of pentacene molecules, two to four monolayers in thickness, are grown via in situ sublimation on silicon substrates in the ultrahigh vacuum chamber of a low-energy electron microscope. It is observed that the diffraction pattern of the pentacene layers fades upon irradiation with low-energy electrons. The damage cross section is found to increase by more than an order of magnitude for electron energies from 0 to 10 eV and by another order of magnitude from 10 to 40 eV. Close to 0 eV, damage is virtually nil. Creation of chemically reactive atomic centers after electron attachment or impact ionization is thought to trigger chemical reactions between neighboring molecules that gradually transform the layer into a disordered carbon nanomembrane. Additionally, diminishing spectroscopic features related to the unoccupied band structure of the layers, accompanied by loss of definition in real-space images, and an increase in the background intensity of diffraction images during irradiation point to chemical changes and formation of a disordered layer.

Transmission electron microscopy at very low energy is a promising way to avoid damaging delicate biological samples with the incident electrons, a known problem in conventional transmission electron microscopy. For imaging in the 0-30 eV range, we added a second electron source to a low energy electron microscopy (LEEM) setup, enabling imaging and spectroscopy in both transmission and reflection mode at nanometer (nm) resolution. The latter is experimentally demonstrated for free-standing graphene. Exemplary eV-TEM micrographs of gold nanoparticles suspended on graphene and of DNA origami rectangles on graphene oxide further establish the capabilities of the technique. The long and short axes of the DNA origami rectangles are discernable even after an hour of illumination with low energy electrons. In combination with recent developments in 2D membranes, allowing for versatile sample preparation, eV-TEM is paving the way to damage-free imaging of biological samples at nm resolution.

In a lot of systems, charge transport is governed by local features rather than being a global property as suggested by extracting a single resistance value. Consequently, techniques that resolve local structure in the electronic potential are crucial for a detailed understanding of electronic transport in realistic devices. Recently, we have introduced a new potentiometry method based on low-energy electron microscopy (LEEM) that utilizes characteristic features in the reflectivity spectra of layered materials [1]. Performing potentiometry experiments in LEEM has the advantage of being fast, offering a large field of view and the option to zoom in and out easily, and of being non-invasive compared to scanning-probe methods. However, not all materials show clear features in their reflectivity spectra. Here we, therefore, focus on a different version of low-energy electron potentiometry (LEEP) that uses the mirror mode transition, i.e. the drop in electron reflectivity around zero electron landing energy when they start to interact with the sample rather than being reflected in front of it. This transition is universal and sensitive to the local electrostatic surface potential (either workfunction or applied potential). It can consequently be used to perform LEEP experiments on a broader range of material compared to the method described in Ref [1] . We provide a detailed description of the experimental setup and demonstrate LEEP on workfunction-related intrinsic potential variations on the Si(111) surface and for a metal-semiconductor-metal junction with external bias applied. In the latter, we visualize the Schottky effect at the metal-semiconductor interface. Finally, we compare how robust the two LEEP techniques discussed above are against image distortions due to sample inhomogeneities or contamination.

For many complex materials systems, low-energy electron microscopy (LEEM) offers detailed insights into morphology and crystallography by naturally combining real-space and reciprocal-space information. Its unique strength, however, is that all measurements can easily be performed energy-dependently. Consequently, one should treat LEEM measurements as multi-dimensional, spectroscopic datasets rather than as images to fully harvest this potential. Here we describe a measurement and data analysis approach to obtain such quantitative spectroscopic LEEM datasets with high lateral resolution. The employed detector correction and adjustment techniques enable measurement of true reflectivity values over four orders of magnitudes of intensity. Moreover, we show a drift correction algorithm, tailored for LEEM datasets with inverting contrast, that yields sub-pixel accuracy without special computational demands. Finally, we apply dimension reduction techniques to summarize the key spectroscopic features of datasets with hundreds of images into two single images that can easily be presented and interpreted intuitively. We use cluster analysis to automatically identify different materials within the field of view and to calculate average spectra per material. We demonstrate these methods by analyzing bright-field and dark-field datasets of few-layer graphene grown on silicon carbide and provide a high-performance Python implementation.

Photo electron emission microscopy (PEEM), going back to the earliest days of electron microscopy, and low-energy electron microscopy (LEEM), successfully deployed since the late 1980s, are examples of cathode lens microscopy in which the sample itself is an integral part of the image forming system. While applications have naturally gravitated towards the acquisition of images to elucidate structure and structural evolution, recent years have also seen a rapidly expanding range of spectroscopic capabilities. These address, for example, the occupied and unoccupied electronic band structures of materials, electrical transport in 2-D systems, crystal growth and 2-D strain, inelastic electron energy loss mechanisms, as well as radiation damage in organic materials during low-energy electron irradiation. In this chapter, we discuss applications of these new spectroscopic methods, as well as recent instrumental developments that further expand the potential uses of cathode lens microscopy.

Graphene on silicon carbide (SiC) bears great potential for future graphene electronic applications because it is available on the wafer scale and its properties can be custom tailored by inserting various atoms into the graphene/SiC interface. It remains unclear, however, how atoms can cross the impermeable graphene layer during this widely used intercalation process. Here we demonstrate that in contrast to the current consensus, graphene layers grown in argon atmosphere on SiC are not homogeneous, but instead are composed of domains of different crystallographic stacking as they have been observed in other systems. We show that these domains are intrinsically formed during growth and that dislocations between domains dominate the (de)intercalation dynamics. Tailoring these dislocation networks, e.g., through substrate engineering, will increase the control over the intercalation process and could open a playground for topological and correlated electron phenomena in two-dimensional superstructures.

For many applications, it is important to measure the local work function of a surface with high lateral resolution. Low-energy electron microscopy is regularly employed to this end since it is, in principle, very well suited as it combines high-resolution imaging with high sensitivity to local electrostatic potentials. For surfaces with areas of different work function, however, lateral electrostatic fields inevitably associated with work function discontinuities deflect the low-energy electrons and thereby cause artifacts near these discontinuities. We use ray-tracing simulations to show that these artifacts extend over hundreds of nanometers and cause an overestimation of the true work function difference near the discontinuity by a factor of 1.6 if the standard image analysis methods are used. We demonstrate on a mixed-terminated strontium titanate surface that comparing LEEM data with detailed ray-tracing simulations leads to much a more robust estimate of the work function difference.

The effects of exposure to ionizing radiation are central in many areas of science and technology, including medicine and biology. Absorption of UV and soft-x-ray photons releases photoelectrons, followed by a cascade of lower energy secondary electrons with energies down to 0 eV. While these low energy electrons give rise to most chemical and physical changes, their interactions with soft materials are not well studied or understood. Here, we use a low energy electron microscope to expose thin organic resist films to electrons in the range 0--50 eV, and to analyze the energy distribution of electrons returned to the vacuum. We observe surface charging that depends strongly and nonlinearly on electron energy and electron beam current, abruptly switching sign during exposure. Charging can even be sufficiently severe to induce dielectric breakdown across the film. We provide a simple but comprehensive theoretical description of these phenomena, identifying the presence of a cusp catastrophe to explain the sudden switching phenomena seen in the experiments. Surprisingly, the films undergo changes at all incident electron energies, starting at $\ensuremath{\sim}0\text{ }\text{ }\mathrm{eV}$.

We measure the role of structural defects, including grain boundaries and step edges, in determining the electrical transport characteristics of polycrystalline graphene monolayers synthesized on C-face SiC(0 0 0 ) by thermal decomposition. A combination of multi-probe scanning tunneling microscopy/potentiometry and low-energy electron microscopy allows the transport properties of individual grain boundaries to be correlated with their misorientation and atomic-level structure, without any device fabrication. We find that different types of grain boundary show dramatically different transport properties, and that boundaries can change structure and resistivity along their length. Boundary regions made up of dislocation superlattices separated by continuous graphene exhibit relatively low resistivity which is comparable to the resistivity of the graphene sheet itself. Other grain boundaries display trench structures with a resistivity 1–2 orders of magnitude greater and sufficient to dominate transport through the polycrystalline sheet. We also measure the transport properties of step edges and monolayer-bilayer boundaries on C-face graphene and compare them to Si-face graphene. Such measurements offer a guideline for optimizing graphene growth on SiC to improve its electronic properties.

We have used in situ low-energy electron microscopy (LEEM) to correlate the atomic and electronic structure of graphene films on polycrystalline Ni with nm-scale spatial resolution. Spatially resolved electron scattering measurements show that graphene monolayers formed by carbon segregation do not support the p-plasmon of graphene, indicating strong covalent bonding to the Ni. Graphene bilayers have the Bernal stacking characteristic of graphite and show the expected plasmon loss at 6.5 eV. The experimental results, in agreement with first-principles calculations, show that the p-band structure of free-standing graphene appears only in films with a thickness of at least two layers and demonstrate the sensitivity of the plasmon loss to the electronic structure.

The problem of separating single-wall carbon nanotubes (CNTs) by diameter and/or chirality is one of the greatest impediments toward the widespread application of these promising materials in nanoelectronics. In this paper, we describe a novel physical-chemical method for diameter-selective CNT separation that is both simple and effective and that allows up-scaling to large volumes at modest cost. Separation is based on size-selective noncovalent matching of an appropriate anchor molecule to the wall of the CNT, enabling suspension of the CNTs in solvents in which they would otherwise not be soluble. We demonstrate size-selective separation in the 1-2 nm diameter range using easily synthesized oligo-acene adducts as a diameter-selective molecular anchor. CNT field effect transistors fabricated from diameter-selected CNTs show markedly improved electrical properties as compared to nonselected CNTs.

The nucleation and growth of pentacene thin films are controlled largely by the energies associated with the interfaces. We have used low-energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM) to investigate the nucleation and growth of pentacene thin films on Si(001) surfaces modified with two different molecular monolayers. Clean Si(001)-(2 × 1) surfaces were modified with either 1,5-cyclooctadiene or 1-dodecene prior to pentacene growth to study the effects of exposed π bonds at the interface, orientation of those π bonds relative to each other, and rigidity of the molecular layer on pentacene nucleation, growth, and crystalline orientation. Both molecular monolayers weaken the substrate−pentacene interaction sufficiently to allow for low pentacene nucleation density and good pentacene diffusion, leading to the growth of pentacene grains as large as 100 μm. Pentacene grows epitaxially on both functionalized surfaces, adopting an orthorhombic unit cell that follows the orientatio...

Extreme Ultra Violet (EUV) lithography is a next generation lithographic technique using 13.5 nm wavelength light (91.7eV photon energy) to define sub-20 nm features. This high energy radiation generates lower energy electrons (LEEs) after being absorbed. The mean free path of LEEs increases rapidly below ca. 30 eV allowing them to migrate several nanometers from their point of origin. As LEEs can still have sufficient energy to react with the surrounding resist, this may give rise to pattern blurring, posing a challenge for sub 10 nm features. Here, we introduce Low Energy Electron Microscopy (LEEM) as an extremely useful technique to investigate the interactions of LEEs with EUV resists. Using LEEM we can expose the resist with precise electron energies and doses. We also report the initial results of LEE exposures on poly(methyl methacrylate) PMMA. We have studied the LEE-PMMA interaction depth as a function of electron energy; a distinct exposure threshold is found at ~15 eV, below which the resist responds only very weakly to electron exposure.

Dynamic processes at the solid–liquid interface are of key importance across broad areas of science and technology. Electrochemical deposition of copper, for example, is used for metallization in integrated circuits, and a detailed understanding of nucleation, growth and coalescence is essential in optimizing the final microstructure. Our understanding of processes at the solid–vapour interface has advanced tremendously over the past decade due to the routine availability of real-time, high-resolution imaging techniques yielding data that can be compared quantitatively with theory1,2,3. However, the difficulty of studying the solid–liquid interface leaves our understanding of processes there less complete. Here we analyse dynamic observations—recorded in situ using a novel transmission electron microscopy technique—of the nucleation and growth of nanoscale copper clusters during electrodeposition. We follow in real time the evolution of individual clusters, and compare their development with simulations incorporating the basic physics of electrodeposition during the early stages of growth. The experimental technique developed here is applicable to a broad range of dynamic phenomena at the solid–liquid interface.

We observe the growth of crystalline SiC nanoparticles on Si(001) at 900 °C using in situ electron microscopy. Following nucleation and growth of the SiC, there is a massive migration of Si, forming a crystalline Si mound underneath each nanoparticle that lifts it 4–5 nm above the initial growth surface. The volume of the Si mounds is roughly five to seven times the volume of the SiC nanoparticles. We propose that relaxation of strain drives the mound formation. This new mechanism for relieving interfacial strain, which involves a dramatic restructuring of the substrate, is in striking contrast to the familiar scenario in which only the deposited material restructures to relieve strain.

We have used low-energy electron microscopy to investigate the thermal fluctuation of phase boundaries separating 7×7 and 1×1 regions of the Si(111) surface near the phase transition temperature of 1135 K. We find that the fluctuations are described by a phase boundary stiffness of 180±30 meV/nm. Analysis of the meandering of the phase boundary is used to characterize the elastic repulsion between phase boundaries. Our estimate of the surface stress difference between the phases, 0.06±0.01 eV/A2, is consistent with the measurement of Twesten and Gibson [R. D. Twesten and J. M. Gibson, Phys. Rev. B 50, 17628 (1994)].

We present results that demonstrate how interfacial reactions between a metal film and substrate during deposition affect microstructural evolution. In particular, we investigate Ti films deposited on amorphous SiO2 using ultrahigh vacuum transmission electron microscopy. Ti films were deposited in situ at room temperature and were examined using Auger electron spectroscopy and transmission electron microscopy. An initial [hk0] preferred orientation developed in films up to 2.5 nm in thickness. Films between 2.5 and 5.0 nm developed a [001] preferred orientation that persisted in films up to 20.0 nm thick. These data, in conjunction with Auger electron spectra and dark-field microscopy, suggest that growth of Ti films on SiO2 is directly affected by reactions at the Ti/SiO2 interface and that this reaction is responsible for the observed change in preferred orientation.

The equilibrium thermal roughening of thin Ge layers (one and two monolayers) deposited on Si(001) has been investigated with low-energy electron microscopy. A Ge-coverage-dependent roughening is observed. For two monolayers, the temperature at which imaging contrast is lost due to surface roughness is $900\ifmmode\pm\else\textpm\fi{}25\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, between the roughening temperatures of Ge(001) and Si(001). Lower Ge coverages move this temperature closer to that of Si(001). The roughening is confined to the Ge overlayers. It is believed that this phenomenon represents a new type of surface roughening transition that should be generally applicable for heteroepitaxial films.

The rate of misfit strain relaxation in semiconductor heterostructures is controlled by the kinetics of misfit dislocation nucleation, propagation and interaction. Although there have been a number of detailed theoretical studies of the dislocation–dislocation interaction process in these systems, there exist limited experimental data regarding the regime of epilayer thickness and composition where these interactions affect strain relaxation. Because of its high spatial and temporal resolution, in-situ transmission electron microscopy is an ideal experimental tool with which to observe dislocation interactions. In this work, the unique capabilities of an ultrahigh vacuum transmission electron microscope have been exploited to permit direct, real-time observation of dislocation interactions. This microscope is equipped with in-situ chemical vapour deposition capabilities which allow imaging of dislocation motion during both growth and annealing of SiGe/Si (001) heterostructures. This has permitted...

▪ Abstract In situ Ultra-High Vacuum (UHV) electron microscopy, including Transmission Electron Microscopy (TEM) at 300 keV electron energy and Low-Energy Electron Microscopy (LEEM) at 0-30 eV electron energy, has advanced enormously over the last decade. Growth of thin films such as epitaxial Si1−xGex alloy thin films on Si substrates has become routine, allowing high-resolution video-rate studies of processes such as misfit dislocation injection and interaction, surface roughening and faceting, self-assembly of quantum dots, and shape transitions in such quantum dots. We review results obtained in the SiGe/Si system in the last five years. In addition we discuss new directions in in situ electron microscopy as they apply to thin film formation in a range of materials and environments. [ Erratum ]

For surface science, the 1980s were the decade in which the microscopes arrived. The scanning tunneling microscope (STM) was invented in 1982. Ultrahigh vacuum transmission electron microscopy (UHVTEM) played a key role in resolving the structure of the elusive Si(111)-7 × 7 surface. Scanning electron microscopy (SEM) as well as reflection electron microscopy (REM) were applied to the study of growth and islanding. And low-energy electron microscopy (LEEM), invented some 20 years earlier, made its appearance with the work of Telieps and Bauer.LEEM and TEM have many things in common. Unlike STM and SEM, they are direct imaging techniques, using magnifying lenses. Both use an aperture to select a particular diffracted beam, which determines the nature of the contrast. If the direct beam is selected (no parallel momentum transfer), a bright field image is formed, and contrast arises primarily from differences in the scattering factor. A dark field image is formed with any other beam in the diffraction pattern, allowing contrast due to differences in symmetry. In LEEM, phase contrast is the third important mechanism by which surface and interface features such as atomic steps and dislocations may be imaged. One major difference between TEM and LEEM is the electron energy: 100 keV and above in TEM, 100 eV and below in LEEM. In LEEM, the imaging electrons are reflected from the sample surface, unlike TEM where the electrons zip right through the sample, encountering top surface, bulk, and bottom surface. STM and TEM are capable of ~2 Å resolution, while LEEM and SEM can observe surface features (including atomic steps) with -100 Å resolution.

We have developed an electron lithography method, Hot Electron Emission Lithography (HEEL), which is capable of printing integrated circuits with an exposure time of only a few seconds. The basic design of the mask, manufactured by standard MOS technology, will be discussed. Patterns printed into e-beam resist by a 1:1 projection system show the applicability of the mask for lithography purposes. The minimum feature size projected so far is 160 nm in a system capable of 90 nm resolution. Further improvements in resolution to 50 nm are possible

We describe observations of the growth kinetics of nanosize islands in two heteroepitaxial systems, Ge/Si and CoSi 2 /Si. Growth was carried out during continuous observation in a modified ultra high vacuum transmission electron microscope. Ge islands were grown on Si(100) at 650°C using chemical vapour deposition and CoSi 2 islands were grown by reactive epitaxy by evaporating Co onto a Si(111) substrate heated to 900°C. By analysing real-time observations of the development of individual islands, we show how the factors controlling island nucleation, growth and relaxation in these systems can be identified and understood. We discuss how the growth of these “quantum dots” may be controlled for novel electronic and optoelectronic applications.

We demonstrate that strained GeSi/Si heterostructures act as a model system for the study of the interaction of propagating dislocations with point, line and planar defects. In such heterostructures, the effective stress acting on a propagating dislocation may be varied from tens of MPa to of order 1 GPa, and the length of the propagating dislocation segment may be varied from nm to hundreds of nm. Results are presented from the interaction of strain-relieving dislocations with GeSi free surfaces, with point defects generated by ion implantation, and with other dislocations.

The in situ transmission electron microscope allows us to visualise processes occurring at surfaces and interfaces in real time and is therefore capable of providing detailed, quantitative information about reaction mechanisms. We have used a UHV TEM equipped with in situ growth capabilities to study the process of chemical vapour deposition of Ge on Si(100), with particular emphasis on the formation of self-assembled, nanosize Ge islands. Video-rate image acquisition enables us to track the development of individual islands from nucleation onwards and to observe the introduction of dislocations as the strained islands relax. For islands less than 80 nm in diameter, which are coherently strained, we observe an interesting coarsening process during growth. This coarsening results in a bimodal distribution of island sizes at certain times and a narrow size distribution at later times. We explain the phenomenon by a model in which coarsening occurs among a population of islands for which the equilibrium island shape depends on the size. Numerical simulations of coarsening in the presence of a shape transition are in good agreement with experiment. As the islands grow larger, dislocations form and we observe rapid shape changes associated with dislocation introduction. These changes can also be understood by considering a strain-dependent island shape. The insight that these results provide into the understanding of island growth and evolution can be used to develop arrays of uniformly sized islands ("quantum dots") for a variety of potential applications.

We describe here real-time, in situ observations of the formation of nanosize germanium (Ge) islands on silicon (Si). The deposition of Ge onto electron-transparent Si(100) takes place in a UHV transmission electron microscope that has been modified to allow chemical vapor deposition to be carried out in the polepiece. We recorded the growth process at video rate and were therefore able to follow the evolution of individual islands. As the islands grew, we observed a coarsening process similar to classical Ostwald ripening, but which leads at certain times to a bimodal distribution of island sizes. We show that this phenomenon can be understood using a model in which a conventional coarsening process is modified by a transition between two different island shapes.

We have measured the velocity of misfit dislocation threading segments in real time during ultrahigh vacuum (UHV) chemical vapor deposition heteroepitaxial growth of thin SiGe epilayers on Si (001) using ultrahigh vacuum transmission electron microscopy. We observe no measurable difference in dislocation velocities during growth and during post-growth annealing of samples with an atomically clean surface, in contrast to previous observations in the InGaAs/GaAs (001) system. However, dislocations are seen to move approximately three times slower during growth and post-growth UHV annealing than during annealing of samples which have a native oxide present on the surface. We have used post-growth depositions of arsenic and oxygen to investigate the effect of surface condition on dislocation velocities, and discuss possible causes for the increase in dislocation velocities in the presence of a native oxide. These systematic studies suggest a hitherto unappreciated interaction between moving dislocations and t...

The size distribution of self-assembled heteroepitaxial islands is critical to their application as quantum dots in novel devices. In situ, real time UHV transmission electron microscopy studies of Ge island growth on Si(001) show that island coarsening occurs even during growth. With increasing volume, a shape transition from pyramids to domes gives rise to an abrupt change in chemical potential. This leads to a bifurcation in the size distribution and ultimately to a narrow size range. Simulations of coarsening in the presence of a shape transition are in good agreement with experimental results.

Silicon nanowires hold great promise as components of tiny electronic devices, but the usual method of growing them is poorly understood. New work shows that excessive cleanliness can actually stunt a nanowire's growth. They are made by the ‘vapour–liquid–solid’ method, in which a tiny liquid droplet of a metal such as gold absorbs silicon atoms from a gaseous precursor molecule. As the droplet saturates with silicon, it grows a solid, cylindrical silicon crystal whose diameter is determined by the size of the droplet. But in conditions of extreme cleanliness, gold atoms from the droplet can migrate over the surface of the growing nanowire, resulting in misshapen structures. Interest in nanowires continues to grow, fuelled in part by applications in nanotechnology1,2,3,4,5. The ability to engineer nanowire properties makes them especially promising in nanoelectronics6,7,8,9. Most silicon nanowires are grown using the vapour–liquid–solid (VLS) mechanism, in which the nanowire grows from a gold/silicon catalyst droplet during silicon chemical vapour deposition10,11,12,13. Despite over 40 years of study, many aspects of VLS growth are not well understood. For example, in the conventional picture the catalyst droplet does not change during growth, and the nanowire sidewalls consist of clean silicon facets10,11,12,13. Here we demonstrate that these assumptions are false for silicon nanowires grown on Si(111) under conditions where all of the experimental parameters (surface structure, gas cleanliness, and background contaminants) are carefully controlled. We show that gold diffusion during growth determines the length, shape, and sidewall properties of the nanowires. Gold from the catalyst droplets wets the nanowire sidewalls, eventually consuming the droplets and terminating VLS growth. Gold diffusion from the smaller droplets to the larger ones (Ostwald ripening) leads to nanowire diameters that change during growth. These results show that the silicon nanowire growth is fundamentally limited by gold diffusion: smooth, arbitrarily long nanowires cannot be grown without eliminating gold migration.

One-dimensional (1D) and two-dimensional (2D) periodic square-wave gratings fabricated on Si(001) with repeat spacings of 4 and 5 μm and amplitudes of 0.1–0.25 μm were annealed in ultrahigh vacuum at temperatures between 900 and 1200 °C. The surface profiles of the gratings were determined by atomic force microscopy in air after cooling to room temperature. The microscopic structures of the steps and terraces of the gratings were investigated in situ using low-energy electron microscopy. The morphology of the gratings changes with annealing temperature. At 950 °C, both 1D and 2D gratings have surface profiles resembling truncated sinusoids, with large flat regions at the grating extrema. Between 1050 and 1100 °C, the flats at the minima of the 2D gratings expand whereas the flats at the maxima shrink. This asymmetry does not occur for the 1D gratings in this temperature range. At 1200 °C, the profiles of both the 1D and 2D gratings become sinusoidal, with no flats at the extrema. The different morphologies and their temperature dependence can be explained on the basis of the thermodynamics of the surface and the role of surface diffusion and evaporation.One-dimensional (1D) and two-dimensional (2D) periodic square-wave gratings fabricated on Si(001) with repeat spacings of 4 and 5 μm and amplitudes of 0.1–0.25 μm were annealed in ultrahigh vacuum at temperatures between 900 and 1200 °C. The surface profiles of the gratings were determined by atomic force microscopy in air after cooling to room temperature. The microscopic structures of the steps and terraces of the gratings were investigated in situ using low-energy electron microscopy. The morphology of the gratings changes with annealing temperature. At 950 °C, both 1D and 2D gratings have surface profiles resembling truncated sinusoids, with large flat regions at the grating extrema. Between 1050 and 1100 °C, the flats at the minima of the 2D gratings expand whereas the flats at the maxima shrink. This asymmetry does not occur for the 1D gratings in this temperature range. At 1200 °C, the profiles of both the 1D and 2D gratings become sinusoidal, with no flats at the extrema. The different morphologie...

We have used low energy electron microscopy to study the time dependence of atomic height islands and holes at the extrema of a fabricated biperiodic grating on Si(001). From the pairwise disappearance of these islands and holes as the grating thermally smooths we conclude that steps on Si(001) are remarkably permeable, i.e., adatoms can cross steps without becoming incorporated into them. Comparison of the time dependence of the island areas with model calculations shows that the adatom incorporation probability is actually much smaller than the crossing probability.

High-temperature epitaxy of PtSi/Si(0 0 1) interfaces has been investigated by ultra-high-vacuum transmission electron microscopy for deposition temperatures up to 850°C. At 600°C continuous, polycrystalline, epitaxial films are observed with rectangular grains (10 × 30 nm), with the predominant, preferred orientation PtSi(1 1 0). By 750°C the grains form islands which elongate in the PtSi[0 0 1] direction parallel the Si[1 1 0]. A strong shape anisotropy develops with length-to-width ratios of up to 1 0 0. Interface faceting is detected and observed in all islands by 810°C and the preferred orientation changes to predominantly (1 2 0). The alignment of the PtSi(0 0 2) planes with the Si(2 2 0) planes is preserved at all temperatures and orientations.

We have used low-energy electron microscopy to study two-dimensional island ripening on Si(001). By studying the behavior of individual islands compared to their surroundings, we are able to quantify the step-edge attachment and terrace diffusion processes that are responsible for the ripening. By comparing the time dependence of specific configurations of islands to simulations, we find correlations in the rate of change of an island's area with the sizes of neighboring islands, implying that the chemical potential of the adatom sea is not uniform as classical theories of Ostwald ripening assume. From measurements of the time dependence of each island, we chart out these nonuniformities and relate them to adatom diffusion coefficients. \textcopyright{} 1996 The American Physical Society.

We have used the UHV-TEM to study the growth of Ge on Si(111) in situ, from the deposition of the first monolayer to complete relaxation by the introduction of dislocations. We show that, at 650°C, the growth of Ge islands is dominated by steps on the surface. Indeed, islands nucleated on steps that run along the 〈 11 2 ¯ 〉 direction have a very high aspect ratio, while other islands are roughly triangular in shape. Dislocations form initially exclusively by coalescence of these smaller islands. Further growth involves a complicated competition between surrounding strained islands and the relaxed island. At lower temperature (350°C), steps still are a preferred site for island formation, but the limited surface diffusion makes it possible for islands to also nucleate on flat terraces. In this case, an island grows by incorporating dislocations at its edge, which results in a completely relaxed island from the start, and a very uniform network of dislocations. These in situ electron microscopy studies reveal a much more complicated and rich growth process than previously imagined, and tie previously obtained results together into a single picture.

This paper gives a brief review of low-energy electron microscopy (LEEM) as used for in situ studies of surface dynamical processes. The capabilities of LEEM are illustrated with two examples. One is a kinetic instability observed during growth of the first layer of CaF 2 on Si (111). The second concerns the nucleation of misfit dislocations during the growth of thicker, epitaxial CaF 2 films on Si (111), as the critical thickness is exceeded. Both examples highlight the importance of real time, in situ observations of surface dynamical processes.

Using a simple optical technique, we have measured the change in surface stress of Si(001) during the adsorption of a monolayer of arsenic and during the epitaxial growth of germanium with arsenic as a surfactant. Tensile surface stress increases nearly linearly with arsenic coverage until it attains a value of 1400 ± 100 dyn/cm at a full monolayer. During germanium deposition on an arsenic terminated surface, (compressive) surface stress increases at a rate of −1270 dyn/cm per ML (monolayer) between 3 and 8 or 9 ML, the known critical film thickness for defect formation; the stress per monolayer found above 10 ML is substantially smaller.

The high carrier mobility of graphene is key to its applications, and understanding the factors that limit mobility is essential for future devices. Yet, despite significant progress, mobilities in excess of the 2×10(5) cm(2) V(-1) s(-1) demonstrated in free-standing graphene films have not been duplicated in conventional graphene devices fabricated on substrates. Understanding the origins of this degradation is perhaps the main challenge facing graphene device research. Experiments that probe carrier scattering in devices are often indirect, relying on the predictions of a specific model for scattering, such as random charged impurities in the substrate. Here, we describe model-independent, atomic-scale transport measurements that show that scattering at two key defects--surface steps and changes in layer thickness--seriously degrades transport in epitaxial graphene films on SiC. These measurements demonstrate the strong impact of atomic-scale substrate features on graphene performance.

We have determined the growth mode of graphene on SiC(0001) and $\mathrm{SiC}(000\overline{1})$ using ultrathin, isotopically labeled $\mathrm{Si}^{13}\mathrm{C}$ ``marker layers'' grown epitaxially on the $\mathrm{Si}^{12}\mathrm{C}$ surfaces. Few-layer graphene overlayers were formed via thermal decomposition at elevated temperature. For both surface terminations (Si face and C face), we find that the $^{13}\mathrm{C}$ is located mainly in the outermost graphene layers, indicating that, during decomposition, new graphene layers form underneath existing ones.

Depth profiling of graphene with high-resolution ion beam analysis is a practical method for analysis of monolayer thicknesses of graphene. Not only is the energy resolution sufficient to resolve graphene from underlying SiC, but by use of isotope labeling it is possible to tag graphene generated from reacted ethylene. Furthermore, we are able to analyze graphene supported by oxidized Si(100) substrates, allowing the study of graphene films grown by chemical vapor deposition on metal and transfered to silicon. This introduces a powerful method to explore the fundamentals of graphene formation.

In this Letter we show that a coherent low energy electron beam (100 eV) can be used to obtain real space images of structures and defects buried deep below the surface of the sample. The elastic strain fields of such buried structures, extending to the free surface, are found to give rise to localized phase shifts in the reflected electron waves, resulting in excellent image contrast under slight objective lens defocus conditions. We can now image the formation and evolution of buried interfaces and defects in situ, and in real time. Because of the very low electron energies used, this imaging method is nondestructive.

Medium‐energy ion scattering (MEIS) has been successfully applied for many years as a technique for structural analysis of solids. Advantages over competing techniques include superb depth resolution (5–10 A), quantitative information (well‐known cross sections), and ease of interpretation. A weakness of the technique is the lack of sensitivity to light elements. We have adapted the technique to detect light elements by elastic recoil detection analysis (ERDA). This has been used to analyze samples containing hydrogen and boron, with depth resolution of ≊10 A, comparable to conventional MEIS. This is an order of magnitude improvement over conventional ERDA.