Frances M. Ross

The influence of the surface migration of gold on the growth of silicon nanowires

Interest in nanowires continues to grow, fuelled in part by applications in nanotechnology. The ability to engineer nanowire properties makes them especially promising in nanoelectronics. 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 deposition. 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 facets. 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.

Electron microscopy of specimens in liquid

Imaging samples in liquids with electron microscopy can provide unique insights into biological systems, such as cells containing labelled proteins, and into processes of importance in materials science, such as nanoparticle synthesis and electrochemical deposition. Here we review recent progress in the use of electron microscopy in liquids and its applications. We examine the experimental challenges involved and the resolution that can be achieved with different forms of the technique. We conclude by assessing the potential role that electron microscopy of liquid samples can play in areas such as energy storage and bioimaging.

Formation of Compositionally Abrupt Axial Heterojunctions in Silicon-Germanium Nanowires

Sharp Nanowires The potential for using nanowires in devices can be limited by the ability to synthesize them from two or more materials while maintaining compositional purity at the interfaces. Instead of using liquid droplets at the eutectic point when the melting point is at a minimum, Wen et al. (p. 1247) show that generating the wires at solid alloy catalysts allows fabrication of silicon germanium wires with atomically sharp interfaces. The system works well because an AlAu alloy composition was chosen in which Si and Ge have a low solubility but which have a high enough eutectic temperature so that nanowire growth is not limited by the reactivity of the Si and Ge precursors. A solid alloy catalyst is used to synthesize atomically sharp interfaces in silicon-germanium nanowires. We have formed compositionally abrupt interfaces in silicon-germanium (Si-Ge) and Si-SiGe heterostructure nanowires by using solid aluminum-gold alloy catalyst particles rather than the conventional liquid semiconductor–metal eutectic droplets. We demonstrated single interfaces that are defect-free and close to atomically abrupt, as well as quantum dots (i.e., Ge layers tens of atomic planes thick) embedded within Si wires. Real-time imaging of growth kinetics reveals that a low solubility of Si and Ge in the solid particle accounts for the interfacial abruptness. Solid catalysts that can form functional group IV nanowire-based structures may yield an extended range of electronic applications.

Formation of a stratified lanthanum silicate dielectric by reaction with Si(001)

We have characterized the structure and electrical properties of lanthanum silicate layers formed on Si(001) by reaction of lanthanum oxide with the substrate. Postoxidation of the deposited films results in the formation of a stacked dielectric with a lanthanum silicate layer atop an interfacial layer of SiO2. This structure combines the interfacial properties of SiO2 with the large permittivity of lanthanum silicate. Although the resulting film has leakage properties far superior to an equivalent thickness of SiO2, there is evidence of significant quantities of ionic charge that must be eliminated before use in electronic applications.

Kinetics of Individual Nucleation Events Observed in Nanoscale Vapor-Liquid-Solid Growth

We measured the nucleation and growth kinetics of solid silicon (Si) from liquid gold-silicon (AuSi) catalyst particles as the Si supersaturation increased, which is the first step of the vapor-liquid-solid growth of nanowires. Quantitative measurements agree well with a kinetic model, providing a unified picture of the growth process. Nucleation is heterogeneous, occurring consistently at the edge of the AuSi droplet, yet it is intrinsic and highly reproducible. We studied the critical supersaturation required for nucleation and found no observable size effects, even for systems down to 12 nanometers in diameter. For applications in nanoscale technology, the reproducibility is essential, heterogeneity promises greater control of nucleation, and the absence of strong size effects simplifies process design.

Controlling nanowire structures through real time growth studies

In situ electron microscopy can be used to visualize the physical processes that control the growth of Si and Ge nanowires through the vapor–liquid–solid mechanism. Images and movies are recorded in a transmission electron microscope that has capabilities for depositing catalysts onto a sample and for introducing chemical vapor deposition precursor gases while the sample remains under observation. This technique allows us to measure nucleation, catalyst stability, surface structure and growth kinetics, in some cases confirming existing models and in other cases producing unexpected results and suggesting approaches toward growing novel structures. We will show that nanowire formation provides a unique window into the fundamentals of crystal growth as well as an opportunity to fabricate precisely controlled structures for novel applications.

Opportunities and challenges in liquid cell electron microscopy

Advances in seeing small things Electron microscopes, particularly those with aberration correction, can view materials at the subnanometer scale. Additional improvements make it possible to obtain images at lower electron doses, thus minimizing the damage to the sample. However, for a number of materials, particularly those of biological origin, samples need to be imaged in solution. Ross reviews recent advances that have made it possible to do liquid cell electron microscopy, which opens up the possibility of studying problems such as the changes inside a battery during operation, the growth of crystals from solution, or biological molecules in their native state. Science, this issue p. 10.1126/science.aaa9886 BACKGROUND Transmission electron microscopy offers structural and compositional information with atomic resolution, but its use is restricted to thin, solid samples. Liquid samples, particularly those involving water, have been challenging because of the need to form a thin liquid layer that is stable within the microscope vacuum. Liquid cell electron microscopy is a developing technique that allows us to apply the powerful capabilities of the electron microscope to the imaging and analysis of liquid specimens. We can examine liquid-based processes in materials science and physics that are traditionally inaccessible to electron microscopy, and image biological structures at high resolution without the need for freezing or drying. The changes that occur inside batteries during operation, the attachment of atoms during the self-assembly of nanocrystals, and the structures of biological materials in liquid water are examples in which a microscopic view is providing unique insights. ADVANCES The difficulty of imaging water and other liquids was recognized from the earliest times in the development of transmission electron microscopy. Achieving a practical solution, however, required the use of modern microfabrication techniques to build liquid cells with thin but strong windows. Usually made of silicon nitride on a silicon support, these liquid cells perform two jobs: They separate the liquid from the microscope vacuum while also confining it into a layer that is thin enough for imaging with transmitted electrons. Additional functionality such as liquid flow, electrodes, or heating can be incorporated in the liquid cell. The first experiments to make use of modern liquid cells provided information on electrochemical deposition, nanomaterials synthesis, diffusion in liquids, and the structure of biological assemblies. Materials and processes now under study include corrosion, biomolecular structure, bubble dynamics, radiation effects, and biomineralization. New window materials such as graphene can improve resolution, and elemental analysis is possible by measuring energy loss or x-ray signals. Advances in electron optics and detectors, and the correlation of liquid cell microscopy data with probes such as fluorescence, have increased the range of information available from the sample. Because the equipment is not too expensive and works in existing electron microscopes, liquid cell microscopy programs have developed around the world. OUTLOOK Liquid cell electron microscopy is well positioned to explore new frontiers in electrochemistry and catalysis, nanomaterial growth, fluid physics, diffusion, radiation physics, geological and environmental processes involving clays and aerosols, complex biomaterials and polymers, and biological functions in aqueous environments. Continuing improvements in equipment and technique will allow materials and processes to be studied under different stimuli—for example, in extreme temperatures, during gas/liquid mixing, or in magnetic or electric fields. Correlative approaches that combine liquid cell electron microscopy with light microscope or synchrotron data promise a deeper study of chemical, electrochemical, and photochemical reactions; analytical electron microscopy will provide details of composition and chemical bonding in water; high-speed and aberration-corrected imaging extend the scales of the phenomena that can be examined. As liquid cell microscopy becomes more capable and quantitative, it promises the potential to extend into new areas, adopt advanced imaging modes such as holography, and perhaps even solve grand challenge problems such as the structure of the electrochemical double layer or molecular movements during biological processes. Schematic diagram of a liquid cell for the transmission electron microscope and its application for imaging phenomena in materials science, life science, and physics. The liquid cell is made from two vacuum-tight electron transparent membranes. In this diagram the membranes are made of silicon nitride (blue) on a silicon support (gray), although other materials are possible. A spacer layer (not shown) keeps the membranes at a controlled separation of about 100 nm to 1 mm. The cell is filled with the liquid of interest, and the liquid may be flowed using an external pump (not shown). The electron beam (pink) passes through the membranes and liquid to allow recording of images, movies, or spectroscopic data for compositional analysis. Several possible experiments are illustrated: growth of nanocrystals in solution, nucleation and growth of bubbles, imaging biological structures such as whole cells or viruses in liquid water, and imaging electrochemical processes at an electrode (yellow) that is built into the liquid cell. The dimensions of the electron beam and the nanoscale objects are exaggerated for clarity. Transmission electron microscopy offers structural and compositional information with atomic resolution, but its use is restricted to thin, solid samples. Liquid samples, particularly those involving water, have been challenging because of the need to form a thin liquid layer that is stable within the microscope vacuum. Liquid cell electron microscopy is a developing technique that allows us to apply the powerful capabilities of the electron microscope to imaging and analysis of liquid specimens. We describe its impact in materials science and biology. We discuss how its applications have expanded via improvements in equipment and experimental techniques, enabling new capabilities and stimuli for samples in liquids, and offering the potential to solve grand challenge problems.

Interface dynamics and crystal phase switching in GaAs nanowires

Controlled formation of non-equilibrium crystal structures is one of the most important challenges in crystal growth. Catalytically grown nanowires are ideal systems for studying the fundamental physics of phase selection, and could lead to new electronic applications based on the engineering of crystal phases. Here we image gallium arsenide (GaAs) nanowires during growth as they switch between phases as a result of varying growth conditions. We find clear differences between the growth dynamics of the phases, including differences in interface morphology, step flow and catalyst geometry. We explain these differences, and the phase selection, using a model that relates the catalyst volume, the contact angle at the trijunction (the point at which solid, liquid and vapour meet) and the nucleation site of each new layer of GaAs. This model allows us to predict the conditions under which each phase should be observed, and use these predictions to design GaAs heterostructures. These results could apply to phase selection in other nanowire systems.

Epitaxial silicon and germanium on buried insulator heterostructures and devices

Future microelectronics will be based upon silicon or germanium-on-insulator technologies and will require an ultrathin (<10 nm), flat silicon or germanium device layer to reside upon an insulating oxide grown on a silicon wafer. The most convenient means of accomplishing this is by epitaxially growing the entire structure on a silicon substrate. This requires a high quality crystalline oxide and the ability to epitaxially grow two dimensional, single crystal films of silicon or germanium on top of this oxide. We describe a method based upon molecular beam epitaxy and solid-phase epitaxy to make such structures and demonstrate working field-effect transistors on germanium-on-insulator layers.

Lateral control of self-assembled island nucleation by focused-ion-beam micropatterning

We demonstrate that the nucleation sites of nanoscale, self-assembled Ge islands on Si(001) can be controlled by patterning the Si surface in situ with a focused ion beam. At low doses of 6000 Ga+ ions per <100 nm spot, the selective growth is achieved without modifying the initial surface topography. At larger doses, topographic effects produced by sputtering and redeposition control the selective nucleation sites. Islands grown on irradiated spots are smaller with higher aspect ratio than islands grown on clean Si(001), suggesting a strong surfactant effect of Ga.