M. C. Reuter

Growth dynamics of pentacene thin films.

The recent demonstration of single-crystal organic optoelectronic devices has received widespread attention. But practical applications of such devices require the use of inexpensive organic films deposited on a wide variety of substrates. Unfortunately, the physical properties of these organic thin films do not compare favourably to those of single-crystal materials. Moreover, the basic physical principles governing organic thin-film growth and crystallization are not well understood. Here we report an in situ study of the evolution of pentacene thin films, utilizing the real-time imaging capabilities of photoelectron emission microscopy. By a combination of careful substrate preparation and surface energy control, we succeed in growing thin films with single-crystal grain sizes approaching 0.1 millimetre (a factor of 20–100 larger than previously achieved), which are large enough to fully contain a complete device. We find that organic thin-film growth closely mimics epitaxial growth of inorganic materials, and we expect that strategies and concepts developed for these inorganic systems will provide guidance for the further development and optimization of molecular thin-film devices.

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.

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.

In situ ultrahigh vacuum transmission electron microscopy studies of hetero-epitaxial growth I. Si(001)/Ge

Abstract We use ultrahigh vacuum transmission electron microscopy (UHV-TEM) to study the growth of Ge on Si(001) in real time at different temperatures and for coverages ranging from the initial monolayers to the development and relaxation of 3D islands. During growth of the first monolayers the surface gradually changes from a disordered missing-dimer structure to a rather well ordered (2 × 8) reconstruction, an evolution clearly resolved by the TEM. As the coverage is increased 3D islands starts to form. The growth and relaxation of these islands are shown to depend significantly on the temperature, e.g. with different dislocations formed at high and low temperatures. We interpret this difference in terms of the brittle-ductile transition in Ge, below which dislocation glide is frozen out. An interesting observation is that islands grown at low temperatures are more fully relaxed than those grown at higher temperatures. At high enough temperature the islands are initially, up to a specific size, coherent with the substrate and further growth occurs in a remarkably oscillatory fashion with the introduction of each (60°-type) dislocation, where the core of the island, of about 2000 A in diameter, remains fully strained. However, in the low-temperature regime the islands grow relaxed from the outset with pure edge dislocations continuously being introduced in the moving edges. For temperatures less than 600°C the transition from 2D to 3D growth occurs via the formation of small and strained 3D islands, so-called “hut clusters”. We monitor the nucleation and characteristics of these clusters and discuss their possible role in the formation of relaxed 3D islands. The different growth mechanisms are discussed in terms of a simple model for the energetics of strain-relaxed islands, leading to a qualitative description of the temperature-dependent growth modes.

Design of a new photo-emission/low-energy electron microscope for surface studies

Abstract We have designed a new photo-emission/low-energy electron microscope (PEEM/LEEM) for surface studies in ultra-high vacuum (UHV). While following the general design concepts developed by Bauer and Telieps, significant innovations are made to enhance the resolution and capabilities of the instrument: a piezoelectrically driven sample stage, a magnetic cathode objective lens, an in-vacuum deflection magnet, and a double-gap, in-vacuum projector lens. Particular attention was paid to integrating the electron-optical requirements and constraints with the stringent vacuum requirements. At the time of writing this paper, construction of the microscope is nearly completed.

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.

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.

Decomposition of interfacial SiO2 during HfO2 deposition

Growth of HfO2 by Hf deposition in an oxidizing ambient is found to cause removal of interfacial SiO2. Medium-energy ion scattering results show that the reaction takes place during growth, and involves transport of oxygen through the HfO2 layer. An examination of the temperature dependence suggests that oxygen vacancy reactions are responsible.

Atomic-Scale Variability and Control of III-V Nanowire Growth Kinetics

Regular Nanowires For a range of nanotechnology applications, semiconductor nanowires will need to be grown with high precision and control. Chou et al. (p. 281) studied the growth of gallium phosphide (GaP) nanowires using chemical vapor deposition within a transmission electron microscope and worked out conditions that could generate regular and predictable wire growth. Fluctuations and defects in III-V nanowire growth can be avoided by growing at a low V/III ratio. In the growth of nanoscale device structures, the ultimate goal is atomic-level precision. By growing III-V nanowires in a transmission electron microscope, we measured the local kinetics in situ as each atomic plane was added at the catalyst-nanowire growth interface by the vapor-liquid-solid process. During growth of gallium phosphide nanowires at typical V/III ratios, we found surprising fluctuations in growth rate, even under steady growth conditions. We correlated these fluctuations with the formation of twin defects in the nanowire, and found that these variations can be suppressed by switching to growth conditions with a low V/III ratio. We derive a growth model showing that this unexpected variation in local growth kinetics reflects the very different supply pathways of the V and III species. The model explains under which conditions the growth rate can be controlled precisely at the atomic level.

Dual-gate silicon nanowire transistors with nickel silicide contacts

The formation of nickel silicide contacts in silicon nanowire transistors and its impact on the electrical device characteristics is investigated. The authors notice that silicide formation at low temperature converts substantial portions of the silicon wire into a metallic source/drain extension. The study also shows the impact of these contacts in a dual-gate field-effect transistor design and discusses the importance of carrier injection from the contacts even for devices with substantial scattering inside the channel