J. Tersoff

Scanning tunneling microscopy

A scanning tunneling microscope (STM) can provide atomic‐resolution images of samples in ultra‐high vacuum, moderate vacuum, gases including air at atmospheric pressure, and liquids including oil, water, liquid nitrogen, and even conductive solutions. This review contains images of single‐crystal metals, metal films, both elemental and compound semiconductors, superconductors, layered materials, adsorbed atoms, and even DNA. A discussion of results on lithography leads into speculations on a bright future in which STMs may not only observe, but also manipulate surfaces, right down to the atomic level.

Scaling of excitons in carbon nanotubes.

Light emission from carbon nanotubes is expected to be dominated by excitonic recombination. Here we calculate the properties of excitons in nanotubes embedded in a dielectric, for a wide range of tube radii and dielectric environments. We find that simple scaling relationships give a good description of the binding energy, exciton size, and oscillator strength.

Dislocations and strain relief in compositionally graded layers

The performance of strained‐layer heterostructures is often limited by threading dislocations. Such defects can be reduced, and in some cases nearly eliminated, by growing a graded buffer layer. Here, we provide a quantitative picture of the role of grading, by calculating the equilibrium distribution of dislocations and residual strain in such compositionally graded films. In layers with graded strain, threading dislocations are subject to greater force and weaker pinning than in uniform layers, helping them to be swept to the edge of the sample.

Electron-Phonon Interaction and Transport in Semiconducting Carbon Nanotubes

We calculate the electron-phonon scattering and binding in semiconducting carbon nanotubes, within a tight binding model. The mobility is derived using a multi-band Boltzmann treatment. At high fields, the dominant scattering is inter-band scattering by LO phonons corresponding to the corners K of the graphene Brillouin zone. The drift velocity saturates at approximately half the graphene Fermi velocity. The calculated mobility as a function of temperature, electric field, and nanotube chirality are well reproduced by a simple interpolation formula. Polaronic binding give a band-gap renormalization of ~70 meV, an order of magnitude larger than expected. Coherence lengths can be quite long but are strongly energy dependent.

Drain voltage scaling in carbon nanotube transistors

Decreasing the oxide thickness in carbon nanotube field-effect transistors (CNFETs) improves the turn-on behavior. However, we demonstrate that this also requires scaling the range of the drain voltage. This scaling is needed to avoid an exponential increase in off-current with drain voltage, due to modulation of the Schottky barriers at both the source and drain contact. We illustrate this with results for bottom-gated ambipolar CNFETs with oxides of 2 and 5 nm, and give an explicit scaling rule for the drain voltage. Above the drain voltage limit, the off-current becomes large and has equal electron and hole contributions. This allows the recently reported light emission from appropriately biased CNFETs.

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.

An off-normal fibre-like texture in thin films on single-crystal substrates

In the context of materials science, texture describes the statistical distribution of grain orientations. It is an important characteristic of the microstructure of polycrystalline films, determining various electrical, magnetic and mechanical properties. Three types of texture component are usually distinguished in thin films: random texture, when grains have no preferred orientation; fibre texture, for which one crystallographic axis of the film is parallel to the substrate normal, while there is a rotational degree of freedom around the fibre axis; and epitaxial alignment (or in-plane texture) on single-crystal substrates, where an in-plane alignment fixes all three axes of the grain with respect to the substrate. Here we report a fourth type of texture—which we call axiotaxy—identified from complex but symmetrical patterns of lines on diffraction pole figures for thin films formed by solid-state reactions. The texture is characterized by the alignment of planes in the film and substrate that share the same d-spacing. This preferred alignment of planes across the interface manifests itself as a fibre texture lying off-normal to the sample surface, with the fibre axis perpendicular to certain planes in the substrate. This texture forms because it results in an interface, which is periodic in one dimension, preserved independently of interfacial curvature. This new type of preferred orientation may be the dominant type of texture for a wide class of materials and crystal structures.

Contact resistance of carbon nanotubes

Electrical contacts to carbon nanotubes typically exhibit high resistance, posing a serious obstacle to their application in electronic devices. One important factor may be their unique electronic structure, which gives weak electronic coupling at the Fermi surface. This suggests some possible ways to reduce contact resistance.

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.

Unexpected scaling of the performance of carbon nanotube Schottky-barrier transistors

We show that carbon nanotube transistors exhibit scaling that is qualitatively different than conventional transistors. The performance depends in an unexpected way on both the thickness and the dielectric constant of the gate oxide. Experimental measurements and theoretical calculations provide a consistent understanding of the scaling, which reflects the very different device physics of a Schottky barrier transistor with a quasi-one-dimensional channel contacting a sharp edge. A simple analytic model gives explicit scaling expressions for key device parameters such as subthreshold slope, turn-on voltage, and transconductance.