Nanotechnology: a barrier falls.
Explore the source record for details and available documents.
Comment↗
Biomedical subjects
J Tersoff
Publications and source records attributed to J Tersoff.
Electrically induced optical emission from a carbon nanotube FET.
Polarized infrared optical emission was observed from a carbon nanotube ambipolar field-effect transistor (FET). An effective forward-biased p-n junction, without chemical dopants, was created in the nanotube by appropriately biasing the nanotube device. Electrical measurements show that the observed optical emission originates from radiative recombination of electrons and holes that are simultaneously injected into the undoped nanotube. These observations are consistent with a nanotube FET model in which thin Schottky barriers form at the source and drain contacts. This arrangement is a novel optical recombination radiation source in which the electrons and holes are injected into a nearly field-free region. Sucha source may form the basis for ultrasmall integrated photonic devices.
Journal Article↗
Influence of supersaturation on surface structure.
Using low-energy electron microscopy, we have investigated the influence of an external flux on the structure of the Si(111) surface during growth and etching at elevated temperatures. We find that varying the adatom supersaturation effectively changes the surface free energies of coexisting 7 x 7 and '1 x 1' regions of the surface. In response, the boundaries separating the phases adopt a new steady-state configuration. The measured configuration can be used to quantitatively determine the difference in free energy between the phases, Deltagamma. The change in Deltagamma provides a measure of the local supersaturation at the surface, and can be interpreted as a change in the phase-transition temperature.
Journal Article↗
Barrierless formation and faceting of SiGe islands on Si(001).
The initial stages of the formation of SiGe islands on Si(001) pose a long-standing puzzle. We show that the behavior can be consistently explained by one simple assumption-that for strained SiGe, (001) is a stable orientation but not a facet orientation. Calculations of energy and morphology reproduce the key features of "prepyramid" and "pyramid" islands, and explain the initial formation and subsequent shape transition. Scanning tunneling microscopy measurements confirm the key assumptions and predictions of the model.
Journal Article↗
Carbon nanotubes as schottky barrier transistors.
We show that carbon nanotube transistors operate as unconventional "Schottky barrier transistors," in which transistor action occurs primarily by varying the contact resistance rather than the channel conductance. Transistor characteristics are calculated for both idealized and realistic geometries, and scaling behavior is demonstrated. Our results explain a variety of experimental observations, including the quite different effects of doping and adsorbed gases. The electrode geometry is shown to be crucial for good device performance.
Journal Article↗
Microscopic dynamics of silicon oxidation.
We study the silicon oxidation process and the dynamic structure of the SiO2-Si (001) interface using a grand canonical Monte Carlo approach. We find that Si-O-Si bridge bonds are the main building blocks of the advancing interface, and we identify a kinetic pathway that continually creates new bridge bonds. Oxidation proceeds by local events, with little evidence of "step flow" in the simulation. Yet the interface remains remarkably smooth and abrupt as it advances.
Journal Article↗
Multiple functionality in nanotube transistors.
Calculations of quantum transport in a carbon nanotube transistor show that such a device offers unique functionality. It can operate as a ballistic field-effect transistor, with excellent characteristics even when scaled to 10 nm dimensions. At larger gate voltages, channel inversion leads to resonant tunneling through an electrostatically defined nanoscale quantum dot. Thus the transistor becomes a gated resonant-tunneling device, with negative differential resistance at a tunable threshold. For the dimensions considered here, the device operates in the Coulomb blockade regime, even at room temperature.
Journal Article↗
Surface stress and thermodynamic nanoscale size selection.
Using the Si(111) surface as an example, we show how temperature can be used to tune the size of domains during a surface phase transition. From analysis of the measured stable domain sizes, we determine key material parameters and clarify the close relationship between nucleation and thermodynamic size selection. More generally, the model we developed describes nanoscale self-assembly processes in contact with a reservoir (for example, liquid- or vapor-phase epitaxy).
Journal Article↗
Ambipolar electrical transport in semiconducting single-wall carbon nanotubes.
Ambipolar electrical transport is reported in single-wall carbon nanotube (SWNT) field-effect transistors. In particular, the properties of SWNT junctions to TiC are discussed in detail. The carbide-nanotube junctions are abrupt and robust. In contrast to planar junctions, these contacts present low resistance for the injection of both p- and n-type carriers--the apparent barrier height of the junction is modified by the gate field. Thus SWNTs offer the novel possibility of ambipolar Ohmic contacts.
Journal Article↗
Facet growth under stress: the limits of strained-layer stability.
A crystal facet is metastable under stress, but the process of growth or sublimation roughens the facet and is expected to render it unstable. This poses a fundamental limit for heteroepitaxial growth of planar layers, e.g., in semiconductor devices. An analysis shows that this facet-growth instability can be suppressed to an arbitrary degree by growing slowly. Moreover, the local stress ("force dipole") inherent in atomic steps introduces a new, purely kinetic effect that dominates at low strain and can render planar growth dynamically stable.
Journal Article↗
Phase coexistence during surface phase transitions.
In contrast to standard thermodynamic models, we observe phase coexistence over an extended temperature range at a first-order surface phase transition. We have measured the domain evolution of the Si(111)-( 7x7) to ( 1x1) phase transition with temperature, using low-energy electron microscopy. Comparison with detailed, quantitative theoretical predictions shows that coexistence is due to long-range elastic and electrostatic domain interactions. Phase coexistence is predicted to be a universal feature of surface phase transitions.
Journal Article↗
Negative differential resistance in nanotube devices
Carbon nanotube junctions are predicted to exhibit negative differential resistance, with very high peak-to-valley current ratios even at room temperature. We treat both nanotube p-n junctions and undoped metal-nanotube-metal junctions, calculating quantum transport through the self-consistent potential within a tight-binding approximation. The undoped junctions in particular may be suitable for device integration.
Journal Article↗
Instability wavelength in strained-alloy epitaxy
Explore the source record for details and available documents.
Journal Article↗
Role of fermi-level pinning in nanotube schottky diodes
At semiconductor-metal junctions, the Schottky barrier height is generally fixed by "Fermi-level pinning." We find that when a semiconducting carbon nanotube is end contacted to a metal (the optimal geometry for nanodevices), the behavior is radically different. Even when the Fermi level is fully "pinned" at the interface, the turn-on voltage is that expected for an unpinned junction. Thus the threshold may be adjusted for optimal device performance, which is not possible in planar contacts. Similar behavior is expected at heterojunctions between nanotubes and semiconductors.
Journal Article↗
Structure and energetics of the Si- SiO2 interface
Using a Monte Carlo approach, we identify low-energy structures for the (001)-oriented Si-SiO2 interface. The optimal interface structure found consists of an ordered array of Si-O-Si "bridges," with low strain energy. This structure explains several puzzling experimental observations.
Journal Article↗
Enhanced instability of strained alloy films due to compositional stresses
A single-component strained film is known to be unstable to the stress-driven morphological instability. Here, we determine how the instability is modified in an alloy film by considering the effect of compositional stresses due to an atomic size difference. We find that the coupling of composition to stress always makes the film more unstable to the formation of stress-driven surface undulations. The destabilization is greatest over a range of intermediate deposition rates.
Journal Article↗
Nonuniform composition profile in In0.5Ga0.5As alloy quantum dots
We use cross-sectional scanning tunneling microscopy to examine the shape and composition distribution of In0.5Ga0.5As quantum dots (QDs) formed by capping heteroepitaxial islands. The QDs have a truncated pyramid shape. The composition appears highly nonuniform, with an In-rich core having an inverted-triangle shape. Thus the electronic properties will be drastically altered, relative to the uniform composition generally assumed in device modeling. Theoretical analysis of the QD growth suggests a simple explanation for the unexpected shape of the In-rich core.
Journal Article↗