22 research outputs found
Quasi-ballistic, nonequilibrium electron distribution in inhomogeneous semiconductor structures
We report on a study of quasi-ballistic transport in deep submicron,
inhomogeneous semiconductor structures, focusing on the analysis of signatures
found in the full nonequilibrium electron distribution. We perform
self-consistent numerical calculations of the Poisson-Boltzmann equations for a
model n(+)-n(-)-n(+) GaAs structure and realistic, energy-dependent scattering.
We show that, in general, the electron distribution displays significant,
temperature dependent broadening and pronounced structure in the high-velocity
tail of the distribution. The observed characteristics have a strong spatial
dependence, related to the energy-dependence of the scattering, and the large
inhomogeneous electric field variations in these systems. We show that in this
quasi-ballistic regime, the high-velocity tail structure is due to pure
ballistic transport, whereas the strong broadening is due to electron
scattering within the channel, and at the source(drain) interfaces.Comment: 4 pages, 2 figure
Spin injection and accumulation in inhomogeneous semiconductors
We present a study of spin transport in charge and spin inhomogeneous
semiconductor systems. In particular, we investigate the propagation of
spin-polarized electrons through a boundary between two semiconductor regions
with different doping concentrations. We use a theoretical and numerical
method, presented in this paper, based on a self-consistent treatment of a
two-component version of the Boltzmann transport equation. We show that
space-charge effects strongly influence the spin transport properties, in
particular giving rise to pronounced spin accumulation and spin density
enhancement.Comment: 4 page
Large variations in the hole spin splitting of quantum-wire subband edges
We study Zeeman splitting of zone-center subband edges in a cylindrical hole
wire subject to a magnetic field parallel to its axis. The g-factor turns out
to fluctuate strongly as a function of wire-subband index, assuming values that
differ substantially from those found in higher-dimensional systems. We analyze
the spin properties of hole-wire states using invariants of the spin-3/2
density matrix and find a strong correlation between g-factor value and the
profile of hole-spin polarization density. Our results suggest possibilities
for confinement engineering of hole spin splittings.Comment: 4 pages, 3 figures, RevTex4, to appear in PR
Metal Hydrides Form Halogen Bonds: Measurement of Energetics of Binding
The formation of halogen bonds from iodopentafluorobenzene and 1-iodoperfluorohexane to a series of bis(η5-cyclopentadienyl)metal hydrides (Cp2TaH3, 1; Cp2MH2, M = Mo, 2, M = W, 3; Cp2ReH, 4; Cp2Ta(H)CO, 5; Cp = η5-cyclopentadienyl) is demonstrated by 1H NMR spectroscopy. Interaction enthalpies and entropies for complex 1 with C6F5I and C6F13I are reported (ÎH° = â10.9 ± 0.4 and â11.8 ± 0.3 kJ/mol; ÎS° = â38 ± 2 and â34 ± 2 J/(mol·K), respectively) and found to be stronger than those for 1 with the hydrogen-bond donor indole (ÎH° = â7.3 ± 0.1 kJ/mol, ÎS° = â24 ± 1 J/(mol·K)). For the more reactive complexes 2â5, measurements are limited to determination of their low-temperature (212 K) association constants with C6F5I as 2.9 ± 0.2, 2.5 ± 0.1, <1.5, and 12.5 ± 0.3 Mâ1, respectively
Modelling of Quantum Transport in Nanostructures
In this thesis, theoretical studies of the transport properties of three nanoscale systems: one-dimensional (1D) quantum wires (QWRs), zero-dimensional (0D), laterally confined, double-barrier resonant tunnelling structures (DBRTSs) and three-terminal ballistic junctions (TBJs), have been performed. In the first part of the thesis, an overview of the realization and properties of such systems is given along with a description of modelling tools used in the calculations. The second and main part of the thesis contains the original research results, summarized into seven papers. The conductance of QWRs with corrugated boundaries is investigated in Paper I with respect to the nature of the boundary roughness, geometrical parameters of the QWR and temperature. It is shown that, due to the structural imperfections, the conductance exhibits rapid fluctuations, strong, broad dips between adjacent conductance plateaus at very low temperatures and, in general, a suppression of the conductance below the values expected for an ideal QWR. The results agree with existing experimental results. Experimental studies of the transport properties of 0D quantum dots obtained by laterally confining vertical DBRTSs by means of metallic gates have shown complex, gate-dependent fine structure in the measured current-voltage (I-V) characteristics. The origin of this fine structure is theoretically studied and explained (Papers II-V) in terms of quasi-1D-0D-1D systems with a tunable lateral confinement. It is shown that, due to the low dimensionality of the emitter, dot and collector regions, complex fine structure, which is strongly dependent on Fermi energy, source-drain voltage, and gate voltage, is formed in the I-V characteristics, which may explain the experimentally observed results. A tentative comparison between experiments and theory is made in Paper IV. Three-terminal junction systems have very recently emerged as excellent candidates for use as building blocks in the formation of nanoscale electronic devices. A general formalism for the calculation of electron transport through three- terminal quantum structures is presented in Paper VI. Using this method, the transport through Y-shaped TBJ structures is studied in Paper VII. Quantum effects are shown to influence the transport properties of TBJs at low temperatures, possibly enabling new device functionality