134 research outputs found
Unipolar and bipolar operation of InAs/InSb nanowire heterostructure field-effect transistors
We present temperature dependent electrical measurements on n-type InAs/InSb nanowireheterostructurefield-effect transistors. The barrier height of the heterostructure junction is determined to be 220 meV, indicating a broken bandgap alignment. A clear asymmetry is observed when applying a bias to either the InAs or the InSb side of the junction. Impact ionization and band-to-band tunneling is more pronounced when the large voltage drop occurs in the narrow bandgapInSb segment. For small negative gate-voltages, the InSb segment can be tuned toward p-type conduction, which induces a strong band-to-band tunneling across the heterostructucture junction.This work was carried out within the Nanometer Structure
Consortium at Lund University and was supported by
the Swedish Research Council (VR), the Swedish Foundation
for Strategic Research (SSF), and the Knut and Alice
Wallenberg Foundation
Increased absorption in InAsSb nanowire clusters through coupled optical modes
Nanowires can act as efficient light absorbers where waveguide modes are resonant to specific wavelengths. This resonant wavelength can easily be tuned by the nanowire dimensions, but the absorption of infrared radiation requires diameters of hundreds of nm, which is difficult to achieve using epitaxial growth. Here, we demonstrate that infrared absorption in InAsSb nanowires with the diameters of only 140 nm grown on Si substrates can be enhanced resonantly by placing them closely packed in clusters of different sizes. We find that coating the nanowires with a dielectric to optically connect them results in an efficient absorption diameter far exceeding the diameter of the constituent nanowires and that the cut-off wavelength is redshifted with an increasing cluster diameter. Numerical simulations are in agreement with the experimental results and demonstrate that if nanowires are positioned in clusters, a peak absorptance of 20% is possible at 5.6 μm with only 3% surface coverage. This absorptance is 200 times higher than for wires placed in an equidistant pattern. Our findings have direct implications for the design of efficient nanowire based photodetectors and solar cells
N-representability of two-electron densities and density matrices and the application to the few-body problem
We have found a (dense) basis for the N-representable, two-electron
densities, in which all N-representable two-electron densities can be expanded,
using positive coefficients. The inverse problem of finding a representative
wavefunction, giving a prescribed two-electron density, has also been solved.
The two-electron densities are found to lie in a convex set in a vector space.
We show that density matrices are more complicated objects than densities, and
density matrices do not seem to lie in a convex set. An algorithm to compute
the ground-state energy of a few-particle system is proposed, based on the
obtained results, where the correlation is treated exactly
Measurement of line widths and permanent electric dipole moment change of the Ce 4f-5d transition in Y_2SiO_5 for a qubit readout scheme in rare-earth ion based quantum computing
In this work the inhomogeneous (zero-phonon line) and homogeneous line
widths, and one projection of the permanent electric dipole moment change for
the Ce 4f-5d transition in Y_2SiO_5 were measured in order to investigate the
possibility for using Ce as a sensor to detect the hyperfine state of a
spatially close-lying Pr or Eu ion. The experiments were carried out on Ce
doped or Ce-Pr co-doped single Y_2SiO_5 crystals. The homogeneous line width
was measured to be about 3 MHz, which is essentially limited by the excited
state lifetime. Based on the line width measurements, the oscillator strength,
absorption cross section and saturation intensity were calculated to be about
9*10^-7, 5*10^-19 m^2 and 1*10^7 W/m^2, respectively. One projection of the
difference in permanent dipole moment, Delt_miu_Ce, between the ground and
excited states of the Ce ion was measured as 6.3 * 10^-30 C*m, which is about
26 times as large as that of Pr ions. The measurements done on Ce ions indicate
that the Ce ion is a promising candidate to be used as a probe to read out a
single qubit ion state for the quantum computing using rare-earth ions.Comment: 9 figures, 8 page
Increased absorption in InAsSb nanowire clusters through coupled optical modes
Nanowires can act as efficient light absorbers where waveguide modes are resonant to specific wavelengths. This resonant wavelength can easily be tuned by the nanowire dimensions, but the absorption of infrared radiation requires diameters of hundreds of nm, which is difficult to achieve using epitaxial growth. Here, we demonstrate that infrared absorption in InAsSb nanowires with the diameters of only 140 nm grown on Si substrates can be enhanced resonantly by placing them closely packed in clusters of different sizes. We find that coating the nanowires with a dielectric to optically connect them results in an efficient absorption diameter far exceeding the diameter of the constituent nanowires and that the cut-off wavelength is redshifted with an increasing cluster diameter. Numerical simulations are in agreement with the experimental results and demonstrate that if nanowires are positioned in clusters, a peak absorptance of 20% is possible at 5.6 μm with only 3% surface coverage. This absorptance is 200 times higher than for wires placed in an equidistant pattern. Our findings have direct implications for the design of efficient nanowire based photodetectors and solar cells
Single-nanowire, low-bandgap hot carrier solar cells with tunable open-circuit voltage
Compared to traditional pn-junction photovoltaics, hot carrier solar cells
offer potentially higher efficiency by extracting work from the kinetic energy
of photogenerated "hot carriers" before they cool to the lattice temperature.
Hot carrier solar cells have been demonstrated in high-bandgap ferroelectric
insulators and GaAs/AlGaAs heterostructures, but so far not in low-bandgap
materials, where the potential efficiency gain is highest. Recently, a high
open-circuit voltage was demonstrated in an illuminated wurtzite InAs nanowire
with a low bandgap of 0.39 eV, and was interpreted in terms of a
photothermoelectric effect. Here, we point out that this device is a hot
carrier solar cell and discuss its performance in those terms. In the
demonstrated devices, InP heterostructures are used as energy filters in order
to thermoelectrically harvest the energy of hot electrons photogenerated in
InAs absorber segments. The obtained photovoltage depends on the
heterostructure design of the energy filter and is therefore tunable. By using
a high-resistance, thermionic barrier an open-circuit voltage is obtained that
is in excess of the Shockley-Queisser limit. These results provide
generalizable insight into how to realize high voltage hot carrier solar cells
in low-bandgap materials, and therefore are a step towards the demonstration of
higher efficiency hot carrier solar cells
GaAsP Nanowires Grown by Aerotaxy
We have grown GaAsP nanowires with high optical and structural quality by Aerotaxy, a new continuous gas phase mass production process to grow III-V semiconductor based nanowires. By varying the PH3/AsH3 ratio and growth temperature, size selected GaAs1-xPx nanowires (80 nm diameter) with pure zinc-blende structure and with direct band gap energies ranging from 1.42 to 1.90 eV (at 300 K), (i.e., 0 ≤ x ≤ 0.43) were grown, which is the energy range needed for creating tandem III-V solar cells on silicon. The phosphorus content in the NWs is shown to be controlled by both growth temperature and input gas phase ratio. The distribution of P in the wires is uniform over the length of the wires and among the wires. This proves the feasibility of growing GaAsP nanowires by Aerotaxy and results indicate that it is a generic process that can be applied to the growth of other III-V semiconductor based ternary nanowires
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