The incorporation of Au during vapor−liquid−solid nanowire growth might inherently limit the performance of nanowire-based devices. Here, we assess the material quality of Au-assisted and Au-free ...grown GaAs/(Al,Ga)As core−shell nanowires using photoluminescence spectroscopy. We show that at room temperature, the internal quantum efficiency is systematically much lower for the Au-assisted nanowires than for the Au-free ones. In contrast, the optoelectronic material quality of the latter is comparable to that of state-of-the-art planar double heterostructures.
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Collective electron transport causes a weakly coupled semiconductor superlattice under dc voltage bias to be an excitable system with 2N+2 degrees of freedom: electron densities and fields at N ...superlattice periods plus the total current and the field at the injector. External noise of sufficient amplitude induces regular current self-oscillations (coherence resonance) in states that are stationary in the absence of noise. Numerical simulations show that these oscillations are due to the repeated nucleation and motion of charge dipole waves that form at the emitter when the current falls below a critical value. At the critical current, the well-to-well tunneling current intersects the contact load line. We have determined the device-dependent critical current for the coherence resonance from experiments and numerical simulations. We have also described through numerical simulations how a coherence resonance triggers a stochastic resonance when its oscillation mode becomes locked to a weak ac external voltage signal. Our results agree with the experimental observations.
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Compact solid-state lamps based on light-emitting diodes (LEDs)
are of current technological interest as an alternative to conventional light
bulbs. The brightest LEDs available so far emit red light ...and exhibit higher
luminous efficiency than fluorescent lamps. If this luminous efficiency could
be transferred to white LEDs, power consumption would be dramatically reduced,
with great economic and ecological consequences. But the luminous efficiency
of existing white LEDs is still very low, owing to the presence of electrostatic
fields within the active layers. These fields are generated
by the spontaneous and piezoelectric polarization along the 0001 axis of
hexagonal group-III nitrides-the commonly used materials for light generation. Unfortunately, as this crystallographic orientation corresponds
to the natural growth direction of these materials deposited on currently
available substrates. Here we demonstrate that the epitaxial
growth of GaN/(Al,Ga)N on tetragonal LiAlO2 in a non-polar direction
allows the fabrication of structures free of electrostatic fields, resulting
in an improved quantum efficiency. We expect that this approach will pave
the way towards highly efficient white LEDs.
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Vertical GaN nanowires are grown in a self-induced way on a sputtered Ti film by plasma-assisted molecular beam epitaxy. Both in situ electron diffraction and ex situ ellipsometry show that Ti is ...converted to TiN upon exposure of the surface to the N plasma. In addition, the ellipsometric data demonstrate this TiN film to be metallic. The diffraction data evidence that the GaN nanowires have a strict epitaxial relationship to this film. Photoluminescence spectroscopy of the GaN nanowires shows excitonic transitions virtually identical in spectral position, line width, and decay time to those of state-of-the-art GaN nanowires grown on Si. Therefore, the crystalline quality of the GaN nanowires grown on metallic TiN and on Si is equivalent. The freedom to employ metallic substrates for the epitaxial growth of semiconductor nanowires in high structural quality may enable novel applications that benefit from the associated high thermal and electrical conductivity as well as optical reflectivity.
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We demonstrate the frequency stabilization of a terahertz quantum-cascade laser (QCL) to the Lamb dip of the absorption line of a D
O rotational transition at 3.3809309 THz. To assess the quality of ...the frequency stabilization, a Schottky diode harmonic mixer is used to generate a downconverted QCL signal by mixing the laser emission with a multiplied microwave reference signal. This downconverted signal is directly measured by a spectrum analyzer showing a full width at half maximum of 350 kHz, which is eventually limited by high-frequency noise beyond the bandwidth of the stabilization loop.
In the present paper we focus on studies of shape coexistence in even-mass nuclei in the neutron-deficient Pb region. They are based on experiments carried out using tagging techniques in the ...Accelerator Laboratory of the University of Jyväskylä, Finland. Excited states in many of these nuclei can only be accessed via fusion-evaporation reactions employing high-intensity stable-ion beams. The key features in these experiments are high selectivity, clean spectra and instrumentation that enables high count rates. We review three spectroscopic highlights in this region.
There is strong circumstantial evidence that certain heavy, unstable atomic nuclei are 'octupole deformed', that is, distorted into a pear shape. This contrasts with the more prevalent rugby-ball ...shape of nuclei with reflection-symmetric, quadrupole deformations. The elusive octupole deformed nuclei are of importance for nuclear structure theory, and also in searches for physics beyond the standard model; any measurable electric-dipole moment (a signature of the latter) is expected to be amplified in such nuclei. Here we determine electric octupole transition strengths (a direct measure of octupole correlations) for short-lived isotopes of radon and radium. Coulomb excitation experiments were performed using accelerated beams of heavy, radioactive ions. Our data on (220)Rn and (224)Ra show clear evidence for stronger octupole deformation in the latter. The results enable discrimination between differing theoretical approaches to octupole correlations, and help to constrain suitable candidates for experimental studies of atomic electric-dipole moments that might reveal extensions to the standard model.
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The realization of semiconductor structures with stable excitons at room temperature is crucial for the development of excitonics and polaritonics. Quantum confinement has commonly been employed for ...enhancing excitonic effects in semiconductor heterostructures. Dielectric confinement, which gives rises to much stronger enhancement, has proven to be more difficult to achieve because of the rapid nonradiative surface/interface recombination in hybrid dielectric-semiconductor structures. Here, we demonstrate intense excitonic emission from bare GaN nanowires with diameters down to 6 nm. The large dielectric mismatch between the nanowires and vacuum greatly enhances the Coulomb interaction, with the thinnest nanowires showing the strongest dielectric confinement and the highest radiative efficiency at room temperature. In situ monitoring of the fabrication of these structures allows one to accurately control the degree of dielectric enhancement. These ultrathin nanowires may constitute the basis for the fabrication of advanced low-dimensional structures with an unprecedented degree of confinement.
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