344 research outputs found
Photoconductance of a submicron oxidized line in surface conductive single crystalline diamond
We report on sub-bandgap optoelectronic phenomena of hydrogen-terminated
diamond patterned with a submicron oxidized line. The line acts as an energy
barrier for the two-dimensional hole gas located below the hydrogenated diamond
surface. A photoconductive gain of the hole conductivity across the barrier is
measured for sub-bandgap illumination. The findings are consistent with
photogenerated electrons being trapped in defect levels within the barrier. We
discuss the spatial and energetic characteristics of the optoelectronic
phenomena, as well as possible photocurrent effects
Phosphorus donors in highly strained silicon
The hyperfine interaction of phosphorus donors in fully strained Si thin
films grown on virtual SiGe substrates with is
determined via electrically detected magnetic resonance. For highly strained
epilayers, hyperfine interactions as low as 0.8 mT are observed, significantly
below the limit predicted by valley repopulation. Within a Green's function
approach, density functional theory (DFT) shows that the additional reduction
is caused by the volume increase of the unit cell and a local relaxation of the
Si ligands of the P donor.Comment: 12 pages, 3 figure
Optical characterization of Mg-doped GaN films grown by metalorganic chemical vapor phase deposition
Scanning electron microscopy, micro-Raman, and photoluminescence (PL) measurements are reported for Mg-doped GaN films grown on (0001) sapphire substrates by low-pressure metalorganic chemical vapor phase deposition. The surface morphology, structural, and optical properties of GaN samples with Mg concentrations ranging from 1019 to 1021 cm−3 have been studied. In the scanning micrographs large triangular pyramids are observed, probably due to stacking fault formation and three-dimensional growth. The density and size of these structures increase with the amount of magnesium incorporated in the samples. In the photoluminescence spectra, intense lines were found at 3.36 and 3.31 eV on the triangular regions, where the presence of cubic inclusions was confirmed by micro-Raman measurements. The excitation dependence and temperature behavior of these lines enable us to identify their excitonic nature. From our study we conclude that the interface region between these defects and the surrounding wurtzite GaN could be responsible for PL [email protected] ; [email protected] ; [email protected]
First measurements with a new -electron detector for spectral shape studies
The shape of the spectrum corresponding to the electrons emitted in
decay carries a wealth of information about nuclear structure and fundamental
physics. In spite of that, few dedicated measurements have been made of
-spectrum shapes. In this work we present a newly developed detector for
electrons based on a telescope concept. A thick plastic scintillator is
employed in coincidence with a thin silicon detector. First measurements
employing this detector have been carried out with mono-energetic electrons
from the high-energy resolution electron-beam spectrometer at Bordeaux. Here we
report on the good reproduction of the experimental spectra of mono-energetic
electrons using Monte Carlo simulations. This is a crucial step for future
experiments, where a detailed Monte Carlo characterization of the detector is
needed to determine the shape of the -electron spectra by deconvolution
of the measured spectra with the response function of the detector. A chamber
to contain two telescope assemblies has been designed for future -decay
experiments at the Ion Guide Isotope Separator On-Line facility in
Jyv\"askyl\"a, aimed at improving our understanding of reactor antineutrino
spectra
Long-range potential fluctuations and 1/f noise in hydrogenated amorphous silicon
We present a microscopic theory of the low-frequency voltage noise (known as
"1/f" noise) in micrometer-thick films of hydrogenated amorphous silicon. This
theory traces the noise back to the long-range fluctuations of the Coulomb
potential produced by deep defects, thereby predicting the absolute noise
intensity as a function of the distribution of defect activation energies. The
predictions of this theory are in very good agreement with our own experiments
in terms of both the absolute intensity and the temperature dependence of the
noise spectra.Comment: 8 pages, 3 figures, several new parts and one new figure are added,
but no conceptual revision
Exciton bimolecular annihilation dynamics in supramolecular nanostructures of conjugated oligomers
We present femtosecond transient absorption measurements on -conjugated
supramolecular assemblies in a high pump fluence regime.
Oligo(\emph{p}-phenylenevinylene) monofunctionalized with
ureido-\emph{s}-triazine (MOPV) self-assembles into chiral stacks in dodecane
solution below 75C at a concentration of M. We
observe exciton bimolecular annihilation in MOPV stacks at high excitation
fluence, indicated by the fluence-dependent decay of B-exciton
spectral signatures, and by the sub-linear fluence dependence of time- and
wavelength-integrated photoluminescence (PL) intensity. These two
characteristics are much less pronounced in MOPV solution where the phase
equilibrium is shifted significantly away from supramolecular assembly,
slightly below the transition temperature. A mesoscopic rate-equation model is
applied to extract the bimolecular annihilation rate constant from the
excitation fluence dependence of transient absorption and PL signals. The
results demonstrate that the bimolecular annihilation rate is very high with a
square-root dependence in time. The exciton annihilation results from a
combination of fast exciton diffusion and resonance energy transfer. The
supramolecular nanostructures studied here have electronic properties that are
intermediate between molecular aggregates and polymeric semiconductors
Effect of hydrogen on ground state structures of small silicon clusters
We present results for ground state structures of small SiH (2 \leq
\emph{n} \leq 10) clusters using the Car-Parrinello molecular dynamics. In
particular, we focus on how the addition of a hydrogen atom affects the ground
state geometry, total energy and the first excited electronic level gap of an
Si cluster. We discuss the nature of bonding of hydrogen in these
clusters. We find that hydrogen bonds with two silicon atoms only in SiH,
SiH and SiH clusters, while in other clusters (i.e. SiH,
SiH, SiH, SiH, SiH and SiH) hydrogen is bonded
to only one silicon atom. Also in the case of a compact and closed silicon
cluster hydrogen bonds to the cluster from outside. We find that the first
excited electronic level gap of Si and SiH fluctuates as a function
of size and this may provide a first principles basis for the short-range
potential fluctuations in hydrogenated amorphous silicon. Our results show that
the addition of a single hydrogen can cause large changes in the electronic
structure of a silicon cluster, though the geometry is not much affected. Our
calculation of the lowest energy fragmentation products of SiH clusters
shows that hydrogen is easily removed from SiH clusters.Comment: one latex file named script.tex including table and figure caption.
Six postscript figure files. figure_1a.ps and figure_1b.ps are files
representing Fig. 1 in the main tex
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