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 Si$_{1-x}$Ge$_x$ substrates with $x\leq 0.3$ 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 β\beta-electron detector for spectral shape studies

The shape of the spectrum corresponding to the electrons emitted in $\beta$ decay carries a wealth of information about nuclear structure and fundamental physics. In spite of that, few dedicated measurements have been made of $\beta$-spectrum shapes. In this work we present a newly developed detector for $\beta$ 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 $\beta$-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 $\beta$-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 $\pi$-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 75$^{\circ}$C at a concentration of $4\times 10^{-4}$ M. We observe exciton bimolecular annihilation in MOPV stacks at high excitation fluence, indicated by the fluence-dependent decay of $1^1$B$_{u}$-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 Si$_{n}$H (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$_{n}$ cluster. We discuss the nature of bonding of hydrogen in these clusters. We find that hydrogen bonds with two silicon atoms only in Si$_{2}$H, Si$_{3}$H and Si$_{5}$H clusters, while in other clusters (i.e. Si$_{4}$H, Si$_{6}$H, Si$_{7}$H, Si$_{8}$H, Si$_{9}$H and Si$_{10}$H) 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$_{n}$ and Si$_{n}$H 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 Si$_{n}$H clusters shows that hydrogen is easily removed from Si$_{n}$H 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