Agglomeration of As Antisites in As-Rich Low-Temperature GaAs: Nucleation without a Critical Nucleus Size

To investigate the early stages of nucleation and growth of As precipitates in GaAs grown at low substrate temperature, we make use of a self-consistent-charge density-functional based tight-binding method. Since a pair of As antisites already shows a significant binding energy which increases when more As antisites are attached, there is no critical nucleus size. Provided that all excess As has precipitated, the clusters may grow in size since the binding energies increase with increasing agglomeration size. These findings close the gap between experimental investigation of point defects and the detection of nanometer-size precipitates in transmission electron microscopy.Peer reviewe

The influence of laser relative intensity noise in the Laser Interferometer Space Antenna

LISA is an upcoming ESA mission that will detect gravitational waves in spaceby interferometrically measuring the separation between free-falling testmasses at picometer precision. To reach the desired performance, LISA willemploy the noise reduction technique time-delay interferometry (TDI), in whichmultiple raw interferometric readouts are time shifted and combined into thefinal scientific observables. Evaluating the performance in terms of these TDIvariables requires careful tracking of how different noise sources propagatethrough TDI, as noise correlations might affect the performance in unexpectedways. One example of such potentially correlated noise is the relativeintensity noise (RIN) of the six lasers aboard the three LISA satellites, whichwill couple into the interferometric phase measurements. In this article, wecalculate the expected RIN levels based on the current mission architecture andthe envisaged mitigation strategies. We find that strict requirements on thetechnical design reduce the effect from approximately 8.7 pm/rtHz perinter-spacecraft interferometer to that of a much lower sub-1 pm/rtHz noise,with typical characteristics of an uncorrelated readout noise after TDI. Ourinvestigations underline the importance of sufficient balanced detection of theinterferometric measurements.<br

Clustering of vacancy defects in high-purity semi-insulating SiC

Positron lifetime spectroscopy was used to study native vacancy defects in semi-insulating silicon carbide. The material is shown to contain (i) vacancy clusters consisting of 4--5 missing atoms and (ii) Si vacancy related negatively charged defects. The total open volume bound to the clusters anticorrelates with the electrical resistivity both in as-grown and annealed material. Our results suggest that Si vacancy related complexes compensate electrically the as-grown material, but migrate to increase the size of the clusters during annealing, leading to loss of resistivity.Comment: 8 pages, 5 figure

Microscopic identification of native donor Ga-vacancy complexes in Te-doped GaAs

Native vacancies in Te-doped (5×1016–5×1018cm−3) GaAs were investigated by means of positron lifetime and Doppler-broadening coincidence spectroscopy. The experimental data were related to theoretical calculations of the positron lifetime and the annihilation momentum distribution. Monovacancies were observed in all Te-doped GaAs samples under study. It will be shown that they can directly be identified to be Ga-vacancy–TeAs-donor complexes. These complexes are the dominating type of vacancy defects in the doping range under observation.Peer reviewe

Neuronal ribosomes exhibit dynamic and context-dependent exchange of ribosomal proteins

Owing to their morphological complexity and dense network connections, neurons modify their proteomes locally, using mRNAs and ribosomes present in the neuropil (tissue enriched for dendrites and axons). Although ribosome biogenesis largely takes place in the nucleus and perinuclear region, neuronal ribosomal protein (RP) mRNAs have been frequently detected remotely, in dendrites and axons. Here, using imaging and ribosome profiling, we directly detected the RP mRNAs and their translation in the neuropil. Combining brief metabolic labeling with mass spectrometry, we found that a group of RPs rapidly associated with translating ribosomes in the cytoplasm and that this incorporation was independent of canonical ribosome biogenesis. Moreover, the incorporation probability of some RPs was regulated by location (neurites vs. cell bodies) and changes in the cellular environment (following oxidative stress). Our results suggest new mechanisms for the local activation, repair and/or specialization of the translational machinery within neuronal processes, potentially allowing neuronal synapses a rapid means to regulate local protein synthesis

Do Arsenic Interstitials Really Exist in As-Rich GaAs?

To investigate the lattice distortion caused by point defects in As-rich GaAs, we make use of a self-consistent-charge density-functional based tight-binding method. Both relevant defects, the As antisite and the As interstitial, cause significant lattice distortion. In contrast to As interstitials, isolated As antisites lead to lattice strain as well as displacement of nearest neighbor As lattice atoms into the ⟨110⟩ channels, in excellent agreement with experiments. Therefore, our result gives powerful evidence for As antisites being the dominating defect in as-grown As-rich GaAs.Peer reviewe