18 research outputs found
Growth mechanisms in molecular beam epitaxy for GaN-(In,Ga)N core-shell nanowires emitting in the green spectral range
Using molecular beam epitaxy, we demonstrate the growth of (In,Ga)N shells
emitting in the green spectral range around very thin (35 nm diameter) GaN core
nanowires. These GaN nanowires are obtained by self-assembled growth on TiN. We
present a qualitative shell growth model accounting for both the
three-dimensional nature of the nanostructures as well as the directionality of
the atomic fluxes. This model allows us, on the one hand, to optimise the
conditions for high and homogeneous In incorporation and, on the other hand, to
explain the influence of changes in the growth conditions on the sample
morphology and In content. Specifically, the impact of the V/III and In/Ga flux
ratios, the rotation speed and the rotation direction are investigated.
Notably, with In acting as surfactant, the ternary (In,Ga)N shells are much
more homogeneous in thickness along the NW length than their binary GaN
counterparts
Sequential directional deposition of one-sided (In,Ga)N shells on GaN nanowires by molecular beam epitaxy
Capitalizing on the directed nature of the atomic fluxes in molecular beam
epitaxy, we propose and demonstrate the sequential directional deposition of
lateral (In,Ga)N shells on GaN nanowires. In this approach, a sub-monolayer
thickness of each constituent atomic species, i.e. Ga, In, and N, is deposited
subsequently from the same direction by rotating the sample and operating the
shutters accordingly. Using multiple iterations of this process, we achieve the
growth of homogeneous shells on a single side facet of the nanowires. For
higher In content and thus lattice mismatch, we observe a strain-induced
bending of the nanowire heterostructures. The incorporation of In and the
resulting emission spectra are systematically investigated as a function of
both the growth temperature and the In/Ga flux ratio
Electroluminescence and current-voltage measurements of single (In,Ga)N/GaN nanowire light-emitting diodes in the nanowire ensemble
We present the combined analysis of the electroluminescence (EL) as well as
the current-voltage (I-V) behavior of single, freestanding (In,Ga)N/GaN
nanowire (NW) light-emitting diodes (LEDs) in an unprocessed, self-assembled
ensemble grown by molecular beam epitaxy. The data were acquired in a scanning
electron microscope equipped with a micromanipulator and a luminescence
detection system. Single NW spectra consist of emission lines originating from
different quantum wells, and the width of the spectra increases with decreasing
peak emission energy. The corresponding I-V characteristics are described well
by the modified Shockley equation. The key advantage of this measurement
approach is the possibility to correlate the EL intensity of a single NW LED
with the actual current density in this NW. This way, the external quantum
efficiency (EQE) can be investigated as a function of the current in a single
NW LED. The comparison of the EQE characteristic of single NWs and the ensemble
device allows a quite accurate determination of the actual number of emitting
NWs in the working ensemble LED and the respective current densities in its
individual NWs. This information is decisive for a meaningful and comprehensive
characterization of a NW ensemble device, rendering the measurement approach
employed here a very powerful analysis tool
Excitonic and deep-level emission from N- and Al-polar homoepitaxial AlN grown by molecular beam epitaxy
Using low-temperature cathodoluminescence spectroscopy, we study the
properties of N- and Al-polar AlN layers grown by molecular beam epitaxy on
bulk AlN{0001}. Compared to the bulk AlN substrate, layers of both polarities
feature a suppression of deep-level luminescence, a total absence of the
prevalent donor with an exciton binding energy of 28 meV, and a much increased
intensity of the emission from free excitons. The dominant donor in these
layers is characterized by an associated exciton binding energy of 13 meV. The
observation of excited exciton states up to the exciton continuum allows us to
directly extract the free exciton binding energy of 57 meV
Accurate and scalable variant calling from single cell DNA sequencing data with ProSolo
Accurate single cell mutational profiles can reveal genomic cell-to-cell heterogeneity. However, sequencing libraries suitable for genotyping require whole genome amplification, which introduces allelic bias and copy errors. The resulting data violates assumptions of variant callers developed for bulk sequencing. Thus, only dedicated models accounting for amplification bias and errors can provide accurate calls. We present ProSolo for calling single nucleotide variants from multiple displacement amplified (MDA) single cell DNA sequencing data. ProSolo probabilistically models a single cell jointly with a bulk sequencing sample and integrat
Helmholtz Open Science Workshop „Zugang zu und Nachnutzung von wissenschaftlicher Software“ #hgfos16, Report; November 2016
Der Report des Helmholtz Open Science Workshops „Zugang zu und Nachnutzung von wissenschaftlicher Software“ #hgfos16 behandelt die Themen Standards und Qualitätssicherung; Reproduzierbarkeit; Lizenzierung und weitere rechtliche Aspekte; Zitation und Anerkennung; Sichtbarkeit und Modularität; Geschäftsmodelle; Personal,
Ausbildung, Karrierewege. Diese Themen sind eng miteinander verzahnt. Für jeden Themenbereich werden jeweils die Relevanz, Fragestellungen, Herausforderungen, mögliche Lösungsansätze und Handlungsempfehlungen betrachtet
Scientific Software – the role of best practices and recommendations
In Geosciences – like in most other communities – scientific work strongly depends on software. For big data
analysis, existing (closed or open source) program packages are often mixed with newly developed codes. Different versions of software components and varying configurations can influence the result of data analysis. This often makes reproducibility of results and reuse of codes very difficult. Policies for publication and documentation of used and newly developed software, along with best practices, can help tackle this problem.
Within the Helmholtz Association a Task Group “Access to and Re-use of scientific software” was implemented
by the Open ScienceWorking Group in 2016. The aim of the Task Group is to foster the discussion about scientific software in the Open Science context and to formulate recommendations for the production and publication of scientific software, ensuring open access to it. As a first step, a workshop gathered interested scientists from institutions across Germany. The workshop brought together various existing initiatives from different scientific communities to analyse current problems, share established best practices and come up with possible solutions. The subjects in the working groups covered a broad range of themes, including technical infrastructures, standards and quality assurance, citation of software and reproducibility. Initial recommendations are presented and discussed in the talk. They are the foundation for further discussions in the Helmholtz Association and the Priority Initiative “Digital Information” of the Alliance of Science Organisations in Germany. The talk aims to inform about the activities and to link with other initiatives on the national or international level
Intersubband Optoelectronics Using III-Nitride Semiconductors
International audienceThe terms "intersubband" (ISB) or "intraband" refer to electronic transitions between confined states in either the conduction band or the valence band of semiconductor heterostructures. In order to observe ISB absorption, the first electronic level must be populated. However, high doping levels may affect the transition energy. The ISB absorption energy exhibits a significant blue shift due to many-body effects, mostly related to the exchange interaction and the depolarization shift. The presence of internal electric fields in polar materials increases the design complexity of ISB devices. Nanowire heterostructures offer a unique situation for devices requiring low defect density in the active region and the combination of materials with large lattice mismatch. The demand for increasing bandwidth in optical communication networks impels the development of all-optical switches at 1.55 µm, which should be capable of sustaining high repetition rates with low switching energy and high modulation depth
Sequential directional deposition of one-sided (In,Ga)N shells on GaN nanowires by molecular beam epitaxy
Capitalizing on the directed nature of the atomic fluxes in molecular beam epitaxy, we propose and demonstrate the sequential directional deposition of lateral (In,Ga)N shells on GaN nanowires. In this approach, a sub-monolayer thickness of each constituent atomic species, i.e., Ga, In, and N, is deposited subsequently from the same direction by rotating the sample and operating the shutters accordingly. Using multiple iterations of this process, we achieve the growth of homogeneous shells on a single side facet of the nanowires. For higher In content and thus lattice mismatch, we observe a strain-induced bending of the nanowire heterostructures. The incorporation of In and the resulting emission spectra are systematically investigated as a function of both the growth temperature and the In/Ga flux ratio
Accurate and scalable variant calling from single cell DNA sequencing data with ProSolo.
Accurate single cell mutational profiles can reveal genomic cell-to-cell heterogeneity. However, sequencing libraries suitable for genotyping require whole genome amplification, which introduces allelic bias and copy errors. The resulting data violates assumptions of variant callers developed for bulk sequencing. Thus, only dedicated models accounting for amplification bias and errors can provide accurate calls. We present ProSolo for calling single nucleotide variants from multiple displacement amplified (MDA) single cell DNA sequencing data. ProSolo probabilistically models a single cell jointly with a bulk sequencing sample and integrates all relevant MDA biases in a site-specific and scalable-because computationally efficient-manner. This achieves a higher accuracy in calling and genotyping single nucleotide variants in single cells in comparison to state-of-the-art tools and supports imputation of insufficiently covered genotypes, when downstream tools cannot handle missing data. Moreover, ProSolo implements the first approach to control the false discovery rate reliably and flexibly. ProSolo is implemented in an extendable framework, with code and usage at: https://github.com/prosolo/prosolo