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We report on nanoscopic exploration of the luminescence from individual InP quantum dots (QDs) by means of highly spatially resolved cathodoluminescence (CL) spectroscopy directly performed in a scanning transmission electron microscope (STEM). A 7-fold layer stack with high-density InP quantum dots is embedded as an active medium membrane in an external-cavity surface-emitting laser. We characterize the vertical transfer of carriers within the periodic separate confinement heterostructure and determine the capture efficiency of carriers from the cladding layer into the quantum dot layers. Benefiting from the nanoscale resolution of our STEM-CL, we perform single-dot spectroscopy on single isolated QDs in the STEM lamella resolving the details of the excitonic structure of individual quantum dots. Executing highly spatially resolved spectrum line scans within the QD layers, we directly visualize the lateral transport

Land tenure security is increasingly recognized as a foundational element for advancing global sustainable development agendas, but questions remain about how it affects human well-being and environmental outcomes. We identify 117 studies that aimed to estimate the causal effect of land tenure security interventions on these outcomes. Approximately two-thirds of these studies reported positive links between improved tenure security and human well-being or environmental outcomes. Close to half of the studies that examined social and environmental outcomes reported positive impacts on both. The majority of studies assessed government-implemented interventions that statutorily recognized rights through land titling and formalization in the 1990s and 2000s. More research is needed to bolster the body of evidence on the effects of non-technical interventions (for example, capacity building and awareness raising) and the devolution of rights to inform future land policy efforts and accelerate sustainable development. A meta-analysis of 117 studies to determine the impacts of land tenure security on socio-environmental outcomes and sustainable development.

Track engineers face increasing cost pressure and budget restrictions in their work today. This leads to growing difficulty in legitimizing crucial maintenance and renewal measures. As a result, infrastructure managers must ensure they invest all available financial resources as sustainably and efficiently as possible. These boundary conditions require an objective tool enabling both a component-specific condition evaluation and preventive maintenance with renewal planning. The present research introduces such a tool for railway tracks based on innovative track data analyses. This tool includes time-series analyses for predicting future quality behavior. Consequently, the technical necessity of maintenance actions can be derived for every specific track section. In addition, these technical evaluations are combined with economic and operational considerations to plan reasonable maintenance lengths for different track components in the next few years. In a further step, business evaluations by means of annuity monitoring are executed to determine whether ongoing track maintenance or complete track renewal is the most economical solution. This methodology also allows calculating the economic damage caused by neglecting the ideal point in time for reinvestment. On the basis of this economic damage, it is possible to rank projects by priority in the case of insufficient budgets and to ensure that all available resources are invested in the most reasonable manner possible. Furthermore, such analyses clearly show that when a specific degradation level of railway track is reached track renewal is more economic in relation to life-cycle costs than ongoing maintenance.

The current inspection and measurement methodologies for turnouts are time driven and manual and thus based on unloaded measurements. However, these measurements are scarcely usable for behaviour prognosis. Analysis of technologies available on the market shows a possibility for a loaded and automatic inspection by combining already existing technologies. Standard Elements are currently in use as a common tool for assessing the life cycle of turnouts. Those Standard Elements include all maintenance tasks within the entire service life of turnouts, generated and based on experiences as well as on statistical data over an entire network and a long-time span. The economic justification of a maintenance task needs both, the initial investment and the remaining life span to be considered. However, the reasons for the end of service life and therefore lifetime limiting components must be analysed separately. This was done by numerous questionnaires and expert interviews, which have shown that the ballast and the sleepers are the overall reasons for the necessity of a reinvestment.

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With gold prices skyrocketing and demand for other minerals on the rise, mining poses a growing threat to communities and ecosystems around the world. Conducting geospatial analysis of the Amazon biogeographic region, this report estimates, for the first time, the full extent of legal, large-scale mining concessions and illegal mining operations on Indigenous territories within the rainforest. It finds that, together, legal and illegal mining now cover more than 20% of Indigenous lands, endangering hundreds of Indigenous communities and critical ecosystems across 450,000 square kilometers. Mining, the study also shows, is polluting at least 30 Amazonian rivers and eroding communities’ proven ability to prevent deforestation. From 2000 to 2015, Indigenous lands with mining activities had higher incidences of tree cover loss than those without mining. In Bolivia, Ecuador and Peru, for example, forest loss was at least three times higher in Indigenous territories with mining operations – both legal and illegal – than those without; and one to two times higher in Colombia and Venezuela. Yet national laws often favor companies over Indigenous communities, the study’s comprehensive legal assessment of six Amazonian countries, including Bolivia, Brazil, Colombia, Ecuador, Guyana and Peru, reveals. Although these countries all grant important legal protections to communities, these land rights are insufficient and leave most Indigenous Peoples vulnerable to mining. Government oversight of mining activities across Indigenous lands, enforcement of existing legal protections and compliance with international environmental and social safeguards also remain weak in practice, creating tensions between Indigenous communities and miners that often escalate into violent conflict. Protecting Indigenous lands, including the sustainable development and environmental benefits they generate, will require a radical shift in governments’ and companies’ approaches to mining. Amazonian countries should strengthen Indigenous Peoples’ land and natural resource rights; establish and enforce social and environmental safeguards aligned with international standards; more consistently monitor mining operations across Indigenous lands; and step up efforts to identify and prosecute those who facilitate illegal mining practices. Companies, the report recommends, should comply with these stricter safeguards as well as invest in ecological restoration and reforestation of old mining sites.

Microcavities with InGaN quantum wells or GaN-based quantum dots as active medium are building blocks of electrically-driven, low-threshold surface-emitting lasers or single photon emitters in the visible-to-UV spectral range. In this chapter, we highlight essential developments in epitaxial growth techniques of such nitride-based microcavities and their active regions. Modern analytical techniques for structural and optical characterization of these complex heterostructures as presented in this chapter are essential to solve remaining challenges.

Indigenous communities (ICs) have emerged as important players in global efforts to reduce forest carbon emissions, in part because they are viewed as conscientious stewards of the forest lands to which they have legal title. Yet ICs tend to be located in remote areas where deforestation would be limited regardless of who manages them. Therefore, to determine whether IC management actually cuts forest carbon emissions, it is important to control for such confounding factors. To that end, we use propensity score matching and regression to analyze the effects on 2001–2013 deforestation and forest carbon emissions of IC management in the Amazon regions of Bolivia, Brazil, Ecuador and Colombia. We find that IC management reduces both deforestation and forest carbon emissions in Bolivia, Brazil and Colombia. We are not able to discern a statistically significant effect in Ecuador. These findings suggest that IC management can, in fact, help combat climate change.

We present a pulsed reactive magnetron sputter process for high quality AlN on Si (1 1 1) beneficially avoiding any high-temperature growth. Initially, metallic aluminium with a nominal thickness of about one monolayer is deposited at a substrate temperature around 850 °C in an Ar plasma followed by sputtering in an Ar/N plasma. For 250 nm thick AlN layers a surface roughness below 0.2 nm rms is obtained as determined by atomic force microscopy (AFM). Using an Al nucleation step prior to AlN growth substantially improves the crystalline properties of AlN. The FWHM values of the AlN (0 0 0 2) and the AlN (1 0 1 0) diffraction peaks with 0.45° and 0.86°, respectively, are comparable to state-of-the-art AlN on Si layers grown by metalorganic vapour phase epitaxy (MOVPE). Two different N-precursor gases, namely N2 and NH3, lead to distinct layer qualities as revealed by atomic force microscopy and transmission electron microscopy. Only with NH3 substantial lateral growth can be achieved at T = 850 °C which is mandatory to obtain smooth surface morphologies. In MOVPE such lateral AlN growth is typically only achieved at high growth temperatures (T > 1000 °C).

The exfoliation of graphite particles during the treatment in the vibratory disk mill containing commercial sugar was investigated. During the collision of the grinding ring with the grinding chamber wall an intensive stressing of graphite takes place. After treatment in the mill, the resulting dark paste was solved in water and subjected to 10 min sonication. The suspension was centrifuged to remove the not exfoliated multilayered particles. Analysis of Raman spectra shows the presence of graphite sheets in suspension. The represented method of exfoliation is environmentally friendly and scalable.

This work reports on compositionally graded ( 0 0 0 1 ¯ ) N-polar In x Ga 1−x N layers. The InGaN grades with different final In compositions x f up to 0.25 were grown by plasma-assisted molecular beam epitaxy on vicinal GaN base layers with a miscut angle of 4° towards the m -direction. When increasing x f the surface morphology evolved from an interlacing finger structure, attributed to the Ehrlich-Schwobel effect, towards fully strain-relaxed columnar features. Regardless of the crystal morphology and the strain state each graded sample exhibited a bright photoluminescence signal at room temperature spanning the whole visible range. Cross-sectional nanoscale cathodoluminescence evidenced a red-shift of the luminesced signal from 420 to 580 nm along the grade and also showed strong lateral emission inhomogeneities.

The dynamics of magnetic nanoparticles in rotating magnetic fields is studied both experimentally and theoretically. The experimental investigation is focused on the conversion of the magnetic forces to a mechanical torque acting on a ferrofluid confined in a spherical cavity in a rotating magnetic field. Polydispersity usually present in diluted ferrofluids is shown to play a crucial role in the torque conversion. Important features observed experimentally are reproduced theoretically in studies on the dynamics of particles with uniaxial magnetic anisotropy in the presence of thermal noise. The phase lag between the rotating magnetic field and the induced rotating magnetization, as well as the corresponding torque which is transferred to the carrier fluid because of the mutual coupling between both, is analyzed as a function of the particle size. It is shown that for large particles the magnetic moment is locked to the anisotropy axis. On lowering the particle radius, Neel relaxation becomes increasingly important. Illustrative numerical calculations demonstrating this behavior are performed for magnetic parameters typical for iron oxide.

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In this work we show successful metalorganic vapor phase epitaxy (MOVPE) of an AlN/AlGaN distributed Bragg reflector (DBR) that is wavelength matched to GaN quantum dots (QDs) in an AlGaN lambda cavity on top. Full insight into the growth of these structures enables the epitaxy of resonant cavity deep UV single photon emitters. The DBR was grown on an AlN/sapphire template. In order to obtain a high reflectivity as well as a sufficiently large stopband width, the refractive index contrast needs to be maximized. Additionally, the absorption of QD emission in the high gallium containing layer needs to be minimized. A compromise was found for nominal Al-concentration of 70 % in the AlGaN layers. The resulting DBR splits up into self-organized AlN/Al(X)Ga(1-X)N/Al(Y)Ga(1-Y)N trilayers, which add up to desired lambda/2-periods. Therefore, the stopband at 272 nm with a width of 6 nm shows a maximum reflectivity of 99.7 %. GaN QDs were obtained by growth of GaN on AlGaN for 10 s with a V/III-ratio of 30 followed by a growth interruption of 30 s. The QDs exhibit sharp emission lines with a FWHM down to 1 meV in µ-PL measurements. The main intensity of the QD ensemble emission is in the range of 250 nm to 275 nm. Finally, spatially resolved low temperature CL measurements show resonant DBR-enhanced GaN QD emission at 271 nm showing successful wavelength match between a AlN/AlGaN deep UV DBR and GaN QDs in an AlGaN lambda-cavity on top.

Anlagenmanagement versucht bei Produktionsanlagen den gesamten Lebenszyklus, beginnend mit der Planung und Beschaffung, dem anschliesenden Betrieb und der begleitenden Instandsetzung bis zum Ende der Nutzungsdauer der Anlage zu optimieren. Damit sind integrierte Instandsetzungskonzepte im Sinn von „Total Productive Maintenance“ sowie Qualitatsmanagement im Sinn von „Total Quality Management“ notwendige Bestandteile des Anlagenmanagements. Anlagenmanagement beschaftigt sich mit der Frage welche Anlagenauspragung zu wahlen sei. Daraus folgt, dass neben der Instandsetzung auch die Auswahl des Anlagentyps ein integraler Teil des Anlagenmanagements ist. Auf Lebenszykluskosten (LCC) basierende Investitionsentscheidungen und nachfolgende ebenso LCC basierte Instandsetzungsstrategien entsprechen damit weitgehend den Anforderungen des Anlagenmanagements, wobei Lebenszykluskostenbetrachtungen auf RAMS Analysen aufbauen. Erst die Beantwortung der Fragen nach Reliability (Haufigkeit und Art der Fehler), Availability (Verfugbarkeit unter Berucksichtigung der auftretenden Fehler und ihrer Instandhaltbarkeit), Maintainability (Optionen zur Instandsetzung) und Safety (Konsequenzen der Fehler) erlauben das Verhalten einer Anlage und die erforderlichen Instandsetzungsmasnahmen fur die gesamte Nutzungsdauer abzuschatzen und in Form der Lebenszykluskosten zu bewerten.

Freshly introduced a-screw dislocations in gallium nitride are an effective source of ultraviolet radiation, characterized by intense emission of narrow luminescence doublet lines in the spectral range of 3.1-3.2 eV. Furthermore, an additional narrow spectral line with an energy of 3.3 eV has been found at the points of intersection of such dislocations, where extended dislocation nodes were formed. In this communication, we report on the spectral properties of the characteristic luminescence of such nodes, which were obtained for the (0001) gallium nitride samples with dislocations introduced by nanoindentation. The spectral position of the dislocation-related luminescence doublet experiences a redshift with increasing distance from the indentation site. It follows the spectral shift of the excitonic near-bandgap emission, associated with stress relaxation. The luminescence of the intersection points exhibits a similar tendency. At certain local positions, its doublet fine structure is observed, which has a spectral linewidth of the order of or even less than that of the exciton. In this case, the spectral splitting between components of the doublet varies irregularly depending on the position of the exciton (i.e., on the mechanical stress). We see a clear indication of quantum dot-like emission. The fine structure of the luminescence of the intersection points can be easily explained by the energy dependence of emission on their size, as well as on their density, in particular, by the formation of paired nodes, which were previously observed in experiments in a transmission electron microscope.

We report on metalorganic vapor phase epitaxy (MOVPE) of distributed Bragg reflectors (DBR) applying a periodic modulation of the GaN doping concentration only. The doping modulation changes the refractive index of GaN via the Burstein-Moss-effect. MOVPE growth of highly doped GaN:Ge and modulation of the dopant concentration by at least two orders of magnitude within few nanometers is required to achieve a refractive index contrast of 2–3%. Such modulation characteristic is achieved despite the presence of Ge memory effects and incorporation delay. We realized DBRs with up to 100 layer pairs by combining GaN:Ge with a nominal doping concentration of 1.6×1020 cm−3 as low-refractive index material with unintentionally doped GaN as high-refractive index layer. Scanning transmission electron microscope images reveal DBR structures with abrupt interfaces and homogenous layer thicknesses in lateral and vertical direction. Reflectance measurements of DBRs designed for the blue and near UV-spectral region show a narrow stopband with a maximum reflectivity of 85% at 418 nm and even 95% at 370 nm. InGaN/GaN multi-quantum well structures grown on top of such DBRs exhibit narrow emission spectra with linewidths below 3 nm and significantly increased emission intensity.

A uniform array of gallium nitride core-shell microrod (MR) light-emitting diode (LED) structures was grown by metalorganic vapor phase epitaxy. Defects and the quantum well (QW) luminescence in an individual rod were investigated by scanning tunneling electron microscopy (STEM) and STEM cathodoluminescence. Luminescence with different wavelength was detected from the quantum wells on the semipolar tip facets and the nonpolar sidewalls of the MRs. Furthermore, the MR array is processed into LED chips. The electro-optical characteristics of the devices are analyzed. Two separate emission bands are distinguished, which are attributed to the QWs on the semipolar tip facets and the nonpolar sidewalls, respectively. To obtain white LEDs, micrograin phosphors were developed which fit in between individual MRs. By using electrophoretic particle deposition, these phosphors are deposited onto the MR LED chips. Color coordinates, color temperature, and device efficiency are evaluated. Blue (top) and phosphor-converted white (bottom) microrod LEDs on 4″ wafer.

We report the growth of green emitting InGaN quantum wells (QWs) by metal-organic vapor-phase epitaxy (MOVPE) on three-dimensional GaN templates. The {10 (1) over bar1} facets of GaN pyramids, fabricated by selective-area growth (SAG), reduce the influence of the quantum-confined Stark effect (QCSE) on the emission properties of the QW. The luminescence properties of a QW are a good indicator for the crystalline quality of the GaN layer beneath. Especially the presence of voids inside the pyramids, as well as stacking faults (SFs) and threading dislocations (TDs) in the wing region strongly influence the strain situation and the incorporation of In into the overlying InGaN layer. Thus, the crystal quality of the GaN pyramid has a strong influence on the efficiency and the emission properties of the active region. Therefore, a low-temperature nucleation on the GaN buffer in conjunction with a decreasing Ga-flux taking the decreasing c-plane growth surface of the pyramid into account was introduced. The low-temperature photoluminescence (PL) properties of the InGaN QW reveal the differences between the standard formation of the pyramid and this modified growth. Cathodoluminescence (CL) and transmission electron microscopy (TEM) measurements confirm the differences between the two growth modes in the crystal quality of the inner part of the pyramid. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Adequate railway track condition is a prerequisite for safe and reliable railway operation. Many track quality indices (TQIs) have been developed with the aim of assessing the track condition holistically. These indices combine measurement signals of some or all relevant geometry parameters with different mathematical models. In this paper, a selection of important TQIs is evaluated. Using measurement data of a five kilometer track section, the indices are calculated and their properties are discussed. This study reveals that all indices exhibit drawbacks to varying degrees. As a consequence, a new index has been developed&mdash, the track quality index of Graz University of Technology (TUG_TQI). Its favorable characteristics are presented by means of the above-mentioned test section. The TUG_TQI combines all relevant track geometry parameters, which are normalized beforehand to eliminate over or underrepresentation of different parameters. Thus, the index reliably describes the overall geometrical track quality.

AlN/GaN/AlN quantum disks (Q-disks) embedded in self-assembled hexagonally shaped GaN nanowires have been grown on the Si (111) substrate by plasma-assisted molecular beam epitaxy. To correlate the structural and optical properties of individual Q-disks inside the nanowire, highly spatially resolved cathodoluminescence (CL) spectroscopy in a scanning transmission electron microscope has been performed at 18 K. CL spectrum linescans along a single nanowire clearly identify the emission from the GaN base around 354 nm as well as two recombination channels at 347 nm and 330 nm directly correlated with the AlN/GaN/AlN Q-disk. A detailed characterization of these individual quantum objects is presented.

Investigating closely stacked GaN/AlN multiple quantum wells (MQWs) by means of cathodoluminescence spectroscopy directly performed in a scanning transmission electron microscope, we have reached an ultimate spatial resolution of σ CL = 1.8 nm . The pseudomorphically grown MQWs with high interface quality emit in the deep ultraviolet spectral range. Demonstrating the capability of resolving the 10.8 nm separated, ultra-thin quantum wells, a cathodoluminescence profile was taken across individual ones. Applying a diffusion model of excitons generated by a Gaussian-broadened electron probe, the spatial resolution of cathodoluminescence down to the free exciton Bohr radius scale has been determined.

Mapping individual radiative recombination channels at the nanoscale in direct correlation with the underlying crystal structure and composition of III–V semiconductor nanostructures requires unprecedented highly spatially resolved spectroscopy methods. Here, we report on a direct one-by-one correlation between the complex radial structure and the distinct carrier recombination channels of single GaAs-AlGaAs core-multishell nanowire heterostructures using low temperature cathodoluminescence spectroscopy directly performed in a scanning transmission electron microscope. Based on an optimized focused ion beam fabrication of the optically active specimen, we directly visualize the radial luminescence evolution and identify four distinct emission lines, i.e., the near band edge and defect luminescence of the GaAs core (819 nm, 837 nm), the emission of the single embedded GaAs quantum well (QW, 785 nm), and the AlGaAs shell luminescence correlated with alloy fluctuations (650–674 nm). The detailed radial luminescence profiles are anticorrelated between QW luminescence and core emission, illustrating the radial carrier transport of the core-shell system. We inspected in detail the low-temperature capture of excess carriers in the quantum well and barriers.

We systematically studied the desorption induced GaN/AlN quantum dot formation using cathodoluminescence spectroscopy directly performed in a scanning transmission electron microscope (STEM). The GaN films were grown by metal organic vapor phase epitaxy (MOVPE) on top of an AlN/sapphire-template. After the deposition of a few monolayers GaN at 960°C a growth interruption (GRI) without ammonia supply was applied to allow for quantum dot formation. A sample series with GRI durations from 0 s to 60 s was prepared to analyze the temporal evolution systematically. Each quantum dot (QD) structure was capped with AlN grown at 1195°C. Without GRI the cross-sectional STEM images of the reference sample reveal a continuous GaN layer with additional hexagonally-shaped truncated pyramids of 20 nm height and ~100 nm lateral diameter covering dislocation bundles. Spatially averaged spectra exhibit a broad emission band between 260 nm and 310 nm corresponding to the continuous GaN layer. The truncated pyramids exhibit only drastically reduced CL intensity in panchromatic images. Growth interruption leads to desorption of GaN resulting in smaller islands without definite form located in close vicinity to threading dislocations. Now the emission band of the continuous GaN layer is shifted to shorter wavelengths indicating a reduction of GaN layer thickness. By applying 30 s GRI these islands exhibit quantum dot emission in the spectral range from 220 nm to 310 nm with ultra narrow line widths. For longer growth interruptions the QD ensemble luminescence is shifted to lower wavelengths accompanied by intensity reduction indicating a reduced QD density.

The optical and crystalline properties of a c-plane GaN-based LED structure with embedded semipolar InGaN quantum wells (QW) were investigated using highly spatially resolved cathodoluminescence spectroscopy (CL) directly performed in a scanning transmission electron microscope (STEM). Direct correlation of the cross-sectional STEM image with the simultaneously recorded spatially resolved CL mapping at room-temperature reveals the most intense emission coming from the semipolar InGaN QWs. We observe an inhomogeneous wavelength distribution due to local indium fluctuations and varying QW thickness. In contrast, the donor-acceptor pair recombination (DAP) becomes the dominating luminescence process at 16 K resulting in a superposition of the DAP luminescence and the InGaN QW emission.

In GaN, the basal plane stacking fault type I1 is a two-dimensional defect characterized by a cubic inclusion within the wurtzite structure. Excitons are bound at the BSF I1 similar to the localization in a quantum well heterostructure leading to an efficient radiative recombination. In this study, we present the optical and structural properties of basal plane stacking faults occurrent in silicon doped a-plane GaN layer by means of highly spatially resolved cathodoluminescence spectroscopy performed in a scanning transmission electron microscope. Drastically reduced panchromatic intensity in the vicinity of partial dislocations terminating the stacking faults, points to their non-radiative character. Originating from intersection of two-dimensional defects, the emission at 379.6 nm could be attributed to optically active stair-rod dislocations.

GaN based laser diodes with semipolar quantum wells are typically grown on free-standing pseudo-substrates of small size. We present an approach to create a distributed-feedback (DFB) laser with semipolar quantum wells (QWs) on c-oriented templates. The templates are based on 2-inch sapphire wafers, the method could easily be adapted to larger diameters which are available commercially. GaN nanostripes with triangular cross-section are grown by selective area epitaxy (SAE) and QWs are grown on their semipolar side facets. The nanostripes are completely embedded and can be sandwiched inside a waveguide. For optical pumping, open waveguide structures with only a bottom cladding are used. Using nanoimprint lithography, stripe masks with 250 nm periodicity were fabricated over the whole wafer area. The periodicity corresponds to a 3rd order DFB structure for a laser emitting in the blue wavelength regime. These samples were analyzed structurally by high-resolution transmission electron microscopy (HRTEM), and spatio-spectrally by cathodoluminescence (CL) inside a scanning transmission electron microscope (STEM). Samples with an undoped cap are pumped optically for stimulated emission. To prove the feasibility of realizing a 2nd order DFB structure with this approach, stripes with a 170 nm periodicity are fabricated by electron beam lithography and SAE.

We report on lattice-matched AlInN/GaN distributed Bragg reflectors (DBRs) with improved overgrowth of the AlInN layers. Typically, AlInN layers are exposed to high temperatures when changing to GaN growth conditions. Uncapped AlInN surfaces suffer from In desorption leading to formation of 2 nm thick AlN interface layers with micro-cracks at the AlInN surface. Capping the AlInN with 5 nm GaN at the same temperature and subsequent overgrowth with GaN at high temperatures resolves the In-desorption problem and DBRs with improved interface quality and smooth surfaces are achieved. Optical properties of high-reflectivity DBR structures such as maximum reflectivity and bandwidth are virtually unaffected whether or not unintentionally-grown AlN interlayers are present.

We report the influence of {111} stacking faults on the cathodoluminescence (CL) emission characteristics of cubic GaN (c-GaN) films and cubic GaN/AlN multi-quantum wells. Transmission electron microscopy (TEM) measurements indicate that stacking faults (SFs) on the {111} planes are the predominant crystallographic defects in epitaxial films, which were grown on 3C-SiC/Si (001) substrates by plasma-assisted molecular beam epitaxy. The correlation of the SFs and the luminescence output is evidenced with a CL setup integrated in a scanning TEM (STEM). By comparing the STEM images and the simultaneously measured CL signals it is demonstrated that SFs in these films lead to a reduced CL emission intensity. Furthermore, the CL emission intensity is shown to increase with increasing film thickness and decreasing SF density. This correlation can be connected to the reduction of the full width at half maximum of X-ray diffraction rocking curves with increasing film thickness of c-GaN films. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Despite the strong interest in optoelectronic devices working in the deep ultraviolet range, no suitable low cost, large-area, high-quality AlN substrates have been available up to now. The aim of this work is the selective area growth of AlN nanocolumns by plasma assisted molecular beam epitaxy on polar (0001) and semi-polar (11-22) GaN/sapphire templates. The resulting AlN nanocolumns are vertically oriented with semi-polar {1-103} top facets when grown on (0001) GaN/sapphire, or oriented at 58° from the template normal and exposing {1-100} non-polar top facets when growing on (11-22) GaN/sapphire, in both cases reaching filling factors ≥80%. In these kinds of arrays each nanostructure could function as a building block for an individual nano-device or, due to the large filling factor values, the overall array top surfaces could be seen as a quasi (semi-polar or non-polar) AlN pseudo-template.

We present a nanometer-scale correlation of the structural, optical, and electronic properties of InGaN/GaN core-shell microrod LEDs: The microrods were fabricated by MOVPE on a GaN/sapphire template covered with an SiO2-mask. Through the mask openings, Si-doped n-GaN cores were grown with high SiH4 flow rate at the base. Subsequently, the SiH4 flow rate was reduced towards the microrod tip to maintain a high surface quality. The Si-doped GaN core was finally encased by an InGaN single quantum well (SQW) followed by an intrinsic GaN layer and a thick Mg-doped p-GaN shell. Highly spatially resolved cathodoluminescence (CL) directly performed in a scanning transmission electron microscope (STEM) was applied to analyze the free-carrier concentration within the Si-doped GaN core and the luminescence properties of the individual functional layers. The CL was supported by Raman spectroscopy directly carried out at the same microrod on the thin TEM-lamella. The cross-sectional CL of a single microrod resolves the emission of the single layers. CL and Raman measurements reveal a high free-carrier concentration of 7x1019 cm 3 in the bottom part and a decreasing doping level towards the tip of the microrod. Moreover, structural investigations exhibit that initial Si-doping of the core has a strong influence on the formation of extended defects in the overgrown shells. However, we observe the most intense emission coming from the InGaN QW on the non-polar side walls, which shows a strong red shift along the facet in growth direction due to an increased QW thickness accompanied by an increased indium concentration right at the intersection of generated defects and InGaN QW, a red shifted emission appears, which indicates indium clustering.

This work reports on the growth and characterization of InGaN/GaN core–shell structures by plasma-assisted molecular beam epitaxy on an ordered array of top-down patterned GaN microrods fabricated on a GaN/sapphire substrate. Growth of InGaN/GaN is conformal, with axial and radial growth components leading to core–shell structures with clear hexagonal facets. The radial InGaN growth (shell) was confirmed by spatially resolved cathodoluminescence performed using scanning electron microscopy as well as scanning transmission electron microscopy. The In content of the InGaN shell is controlled by means of the growth temperature and the III/V ratio.

Using cathodoluminescence spectroscopy directly performed in a scanning transmission electron microscope at liquid helium temperature, the structural and optical properties of GaN nanocolumns, in particular extended defects like stacking faults and dislocations, have been characterized. The influence of the crystalline real structure on the emission properties using the capability of addressing individual stacking faults is comprehensively examined.

Using cathodoluminescence spectroscopy directly performed in a scanning transmission electron microscope at liquid helium temperatures, the optical and structural properties of a 62 InGaN/GaN multiple quantum well embedded in an AlInN/GaN based microcavity are investigated at the nanometer scale. We are able to spatially resolve a spectral redshift between the individual quantum wells towards the surface. Cathodoluminescence spectral linescans allow directly visualizing the critical layer thickness in the quantum well stack resulting in the onset of plastic relaxation of the strained InGaN/GaN system. (C) 2014 AIP Publishing LLC.

An In0.7Ga0.3N layer with a thickness of 300 nm deposited on GaN/sapphire template by molecular beam epitaxy has been investigated by highly spatially resolved cathodoluminescence (CL). High crystal film quality without phase separation has been achieved. The InGaN layer shows intense emission in the IR spectral region. The lateral as well as the vertical luminescence distribution is used to probe the In composition ([In]) homogeneity: the thick InGaN film exhibits laterally a rather homogeneous emission intensity at 1.04 eV (∼1185 nm) with a FWHM of only 63 meV. Carrier localization into regions of enhanced In concentration originating from compositional fluctuations is revealed. The evolution of emission in growth direction has been explored by a cross-sectional CL linescan showing a slight spectral redshift from the bottom to the surface of the InGaN film corresponding to an increase of [In] of only 0.5% within the layer thickness of 300 nm.

Nitride-based three-dimensional core-shell nanorods (NRs) are promising candidates for the achievement of highly efficient optoelectronic devices. For a detailed understanding of the complex core-shell layer structure of InGaN/GaN NRs, a systematic determination and correlation of the structural, compositional, and optical properties on a nanometer-scale is essential. In particular, the combination of low-temperature cathodoluminescence (CL) spectroscopy directly performed in a scanning transmission electron microscope (STEM), and quantitative high-angle annular dark field imaging enables a comprehensive study of the nanoscopic attributes of the individual shell layers. The investigated InGaN/GaN core-shell NRs, which were grown by metal-organic vapor-phase epitaxy using selective-area growth exhibit an exceptionally low density of extended defects. Using highly spatially resolved CL mapping of single NRs performed in cross-section, we give a direct insight into the optical properties of the individual core-shell layers. Most interesting, we observe a red shift of the InGaN single quantum well from 410 to 471 nm along the nonpolar side wall. Quantitative STEM analysis of the active region reveals an increasing thickness of the single quantum well (SQW) from 6 to 13 nm, accompanied by a slight increase of the indium concentration along the nonpolar side wall from 11% to 13%. Both effects, the increased quantum-well thickness and the higher indium incorporation, are responsible for the observed energetic shift of the InGaN SQW luminescence. Furthermore, compositional mappings of the InGaN quantum well reveal the formation of locally indium rich regions with several nanometers in size, leading to potential fluctuations in the InGaN SQW energy landscape. This is directly evidenced by nanometer-scale resolved CL mappings that show strong localization effects of the excitonic SQW emission.

In this contribution we present our recently developed “reverse-reaction-growth” scheme in molecular beam epitaxy to fabricate 1D GaAs nanowires (NWs) with diameters down to 7 nm. We observe the presence of strong quantum confinement phenomena, opening the path towards a true 1D NW platform. The introduction of crystal defects in the NW effectively leads to bright and spectrally sharp crystal phase quantum dots (CPQDs). By directly correlated transmission electron microscopy, cathodoluminescence, and photoluminescence spectroscopy, we examine the microscopic nature and properties of these CPQDs. We identify individual twin defects (single monolayer inclusions of wurtzite in a zincblende crystal) as the predominant source for CPQD emission.

Propenoxid ist ein wichtiges Zwischenprodukt der chemischen Industrie. Es wird unter anderem zur Herstellung von verschiedenen Kunststoffen benotigt. Da die aktuell eingesetzten Verfahren zur Produktion von Propenoxid entweder grose Mengen unerwunschter Nebenprodukte erzeugen oder in Bezug auf die eingesetzten Ausgangsstoffe sehr kostspielig sind, ist es aus okonomischer und okologischer Sicht wunschenswert, Propen auf direktem Weg durch Luftsauerstoff zu epoxidieren. Wie gezeigt werden konnte, ist dies mit getragerten Goldkatalysatoren moglich. Dieser Beitrag geht der Frage nach, inwiefern titansubstituierte Alumophosphate als Trager fur solche Katalysatoren geeignet sind.

We report on metal organic vapor phase epitaxy of semi-polar growth of nearly (106) oriented GaN films on Si(112) and (105) and (104) GaN on Si(113). We analyze the GaN crystallites by field emission-scanning electron microscopy (FE-SEM), scanning transmission electron microscopy (STEM), photoluminescence (PL), and cathodoluminescence (CL). A correlation between optical properties and microstructure is presented. Our studies reveal a significant reduction of basal plane stacking faults (BSFs) in semi-polar GaN grown on planar Si(112) by applying a low temperature (LT) AlN interlayer. We find that the insertion of the LT-AlN interlayer can eliminate the stacking faults in the upper GaN layer, when the LT-AlN interlayer is inserted on a smooth GaN buffer. The LT-AlN interlayer results in lattice relaxation due to misfit dislocation formation at the GaN/LT–AlN interface. In comparison, GaN grown on Si(113) with same growth conditions and a rough GaN surface does not show any BSF reduction while it is reduced for a smooth GaN layer (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)