In this work we investigate the epitaxial growth of boron containing (Al,Ga)N layers and superlattices for applications in the active region of UV‐LEDs. For AlBN layers containing 5% of boron as ...quantified by secondary ion mass spectrometry, columnar growth has been observed. Transmission electron microscopy (TEM) studies revealed B‐lean, crystalline columns, separated by highly boron containing amorphous intermediate regions. However, in the first few nanometers of AlBN growth, no evidence of columnar growth can be found. Indeed, first layers of AlBGaN/AlN superlattice stacks do not show any sign of columnar growth. However, for increasing thickness, 3D‐like growth develops through formation of pyramidal structures. TEM‐based selective area electron diffraction analysis does not show evidence of lattice tilting, proving that the observed inclined facets are not a result of lattice twinning. Thin AlBGaN layers with different thickness containing 1% boron show similar photoluminescence (PL) spectra with slightly reduced intensity as compared to similar B‐free AlGaN layers. Although small grainy structures develop and increase in density for AlBGaN layers with a thickness of 10 nm and more, the PL intensity increases steadily with layer thickness, showing that these grains do not drastically deteriorate their luminescence properties.
In this article, Rettig et al. focus on the development of 3D growth in boron containing AlGaN layers. Thin layers of this material grow smoothly in 2D mode, as evidenced by a flat interface of an AlBGaN/AlN superlattice structure. However, for thicker layers tilted facets occur, resulting in inclined interfaces of the AlBGaN/AlN superlattice.
The 79 North Glacier (79NG) features Greenland's largest floating ice tongue. Even though its extent has not changed significantly in recent years, observations have indicated a major thinning of the ...ice tongue from below. Both ocean warming and an increase in subglacial discharge from the ice sheet induced by atmospheric warming could increase the basal melt; however, available observations alone cannot tell which of these is the main driver. Here, we employ a global simulation which explicitly resolves the ocean circulation in the cavity with 700 m resolution to disentangle the impact of the ocean and atmosphere. We find that the interannual variability of basal melt below 79NG over the past 50 years is mainly associated with changes in the temperature of the Atlantic Intermediate Water inflow, which can be traced back across the Northeast Greenland continental shelf to the eastern Fram Strait with a lag of 3 years.
•Etched trenches in GaN overgrown with flat layer.•Best overgrowth conditions found by varying pressure, growth rate, V-III ratio and temperature.•Fair electrical results of HEMTs grown on regrown ...layer.
In this work we present studies how to fill epitaxially trenches in a GaN layer in order to form a current aperture for use in 3D high power field effect transistors. By optimising various growth parameters in our metal organic vapour phase epitaxy, we achieved an almost complete and smooth filling of 5 μm wide and 500 nm deep trenches etched into a GaN:Mg layer by reactive-ion etching. By using GaN:Si marker layers, the filling process could be excellently monitored. Eventually, a high electron mobility transistor structure was deposited on top of the optimized structures enabling an electrical assessment of this process.
On the Damping Time Scale of EVP Sea Ice Dynamics Danilov, Sergey; Koldunov, Nikolay V.; Sidorenko, Dmitry ...
Journal of advances in modeling earth systems,
October 2021, 2021-10-00, 20211001, 2021-10-01, Letnik:
13, Številka:
10
Journal Article
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We propose to make the damping time scale, which governs the decay of pseudo‐elastic waves in the Elastic Viscous Plastic (EVP) sea‐ice solvers, independent of the external time step and large enough ...to warrant numerical stability for a moderate number of internal time steps. A necessary condition is that the forcing on sea ice varies slowly on the damping time scale, in which case an EVP solution may still approach a Viscous Plastic one, but on a time scale longer than a single external time step. In this case, the EVP method becomes very close to the recently proposed modified EVP (mEVP) method in terms of stability and simulated behavior. In a simple test case dealing with sea ice breaking under the forcing of a moving cyclone, the EVP method with an enlarged damping time scale can simulate linear kinematic features which are very similar to those from the traditional EVP implementation, although a much smaller number of internal time steps is used. There is more difference in sea‐ice thickness and linear kinematic features simulated in a realistic Arctic configuration between using the traditional and our suggested choices of EVP damping time scales, but it is minor considering model uncertainties associated with choices of many other parameters in sea‐ice models.
Plain Language Summary
Numerical simulations of sea ice in ocean or climate models most frequently rely on the Elastic Viscous Plastic (EVP) method. Computational expenses of this method depend on its internal time step which is limited by the requirements of numerical stability. It is shown that some adjustment of dissipation time scale in the EVP method allows the time scale to be increased, thereby reducing the numerical costs without sacrificing the results of simulations.
Key Points
Elastic Viscous Plastic (EVP) stability can be improved by proper selection of the time scale governing the decay of elastic waves
A significant reduction of the required number of EVP sub‐cycles can be achieved on high‐resolution meshes
EVP and the modified EVP simulate very similar sea‐ice thickness and concentration
The non‐local model of mixing based on internal wave breaking, IDEMIX, is implemented as an enhancement of a turbulent kinetic energy closure model in three non‐eddy resolving general circulation ...ocean models that differ in the discretization and choice of computational grids. In IDEMIX internal wave energy is generated by an energy flux resulting from near‐inertial waves induced by wind forcing at the surface, and at the bottom, by an energy flux that parameterizes the transfer of energy between baroclinic and barotropic tides. In all model simulations with IDEMIX, the mixing work is increased compared to the reference solutions without IDEMIX, reaching values in better agreement with finestructure observations. Furthermore, the horizontal structure of the mixing work is more realistic as a consequence of the heterogeneous forcing functions. All models with IDEMIX simulate deeper thermocline depths related to stronger shallow overturning cells in the Indo‐Pacific. In the North Atlantic, deeper mixed layers in simulations with IDEMIX are associated with an increased Atlantic overturning circulation and an increase of northward heat transports toward more realistic values. The response of the deep Indo‐Pacific overturning circulation and the weak bottom cell of the Atlantic to the inclusion of IDEMIX is incoherent between the models, suggesting that additional unidentified processes and numerical mixing may confound the analysis. Applying different tidal forcing functions leads to simulation differences that are small compared to differences between the different models or between simulations with IDEMIX and without IDEMIX.
Plain Language Summary
Waves in the ocean interior play a fundamental role for ocean dynamics since they can carry energy over long distances and, once they break, lead to turbulent mixing. This turbulent mixing can cause dense water masses to rise from the deep ocean with a direct impact on large‐scale currents. The wave dynamics occur on spatial scales that cannot be resolved in global ocean or climate models. To account for these processes, we apply the new parameterization IDEMIX that describes internal wave generation, propagation, and mixing. Using three different ocean models with and without IDEMIX ensures that we can identify model‐specific effects of the parameterization and discriminate them from those independent of the model. We find that the simulated mixing patterns agree better with observations once IDEMIX is applied. Large‐scale currents and the vertical temperature distribution are substantially affected by the internal wave parameterization. Whether this leads to an improved agreement with observed currents and water mass properties depends on the specific model and on numerical effects. In most cases, simulations with IDEMIX are not very sensitive to details of how the internal wave model is driven by tidal energy input.
Key Points
the IDEMIX closure for a consistent representation of internal wave‐induced mixing is evaluated in three state‐of‐the‐art ocean models
only in simulations with IDEMIX can the models reproduce the magnitude and spatial variability of the observed mixing work
most changes with IDEMIX can be attributed to stronger mixing, but some effects are confounded by other processes and numerical mixing
Sea ice dynamics determine the drift and deformation of sea ice. Nonlinear physics, usually expressed in a viscous‐plastic rheology, makes the sea ice momentum equations notoriously difficult to ...solve. At increasing sea ice model resolution the nonlinearities become stronger as linear kinematic features (leads) appear in the solutions. Even the standard elastic‐viscous‐plastic (EVP) solver for sea ice dynamics, which was introduced for computational efficiency, becomes computationally very expensive, when accurate solutions are required, because the numerical stability requires very short, and hence more, subcycling time steps at high resolution. Simple modifications to the EVP solver have been shown to remove the influence of the number of subcycles on the numerical stability. At low resolution appropriate solutions can be obtained with only partial convergence based on a significantly reduced number of subcycles as long as the numerical procedure is kept stable. This previous result is extended to high resolution where linear kinematic features start to appear. The computational cost can be strongly reduced in Arctic Ocean simulations with a grid spacing of 4.5 km by using modified and adaptive EVP versions because fewer subcycles are required to simulate sea ice fields with the same characteristics as with the standard EVP.
Key Points
We explore the performance of new options for EVP solvers at high resolution (4.5 km)
A significant reduction of the required number of EVP subcycles leads to a sixfold speedup of the sea ice dynamics
This speedup does not lead to a deterioration of the simulated sea ice
The Atlantic meridional overturning circulation (AMOC) is one of the most important characteristics of an ocean model run. Using the depth (z) and density frameworks, we analyze how the sinking and ...diapycnal transformations defining the AMOC as well as AMOC strength and variability react to mesh refinement from low to higher resolution in two model runs driven by the CORE‐II forcing. Both runs can represent the key locations of sinking and diapycnal transformations behind AMOC, that is, northeastern North Atlantic. Although their spatial patterns do not change significantly with resolution in both frameworks as the consequence of the same atmospheric forcing, the quantitative differences, reaching several sverdrups, are seen in different locations between two model runs for both frameworks. In particular, the refinement leads to the strongest differences in the vertical transport and diapycnal transformations in the latitude range between 30°N and 55°N. The z framework emphasizes the role of localized upwelling around the Gulf Stream separation site, whereas the density framework emphasizes the contribution of (spurious) diapycnal mixing around the Grand Banks. Both effects are reduced in the higher‐resolution run, leading to higher AMOC south of 26°N as compared to the low‐resolution run, despite the AMOC maxima, located at high latitudes, are higher in the low‐resolution run. We suggest that both AMOC frameworks should be used routinely in standard analyses, including forthcoming intercomparison projects.
Plain Language Summary
In various international programs such as the Climate Model Intercomparison Project (CMIP), climate models are used to assess the past, present, and future climate. The Atlantic meridional overturning circulation (AMOC) is one of the most important characteristics of an ocean model simulation. Commonly, it is computed as a stream function of zonally averaged flow along the constant depth (z‐AMOC). However, there are shortcomings related to the inclination of density surfaces in reality, which may lead to the appearance of artificial circulation cells. In order to eliminate these artifacts, it is essential to compute the AMOC along constant density surfaces (ϱ‐AMOC). That is why recent studies underlined the importance of the ϱ framework for the AMOC analysis. However, neither the CMIP data nor the native output of most of the ocean circulation models is sufficient for the straightforward computation of ϱ‐AMOC. Hence, ϱ‐AMOC remains important but rarely computed diagnostics. In this paper we analyze the fundamental differences between both representations of AMOC in order to better understand the role of the spatial resolution of numerical models in representing AMOC formation, strength, and variability. We suggest that the ϱ‐AMOC and water mass transformation framework should be used routinely in standard analyses, including forthcoming intercomparison projects.
Key Points
AMOC in depth and density coordinates manifests same physics differently
Stronger interior transformation at low model resolution causes the difference in AMOC to high resolution
FESOM, a representative of ocean climate models, is able to adequately simulate the locations of sinking and diapycnal transformations behind AMOC
A stack of five Al(Ga)N-based quantum wells is investigated by combined laterally and depth resolved cathodoluminescence (CL) spectroscopy in order to distinguish lateral and vertical inhomogeneities ...of these wells. Transmission electron microscopy (TEM) micrographs provide data for the real sample structure, which enters into the Monte-Carlo simulation of the depth-resolved CL measurements to refine the depth resolution. The comparison of these CL measurements to the results of electron energy loss spectra (EELS) allows to identify local thickness variations of the lower three quantum wells to be the origin of two different luminescence contributions to the overall spectrum. The differentiation of the two groups of quantum wells by depth-resolved CL is demonstrated.
Using a new high-temperature chemical vapor deposition (CVD)-based growth process for high-quality zinc oxide (ZnO) layers, the initial stages for heteroepitaxial growth on sapphire substrates with ...an aluminum nitride nucleation layer was investigated. A series of samples were grown with various supplies of zinc vapor, which can easily be controlled by the amount of the precursor gas methane (CH4) used to reduce the ZnO powder. In the substrate region, the zinc vapor was reoxidized by pure oxygen, which initially led to the formation of ZnO islands on the substrate, and for longer growth duration to the desired highly crystalline ZnO layers. To determine the details about this initial layer formation process, atomic force microscopy and scanning electron microscopy were used. We find that the ZnO microcrystals coalesce very fast and form a smooth and closed layer after a growth time of 10 min only. Electron backscatter diffraction measurements on this early stage of the layer formation show a perfect c-orientation of the ZnO microcrystals. Also high-resolution X-ray diffraction measurements support the perfect alignment of the ZnO layer and show a drastic increase in crystal quality over growth time. This increase in crystal quality is also demonstrated by low-temperature photoluminescence (PL) measurements, in which the spectra show sharp lines for the donor-bound excitons already for the ZnO microcrystals. The PL spectra also show clearly that the tendency of forming basal plane stacking faults is quite high when the ZnO microcrystals are starting to coalesce but anneal out very fast after coalescence.
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