Structural Analysis of Low Defect Ammonothermally Grown GaN Wafers by Borrmann Effect X-ray Topography

X-ray topography defect analysis of entire 1.8-inch GaN substrates, using the Borrmann effect, is presented in this paper. The GaN wafers were grown by the ammonothermal method. Borrmann effect topography of anomalous transmission could be applied due to the low defect density of the substrates. It was possible to trace the process and growth history of the GaN crystals in detail from their defect pattern imaged. Microscopic defects such as threading dislocations, but also macroscopic defects, for example dislocation clusters due to preparation insufficiency, traces of facet formation, growth bands, dislocation walls and dislocation bundles, were detected. Influences of seed crystal preparation and process parameters of crystal growth on the formation of the defects are discussed.

Applications of Continuous Snowpack Temperature Monitoring

Predicting metamorphism within seasonal snowpacks is critical for avalanche forecasting and runoff timing as it relates to water supply management. Snowpack temperature gradients play a key role in snow metamorphism, and their magnitude controls how snow strength changes; therefore, they are of interest to avalanche forecasters. Before major melt, the snowpack must warm to isothermal conditions at 0°C. Measuring this transition from warming to the ripening phase could help improve our current models for runoff timing. Measuring snowpack temperature gradients is currently a non-automated process that requires disturbance of the snow profile, and only gives a snapshot in time of the temperature conditions. Here we demonstrate an automated method to monitor in situ snowpack temperature using a thermocouple array, co-located with the Banner Summit SNOTEL site in central Idaho. Showing the location and duration of critical temperature gradients helps avalanche forecasters detect warning signs related to possible facet formation. During the 2019 winter, we observed large temperature gradients in the bottom 20cm of the snowpack, with the gradient falling below critical (< 0.1°C/cm) by early January. Critical gradients were observed near the surface throughout the winter, and temperatures were within ±0.06°C of the melting point when the snowpack became isothermal in the spring. We anticipate this dataset will inform snowpack energy balance models and aid in the prediction of avalanche hazards and runoff timing.

Cellular Automaton Modeling of Silicon Facet Formation during Directional Solidification

Silicon facet formation during directional solidification is simulated by cellular automaton (CA) modeling in which anisotropic interfacial energy and kinetics are considered. Numerical simulations were performed with different anisotropy strengths of interfacial energy and they show good agreement with analytical equilibrium shapes obtained by the Gibbs-Thomson equation. We also compare our results of anisotropic kinetics with in situ observation experiments and the results of the phase model to verify the accuracy of our model. Simulation results of facet formation show that perturbation is promoted to the corner by the negative temperature gradient of the interface and the heat accumulation location leads to the disappearance of small corners.