Comparison of Achievable Contrast Features in Computed Tomography Observing the Growth of a 4H-SiC Bulk Crystal

Abstract

Today the physical vapor transport process is regularly applied for the growth of bulk SiC crystals. Due to the required high temperature of up to 2400 °C, and low gas pressure of several Mbar inside the crucible, the systems are encapsulated by several layers for heating, cooling and isolation inhibiting the operator from observing the growth. Also, the crucible itself is fully encapsulated to avoid impurities from being inserted into the crystal or disturbing the temperature field distribution. Thus, once the crucible has been set up with SiC powder and the seed crystal, the visible access to the progress of growth is limited. In the past, X-ray radiography has allowed this limitation to be overcome by placing the crucible in between an X-ray source and a radiographic film. Recently these two-dimensional attenuation signals have been extended to three-dimensional density distribution by the technique of computed tomography (CT). Beside the classic X-ray attenuation signal dominated by photoelectric effect, Compton effect and Rayleigh scattering, X-ray diffraction resulting in the crystalline structure of the 4H-SiC superimposes the reconstructed result. In this contribution, the achievable material contrast related to the level of X-ray energy and the absorption effects is analyzed using different CT systems with energies from 125 kV to 9 MeV. Furthermore the X-ray diffraction influence is shown by the comparison between the advanced helical-CT method and the classical 3D-CT.

High-Energy Computed Tomography as a Prospective Tool for In Situ Monitoring of Mass Transfer Processes inside High-Pressure Reactors—A Case Study on Ammonothermal Bulk Crystal Growth of Nitrides including GaN

“…A third aspect to consider, especially upon scale-up, is the role of Bragg diffraction, which will be caused by the presence of large single crystals. Such effects were observed in the PVT growth of 3” SiC bulk crystals [ 17 ]. Depending on the energy of the diffracted radiation, it will be blocked by the autoclave walls and furnace to a smaller or larger extent.…”

“…Compared to a PVT growth of SiC [ 18 ], the simultaneous growth of many crystals with the ammonothermal method adds to the complexity of estimating the scalability of in situ monitoring via CT. In the example of SiC PVT growth, the information inside the crystals is lost eventually, though the outer dimensions can still be visualized (in analogy to a shadowgraph) [ 17 ]. In the case of the ammonothermal method, however, it would be particularly desirable to achieve total transmission because this would prevent seeds from blocking the X-rays from reaching inner seeds and exiting the setup thereafter.…”

“…In the field of crystal growth, this technique has already successfully been applied for evaluating the growth kinetics of SiC during bulk growth via the physical vapor transport (PVT) method [ 16 ]. For PVT growth of SiC, the applicability of computed tomography has been estimated to be practical for 6” or even 8” in tracking the movement of the growing crystal surface, though information on density variations inside the crystals (such as voids) is not obtainable at X-ray energies convenient for shielding [ 17 ]. Due to the somewhat high acceleration voltages and the continuous X-ray exposure in the order of minutes, the main application space of computed tomography in SiC is expected to be in research and development rather than in-line process monitoring [ 18 ].…”

Influence analysis of complex crack geometric parameters on mechanical properties of soft rock

“…When X-rays emitted from an emission source pass through an object, the intensity of the X-rays is attenuated to some extent; the attenuation is influenced by the thickness and internal structure of the specimen. The attenuation characteristics follow the Lambert-Beer law, which is expressed as follows (Salamon et al 2019;Zhou et al 2016):…”