Papers by Keyword: X-Ray Topography

Abstract: The structural changes of a type of polydiacetylene single crystal obtained via the physical vapor transport technique induced by the formation of polymeric backbone chains upon ultraviolet irradiation are discussed in this paper. On the basis of the results of X-ray diffractometry, X-ray topography, and atomic force microscopy, the structural changes were confirmed to be due to the formation of backbone chains along the basal (010) plane. The high periodicity between equivalent (010) planes is maintained regardless of the formation of backbone chains. The observation of the surface morphology on the basal (010) plane revealed that many polymeric bundles were formed in both the [001] and [101] directions.

Abstract: Synchrotron monochromatic beam X-ray topography (SMBXT), synchrotron white beam X-ray topography (SWBXT) and high-resolution X-ray topography (HRXRT) were used to characterize a series of wafers sliced from two PVT-grown 4H-SiC boules under similar growth conditions. A unique spoke-shaped distribution of the threading screw/mixed dislocations (TSDs/TMDs) density map can be observed from wafers sliced from later stages of growth of both boules. Systematic sequential analysis of the SMBXT grazing incidence images and HRXRT reflection images of the wafers reveals the spoke patterns are formed due to continuous deflection process of TSDs/TMDs by thin layer of polytypes that propagate along step flow direction and expand vertically, leading to TSD density difference across the wafer. Regions with high TSD densities have higher growth rate, resulting in a ridge and valley structure. Generation of macrosteps in the valley regions due to regular step structure deflect more TSDs/TMDs that then form mixed type (Shockley+Frank) stacking faults.

Abstract: Silicon carbide is a leading wide-bandgap semiconductor for high-voltage power electronics. For 6.5–10 kV operation, thick epitaxial layers (≥60 µm) are required to sustain depletion width and maintain uniform electric fields, placing a premium on low extended-defect densities in both substrate and epilayer. Thick epitaxial 4H-SiC layers of 60 µm and 110 µm were grown on 6-inch substrates in a multi-wafer warm-wall reactor and evaluated by synchrotron X-ray topography in grazing-incidence (22-4 16) and transmission (11-20) geometries. Transmission imaging showed substrate dislocation content near the lower bound typically reported for 6-inch wafers. Notably, grazing-incidence topography (penetration depth >40 µm) revealed no basal-plane dislocations propagating into the epilayers, consistent with efficient dislocation conversion at the substrate–epilayer interface. The 3C-SiC inclusion density was ~30 per 6-inch wafer for 60 µm epilayers and ~60 per wafer for 110 µm epilayers; the average micropipes density varies from 0 to 5 for both 60 and 110 um epiwafers. Threading dislocation densities—screw, edge, and mixed—were on the order of 1.0–2.0 × 10³ cm⁻². These results establish thick 4H-SiC epilayers with suppressed basal-plane propagation and substantially reduced extended-defect content, providing a strong basis for reliable 6.5–10 kV device fabrication.

Abstract: Silicon carbide (SiC) is valued for high-power and high-frequency devices, but its performance is limited by crystalline defects. We report a newly observed defect arrangement, termed the “galaxy” defect, in wafers from a PVT-grown 6-inch 4° off-axis boule. Optical microscopy revealed dense clusters of micron-sized inclusions, while synchrotron X-ray topography (XRT) showed associated dislocation networks. Transmission synchrotron XRT indicated threading dislocation clusters, and grazing images revealed high densities of basal plane dislocations, deflected Frank partials, and threading-edge-dislocation low-angle grain boundaries (TED-LAGBs). The defect evolved as growth progressed, producing increasingly complex dislocation structures. Based on the observation, we proposed a mechanism for the evolution of the defect involving the generation, evolution, and interaction between the inclusions and dislocations.

Abstract: Micropipe defects in silicon carbide (SiC) materials significantly degrade the performance of SiC materials and their applications in semiconductor devices. In this study, systematic methods were utilized to characterize different micropipes in 4H-SiC. X-ray topography was employed to investigate the morphology of micropipe defects in SiC substrates and quantify their associated lattice distortion fields. Meanwhile, white light interferometry mode microscopy and inner stain were utilized to thoroughly characterize their properties. It was found that micropipes were accompanied with different size and distortion areas in SiC substrate. This work will be served as a refined characterization of micropipes and give guidance for device application for SiC substrate.

Abstract: Results from optical defect inspections, and X-ray topography, on wafers from entire 4H-SiC ingots provide a clear visualization on the positional dependance of bulk inclusions in ingots with respect to growth stages, looking to both density and size. It is also clear while studying the superpositioning of Laue–Bragg interference densities that the different categories of said defectivity generate new crystallographic defects, dislocations. These in turn lead to significant reductions in usability of wafers, and the lack of tracing such defects, cause an increased difficulty to predict the final device yield, as is displayed by growing epitaxial layers on materials heavily affected by bulk inclusions.

Abstract: Several 1.2kV 4H-SiC devices of various cell architectures have been successfully fabricated by employing different P+ implantation conditions, resulting in varying levels of Basal Plane Dislocation (BPD) densities across the different device designs. It was found that by utilizing devices designed with an orthogonal P+ source layout as opposed to the traditional P+ stripe pattern, the long-term reliability under sustained 3rd Quadrant current stress conduction can be greatly improved even in devices with medium BPD densities. In addition, the use of the unipolar current of the JBSFET can further enhance long-term reliability under sustained 3^rd Quadrant current stress by mitigating stacking fault expansion, even in devices with a high BPD density.

Abstract: 4H-SiC with 180 μm epilayer was subjected to UV exposure. Stacking fault expanded from basal plane dislocation (BPD) loop generated during growth in the epilayer was observed by UV Photoluminescence Imaging (UVPL) and X-ray Topograph (XRT) techniques. Interactions between partial dislocation, emanating from the BPD loop and gliding via recombination-enhanced dislocation glide mechanism, and threading screw/mix dislocations are detected and analyzed, where stacking faults migrate to different basal plane after the interactions. Such migration increases the faulted volume that can severely degrade reliability and performance of high power SiC devices by increasing reverse leakage current and on-state resistance and could eventually lead to device failure.

Abstract: Prismatic slip systems are the secondary slip systems in Silicon carbide (4H-SiC) crystals. The previously proposed radial thermal model of the PVT growth process for SiC crystals, which predicts the occurrence of slip in different prismatic planes as a function of the radial position in the boule, has been shown to generally work well. Recent observations of growth interface nucleation of prismatic slip necessitated updating the thermal model to incorporate the effects of the curvature of the growth interface. A 3D finite element model has been developed to include the growth interface curvature complexity. The model predicts high dislocation densities due to prismatic slip near the peripheral regions dropping to zero near the center for wafers from sections of the boule grown with a flatter interface and a less dense distribution of prismatic slip dislocations that extends to the center for wafers from boule sections grown with a more convex interface. Additionally, due to such an interface-initiated prismatic slip, , the asymmetrical step configuration produced by off-axis growth results in an asymmetrical distribution of prismatic slip. The studies suggest that a reduced surface curvature is necessary to suppress the prevalence of interface-related prismatic slip generation.

Abstract: Axial sliced samples from 4H-SiC boules grown by PVT method were characterized by lab-based X-ray topography systems. Valuable information about dislocation behaviors during crystal growth was revealed. TSD/TMD propagation during PVT growth was studied. The different TSD/TMD propagation directions inside and outside the facet region were identified as the direct cause of the reduced TSD/TMD density at the facet boundary on some c-plane wafers. A defect reduction mechanism was proposed based on this discovery. It was observed that TSDs/TMDs were emitted by a void structure formed during crystal growth, which explained the elevated TSD/TMD density in the center regions of some wafers. The formation mechanism of such defect is different from the previous studies and remains under investigation.