In situ bow reduction during sublimation growth of cubic silicon carbide

Sublimation growth of cubic silicon carbide (3C–SiC) with diameters of 50 and 100 mm was performed on freestanding homoepitaxial grown seeds. For both seeds and sublimation grown crystals, two different relaxation axes with varying curvature could be observed with the higher bent axis aligned perpendicular to the original wafer flat. A general reduction in the wafer bow independent of the starting curvature and size of the seeds could be observed. Using the X-ray imaging, we could observe in situ that the bow reduction is linked to the growth of new material and cannot be initiated by heat up or cool down processes alone. Raman spectroscopy of the grown crystals revealed that the observed flattening goes along with a tensing of the seeding layers while the surface of the crystals remains free of a stress gradient. A slight concave bending of lattice planes along the main relaxation axis could be observed by high-resolution XRD rocking curve measurements while for the lower bent axis, no lattice plane bending occurred. Full width half maximum values of the (002) reflection showed values as low as 67 arcseconds proofing the possibility to grow large-area, high-quality 3C–SiC using sublimation growth

Analysis of the Basal Plane Dislocation Density and Thermomechanical Stress during 100 mm PVT Growth of 4H-SiC

Basal plane dislocations (BPDs) in 4H silicon carbide (SiC) crystals grown using the physical vapor transport (PVT) method are diminishing the performance of SiC-based power electronic devices such as pn-junction diodes or MOSFETs. Therefore, understanding the generation and movement of BPDs is crucial to grow SiC suitable for device manufacturing. In this paper, the impact of the cooldown step in PVT-growth on the defect distribution is investigated utilizing two similar SiC seeds and identical growth parameters except for a cooldown duration of 40 h and 70 h, respectively. The two resulting crystals were cut into wafers, which were characterized by birefringence imaging and KOH etching. The initial defect distribution of the seed wafer was characterized by synchrotron white beam X-ray topography (SWXRT) mapping. It was found that the BPD density increases with a prolonged cooldown time. Furthermore, small angle grain boundaries based on threading edge dislocation (TED) arrays, which are normally only inherited by the seed, were also generated in the case of the crystal cooled down in 70 h. The role of temperature gradients inside the crystal during growth and post-growth concerning the generation of shear stress is discussed and supported by numerical calculations

Multiple novel prostate cancer susceptibility signals identified by fine-mapping of known risk loci among Europeans

Genome-wide association studies (GWAS) have identified numerous common prostate cancer (PrCa) susceptibility loci. We have fine-mapped 64 GWAS regions known at the conclusion of the iCOGS study using large-scale genotyping and imputation in 25 723 PrCa cases and 26 274 controls of European ancestry. We detected evidence for multiple independent signals at 16 regions, 12 of which contained additional newly identified significant associations. A single signal comprising a spectrum of correlated variation was observed at 39 regions; 35 of which are now described by a novel more significantly associated lead SNP, while the originally reported variant remained as the lead SNP only in 4 regions. We also confirmed two association signals in Europeans that had been previously reported only in East-Asian GWAS. Based on statistical evidence and linkage disequilibrium (LD) structure, we have curated and narrowed down the list of the most likely candidate causal variants for each region. Functional annotation using data from ENCODE filtered for PrCa cell lines and eQTL analysis demonstrated significant enrichment for overlap with bio-features within this set. By incorporating the novel risk variants identified here alongside the refined data for existing association signals, we estimate that these loci now explain ∼38.9% of the familial relative risk of PrCa, an 8.9% improvement over the previously reported GWAS tag SNPs. This suggests that a significant fraction of the heritability of PrCa may have been hidden during the discovery phase of GWAS, in particular due to the presence of multiple independent signals within the same regio

Atlas of prostate cancer heritability in European and African-American men pinpoints tissue-specific regulation

Although genome-wide association studies have identified over 100 risk loci that explain ∼33% of familial risk for prostate cancer (PrCa), their functional effects on risk remain largely unknown. Here we use genotype data from 59,089 men of European and African American ancestries combined with cell-type-specific epigenetic data to build a genomic atlas of single-nucleotide polymorphism (SNP) heritability in PrCa. We find significant differences in heritability between variants in prostate-relevant epigenetic marks defined in normal versus tumour tissue as well as between tissue and cell lines. The majority of SNP heritability lies in regions marked by H3k27 acetylation in prostate adenoc7arcinoma cell line (LNCaP) or by DNaseI hypersensitive sites in cancer cell lines. We find a high degree of similarity between European and African American ancestries suggesting a similar genetic architecture from common variation underlying PrCa risk. Our findings showcase the power of integrating functional annotation with genetic data to understand the genetic basis of PrCa

Global defect distribution and their influence on the local real structure of 4H-SiC

In this work 4H-SiC, grown by Physical Vapor Transport (PVT) was investigated with Synchrotron White Beam X-ray Topography (SWXRT) in back-reflection geometry, section topography, X-ray Diffraction Laminography (XDL), polarized light microscopy and at last High Resolution X-ray Diffractometry (HRXRD). Full SWXRT back-reflection wafer-mappings were recorded giving a good dislocation overview. The observed defect features - verified with SWXRT - include threading dislocations like Threading Screw Dislocations (TSDs), superscrews like Micropipes (MPs), dislocation networks in form of Basal Plane Dislocations (BPDs) and Small-Angle-Grain- Boundaries (SAGBs) in different portions. The dislocations are inhomogeneously distributed and show global wafer areas of the same principal dislocation arrangement and density which do not change in growth direction, but there are local defect differences visible within the areas in growth direction. Furthermore, there are features on the topographs which cannot be assigned to the diffraction pattern of a specific dislocation type. They appear as large, shapeless white spot which was assigned to either a void, different polytype, differently oriented grain or an arrangement of closely spaced MPs with the complementary results of polarization microscopy. MPs and other comparable threading dislocations assumed to lie approximately parallel to growth direction [0001] with solely a small but constant inclination arising from an edge component was shown to be incomplete. Polarization microscopy Z-stacks of different MPs reveal strong individual fluctuations of propagation direction and inclination angle in the mm range. XDL showed furthermore a differently pronounced strain field around the MP’s core in accordance to the tilt component. Section topography showed that different dislocation types and densities leading to a local difference of section border bowing and orientation contrast arising Delta Theta extensions. Those differences are in accordance with the different global wafer regions shown in the wafer-mappings. For the real structural analysis, High Resolution X-ray Diffractometry (HRXRD) measurements were performed on several wafer areas representing different dislocation types, densities and their interactions. Bond measurements verified a local lattice parameter difference in dependence of the dislocation content. It turned out that SAGBs show the biggest influence on the lattice parameter in c, whereas MPs leading to the least deviation. Reciprocal Space Maps (RSM) show a difference in the crystal lattice strain and tilt conditions depending not only on the dominant dislocation type but also on the rocking direction along different preferential directions of dislocations. It was shown, that MPs with the least inclination towards growth direction leading to a separation into several strain and tilt segments