Studies on Lattice Strain Variation due to Nitrogen Doping by Synchrotron X-ray Contour Mapping Technique in PVT-Grown 4H-SiC Crystals
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Lattice strain in 4H-SiC substrate wafers can have a deleterious effect on the performance of power electronic devices, especially under high-temperature operation. Significant strain can be introduced by lattice parameter change due to the incorporation of impurities in heavily doped 4H-SiC crystals. Synchrotron x-ray topographic contour mapping technique is able to deconvolute the lattice strain component from lattice tilt and thus generate strain maps, which has been incorporated into an anisotropic elasticity model to determine the nitrogen doping concentration in 4H-SiC substrate wafers. In order to further investigate the relationship between lattice strain and doping concentration, the lattice strain variation across the facet and off-facet regions in different 4H-SiC substrate wafers was studied. Hall effect measurements were carried out to measure the nitrogen concentration of 4H-SiC wafers, which shows a decrease in resistivity and Hall mobility with the increase of nitrogen concentration. The result shows that lattice strain within the basal plane is isotropic, while along the growth direction , the strain value is one order magnitude lower. Qualitative study of lattice strain reveals more uniform distribution of strain inside the wafer facet compared to the outside regions. Additionally, wafers with higher nitrogen concentration were found to have larger overall lattice strain variation. Variation of lattice strain due to nitrogen doping was further confirmed by triple axis x-ray rocking curve measurements showing the highest full width at half maximum inside the wafer facet.
Keywords4H-SiC substrate nitrogen concentration lattice strain synchrotron x-ray topography HRXRD
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This study was based upon research conducted at Cornell High Energy Synchrotron Source (CHESS) which is supported by the National Science Foundation and the National Institute of Health/National Institute of General Medical Sciences under NSF award DMR-1332208. Hall effect measurements were carried out at the Center of Functional Nanomaterials (CFN), Brookhaven National Laboratory (DOE Office of Basic Energy Sciences Contract No. DE-AC02-98CH10886). We would like to specially acknowledge Dr. Ken Finkelstein and Dr. Albert Macrander (ANL) for help in experimental setup and useful discussion. Additional support was provided by the Joint Photon Sciences Institute at SBU for travel and subsistence for access to CHESS.
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