Silicon carbide (SiC) is an important semiconductor for high temperature and high voltage applications such as in electric vehicles. The SiC boule is grown by seeded sublimation deposition process where the primary ingredients viz. carbon (C) and silicon (Si) are sublimated at very high temperature. Eddy and Gaskill [1] indicated that the variations in the atomic ratio of Si to C in the vapor phase that arise from feedstock granularity is difficult to minimize during growth. It is also difficult to avoid impurities arising from both in the SiC feedstock and in the graphite-insulation used in the growth reactor. Additionally, radial, and axial thermal profiles in the reactor and boule may cause polycrystallinity, grain boundary formation, and other defects in the lattice, which may lead to macro- and micro-domain formation. Because the understanding of materials’ properties at the extreme growth temperature of ~2200°C is incomplete, the boule growth modeling efforts are based on extrapolated parameters. Thus, an accurate experimental quantification is very important. The evolution of domain walls in single crystalline SiC is therefore very difficult to eliminate, and they stay put in the boule. Once the boule is sliced into substrates, and polished, the substrates are further processed to deposit the epitaxial layers before device fabrication. The imperfections in the boule created during crystal growth thus remain in the wafers. This paper investigates SiC substrates and epitaxial SiC wafers via cameraless T-ray imaging route. T-ray images reveal significant features that were not seen before. “Dendrimer dipole excitation (DDE)” T-ray source was used that generates wider terahertz bandwidth (0.1 THz to ~30 THz) and generate >200 milliwatts of CW power without requiring a femto-second pulsed laser. An instrument system, the T-ray nanoscanning and 3D imager (TN3DI), has been designed around the DDE T-ray source that was used for the present investigations. Here the wafer was directly mounted on the nanoscanner and the whole wafer was scanned for imaging. T-ray images (see Fig. 1) have revealed some patterns in the SiC wafers of the present investigation that were unseen previously. Optical images and T-ray images were compared side-by-side that shows that the T-ray exhibits higher sensitivity and clearer images thus reveals internal intricate pattern not visible by the optical and X-ray technique. Here, we propose that the unique patterns observed from the T-ray images are due to some kind of domain formation in the boule that has remained in the wafers even after slicing and polishing. This hypothesis is further strengthened via T-ray micrograph of different wafers that exhibit many micro-domains on the epi layer of a 100 mm SiC wafer. Thus, the T-ray images have revealed some intricate patterns for the first time that are present in the SiC wafers. Details of the principle and experimental will be presented.
REFERENCES [1] C. R. Eddy Jr. and D. K. Gaskill, “Silicon Carbide as a Platform for Power Electronics,” SCIENCE, 12 JUNE (2009) Vol. 324, 1398–1400. DOI: 10.1126/science.1168704.
