Investigation of Penetration Depth and Defect Image Contrast Formation in Grazing Incidence X-ray Topography of 4H-SiC Wafers

Abstract

Synchrotron X-ray Topography has been shown to be a vital tool for the nondestructive characterization of defects in 4H-SiC crystals [1-2]. Techniques utilizing reflection geometry are particularly useful for discerning defects at different depths below the crystal surface. For example, in studying defects in SiC pin diode structures, which typically comprise a buffer layer homoepitaxially grown on a substrate, with the drift layer grown on top of the buffer layer, it is important to be able to discriminate the depth at which particular defect configurations reside. This is particularly important for the characterization of defects resulting from relaxation processes such as interfacial dislocations and half loop arrays [3]. There are generally two approaches adopted depending of the level of perfection of the crystal. In deformed regions (such as around dislocation cores) the penetration depth is simply determined by photoelectric absorption. In perfect regions it is determined by extinction. Extinction contrast predicts particularly shallow penetration depths in these geometries, whereas kinematical theory predicts much deeper penetration depths. However, even under the kinematical diffraction conditions, the penetration depth calculated theoretically by just considering a dislocation as a single line is much smaller than the value measured experimentally. In our study we compared two models involving different diffracting volumes. In one model, the volume is defined by the misorientation larger than the FWHM of the rocking curve in the vicinity of the dislocation core. In another model, the contrast is naturally formed by simulating the photoelectric absorption profile of the diffraction beam according to the local lattice plane tilt by assuming the dislocation image formation is dominant by orientation contrast. We will present a comparison between measured penetration depths in grazing incidence geometry with these two approaches and develop an optimized model to explain our observations. [1] Muller, G.S., et al., Volume production of high quality SiC substrates and epitaxial layers: Defect trends and device applications. Journal of Crystal Growth, 2012. 352(1): p. 39-42. [2] Dudley, M., et al., Characterization of 100 mm diameter 4H-Silicon carbide crystals with extremely low basal plane dislocation density, Material Science Forum,2001. 65: p. 353-356. [3] Wang, H., Dudley, M., et al., Studies of the Origins of Half-Loop Arrays and Interfacial Dislocations Observed in Homoepitaxial Layers of 4H-SiC. Journal of Electronic Materials, 2015. 44(5): p. 1268-1274