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The range of polishing-induced subsurface damage remaining in a commercially available production grade 4H-SiC (0001) epi-ready substrate was evaluated by the observation from the (-1100) cleavage plane using two kinds of highly strain-sensitive characterization methods. Firstly, the analysis using electron backscattered diffraction (EBSD) with a submicron spatial resolution was conducted on the exposed cross sectional plane. Then, for the further quantitative evaluation excluding the influence of roughness or contamination of the cleavage plane, a synchrotron X-ray micro-diffraction experiment was carried out. The range of the subsurface damage evaluated in those experiments was ensured by confirming none of additional strain inserted at the cleavage, as compared with the damage-free substrate prepared by high temperature thermal etching. As a result, the depth of the residual strained region below polishing-induced scratches at the surface was estimated to be in the range of a few microns, which is much deeper than the previously reported value of 100 nm by cross-sectional transmission electron microscopy. It suggests a potential of EBSD for the conventional tool to characterize even a small amount of strain in SiC single crystal.
It is well-known that SiC crystal deficiencies are delaying the realization of outstandingly superior SiC power electronics. Efforts to date have centered on eradicating micropipes, and 4H-SiC substrates with extremely low micropipe densities have been achieved. Nevertheless, SiC substrates and epilayers still contain several types of dislocations in densities on the order of thousands per square centimetres, which are nearly 100-fold micropipe densities. While not nearly as detrimental to SiC device performance as micropipes, it has recently been demonstrated that dislocations existing in SiC crystals degrade several characteristics of SiC devices, e.g., the forward bias characteristics of SiC pin diodes and the gate oxide reliability of SiC MOSFETs.
This paper reports several dislocation processes occurring during the growth of hexagonal SiC bulk crystals. Particularly, we focus on the dislocation formation and propagation processes in SiC crystals. We have investigated dislocation processes in 4H-SiC bulk crystals grown by the physical vapor transport (PVT) growth method, using defect selective etching and transmission electron microscopy (TEM).
It was found that foreign polytype inclusions introduced a high density of dislocations at the polytype boundary. In the polytype-transformed areas of the crystal, very few medium size hexagonal etch pits due to threading screw dislocations were observed, indicating that the polytype transformation ceased the propagation of threading screw dislocations. The oval-shaped etch pit arrays observed on the etched vicinal (0001)Si surface, indicative of the dislocation multiplication in the basal plane, showed characteristic distribution around micropipes and low angle grain boundaries. Based on the results, we will argue the dislocation behavior in PVT grown SiC crystals, suggesting that dislocation interaction and conversion are relevant processes to understanding the behavior.
The defect formation during sublimation bulk crystal growth of silicon carbide (SiC) is discussed. SiC bulk crystals are produced by seeded sublimation growth (modified-Lely method), where SiC source powder sublimes and is recrystallized on a slightly cooled seed crystal at uncommonly high temperatures (≥2000°C). The crystals contain structural defects such as micropipes (hollow core dislocations), subgrain boundaries, stacking faults and glide dislocations in the basal plane. The type and density of the defects largely depend on the crystal growth direction, and many aspects are different between the growth parallel and perpendicular to the <0001> c-axis. Micropipes are characteristic defects to the c-axis growth, while a large number of stacking faults are introduced during growth perpendicular to the c-axis. We discuss the cause and mechanism of the defect formation
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