| dcterms.abstract | 4H silicon carbide (4H-SiC) is a widely used wide bandgap semiconductor material because of its superior electronic and physical properties in devices for high temperature, high power, and high frequency applications. However, crystallographic defects such as dislocations, inclusions and stacking faults, as well as for inhomogeneous long-range strains and cracks play a detrimental role in the performance of SiC devices. Eliminating these defects or at least reducing their densities is a major effort during physical vapor transport (PVT) growth to obtain high-quality crystals. An enhanced understanding of the properties of these defects and their behavior is therefore necessary to provide feedback to improve crystal growth process. The goal of this study, therefore, is to improve the fundamental understanding of the defects involving the origins, formation mechanisms and their behavior in terms of propagation and multiplication using synchrotron X-ray topography and other characterization techniques. The entire study is divided into 3 sections : 1. Synchrotron X-ray Topography with grazing incidence geometry is useful for discerning defects at different depths below the crystal surface, particularly for 4H-SiC epitaxial wafers. The penetration depths measured from X-ray topographs are much larger than the theoretical values. In order to interpret this discrepancy, we simulate topographic contrast of dislocations based on two of the most basic contrast formation mechanisms – orientation contrast and kinematical contrast. Orientation contrast considers merely the displacement fields associated with dislocations while kinematical contrast also takes the diffraction volume into account. The diffraction volume is defined by the effective misorientation around dislocations and the rocking curve width for particular diffraction vector. Ray Tracing Simulation has been carried out to visualize dislocation contrast for both models, taking into account the photoelectric absorption of X-ray beams inside the crystal. Analysis of X-ray topographs can comparison with ray tracing simulations show that orientation contrast plays the key role in determining both the contrast and effective X-ray penetration depths for all types of dislocations. 2. Effect of heavy nitrogen doping on 4H-SiC bulk crystal quality: Synchrotron white beam X-ray topography studies carried out on 4H-SiC wafers characterized by locally varying doping concentrations reveals the presence of overlapping Shockley stacking faults generated from residual surface scratches in regions of higher doping concentrations after the wafers have been subjected to heat treatment. The fault generation process is driven by the fact that in regions of higher doping concentrations, a faulted crystal containing double Shockley faults is more stable than perfect 4H–SiC crystal at the high temperatures (>1000 °C) that the wafers are subject to during heat treatment. We have developed a model for the formation mechanism of the rhombus shaped stacking faults, and experimentally verified it by characterizing the configuration of the bounding partials of the stacking faults on both surfaces. Using high resolution transmission electron microscopy, we have verified that the enclosed stacking fault is a double Shockley type. The anisotropic strains introduced by nitrogen incorporation in 4H-SiC bulk crystal have been analyzed by X-ray topographic contour mapping and the highest value of strains has been found to be about -4×?10?^(-4) and -2.7×?10?^(-3) along the c-axis and a-axis, respectively. The anisotropic strains originate from the different elastic properties and local electronic environment within the closed packed basal plane and the c-axis direction inside 4H-SiC bulk crystals. The highest nitrogen doping concentration estimated from the strain is found to be about 1.5×?10?^20 ? cm?^(-3), which exceeds the theoretically predicted threshold doping level of 2×?10?^20cm-3 for spontaneously faulting at temperatures greater than 1000?C. The shift and broadening of LO-photon-plasma peak in Raman spectroscopy measurements indicates a significant increase in doping concentration of the heavy doped region inside our wafer crystal which agrees well with our measurements and calculations. 3. The operation of Frank-Read sources (FRS), a fundamentally internal source of BPD nucleation and multiplication, in 4H-SiC substrates during heat treatment has been directly observed by in-situ synchrotron X-ray topography recording during heating inside a double ellipsoidal mirror furnace. A model has been developed to explain the operation of Frank Read sources. Deflection of threading edge dislocations (TEDs) on to the basal planes by macrosteps and re-deflection of resulting basal plane dislocations (BPDs) into TEDs during PVT crystal growth produces a specific configuration with one BPD segment pinned by two TEDs. Under the influence of the large thermal gradient stresses induced by heating in the double ellipsoidal mirror furnace, the BPD segment pinned by the two TEDs can glide and activate the double ended Frank-Read source multiplication process as observed in our experiment. Further, more complicated interactions between dislocations from multiple double-ended Frank Read sources operating on the same basal plane is observed. When an expanding dislocation loop from one FRS encounters a TED pinning point belonging to another FRS, the expansion of the first loop is impeded and forces the loop to bend into a configuration where dislocation segments of opposite senses are lined up next to each other. These segments subsequently annihilate thereby resulting in a swap of the pinning point between the two FRSs. This phenomenon appears to provide a mechanism by which one dislocation directly “passes through” other dislocations on the same basal plane that is typically not expected to occur. | |
