Ouloide Yannick Goue

Synchrotron X-Ray Topography Analysis of Double Shockley Stacking Faults in 4H-SiC Wafers

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.

Using Ray Tracing Simulations for Direct Determination of Burgers Vectors of Threading Mixed Dislocations in 4H-SiC c-Plane Wafers Grown by PVT Method

The presence of threading mixed dislocations (TMDs) (with both edge and screw component) in 4H-SiC crystals grown by PVT method has been reported both from axial slices (wafers cut parallel to the growth axis) and commercial offcut wafers (cut almost perpendicular to the growth axis). In this paper, a systematic method is developed and demonstrated to unambiguously determine the Burgers vectors of TMDs in 4H-SiC commercial offcut wafers using both Synchrotron Monochromatic X-ray Topography (SMBXT) and Ray Tracing Simulations. The principle of this method is that the contrast of dislocations on different reflections varies with the relative orientation of Burgers vectors with respect to the diffraction vectors. Measurements confirm that in commercial offcut wafers the majority of the threading dislocations with screw component are mixed type dislocations.

Effect of Doping Concentration Variations in PVT-Grown 4H-SiC Wafers

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 stacking faults are rhombus-shaped and bound by Shockley partial dislocations. 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 a 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. Our studies show that during heat treatment of the wafer, such double Shockley faults can be generated in regions where dislocation sources are presents (e.g. scratches or low-angle boundaries) and when the nitrogen doping concentration exceeds a certain level.

Direct Determination of Burgers Vectors of Threading Mixed Dislocations in 4H-SiC Grown by PVT Method

In addition to pure threading screw dislocations (TSDs), the presence of threading mixed dislocations (TMDs) (with a component) has been reported both in 4H-SiC axial slices (wafers cut parallel to the growth axis) and in commercial offcut wafers (cut almost perpendicular to the growth axis). In this paper, we first demonstrate a method to quickly distinguish TMDs from TSDs in axial slices via synchrotron white-beam x-ray topography. Since such axial slices are usually not available for commercial purposes, a systematic method is then developed and demonstrated here to unambiguously determine the Burgers vectors of TMDs in 4H-SiC commercial offcut wafers. In this second study, both synchrotron monochromatic-beam x-ray topography and ray-tracing simulation are used. The x-ray topographs were recorded using grazing-incidence geometry. The principle of this method is that the contrast of dislocations on different reflections varies with the relative orientation of Burgers vectors with respect to the diffraction vectors. Measurements confirm that, in a commercial offcut wafer, the majority of the threading dislocations with screw component are mixed-type dislocations.

Optimization of 150 mm 4H SiC Substrate Crystal Quality

Continuous optimization of bulk 4H SiC PVT crystal growth processes has yielded an improvement in 150 mm wafer shape, as well as a marked reduction in stacking fault density. Mean wafer bow and warp decreased by 26% and 14%, respectively, while stacking faults were nearly eliminated from wafers produced using the refined process. These quality enhancements corresponded to an adjustment to key thermal parameters predicted to control intrinsic crystal stresses, and a reduction in crystal dome curvature.

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

Synchrotron x-ray topography in grazing-incidence geometry is useful for discerning defects at different depths below the crystal surface, particularly for 4H-SiC epitaxial wafers. However, the penetration depths measured from x-ray topographs are much larger than theoretical values. To interpret this discrepancy, we have simulated the topographic contrast of dislocations based on two of the most basic contrast formation mechanisms, viz. orientation and kinematical contrast. Orientation contrast considers merely displacement fields associated with dislocations, while kinematical contrast considers also diffraction volume, defined as the effective misorientation around dislocations and the rocking curve width for given diffraction vector. Ray-tracing simulation was carried out to visualize dislocation contrast for both models, taking into account photoelectric absorption of the x-ray beam inside the crystal. The results show that orientation contrast plays the key role in determining both the contrast and x-ray penetration depth for different types of dislocation.

Mapping of Threading Screw Dislocations in 4H n-Type SiC Wafers

X-ray topography shows that selective KOH etching after CVD growth of n-type epilayers on highly N doped 4H SiC substrates can be used to reliably map pure and mixed Threading Screw Dislocations (TSD). The influence of the mapping grid density and the wafer position in the crystal on the average TSD density are investigated. A reliable mapping of TSD contributed to the development of 100mm SiC wafers with average TSD density down to 200 cm-2.

Correlation of Lifetime Mapping of 4H-SiC Epilayers with Structural Defects Using Synchrotron X-Ray Topography

Lifetime maps for two 4H-SiC epi-wafers (samples 1 and 2) were recorded using microwave photoconductive decay (μPCD) measurements and correlated with the type and distribution of structural defects mapped by synchrotron X-ray topography (white beam and monochromatic). Sample 1 showed lower lifetime inside one of its higher doped facet regions and along its edges. The low lifetime in the facet region was associated with the presence of a high density of multi-layered Shockley stacking faults (SFs) and low angle grain boundaries (LAGBs). These stacking faults are likely double Shockley stacking faults (DSSFs) and probably nucleated from scratches present on the substrate surface and LAGBs present in that region, propagating during epilayer growth. In contrast, sample 2 showed a reduced carrier lifetime in the middle region associated with a network of interfacial dislocations (IDs) and half loop arrays (HLAs) originating from 3C inclusions that are generated during epilayer growth. Along the edges of both samples, overlapping triangular defects, microcracks and BPD loops lowered lifetime.

Study of defect structures in 6H-SiC a/m-plane pseudofiber crystals grown by hot-wall CVD epitaxy

Structural perfection of silicon carbide (SiC) single crystals is essential to achieve high-performance power devices. A new bulk growth process for SiC proposed by researchers at NASA Glenn Research Center, called large tapered crystal (LTC) growth, based on axial fiber growth followed by lateral expansion, could produce SiC boules with potentially as few as one threading screw dislocation per wafer. In this study, the lateral expansion aspect of LTC growth is addressed through analysis of lateral growth of 6H-SiC a/m-plane seed crystals by hot-wall chemical vapor deposition. Preliminary synchrotron white-beam x-ray topography (SWBXT) indicates that the as-grown boules match the polytype structure of the underlying seed and have a faceted hexagonal morphology with a strain-free surface marked by steps. SWBXT Laue diffraction patterns of transverse and axial slices of the boules reveal streaks suggesting the existence of stacking faults/polytypes, and this is confirmed by micro-Raman spectroscopy. Transmission x-ray topography of both transverse and axial slices reveals inhomogeneous strains at the seed–epilayer interface and linear features propagating from the seed along the growth direction. Micro-Raman mapping of an axial slice reveals that the seed contains high stacking disorder, while contrast extinction analysis (g·b and g·b×l) of the linear features reveals that these are mostly edge-type basal plane dislocations. Further high-resolution transmission electron microscopy investigation of the seed–homoepilayer interface also reveals nanobands of different SiC polytypes. A model for their formation mechanism is proposed. Finally, the implication of these results for improving the LTC growth process is addressed.