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Silicon carbide (SiC) is an important semiconductor material for a variety of electronic and optoelectronic applications owing to the unique combination of its superior electronic and physical properties. In order to continuously improve the crystal quality and improve device performance, obtaining a clear understanding of the defect types and their distribution and potential influence on device operation is of great importance. In this study, 6H-SiC crystals grown by physical vapor transport (PVT) have been characterized by synchrotron monochromatic beam x-ray topography (SMBXT). By recording six different $$11{\bar{2}},12$$ grazing incidence reflections and analyzing the contrast patterns of threading screw dislocations (TSDs), threading edge dislocations (TEDs), threading mixed dislocations (TMDs), and basal plane dislocations (BPDs) observed in conjunction with ray-tracing simulation, the Burgers vectors of these dislocations have been determined. This successfully demonstrated a direct Burgers vector determination approach for each type of dislocation. Understanding these dislocation types and their distributions in 6H-SiC wafers can provide crucial feedback for pursuing crystal quality enhancement during growth process. High-resolution x-ray diffraction (HRXRD) has been performed on the wafer to carry out the rocking curve analysis of areas with different degrees of lattice distortion.

Six-inch AlInP/GaAs epitaxial wafers grown by organometallic vapor-phase epitaxy (OMVPE) are being developed for light-emitting diodes (LEDs). The surface morphology of the epilayer changes under different growth conditions, where hazy features arise under high growth pressure, preventing their use in device fabricatiion. Applying optical microscopy, atomic force microscopy (AFM), and synchrotron x-ray topography, it is observed that the hazy region is rougher than the clear region and additional lattice distortion exists in the hazy region. The tilt and strain are quantitatively analyzed using 004 reciprocal-space mapping (RSM), where the hazy regions are associated with weakened and broadened peaks surrounding the sharp peak from the clear regions of the epilayer. More importantly, by employing 002 RSM to lower the x-ray penetration depth, it is revealed that the hazy features are formed when a certain thickness is exceeded, indicating that the epilayers do not suffer from lattice distortion at the beginning of the epitaxial growth. The characterization results suggest that changes in the growth conditions at the epilayer–substrate interface may not help to avoid the formation of the hazy features, but that other growth parameters at the bulk epilayer are worth investigation.

With the increasing attention of 4H-silicon carbide (4H-SiC) crystals in the applications of high-power electronics, it has become necessary to further improve the development of the 4H-SiC crystal growth process, especially the initial stage of physical vapor transport (PVT) growth, which is a critical step to obtain high quality SiC crystals with polytype stability and low dislocation density. This paper describes a study on dislocation behavior of large diameter 4H-SiC crystals at the early stages of PVT growth. Synchrotron x-ray topography is applied to 6-inch PVT-grown crystals with a thickness of several hundred microns on 4H-SiC seeds. Grazing-incidence topographs in $$g = 11\overline{2}8$$ and $$g = 1\overline{1}09$$ recorded from both the seed crystal and newly grown layer show the presence of screw-type basal plane dislocations (BPDs) with $$b = 1/3\left[ {11\overline{2}0} \right]$$ at the inner region of the wafers, which is further confirmed by comparing with ray tracing simulated images of these dislocations. Their origins are likely from deflection of threading edge dislocations (TEDs) onto the basal plane by the overgrowth of macro-steps. Pairs of threading screw/mixed dislocations (TSDs/TMDs) are found to be newly generated at the initial growth stage and some are deflected onto the basal plane. A high density of newly generated TEDs is observed in the early-grown crystals, which are either nucleated in pairs at the seed/crystal interface or converted from BPDs in the seed crystal. Furthermore, dislocations with unique shapes are observed and found to be associated with deflection of TMDs and TEDs, which become the major source for BPD generation during the early PVT growth stage. Possible models to explain their formation mechanism are developed.

Residual contrast of threading edge dislocations is observed in synchrotron back-reflection X-ray topographs of 4H-SiC epitaxial wafers recorded using basal plane reflections where both g · b = 0 and g · b × l = 0. The ray-tracing simulation method based on the orientation contrast formation mechanism is applied to simulate images of such dislocations by applying surface relaxation effects. The simulated contrast features match the observed features on X-ray topographs, clearly demonstrating that the contrast is dominated by surface relaxation. Depth profiling indicates that the surface relaxation primarily takes place within a depth of 5 µm below the surface.

Gallium nitride (GaN) substrates grown by different methods were characterized by high-resolution x-ray diffraction and synchrotron x-ray topography. Using the monochromatic beam in the grazing incidence geometry, high-resolution x-ray topographs reveal the various dislocation types present. Dislocation contrasts were correlated with ray-tracing simulation results successfully so that the Burgers vectors of the dislocations could be determined. Ammonothermal-grown GaN substrate wafers show the best quality among all the wafers. These wafers, which are free of basal plane dislocations (BPDs) have threading mixed dislocations (TMDs) dominant among the threading dislocations (TDs). Images of patterned hydride vapor phase epitaxy (HVPE) GaN reveal a starkly heterogeneous distribution of dislocations with large areas containing low threading dislocation densities in between a grid of strain centers with higher threading dislocation densities and BPDs. The strain level of regular HVPE GaN substrates is very high, and the dislocation density is around 105–106 cm−2, which is much higher than 104 cm−2 of ammonothermal samples and dislocation-free areas in the patterned HVPE samples.

The thermoelastic stress, mechanical properties and defect content of bulk 4H n-type SiC crystals were investigated following adjustments to the PVT growth cell configuration that led to a 40% increase in growth rate. The resulting 150 mm wafers were compared with wafers produced from a control process in terms of wafer bow and warp, and dislocation density. Wafer shape was found to be comparable among the processes, indicating minimal impact on internal stress. Threading edge and threading screw dislocation densities increased and decreased, respectively, while basal plane dislocation densities were unaffected by the increase in growth rate. Loss of wafer planar stability was observed in certain cases. The elastic modulus was measured to be in the range of approximately 420-450 GPa for selected stable and unstable wafers, and was found to correspond to resistivity.

Dislocation behavior during the early stages of physical vapor transport (PVT) growth of 6-inch diameter 4H-SiC crystals has been investigated by synchrotron monochromatic beam X-ray topography (SMBXT) in conjunction with ray tracing simulations of dislocation images. Our studies reveal that most of the TSDs/TMDs are replicated into the newly grown layer while most TEDs are generated by either nucleation in pairs at the growth interface or by redirection of BPDs in the seed crystal. Most BPDs in the newly grown layer are of screw type with and this has been verified by comparison with ray tracing simulated images. TEDs with same and opposite sign of Burgers vector are found to be deflected on to the same basal plane by the overgrowth of macro-steps and they glide in the same and opposite directions respectively. TMDs deflected on to the basal plane by macro-steps get dissociated into c and a components, with the a segment undergoing glide to form V-shaped configurations.

4H-SiC substrates and homo-epitaxial layers were obtained using the traditional methods of physical vapor transport and chemical vapor deposition. Defect morphology has been studied using both Synchrotron White Beam X-ray Topography and Monochromatic Beam X-ray Topography. Molten KOH etching method was adopted to further investigate the dislocation behavior mechanisms. Deflected dislocations were observed at the periphery regions in both substrate and epitaxial wafers. 3C polytypes and half loop arrays were observed in the 4H-SiC epitaxial wafer. It is also found that the majority of basal plane dislocations are converted to threading edge dislocations in the epitaxial wafer samples. The proportion of BPD to TED conversion depends on the surface step morphology and growth mode in epitaxial growth which in turn depends on the C/Si ratio. By the optimization of etching time prior to epitaxy and C/Si ratio, high-quality epitaxial wafers with extremely low basal plane dislocations densities (＜0.1 cm-2) was obtained.

In x-ray topography studies, ray tracing simulation has been particularly useful in identifying and characterizing Burgers vectors of dislocations using only one reflection instead of the traditional method of recording at least three reflections and applying $$\vec{g} \cdot \vec{b} = 0$$ and $$\vec{g} \cdot \vec{b} \times \vec{l} = 0$$ criteria. In this study, ray tracing simulation of expected dislocations in (AlxGa(1–x))0.5In0.5P epitaxial layers on GaAs substrates has been carried out by using well-known expressions for displacement fields around dislocations. By comparing the simulated images with observed images on monochromatic x-ray topographs, the Burgers vectors have been characterized. The x-ray topographs from the (AlxGa(1–x))0.5In0.5P epitaxial layers also reveal a unique pattern consisting of a series of circular dark contrast features from inclusions. These dark circles decrease in size as the center of the inclusions is approached. Ray tracing simulated image of an inclusion assuming a spherical strain field matches well with the experimental image, thus providing information on the level of strain around the inclusion.

The inhomogeneous distributions of basal plane dislocations (BPDs) in PVT-grown 4H-SiC crystal boule due to internal stresses cause lattice plane bending, which strongly affect SiC-based device fabrication. The relationship between BPDs and local basal plane bending in 6-inch 4H-SiC substrates has been investigated. Synchrotron monochromatic beam x-ray topography (SMBXT) imaging shows black and white contrast of BPDs with Burgers vectors of opposite signs based on the principle of ray tracing. We have evaluated the net difference of BPDs with black and white contrast along both [11$$ \bar{2}$$0] and [1$$ \bar{1}$$00] radial directions on the Si face across multiple 6-inch diameter 4H-SiC substrates sliced from the same and different boules and predicted the nature (concave/convex) and amount of bending of the basal plane in these wafers. Line scans of 0008 reflection using high resolution x-ray diffractometry (HRXRD) has been carried out along the two directions to verify the nature of bending in these wafers. Results show quite different bending behavior along [11$$ \bar{2} $$0] and [1$$ \bar{1}$$00] directions, indicating that the Si face of 6-inch substrates creates non-isotropic bending on the basal plane. These observations are correlated quite well with net BPD density analysis. The physical shapes of the wafers were also measured to be not flat due to the surface effect. Quantitative analysis of the degree of basal plane bending based on the SMBXT data was carried out and found to be correlating well with the measured tilt angle from HRXRD. Existence of a high stress center was observed in one of the 6-inch wafers resulting in severe bending which is associated with both large bending angles and abrupt changes in lattice constants a and c.

Lattice strain in 4H-SiC substrate wafers can have a deleterious effect on the performance of power electronic devices, especially under high-temperature operation. Significant strain can be introduced by lattice parameter change due to the incorporation of impurities in heavily doped 4H-SiC crystals. Synchrotron x-ray topographic contour mapping technique is able to deconvolute the lattice strain component from lattice tilt and thus generate strain maps, which has been incorporated into an anisotropic elasticity model to determine the nitrogen doping concentration in 4H-SiC substrate wafers. In order to further investigate the relationship between lattice strain and doping concentration, the lattice strain variation across the facet and off-facet regions in different 4H-SiC substrate wafers was studied. Hall effect measurements were carried out to measure the nitrogen concentration of 4H-SiC wafers, which shows a decrease in resistivity and Hall mobility with the increase of nitrogen concentration. The result shows that lattice strain within the basal plane is isotropic, while along the growth direction [0001], the strain value is one order magnitude lower. Qualitative study of lattice strain reveals more uniform distribution of strain inside the wafer facet compared to the outside regions. Additionally, wafers with higher nitrogen concentration were found to have larger overall lattice strain variation. Variation of lattice strain due to nitrogen doping was further confirmed by triple axis x-ray rocking curve measurements showing the highest full width at half maximum inside the wafer facet.

A more sophisticated simulation model is developed based on the principle of ray-tracing to simulate the grazing-incidence synchrotron X-ray topographic contrast of dislocations lying on the basal plane including basal plane dislocations and deflected threading screw and mixed dislocations in off-axis 4H-SiC crystals. The model incorporates effects of surface relaxation as well as the photoelectric absorption to predict dislocation contrast. Compared to conventional ray-tracing images, surface relaxation effects dominate dislocation contrast for diffraction near the crystal surface. The simulated dislocation contrast gradually weakens with increasing depth of the diffracted beam position within the crystal due to photoelectric absorption. The distinctive features of the net simulated dislocation images obtained by aggregating through the effective penetration depth correlate well with contrast features observed on the experimental topographic images. Depth analysis reveals that in some cases the diffracted X-rays from regions below the dislocation can contribute additional contrast features previously not considered.

The contrast features of synchrotron X-ray topographic images of screw-type basal plane dislocations (BPDs) in on-axis 4H-SiC wafers have been studied. Screw BPD images are categorized into two types: one exhibiting a white line bordered by black lines and the other a pure black line contrast. Similar images for off-axis specimens and the corresponding ray-tracing simulations demonstrate that these contrasts can be attributed to the depth of the screw BPDs below the crystal surface. The correlation of the contrast features between simulations and the screw BPD topography images can be used to estimate the depth.

Ammonothermal growth of bulk gallium nitride (GaN) crystals is considered the most suitable method to meet the demand for high quality bulk substrates for power electronics. A non-destructive evaluation of defect content in state-of-the-art ammonothermal substrates has been carried out by synchrotron X-ray topography. Using a monochromatic beam in grazing incidence geometry, high resolution X-ray topographs reveal the various dislocation types present. Ray-tracing simulations that were modified to take both surface relaxation and absorption effects into account allowed improved correlation with observed dislocation contrast so that the Burgers vectors of the dislocations could be determined. The images show the very high quality of the ammonothermal GaN substrate wafers which contain low densities of threading dislocations (TDs) but are free of basal plane dislocations (BPDs). Threading mixed dislocations (TMDs) were found to be dominant among the TDs, and the overall TD density (TDD) of a 1-inch wafer was found to be as low as 5.16 × 103 cm−2.

Gallium nitride substrates grown by the hydride vapor phase epitaxy (HVPE) method using a patterned growth process have been characterized by synchrotron monochromatic beam X-ray topography in the grazing incidence geometry. Images reveal a starkly heterogeneous distribution of dislocations with areas as large as 0.3 mm2 containing threading dislocation densities below 103 cm−2 in between a grid of strain centers with higher threading dislocation densities (>104 cm−2). Basal plane dislocation densities in these areas are as low as 104 cm−2. By comparing the recorded images of dislocations with ray tracing simulations of expected dislocations in GaN, the Burgers vectors of the dislocations have been determined. The distribution of threading screw/mixed dislocations (TSDs/TMDs), threading edge dislocations (TEDs) and basal plane dislocations (BPDs) is discussed with implications for fabrication of power devices.

Synchrotron white beam X-ray topography and monochromatic beam topography have been applied to study GaAs substrates and (AlxGa(1−x))0.5In0.5P epitaxial layers grown on them that are being developed for LEDs. The GaAs substrates are 15° offcut toward [1 1 0] and mainly consist of dislocations in the {1 1 1} slip planes and low angle grain boundaries. The nominally lattice matched (AlxGa(1−x))0.5In0.5P layers contain defects such as threading dislocations replicated from the substrates, inclusions and misfit dislocations generated during organometallic vapor phase epitaxy (OMVPE) growth. The existence of misfit dislocations (MDs) indicates that strain relaxation has taken place to accommodate the in-plane lattice mismatch between epitaxial layer and substrate. The driving force for the glide of threading dislocations and relaxation efficiency of dislocations with different Burgers vectors and glide planes has been analyzed in the wafers with 15° offcut to predict the Burgers vectors and line directions of dislocations that dominate strain relaxation. Burgers vectors have been determined using ray tracing simulations. Reciprocal space maps (RSMs) recorded from the wafers to measure the strain and tilt of the epilayers show both in-plane and out-of-plane lattice mismatch as well as tilt between epilayer and substrate. A modified Nagai′s model has been developed to analyze and account for in-plane and out-of-plane tilt.