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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 [112¯0] and [11¯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 [112¯0] and [11¯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.

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 10 5 –10 6  cm −2 , which is much higher than 10 4 cm −2 of ammonothermal samples and dislocation-free areas in the patterned HVPE samples.

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 2 ¯ 8 and g = 1 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 11 2 ¯ 0 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.

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 g→·b→=0 and g→·b→×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.

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.

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 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.

Synchrotron monochromatic beam X-ray topography (SMBXT) in grazing incidence geometry shows black and white contrast for basal plane dislocations (BPDs) with Burgers vectors of opposite signs as demonstrated using ray tracing simulations. The inhomogeneous distribution of these dislocations is associated with the concave/convex shape of the basal plane. Therefore, the distribution of these two BPD types were examined for several 6-inch diameter 4H-SiC substrates and the net BPD density distribution was used for evaluating the nature and magnitude of basal plane bending in these wafers. Results show different bending behaviors along the two radial directions - [110] and [100] directions, indicating the existence of non-isotropic bending. Linear mapping of the peak shift of the 0008 reflection along the two directions was carried out using HRXRD to correlate with the results from the SMBXT measurements. Basal-plane-tilt angle calculated using the net BPD density derived from SMBXT shows a good correlation with those obtained from HRXRD measurements, which further confirmed that bending in basal plane is caused by the non-uniform distribution of BPDs. Regions of severe bending were found to be associated with both large tilt angles (95% black contrast BPDs to 5% white contrast BPDs) and abrupt changes in a and c lattice parameters i.e. local strain.

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.

In recent years, the demand for high power electronic devices fabricated from silicon carbide (SiC) has been increasing rapidly due to its superior material advantages such as excellent chemical resistance, high thermal conductivity and high breakdown voltage, while other conventional semiconductor materials such as gallium arsenide (GaAs), Silicon (Si) and Indium Phosphide (InP) are considered to have reached their limits. Particularly, the transformation of automotive industry to electric vehicles has been accelerated by the use of SiC, which enables better system efficiencies, while reducing cost and weight with conserving space. However, problems are currently arising from limitations on the device active area and long-term reliability due to the challenges in maintaining and improving crystal quality while increasing the size of commercial SiC substrate wafers. The existence of extended defects in SiC crystals is one of the major factors limiting the device yield and manufacturing efficiency. It is therefore essential to have an enhanced understanding of the nature of these defects, investigate their behavior and assess their effect on the device reliability so as to provide insights to improve crystal growth process. The goal of this study is to provide direct evidence of existing defects in physical vapor transport-grown 4H-SiC bulk crystals using synchrotron X-ray topography technique, determine their origins, reveal their propagation and multiplication behavior during growth. Below is a brief summary of the major contents included in this dissertation:(1) Lattice strain in 4H-SiC substrate wafers can have a deleterious effect on the performance of power electronic devices especially under the high temperature operation. Significant strain can be introduced by lattice parameter change due to the impurity incorporation in heavily doped 4H-SiC crystals. Synchrotron X-ray rocking curve topography method is able to deconvolute 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 this study, variation of lattice strain and lattice tilt on 4H-SiC axial slices and regular substrate wafers has been investigated. Results shows the predominant contribution of tilt component in the (112 ̅0) crystal plane. In order to further investigate the relationship between lattice strain and nitrogen 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 in 4H-SiC wafers, which shows decrease in resistivity and Hall mobility with the increase of nitrogen concentration. 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. Besides, wafer with higher nitrogen concentration was 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 (FWHM) inside the wafer facet.(2) 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. The net difference of BPDs with black and white contrast along both [112 ̅0] and [11 ̅00] radial directions on Si face across multiple 6-inch diameter 4H-SiC substrates sliced from the same and different boules was evaluated to predict 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 also been carried out along the two directions to verify the nature of bending in these wafers. Results show quite different bending behavior along [112 ̅0] and [11 ̅00] directions, indicating that the Si face of 6-inch substrates is characterized by non-isotropic bending on the basal plane. These observations correlate quite well with net BPD density analysis. The physical shapes of the wafers were also 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 correlate well with the measured tilt angle from HRXRD measurements. Existence of 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.(3) Dislocation behavior during the early stages of physical vapor transport (PVT) growth of 6-inch diameter 4H-SiC crystals has been investigated by SMBXT in conjunction with ray-tracing simulations of dislocation images. Our studies reveal that most of the threading screw dislocations/threading mixed dislocations (TSDs/TMDs) are replicated into the newly grown layer while most threading edge dislocations (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 b=1/3[112 ̅0] 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.(4) Surface morphologies on the surface of the 4° off-axis 4H-SiC crystals with 6-inch diameter at the initial PVT growth stage have been investigated by using various microscopic and spectroscopic methods combined with synchrotron X-ray topography technique. Observations reveal significant different morphological features on the heavily doped facet, intermediate and the off-facet regions. Surface hillocks observed at the crystal facet are found to be associated with the formation of TSDs/TMDs. In the intermediate region, formation of the C-shape hillocks is interpreted in terms of the interaction with the macro-steps formed by step bunching at the facet periphery. AFM results show around 20nm height of the macro-steps in the intermediate region. Analysis of Raman spectra reveals the presence of 3C polytypes located on the top of surface hillocks that are characterized with dark features as observed under Nomarski optical microscopy., The proportion of the 3C polytype shows significant point to point variation at these hillocks. In the off-facet region, growth pits are observed and correlated with the topographic contrast of TSDs/TMDs and TEDs. The interaction behavior of the vicinal growth step with a single or pair of opposing sign spiral steps (associated with TSDs) is studied, and possible models are presented.(5) 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 BPDs and deflected TSDs and TMDs 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.

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.