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The contrast of dislocations in 4H-SiC crystals shows distinctive features on grazing-incidence X-ray topographs for diffraction at different positions on the operative rocking curve. Ray-tracing simulations have previously been successfully applied to describe the dislocation contrast at the peak of a rocking curve.The present work shows that the dislocation images observed under weak diffraction conditions can also be simulated using the ray-tracing method. These simulations indicate that the contrast of the dislocations is dominated by orientation contrast. Analysis of the effective misorientation reveals that the dislocation contrast in weak-beam topography is more sensitive to the local lattice distortion, consequently enabling information to be obtained on the dislocation sense which cannot be obtained from the peak.

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.

As one of the most important wide bandgap semiconductors, Gallium Nitride (GaN) has many applications in power electronic devices such as inverters and switches, as well as optoelectronic devices like light emitting diodes.[1] However, high-quality bulk GaN growth is still limited by the availability of growth techniques. Two most popular growth method for GaN are hydride vapor phase epitaxy (HVPE) [2, 3] and ammonothermal [4, 5] method. HVPE method uses foreign substrates, such as sapphire, gallium arsenide or silicon carbide, which gives a large dislocation density due to significant lattice mismatch and difference in coefficients of thermal expansion. However, in ammonothermal method, native seeds were used to minimize the effects of lattice mismatch and a retrograde direction of GaN transport inside the growth system was utilized to exclude most of the contamination and undesirable phases. So, ammonothermal method gives a much better quality. In this study, GaN substrates of both HVPE (Figure 1) and ammonothermal (Figure 2) methods have been characterized by synchrotron monochromatic beam X-ray topography (SMBXT)[6]. The distribution of threading screw/mixed dislocations (TSDs/TMDs), threading edge dislocations (TEDs) and basal plane dislocations (BPDs) is characterized. Ray tracing simulations are adopted to confirm the types of dislocations. The comparison of defect distribution between HVPE and ammonothermal samples has been conducted by both SWBXT and High-resolution X-ray Diffraction (HRXRD). References [1] S.J. Pearton, J.C. Zolper, R.J. Shul, F. Ren, Journal of Applied Physics 86 (1999) 1-78. [2] H.P. Maruska, J. Tietjen, Applied Physics Letters 15 (1969) 327-329. [3] K. Fujito, S. Kubo, H. Nagaoka, T. Mochizuki, H. Namita, S. Nagao, Journal of Crystal Growth 311 (2009) 3011-3014. [4] R. Dwiliński, R. Doradziński, J. Garczyński, L. Sierzputowski, A. Puchalski, Y. Kanbara, K. Yagi, H. Minakuchi, H. Hayashi, Journal of Crystal Growth 310 (2008) 3911-3916. [5] T. Hashimoto, F. Wu, J.S. Speck, S. Nakamura, Journal of Crystal Growth 310 (2008) 3907-3910. [6] B. Raghothamachar, M. Dudley, G. Dhanaraj, X-Ray Topography Techniques for Defect Characterization of Crystals, in: G. Dhanaraj, K. Byrappa, V. Prasad, M. Dudley (Eds.), Springer handbook of crystal growth, Springer Science & Business Media, 2010, p. 1425. Figure 1

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.

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.

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.

Efforts to improve the quality and size of 4H-SiC wafers have resulted in size of 6” and dislocation densities of the order of 103 cm-2. The presence of significant quantities of crystallographic defects in bulk 4H-SiC crystals continues to be a major obstacle for achieving high-quality 4H-SiC substrates. Of particular note is the nucleation and distribution of basal plane dislocations (BPDs). BPDs are deformation-induced and are mostly generated during post-crystal growth cooling or high temperature annealing processes. BPDs have been associated with the formation of Shockley stacking fault leading to an increase in forward voltage degradation and are undesirable. Depending on the PVT growth conditions, internal stresses due to non-uniformity of temperature field cause lattice plane bending. During cooldown, this bending is accommodated by the inhomogeneous distribution of BPDs in a PVT grown 4H-SiC boule and persists in wafers sliced from the boule. Crystal bending affects subsequent wafer processing especially polishing and device fabrication. Therefore, analysis of the BPD distribution (sign and density) enables us to better understand basal plane bending, which provides insight into further improvement of the crystal growth process. Synchrotron Monochromatic Beam X-ray Topography (SMBXT) studies in grazing incidence geometry reveal the appearance of black and white contrast of BPDs which are associated with opposite sign of Burgers vectors. Based on the principle of ray tracing, black contrast BPDs are generated by focusing of diffracted X-rays when the extra half plane is extending away from the imaged surface, while defocusing of X-rays contribute to the generation of white contrast BPDs due to the extra half plane extending towards the surface [1]. The inhomogeneous distribution of the two types of BPD contrast will thus result in concave/convex shape of the basal planes. In our study, we have evaluated the ratio of the black/white contrast of BPDs along 11-20 direction across multiple 6-inch diameter 4H-SiC substrates sliced from the same and different boules and predicted how the lattice is tilted in different substrate wafers. High resolution X-ray diffraction measurements are currently underway to investigate the variation of rocking curve peak shift along 11-20 direction and thus to further verify our result.

Variations in nitrogen doping concentration across the PVT-grown 4H-SiC wafers can induce significant lattice strain, leading to degradation on the performance of SiC-based power devices. A qualitative study on the lattice strain variation in facet and off-facet regions was carried out for 4H-SiC substrates. The lattice strain maps for 4H-SiC wafers were derived from the 11-20, 1-100 (transmission geometry) and 0008 (reflection geometry) using synchrotron double crystal contour mapping method. Results show that the lattice strain within the basal plane is isotropic, while along [0001] direction, lattice strain is one order of magnitude lower, indicating elastic anisotropy of strain due to doping in 4H-SiC crystals. The distribution of lattice strain inside the facet region is more uniform than in the off-facet region. Measurement of nitrogen doping concentration by Hall effect measurements shows over 45% of difference in doping level between heavily doped facet region and off-facet region. Existence of significant lattice distortion in facet region of 4H-SiC substrates was further confirmed by X-ray rocking curve measurements.

Shifts in the spatial distribution of threading dislocations in 150 mm 4H SiC wafers were examined as a response to intentional changes in both the flow of the nitrogen source gas used to control resistivity during bulk crystal growth, and the growth rate. The density of threading edge and screw dislocations was found to be more evenly distributed in wafers produced under a high-growth rate, low-resistivity process. This result corresponded to a flattening of the resistivity distribution, and a ~34% reduction in on-and off-facet resistivity differential. The effect was attributed to regularized 4H island coalescence due to modulation of step terrace width.

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.

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.

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.

In recent years, 4H-SiC has become a widely acknowledged material applied in high power electronics. 4H-SiC-based devices are able to operate at high voltage, high current and high temperature due to its superior physical properties. However, they have yet to reached to the requirements for mass production, due to the fact of SiC’s relatively inferior crystal quality. Over the past few decades, the effect of various dislocations existing in the crystals such as threading screw/mixed dislocations (TSD/TMDs), threading edge dislocations (TEDs) and basal plane dislocations (BPDs) has been studied and revealed to be main source for degrading the reliability of SiC power devices[1-2]. Recently, studies on the early stages of physical vapor transport (PVT) growth of SiC have been reported by several groups regarding to the investigation of the step morphology and lattice plane bending [3-5]. However, dislocation propagation behavior and their formation mechanism at the early growth stages are yet to be studied in detail. This paper describes the investigation of dislocation behavior during the early stages of PVT-grown 6-inch 4H-SiC crystals by synchrotron monochromatic beam x-ray topography (SMBXT) technique. Ray tracing simulation was applied to simulate the dislocation images. Our studies show that most of the TSDs/TMDs are replicated into the newly grown crystal while most TEDs are generated by either nucleation in pairs at the seed/crystal interface or by redirection of BPDs in the seed crystal. Most BPDs in the newly grown layer are of screw type with 1/3[11-20] and this has been verified by comparison with ray tracing simulated images. TEDs with the same and opposite sign of Burgers vector are found to be deflected on to same basal plane by the overgrowth of macro-steps and start to 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. [1] St.G. Muller, J.J. Sumakeris, M.F. Brady, R.C. Glass, H.M. Hobgood, J.R. Jenny, R. Leonard, D.P. Malta, M.J. Paisley, A.R. Powell, V.F. Tsvetkov, S.T. Allen, M.K. Das, J.W. Palmour, and C.H. Carter. Jr., Eur. Phys. J. Appl. Phys. 27, 29 (2004). [2] R. Singh and M. Pecht, IEEE Ind Electron M 2, 19 (2008). [3] E.K. Sanchez, J.Q. Lin, M.De. Graef, M. Skowronski, W.M. Vetter, M. Dudley, J. Appl. Phys. 91 (3) (2002) 1143. [4] C. Ohshige, T. Takahashi, N. Ohtani, M. Katsuno, T. Fujimoto, S. Sato, H. Tsuge, T. Yano, H. Matsuhata, M. Kitabatake, J. Cryst. Growth 1-6, 408 (2014). [5] N. Ohtani, C. Ohshige, M. Katsuno, T. Fujimoto, S. Sato, H. Tsuge, W. Ohashi, T. Yano, H. Matsuhata, M. Kitabatake, J. Cryst. Growth 9-15, 386 (2014). Figure 1

The superior electronic and physical properties of silicon carbide (SiC), such as wide band-gap, high saturated electron velocity, high breakdown electric field strength, and high thermal conductivity, enable successful applications in modern power electronic devices with high power, high frequency, and high temperature operation requirements. [1,2] Therefore, understanding the nature and behavior of different dislocations is of great significance towards crystal quality enhancement. Synchrotron X-ray has been widely adopted for generating topographic images of threading screw dislocations (TSDs) as well as threading edge dislocations (TEDs) and basal plane dislocations (BPDs). Besides, recent study in 4H-SiC revealed the presence of threading mixed dislocations (TMDs) with Burgers vectors nc+ma (where m and n are integers). [3] Since direct TMDs observation methods such as transmission electron microscopy (TEM) and synchrotron white-beam X-ray topography (SWBXT) are not suitable for commercially preferred c-plane-oriented wafers, ray-tracing simulated images of possible dislocation types are compared with X-ray topographs in order to identify the TMDs non-destructively. Except 4H-SiC, ray-tracing simulation has also been proved as an effective approach for dislocation characterization in various materials such as GaN [4], ZnO [5], and AlN [6]. The same principle is implemented in this study for the investigation of dislocations in 6H-SiC wafer. Synchrotron monochromatic beam X-ray topography (SMBXT) analysis was carried out on a PVT grown 6H-SiC wafer for recording its grazing-incidence images of all six different 11-212 reflections (Fig. 1). The distribution of TSDs, TEDs, TMDs, and BPDs are characterized, and the determination of their Burgers vectors is achieved by correlating the simulated results obtained through ray-tracing simulation with real X-ray topographic observations (Fig. 2). This study provides a clear understanding of dislocations in 6H-SiC, which is crucial for quality improvement of the crystal. Reference: [1]: C. Codreanu, et al, Comparison of 3C-SiC, 6H-SiC and 4H-SiC MESFETs Performances, Materials Science in Semiconductor Processing 3 137 (2000). [2]: J. W. Sun, et al, Shockley-Frank Stacking Faults in 6H-SiC, Journal of Applied Physics 111, 113527 (2012). [3]: J. Q. Guo, et al, Direct Determination of Burgers Vectors of Threading Mixed Dislocations in 4H-SiC Grown by PVT Method, Journal of Electronic Materials 45, 4 (2016). [4]: B. Raghothamachar, et al, X-ray Topography Characterization of Gallium Nitride Substrates for Power Device Applications, submitted for publication. [5]: T. Y. Zhou, “Threading Dislocation Characterization and Stress Mapping Depth Profiling via Ray Tracing Technique,” Ph.D. Thesis, SUNY Stony Brook, Advisor: Michael Dudley (2015). [6]: T. Y. Zhou, et, al, Characterization of Threading Dislocations in PVT-Grown AlN Substrates via x-Ray Topography and Ray Tracing Simulation, Journal of Electronic Materials volume 43, 838 (2014). Figure 1

6-inch AlInP/GaAs epitaxial wafers grown by organometallic vapor phase epitaxy (OMVPE) technique are being developed for light emitting diodes (LEDs) applications [1]. Recently, dislocation configurations in the GaAs substrates and AlInP epilayers have been characterized and conditions for relaxation and formation of misfit dislocations were analyzed [2,3]. Apart from defects such as dislocations, epilayer surface reveals a hazy morphology formed when the epitaxial growth pressure is increased. It is observed that the epilayer surface is rougher compared to regular clear regions as revealed by both optical microscopy and atomic force microscopy. Additionally, the size of the hazy regions increases with growth pressure but other growth parameters such as growth temperature, V/III ratio, likely influence the onset of hazy morphology. Previous research on the formation of hazy features reports that these can be caused by the deficiency of phosphorus [4], lowered V/III ratio [5] or stacking faults [6]. However, the hazy features in our case, are different from the ones reported. Synchrotron X-ray topographs of the hazy regions showed blurred contrast, indicating disordered lattice arrangement. Moreover, internal structure of the hazy regions was studied by reciprocal space mapping (RSM), where weakened and broadened peaks around the clear epilayer peak were observed using 004 reflection, indicating continuously varying strain and tilt between the hazy region and the clear region. The thickness of the hazy regions was also analyzed by employing the 002 RSM, where the X-ray penetration depth is lower. Comparison of the 002 and 004 RSMs reveals that at the beginning of epitaxial growth, the epilayers do not suffer from lattice distortion and the formation of the hazy morphology occurs when a critical thickness is exceeded. Reference: [1] C. H. Chen, S. A. Stockman, M. J. Peanasky, C. P. Kuo. Brightness Light Emitting Diodes Semiconductors and Semimetals, SEMICONDUCT SEMIMET, 48 (1997) pp. 97–144 [2] Hongyu Peng, Tuerxun Ailihumaer, Yafei Liu, Balaji Raghothamachar and Michael Dudley, J. Cryst. Growth, 533 (2020) 125458 [3] Hongyu Peng, Tuerxun Ailihumaer, Balaji Raghothamachar and Michael Dudley, J. Electron. Mater. 49, 3472–3480 (2020) [4] H. H. Ryu, M. H. Jeon, J. Y. Leem, H. J. Song, L. P. Sadwick, G. B. Stringfellow, J Mater Sci (2006) 41:8265–8270 [5] D. S. Cao and G. B. Stringfellow, Journal of Electronic Materials, Vol. 20, No. 1, 1991 [6] Yoshihiro Hiraya, Fumiya Ishizaka, Katsuhiro Tomioka and Takashi Fukui, Applied Physics Express 9, 035502 (2016) Figure 1

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.

Lattice strain in 4H-SiC substrate wafers can strongly affect the performance of power electronic devices especially under the high temperature operation. The impurity incorporation in highly doped 4H-SiC crystals can result in significant lattice parameter change. Recently we reported [1] the synchrotron X-ray topographic contour mapping [2] technique to deconvolute lattice parameter variations from lattice tilt and thus generate strain maps. These strain maps have been incorporated into an anisotropic elasticity model to derive the nitrogen doping concentration variation in the 4H-SiC wafer. Doping distribution can be derived using a set of anisotropic strain maps detected by recording series of diffraction contours across the same region. The nondestructive contour mapping method we developed is capable of not only determining the concentration but also the distribution of the doping i.e. a doping map. In order to verify the doping level determined by contour mapping, we prepared samples with different N-doping levels. Contacts were created by depositing Ni using e-beam evaporator followed by rapid annealing. Hall effect measurements have been carried out on these samples to measure the carrier concentration in order to determine the doping values. The measured values are found to be in good agreement with the doping map derived using our contour mapping method. Reference Yu Yang, Jianqiu Guo, Balaji Raghothamachar, Xiaojun Chan, Taejin Kim, Michael Dudley, J. Electron. Mater. 47(2), 938-943 (2018). J. Guo, Y. Yang, B. Raghothamachar, M. Dudley, and S. Stoupin, J. Electron. Mater. 47(2), 903-909 (2018).

Recent advances made towards the improvement of the quality of 4H silicon carbide (4H-SiC) have enabled many successful applications in the power device industry. While defect densities in homoepitaxial active layers have been lowered significantly, there is still a strong interest in understanding the formation mechanism of crystal defects such as dislocations preferably through direct observation. During 4H-SiC homoepitaxy and post-growth annealing, the process of stress relaxation has previously been reported to lead to the formation of misfit dislocations or interfacial dislocations (IDs). [1] The driving force for such a relaxation process is believed to be the lattice misfit induced by the difference in nitrogen doping concentrations across the substrate/epilayer interface. According to van der Merwe [2], at initial stage of epitaxy growth, misfit is accommodated by elastic strain in epilayers; as the growth continues, misfit strain builds up until a critical amount when crystals start to deform plastically with misfit dislocations introduced at the substrate/epilayer interface, relaxing additional elastic strain and minimizing the free energy of the epi-substrate system. Matthews and Blakeslee, [3] from a mechanics point of view, predict the onset of stress relaxation process to be the moment when the mismatch stress applied on a single threading glide dislocation in the epilayer (basal plane dislocation in the case of 4H-SiC homoepitaxy) exceeds the sum of the line tension and the Peierls-Nabarro stress as the epilayer grows thicker. Nevertheless, the direct observation of the stress relaxation process in silicon carbide crystals has not yet been reported. In this paper, we present an in-situ study in which a sample cut from a 150mm commercial n-/n+ 4H-SiC homoepitaxial wafer was subject to high-temperature heat treatment while sequential synchrotron white beam X-ray topographs were recorded simultaneously. The heat treatment was carried out in a halogen lamp furnace and the local temperature was elevated to as high as 1600°C. Figure 1 shows a series of topographs recorded in this manner and the time interval between consecutive topographs is 10 seconds. As time advances, a basal plane dislocation (BPD) in the epitaxial layer is observed to glide from left to right, depositing a straight segment of misfit dislocation right at the substrate/epilayer interface. Figure 2 schematically shows the entire process. Such dynamic observations of relaxation process suggest that the lattice misfit once again exceeds the critical value during high temperature heat treatment. Since the epilayer thickness does not change, it can be inferred that the misfit strain simply increases as temperature is raised. The reason may be that the thermal expansion coefficient of 4H-SiC crystal is a function of doping concentration. Sasaki et. al [4] also confirmed this by measuring the lattice constants of both n+ substrate and n- epilayer at elevated temperatures. [1] M. Dudley, H. Wang, J. Guo, Y. Yang, B. Raghothamachar, J. Zhang, B. Thomas, G.Y. Chung, E. Sanchez, D.M. Hansen, S.G. Mueller, MRS Advances, 1(2), 91-102 (2016). [2] J.H. van der Merwe, J. Appl. Phys. 34, 117 (1963). [3] J.W. Matthews, and A.E. Blakeslee, Journal of Crystal Growth, 27, 118–125 (1974). [4] S. Sasaki, J. Suda, and T. Kimoto, Materials Science Forum, 717-720, 481-484 (2012) Figure 1