Synchrotron X-ray Topography Research Articles

Secondary recrystallization in grain-oriented silicon steel

Grain-oriented silicon steel is a soft magnetic material widely used for transformer cores. Magnetic properties are closely related to the sharpness of Goss texture, which is evolved by secondary recrystallization. An overview of our results will be presented; secondary recrystallization behaviour has been investigated by in situ observation utilizing synchrotron x-ray topography. The results support the selective growth mechanism. Migration of Goss grains is controlled by inhibitor and sharper Goss grains, which have higher frequency of CSL-boundaries to the matrix, start to grow preferentially while the other grains are stagnated by inhibitor. Secondary recrystallization behaviour can be explained based on this CSL-model. At the initial stage, inhibitor gradient through the thickness promotes Goss selective growth from the subsurface layer and at the later stage, inhibitor drop rate dominates the Goss selective growth. Inhibitor and CSL-boundaries synergistically affect Goss selective growth and Goss grain evolves to a coarse grain.

Influence of surface relaxation on the contrast of threading edge dislocations in synchrotron X-ray topographs under the condition of g · b = 0 and g · b × l = 0

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.

X-ray Topography Characterization of GaN Substrates Used for Power Electronic Devices

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.

Immobilization of Partial Dislocations Bounding Double Shockley Stacking Faults in 4h-Sic Observed by in Situ Synchrotron X-Ray Topography

The expansion of double Shockley stacking faults (DSFs) in an n-type 4H-SiC substrate with a nitrogen concentration of 3.9×10 19 cm −3 was investigated using in situ synchrotron X-ray topography. DSF expansion was observed to be suppressed and immobilized above 1590 K, along with the partial dislocation (PD) shape being changed from a straight to zig-zag configuration. For a different heating process (higher heating rate), the PDs could continue to expand, even above 1590 K. Ex situ topography experiments revealed that the DSFs close to the specimen surface expanded widely, although those expanding toward the specimen interior became immobile. One possible mechanism for this immobilization was proposed, where the core structural changes from a Si-core to the C-core by climb motion induced by the interaction between the PDs and point defects (C interstitials).

Slip planes in monoclinic β-Ga2O3 revealed from its {010} face via synchrotron X-ray diffraction and X-ray topography

Dislocations and their arrays in a {010}-oriented β-Ga2O3 substrate were observed using synchrotron X-ray diffraction (XRD) and X-ray topography (XRT). The XRD measurements used spatially-resolved ω-rocking curves to show that dislocation arrays have a linear contrast with a lower diffraction intensity and a broader full-width at half maximum. Further XRT observations at three different vectors resolved these lines as dot contrasts aligned along the four different directions and which correspond to the outcrops of threading dislocations that lay in four different slip planes. The Burgers vectors for these dislocation arrays were determined using the ∙ invisibility criterion. The results indicate there are four slip systems of (1) {001}, (2) {01}, (3) {10}, and (4) {100}. The slip system {10} has not been previously reported.

Synchrotron X-ray topography characterization of high quality ammonothermal-grown gallium nitride substrates

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.

Characterization of Dislocations in 4H-SiC Single Crystals at the Initial Growth Stage by Synchrotron X-ray Topography

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

Synchrotron X-ray Topography Characterization of Commercial GaN Substrates for Power Electronic Applications

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).

Characterization of Hazy Morphology on AlInP/GaAs Epitaxial Wafers Grown by Organometallic Vapor Phase Epitaxy

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

Characterization of Dislocations in 4H-SiC Single Crystals at the Initial Growth Stage by Synchrotron X-ray Topography

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 technique. Ray tracing simulation is applied to simulate the dislocation images. Our studies show that most 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 crystal are of screw type with 1/3 [112‾0], which is verified by comparison with the simulated images. TEDs with the same and opposite sign of Burgers vector are found to be deflected onto the same basal plane by the overgrowth of macro-steps and glide towards to the same or opposite directions respectively. TMDs deflected onto the basal plane get dissociated into a and c components, with the a segment undergoing glide to form V-shaped configurations.

Synchrotron X-ray Topography Characterization of Commercial GaN Substrates for Power Electronic Applications

Commercial GaN substrates from different vendors were characterized by synchrotron X-ray topography (XRT) as well as high resolution X-ray diffraction (HRXRD) to evaluate their defect and strain distributions. Synchrotron monochromatic beam X-ray topographs reveal the various dislocation types and their distribution in the GaN materials grown by different methods. By correlating the dislocation contrast with ray-tracing simulation results, the Burgers vectors of dislocations were successfully determined. The ammonothermal-grown GaN material using native seed show the highest quality with low dislocation densities. Patterned HVPE GaN wafers have heterogeneous distribution of dislocations with large areas of low threading dislocation densities. Regular HVPE GaN substrate and ammonothermal GaN grown on an HVPE GaN seed show a very high level of strain, and the dislocation densities are much higher than ammonothermal and patterned HVPE samples.

Characterization of Hazy Morphology on AlInP/GaAs Epitaxial Wafers Grown by Organometallic Vapor Phase Epitaxy

6-inch AlInP/GaAs epitaxial wafers grown by organometallic vapor phase epitaxy (OMVPE) technique are being developed for light emitting diodes (LEDs) applications. Defects such as dislocations and inclusions have been characterized by synchrotron X-ray topography. Burgers vector and line direction of the dislocations were determined by ray-tracing simulation. Moreover, epilayer surface of the samples grown under higher pressure reveals a hazy morphology, which is rougher compared to regular clear regions. Synchrotron X-ray topographs of the hazy regions showed blurred contrast, indicating disordered lattice arrangement. 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.

Study of dislocations in AlN single-crystal using bright-field synchrotron x-ray topography under a multiple-beam diffraction condition

Dislocations in an AlN single crystal were studied via bright-field synchrotron x-ray topography under a multiple-beam diffraction (MBD) condition. Two-beam approximation conditions using nine different types of diffraction vectors (g) were applied, including five types near the MBD condition, and the Burgers vector (b) of dislocations was identified using the g·b invisibility criterion. Based on the assignment of the Burgers vectors, some of the propagation behaviors of the dislocation were observed, including dislocation reduction via a reaction between threading edge-type dislocations and the generation of a pair of dislocations that have opposite Burgers vectors.

From Wafers to Bits and Back again: Using Deep Learning to Accelerate the Development and Characterization of SiC

A non-destructive, fast and accurate extended defect counting method on large diameter SiC wafers is presented. Photoluminescence (PL) signals from extended defects on 4H-SiC substrates were correlated to the specific etch features of Basal Plane Dislocations (BPDs), Threading Screw Dislocations (TSDs), and Threading Edge Dislocations (TED). For our non-destructive technique (NDT), automated defect detection was developed using modern deep convolutional neural networks (DCNN). To train a robust network, we used our large volume data set from our selective etch method of 4H-SiC substrates, already established based on definitive correlations to Synchrotron X-Ray Topography (SXRT) [1]. The defect locations, classifications and counts determined by our DCNN correlate with the subsequently etch-delineated features and counts. Once our network is sufficiently trained we will no longer need destructive methods to characterize extended defects in 4H-SiC substrates.

Synchrotron X-Ray Topography Study on the Relationship between Local Basal Plane Bending and Basal Plane Dislocations in PVT-Grown 4H-SiC Substrate Wafers

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.

Origin of reverse leakage current path in edge-defined film-fed growth (001) β -Ga2O3 Schottky barrier diodes observed by high-sensitive emission microscopy

We utilized ultra-high sensitive emission microscopy to investigate the origin of reverse leakage current of edge-defined film-fed grown (001) β-Ga2O3 Schottky barrier diodes. In the emission patterns, we observed a partially appearing void, having a typical width and depth of 300 and 83 nm, respectively, with a base angle of 75°, below the Schottky barrier contact on the β-Ga2O3 surface. Simulations show that the electric field at the bottom of the void reached 9 × 106 V/cm at a reverse bias of −60 V and in the case of an ideally flat surface 3.1 × 106 V/cm. This indicates that the reverse leakage current originates from an electric field concentration in the partially appearing voids on the surface. On the other hand, we have confirmed that there is no clear relationship between the leakage current path and the dislocations observed by synchrotron x-ray topography.

Identification of Burgers vectors of dislocations in monoclinic β-Ga2O3 via synchrotron x-ray topography

Dislocations are major structural defects in semiconductor materials, and they have negative impacts on the performance and reliability of electronic devices. The Burgers vector (b→) of a dislocation is one of the most important characteristics that determines its behavior in an operating device. In this study, we used synchrotron x-ray topography (XRT) to perform systematic observations of dislocations in β-Ga2O3, which is a promising wide-bandgap semiconductor for power device applications. By applying the g→⋅b→ invisibility criterion to dislocation contrasts in XRTs taken at multiple reciprocal lattice vectors g→, we identified 12 types of Burgers vectors, including 11 types theoretically predicted by Yamaguchi et al. [Superlattice Microst. 99, 99 (2016)] and b→=⟨100⟩. Some of these Burgers vectors, such as b→=⟨101¯⟩,12⟨132⟩,12⟨13¯2⟩,⟨001⟩, and ⟨100⟩, were confirmed experimentally for the first time.

Temperature dependence of double Shockley stacking fault behavior in nitrogen-doped 4H-SiC studied by in-situ synchrotron X-ray topography

We observed the behavior of double Shockley stacking faults (DSFs) in 4H-SiC crystals with nitrogen concentrations of 1.0 × 1019–2.6 × 1019 cm−3 over an extensive temperature range (1380–1910 K) by in-situ synchrotron X-ray topography. For a nitrogen concentration of 2.6 × 1019 cm−3, the expansion velocity of the DSFs exponentially increased with temperature in the range from 1370 to 1650 K. In contrast, at a nitrogen concentration of 1.0 × 1019 cm−3, this velocity decreased above 1610 K and the DSFs shrank above 1730 K. The DSF energy, the magnitude of which is the driving force on partial dislocation (PD) movement, was quantitatively estimated from the radius of the curvature of bowed-out partial PDs pinned by threading screw dislocations (TSDs), showing a positive temperature dependence and lying in the range from −0.6 to 0.8 mJ/m2 for 1630–1910 K at a nitrogen concentration of 1.0 × 1019 cm−3. The DSF expansion and shrinkage behavior can be understood by the simple temperature and nitrogen concentration dependence of the DSF energy.

Observation of dislocations in β-Ga2O3 single-crystal substrates by synchrotron X-ray topography, chemical etching, and transmission electron microscopy

To reveal dislocations in β-Ga2O3 and categorize them by their Burgers vectors, we have carried out a comprehensive dislocation characterization by synchrotron X-ray topography (XRT), chemical etching, and transmission electron microscopy (TEM). An important type of dislocation confirmed by XRT is the screw-type dislocations extending in the 〈010〉 direction at a density of about 4.2 × 104 cm−2. These dislocations had length in millimeters and seldom bent to other directions. Chemical etching using molten KOH + NaOH at 200 °C formed hillocks with multiple crystallographic facets on the (010) surface. These hillocks showed various geometrical features that indicated the direction of the dislocation lines. TEM observation confirmed that the hillocks were related to dislocations. Finally, the feasibility of using TEM weak-beam dark-field images and large-angle convergent-beam electron diffraction to identify dislocation Burgers vectors was examined, which seemed to be problematic due to the low-symmetry of the monoclinic crystal structure of β-Ga2O3.

Temperature Dependence of Double Shockley Stacking Fault Behavior in Nitrogen-Doped 4h-Sic Studied by In-Situ Synchrotron X-Ray Topography

We observed the behavior of double Shockley stacking faults (DSFs) in 4H-SiC crystals with nitrogen concentrations of 1.0×1019-2.6×1019 cm−3 over an extensive temperature range (1380-1910 K) by in-situ synchrotron X-ray topography. For a nitrogen concentration of 2.6×1019 cm−3, the expansion velocity of the DSFs exponentially increased with temperature for temperatures in the range from 1370 to 1650 K. In contrast, at a nitrogen concentration of 1.0×1019 cm−3, this velocity decreased above 1610 K and the DSFs shrank above 1730 K. The DSF energy, considered to be the driving force for DSF expansion, was quantitatively estimated from the radius of the curvature of bowed-out partial dislocations (PDs) pinned by threading screw dislocations (TSDs), showing a positive temperature dependence and lying in the range from −0.6 to 0.8 mJ/m2 at a nitrogen concentration of 1.0×1019 cm−3. The DSF expansion and shrinkage behavior can be understood by the simple temperature and nitrogen concentration dependence of the DSF energy.