Multiplication of Basal Plane Dislocations via Interaction with c-Axis Threading Dislocations in 4H-SiC

This paper demonstrates the X-ray three-dimensional topography of basal-plane dislocations (BPDs) and threading edge dislocations (TEDs) in 4H-SiC. Cross-sectional imaging shows the propagation of BPDs from a substrate to an epilayer and the conversion of BPDs into TEDs near the epilayer/substrate interface. The strain analysis of TEDs exhibits the image of strains in the order of ±10-5. The observed strain images correlate well to simulation results.

In 4H-SiC, which is currently considered as a most promising candidate for the power devices, four types of threading dislocation (TD) have been identified. Apart from a threading edge dislocation with b = a, it has been shown recently that there are at least three types of TDs whose Burgers vectors b are parallel or nearly parallel to the c axis, i.e. a threading pure screw dislocation (TpSD) with b = [0001], a threading near screw dislocation (TnSD) with and a threading mixed dislocation (TMD) with . These dislocations have been called simply as threading screw dislocations (TSDs). Since the total Burgers vector, and hence the energy, increases in the order of TpSD, TnSD and TMD, it is expected that the occurrence should decrease in the order of TpSD, TnSD and TMD. This is not the case. The reason why high-energy TnSD and TMD are as abundant as low-energy TpSD has been discussed based on the dissociation models. As a result of dissociation, the difference of energy among these three types of TSD is reduced considerably.

Synchrotron X-ray topography was carried out for 4H-SiC crystals grown by high-temperature gas source method, and transmission topography analysis with g= or 0004 was carried out for the cross-sectional samples. Dislocation contrasts extended in the growth direction were observed and the propagation behavior of threading screw dislocations (TSDs), threading edge dislocations (TEDs), basal plane dislocations (BPDs) and stacking faults (SFs) in the facet and step-flow regions were discussed. The propagation of dislocations in the fast grown crystal with a growth rate of 3.1mm/h was also evaluated by cross-sectional topography.

Understanding the electronic properties of dislocations is key to tailoring their properties in silicon carbide (SiC)-based electronic devices. By combining the Kelvin probe force microscopy analysis and first-principles calculations, we investigate the effect of aluminum (Al) dopants and surface oxides on the electronic properties of dislocations in p-type SiC. It is found that dislocations, including threading screw dislocations (TSDs), threading edge dislocations (TEDs), and basal plane dislocations (BPDs) create deep acceptor-like states in p-type SiC. The defect levels move upwards in the order of BPD, TED and TSD, which closely relates with the lattice distortion of dislocations. Interestingly, we find that the defect levels of dislocations are higher than that of Al dopants in p-type SiC. First-principles calculations indicate that the defect formation energies of Al dopants at the cores of dislocations are higher than those in the perfect region of p-type SiC, as a result of the steric effect. This indicates that the concentration of Al acceptors at the dislocation cores is lower than that in the perfect region, resulting in the higher defect level positions of dislocations in p-type SiC. Additionally, we find that surface oxidation reduces the difference of defect-level positions between dislocations and the perfect region of p-type SiC, which paves the way for tailoring the electronic properties of dislocations in p-type SiC.

A quantitative study was performed to investigate the impact of crystallographic dislocation defects, including screw dislocation, basal plane dislocation, and threading edge dislocation, and their locations in the active and JTE region, on the reverse performance of 4H-SiC p–n diodes. It was found that higher leakage current in diodes is associated with basal plane dislocations, while lower breakdown voltage is attributed to screw dislocations. The above influence increases in severity when the dislocation is in the active region than in the JTE region. Furthermore, due to the closed-core nature, the impact of threading edge dislocation on the reverse performance of the p–n diodes is less severe than that of other dislocations although its density is much higher.

It is well-known that SiC wafer quality deficiencies are delaying the realization of outstandingly superior 4H-SiC power electronics. While efforts to date have centered on eradicating micropipes (i.e., hollow core super-screw dislocations with Burgers vector > 2c), 4H-SiC wafers and epilayers also contain elementary screw dislocations (i.e., Burgers vector = Ic with no hollow core) in densities on the order of thousands per cm2, nearly 100-fold micropipe densities. This paper describes an initial study into the impact of elementary screw dislocations on the reverse-bias current-voltage (I-V) characteristics of 4H-SiC p+n diodes. First, Synchrotron White Beam X-ray Topography (SWBXT) was employed to map the exact locations of elementary screw dislocations within small-area 4H-SiC p+n mesa diodes. Then the high-field reverse leakage and breakdown properties of these diodes were subsequently characterized on a probing station outfitted with a dark box and video camera. Most devices without screw dislocations exhibited excellent characteristics, with no detectable leakage current prior to breakdown, a sharp breakdown I-V knee, and no visible concentration of breakdown current. In contrast devices that contained at least one elementary screw dislocation exhibited a 5% to 35% reduction in breakdown voltage, a softer breakdown I-V knee, and visible microplasmas in which highly localized breakdown current was concentrated. The locations of observed breakdown microplasmas corresponded exactly to the locations of elementary screw dislocations identified by SWBXT mapping. While not as detrimental to SiC device performance as micropipes, the undesirable breakdown characteristics of elementary screw dislocations could nevertheless adversely affect the performance and reliability of 4H-SiC power devices.

Electron-hole recombination enhanced glide of Shockley partial dislocations bounding expanding stacking faults and their interactions with threading dislocations in 4H silicon carbide epitaxial layers have been studied using synchrotron white beam X-ray topography and in situ electroluminescence. The mobile silicon-core Shockley partial dislocations bounding the stacking faults are able to cut through threading edge dislocations leaving no trailing dislocation segments in their wake. However, when the Shockley partial dislocations interact with threading screw dislocations, trailing 30o partial dislocation dipoles are initially deposited in their wake due to the pinning effect of the threading screw dislocations. These dipoles spontaneously snap into their screw orientation, regardless the normally immobile carbon-core Shockley partial dislocation components in the dipoles. They subsequently cross slip and annihilate, leaving a prismatic stacking fault in (2-1-10) plane with the displacement vector 1/3[01-10].

Synchrotron White Beam X-ray Topography (SWBXT) imaging of wafers cut parallel to the growth axis from 4H-SiC boules grown using Physical Vapor Transport has enabled visualization of the evolution of the defect microstructure. Here we present observations of the propagation and post-growth mutual interaction of threading growth dislocations with c-component of Burgers vector. Detailed contrast extinction studies reveal the presence of two types of such dislocations: pure c-axis screw dislocations and those with Burgers Vector n1c+n2a, where n1 is equal to 1 and n2 is equal to 1 or 2. In addition, observations of dislocation propagation show that some of the threading dislocations with c-component of Burgers adopt a curved, slightly helical morphology which can drive the dislocations from adjacent nucleation sites together enabling them to respond to the inter-dislocation forces and react. Since all of the dislocations exhibiting such helical configurations have significant screw component, and in view of the fact that such dislocations are typically not observed to glide, it is believed that such morphologies result in large part from the interaction of a non-equilibrium concentration of vacancies with the originally approximately straight dislocation cores during post-growth cooling. Such interactions can lead to complete or partial Burgers vector annihilation. Among the reactions observed are: (a) the reaction between opposite-sign threading screw dislocations with Burgers vectors c and –c wherein some segments annihilate leaving others in the form of trails of stranded loops comprising closed dislocation dipoles; (b) the reaction between threading dislocations with Burgers vectors of -c+a and c+a wherein the opposite c-components annihilate leaving behind the two a-components; (c) the similar reaction between threading dislocations with Burgers vectors of -c and c+a leaving behind the a-component.

Dislocations near surface of 4H-SiC were observed using synchrotron radiation topography in the Bragg case with grazingincidence geometry. Figure is an image of basal-plane dislocation half-loop at g=11-28, =0.15nm on Si-face. The (0001) plane is tilted towards the [-1-120] direction by 8 degrees from the surface. In this condition, lattice defects within 10 m depth are observed. Along this dislocation line, bright contrast at A, dark and bright asymmetric line at B, and dark contrast at C are observed. Absence in contrast can be seen at B at g=1-108, and so that B is a screw dislocation part. We have observed migrations of dark dislocations in specimens after forward-bias degradation effect, in which Si-core dislocations are known to move. Thus we concluded that C is Si-core, A is C-core edge dislocation, and the Burgers vector is 1/3[-1-120]. The observed dark and bright contrast is discussed to be similar effect described by Ando and Kato (1970). By applying this rule we could identify uniquely 6 different Burgers vectors for all basal-plane dislocations and threading edge dislocations at only one diffraction condition.

Silicon carbide (SiC) is the leading wide bandgap semiconductor for high-power and high-temperature electronics, but the high defect density still limits device performance. This study investigates how inclusions, Basal Plane Dislocations (BPDs), and Threading Screw Dislocations (TSDs) in 4H-SiC substrates affect epitaxial defect formation. Twenty 200 mm SiC wafers were analyzed after epitaxial growth in two industrial Chemical Vapor Deposition (CVD) reactors, one using Trichlorosilane/Ethylene (Reactor A) and the other Silane/Propane (Reactor B). Defects were characterized using Candela (KLA), Altair (KLA), XRTmicron LAB (Rigaku), SICA (Lasertec), and Crossbeam (ZEISS) dual-beam SEM system. Statistical correlation showed that the conversion rate of embedded particles decreases with particle depth and increases with particle size. Reactor A exhibited lower propagation rates, indicating better suppression of substrate-related defects. SEM/FIB-EDX analyses suggested that carbon inclusions generate pits while metallic inclusions induce triangular defects. Dislocation analysis confirmed a strong correlation between TSDs and BPDs with carrots and triangular defects. BPD conversion rates were estimated at about 98.3% (Reactor A) and 99.8% (Reactor B). These results emphasize the importance of substrate quality and buffer layer optimization to minimize defect propagation.

Wide bandgap semiconductor, 6H-SiC, is being applied in photoconductive semiconductor switches (PCSS) due to its semi-insulating properties. This material is normally insulating but when illuminated, charge carriers are pumped to the conduction band and the material becomes conductive in proportion to the light intensity. Under the application of voltage and laser, the density as well as the movement of the defects will have an impact on the performance of the device. Characterization and understanding the mechanism of this potentially destructive defect process is of great scientific interest as well to the development of the device. X-ray topography is a powerful, non-destructive technique for the characterization of extended defects in large, single crystals. In this study, threading edge dislocations (TED), threading screw dislocations (TSD) as well as basal plane dislocations (BPD) in 6H-SiC axial samples are revealed using synchrotron rocking curve topography, where the atomic structure information (Burgers vector) of these dislocations are investigated using ray tracing simulation. The understanding of the nature of these dislocations is of great importance as to predict their propagation and movement under the application of voltage and laser, and eventually helps to improve the performance of the device. Figure 1

This short review provides an overview of advancements in the characterization of defects in silicon carbide (SiC) wafers using polarized light observation. SiC, which has a wide bandgap and excellent thermal and electrical properties, is widely used in power devices. However, various defects such as threading screw dislocations (TSD), threading edge dislocations (TED), and basal plane dislocations (BPD) can significantly affect device performance and reliability. Polarized light observation offers a nondestructive method for visualizing these defects and analyzing the stress fields induced within the SiC crystal structure. This paper summarizes recent developments in this technique, including the application of analyzer rotation to enhance the contrast in defect visualization. Furthermore, the development of automated systems for rapid wafer evaluation is discussed, highlighting the role of polarized light observation in improving quality control and production efficiency in SiC power device manufacturing.

Effect of the V/III ratio during buffer layer growth on the yellow and blue luminescence in undoped GaN epilayer has been studied by means of photoluminescence spectroscopy and high resolution X-ray diffraction. It is found that the densities of screw and edge threading dislocations increase with the V/III ratio of the buffer layer, and the intensities of the yellow luminescence (YL) and blue luminescence (BL) emissions also increase dramatically. However, the density ratio of the edge threading dislocation to the screw threading dislocation remains invariant, as well as the intensity ratio of YL emission to BL emission. It can be concluded from these phenomena that the edge threading dislocation and screw threading dislocation can enhance the YL and BL emissions, respectively.

4H-SiC commercial wafers and sublimation grown epitaxial layers with a thickness of 100 μm have been studied concerning crystalline structure. The substrates and the epitaxial layers have been separately investigated by high-resolution x-ray diffraction and synchrotron white beam x-ray topography. The results show that the structural quality was improved in the epitaxial layers in the [112̄0] and [1̄100] directions, concerning domain distribution, lattice plane misorientation, mosaicity, and strain, compared with the substrates. Misoriented domains have merged together to form larger domains while the tilt between the domains was reduced, which resulted in nonsplitting in diffraction curves. If the misorientation in the substrate is large, we can only see a slight decrease in the misorientation in the epilayer. At some positions on the substrates block structures (mosaicity) were observed. ω-rocking curves showed smaller full width at half maximum values and more uniform and narrow peaks, while the curvature was almost the same in grown epilayers compared with the corresponding substrates. We show that threading edge dislocations along the c axis in silicon carbide grown crystals transform to deflected dislocations in the epilayer. A formation mechanism for deflected dislocations and supporting facts are presented. We further show that these deflected dislocations are one possible source for the creation of stacking faults that recently has been reported to cause degradation in processed SiC bipolar diodes.

Discrimination of dislocation in highly doped n-type 4H–SiC by combining electrochemical reaction and molten alkali etching