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Enhanced nitrogen doping in the 〈11 2 ¯ 0〉 directions on the (000 1 ¯ ) facet of 4H-SiC crystals grown by the physical vapor transport (PVT) growth method was investigated using Raman scattering microscopy and atomic force microscopy (AFM). The enhanced nitrogen doping showed either single or double peak structure of nitrogen incorporation in the azimuthal direction around 〈11 2 ¯ 0〉. AFM observations revealed that these two characteristic doping structures stemmed from different propagation morphologies of spiral steps on the (000 1 ¯ ) facet. It was also unveiled that the change in the step-flow direction was always associated with the change of the terrace-width ratio on the facet, and that the interplay between these two parameters determined the nitrogen incorporation on the (000 1 ¯ ) facet. On the basis of these experimental results, the mechanism of the azimuthal dependence of nitrogen incorporation on the (000 1 ¯ ) facet during PVT growth of 4H-SiC crystals is discussed.

The distribution of basal plane dislocations (BPDs) in physical vapor transport (PVT) grown 4H-SiC crystals has been investigated using Raman microscopy and X-ray topography. X-ray topography observations of (1 1 2 ¯ 0) wafers vertically sliced along the growth direction from the crystals revealed that there existed almost periodically arranged layers with a high density of BPDs (bunched BPDs) in PVT-grown 4H-SiC crystals. These layers were characterized using Raman microscopy and high resolution X-ray diffraction, and it was found that large tensile and compressive stresses were associated with the layers. The stresses showed characteristic variations along the basal plane as well as the growth direction. Based on these results, the formation and multiplication processes of BPDs during PVT growth of 4H-SiC crystals are elucidated.

The defect structure at and near the grown crystal/seed interface of 4H-SiC single crystals grown by the physical vapor transport (PVT) growth method has been investigated using Raman microscopy and x-ray topography. To examine the detailed distribution of crystallographic defects across the interface, 4H-SiC wafers containing a beveled growth interface were prepared from nitrogen-doped 4H-SiC single crystal boules; the beveled interface allows more spatially resolved observations of the defect structure near the interface compared to the interface on vertically sliced [e.g., ( 1 1 2 ¯ 0 ) and ( 1 1 ¯ 0 0 )] wafers. Raman microscopy imaging using the longitudinal optical phonon-plasmon coupled (LOPC) mode determined the position of the growth interface and revealed several characteristic features of the interface through the analyses of the scattering intensity, peak position, and width of the LOPC mode peak. X-ray topography revealed that there existed networks of basal plane dislocations at the growth interface, and they extended deep inside the seed crystal. Based on these observations, we discussed the defect formation process at the initial stage of PVT growth and suggested an important role of vacancy injection during PVT growth of SiC.

The defect structure at the growth front of 4H-SiC single crystal boules grown by the physical vapor transport (PVT) growth method has been investigated using X-ray topography. Plan-view observations of the growth front of the boules revealed that basal plane dislocations (BPDs) were often nucleated at the outcrops of threading screw dislocations during growth. The observations also revealed that BPDs introduced in the facet and near-facet regions were hardly multiplied during the PVT growth process, and thus the crystal portions just beneath these regions contained very low densities of BPDs. The cross-sectional observations of the grown boules confirmed this result and further revealed that a number of BPDs were introduced in the crystal portions well distant from the growth front. They are thought to be introduced via dislocation multiplication due to large thermoelastic stresses imposed on the shoulder parts of the grown boules during PVT growth.

The stability of various types of multiple Shockley type basal plane stacking fault in heavily nitrogen-doped 4H-SiC crystals was theoretically investigated on the basis of the quantum well action (QWA) mechanism. The energy levels of confined electrons within multiple Shockley stacking faults (SSFs) were calculated using the simple quantum well model and compared to the experimental values estimated from photoluminescence measurements. The formation energies of the stacking faults were also calculated on the basis of the axial next nearest neighbor Ising (ANNNI) model for SiC polytypes. Based on these physical and electronic parameters, the stability of multiple SSFs in heavily nitrogen-doped 4H-SiC crystals was discussed, and it was revealed that the stacking sequence of multiple SSFs largely affects their stability; SSFs having a cubic stacking sequence of medium thickness (five to six Si-C bilayer thickness) are stable in heavily nitrogen-doped 4H-SiC crystals at the typical growth and device processing temperatures of SiC, i.e., 1500–2600 K.

We investigated the run-to-run fluctuation in growth conditions of physical vapor transport growth of 4H-SiC boules through observations of surface morphology on the (000-1) facet of the boules. The boules, which were grown under the same macroscopic growth conditions, exhibited slightly different surface morphologies. This indicates that some microscopic growth parameters that influence the surface morphology fluctuate between growth runs. We have considered the C/Si ratio of the vapor sublimed from the source material as a major parameter and discussed the associated variations in the physical and surface properties of the grown crystals.

The annealing behavior of electrical resistivities perpendicular and parallel to the basal plane of heavily nitrogen-doped 4H-SiC crystals was investigated. The temperature dependencies of the resistivities exhibited characteristic behaviors after multiple rounds of high-temperature annealing (1100°C, 30 min). High-temperature annealing induced stacking fault formation to various extents in heavily nitrogen-doped 4H-SiC crystals. Based on these results, we discuss the cause and mechanism of the observed annealing-induced changes in electrical resistivities of the crystals.

The defect structure at the growth front of 4H-SiC boules grown using the physical vapor transport (PVT) method has been investigated using high resolution x-ray diffraction and x-ray topography. The crystal parameters such as the c-lattice constant exhibited characteristic variations across the growth front, which appeared to be caused by variation in surface morphology of the as-grown surface of the boules rather than the defect structure underneath the surface. X-ray topography also revealed that basal plane dislocations are hardly nucleated at the growth front during PVT growth of 4H-SiC crystals.

The step structure on the (0001¯)C facet of 4H-SiC boules grown by the physical vapor transport growth method with different nitrogen doping concentrations was examined in various scales, using different types of microscopy, such as differential interference contrast optical microscopy (DICM) and atomic force microscopy (AFM). DICM observations unveiled characteristic macroscopic surface features of the facet dependent on the nitrogen doping concentration. AFM observations revealed the existence of step trains of half unit-cell height (0.5 nm) on the facet and found that their separation was undulated with a characteristic wavelength dependent on the nitrogen doping concentration, the higher the nitrogen concentration, the longer was the undulation wavelength of step separation. Based on these results, we discussed the origin and formation mechanism of the separation-undulated step structure observed on the (0001¯)C facet of nitrogen-doped 4H-SiC boules.

The influence of the facet trace region in the 4H-SiC substrate on the glide and propagation behaviors of basal plane dislocations (BPDs) in a 4H-SiC homoepitaxial layer was investigated using x-ray topography, high-resolution x-ray diffraction, and micro-Raman scattering spectroscopy. The facet trace region in the substrate, which has a higher nitrogen doping concentration than the other regions in the substrate, is of great interest since it could influence the glide and propagation behaviors of BPDs during 4H-SiC homoepitaxial growth through enhanced nitrogen doping concentration in the region. It was found that the epitaxial layer grown on the facet trace region in the substrate significantly suppressed glide motion of BPDs and exhibited a reduced conversion probability of BPDs to threading edge dislocations during the homoepitaxial growth process. Based on these results, the mechanisms that the epilayer grown on a substrate region with a slightly higher nitrogen concentration showed significantly different glide and propagation behaviors of BPDs are discussed.

Step bunching on a vicinal 4H-SiC (0001) epitaxial layer surface was investigated using low-voltage electron scanning microscopy (LVSEM) and electron channeling contrast (ECC) imaging. LVSEM observations revealed that the step bunching resulted in the formation of atomically flat wide (~250 nm) terraces on the surface, and the terraces tended to form in pairs. The two terraces in paired terraces often showed the same electron channeling contrast as each other, and the contrast of the two terraces, either bright or dark, appeared to be determined by the orthogonal misorientation of substrates. On the basis of these results, the formation mechanism of the step-bunched structure on a vicinal 4H-SiC (0001) surface is discussed.

The strain fields in a 4H-SiC homo-epitaxial layer deposited on a nitrogen-doped 4H-SiC substrate were studied using Raman scattering microscopy. The cross-sectional (1-100) and (11-20) surfaces of the epitaxial substrate were examined through the peak shifts of several Raman-active phonon modes for 4H-SiC, and tensile strain was found along the direction of 4° off the c-axis at the epilayer/substrate interface. The effect of the facet trace in the substrate, which has a higher nitrogen concentration than the other parts of the substrate, was also studied. The tensile strain at the epilayer/substrate interface was found to be hardly enhanced for the epilayer deposited on the facet trace.

The stacking fault formation during physical vapor transport growth of heavily nitrogen-doped (mid-1019 cm−3) 4H-SiC crystals was investigated. Low-voltage scanning electron microscopy (LVSEM) observations detected the stacking fault formation on the (000-1) facet of heavily nitrogen-doped 4H-SiC crystals. Stacking faults showed characteristic morphologies, and atomic force microscopy (AFM) studies revealed that these morphologies of stacking faults stemmed from the interaction between surface steps and stacking faults. Based on these results, the stacking fault formation mechanism in heavily nitrogen-doped 4H-SiC crystals is discussed.

An evaluation of p + high pressure and high temperature (HPHT) substrates as the seed crystals of p + substrates for power devices was carried out. The crystallinity measured by Raman spectroscopy showed a poor distribution, such as a standard deviation of 0.44 cm− 1 for the 1332 cm− 1 diamond peak in a 3 × 3 mm2 area, whereas the undoped crystal had a standard deviation of only 0.02 cm− 1. B doping distribution mapping was carried out for the first time using B-related peaks that appear because of crystallographic symmetry breaking due to B incorporation into the diamond lattice. High B concentration was observed in the [ 1 − 11 ] and [ 1 1 − 1 ] growth sectors and low B concentration was observed in the [111] and [ 1 − 1 − 1 ] growth sectors. This result presumably depends on the instrumental pressure and temperature control in the HPHT equipment.

Polarized Raman measurements were performed on P-doped diamond films to investigate the origin of the peaks in the lower wave numbers, which are unknown to date, and a comparative study of several P-doped substrates was conducted. The measurements with four configurations of the polarized geometries were carried out for five samples with different P and H concentration, [H]/[P] ratios, and film thickness. For four highly P-doped films, several Raman peaks were observed for all four configurations; retaining the Oh7 cubic structure of diamond, presumably relate to substitutional sites. For a sample of high [H]/[P] ratio (=1.17) and a very thick film, several other peaks were observed. The peak appearances for four configurations indicate that the origin is not in the Oh7 diamond structure but presumably interstitial sites. In the future, it may be possible to realize effective P doping by the improvement of film synthesis by utilizing the results of Raman polarization measurements, which are simple and nondestructive.

The range of polishing-induced subsurface damage remaining in a commercially available production grade 4H-SiC (0001) epi-ready substrate was evaluated by the observation from the (-1100) cleavage plane using two kinds of highly strain-sensitive characterization methods. Firstly, the analysis using electron backscattered diffraction (EBSD) with a submicron spatial resolution was conducted on the exposed cross sectional plane. Then, for the further quantitative evaluation excluding the influence of roughness or contamination of the cleavage plane, a synchrotron X-ray micro-diffraction experiment was carried out. The range of the subsurface damage evaluated in those experiments was ensured by confirming none of additional strain inserted at the cleavage, as compared with the damage-free substrate prepared by high temperature thermal etching. As a result, the depth of the residual strained region below polishing-induced scratches at the surface was estimated to be in the range of a few microns, which is much deeper than the previously reported value of 100 nm by cross-sectional transmission electron microscopy. It suggests a potential of EBSD for the conventional tool to characterize even a small amount of strain in SiC single crystal.

We have established the grazing-incidence x-ray diffraction (GIXD) method under ultra- high temperature for in-situ crystal growth observation. For this purpose, we have developed the compact furnace which can be mounted on a goniometer used for the GIXD experiment. Using the custom-designed furnace, we have succeeded in controlling very high temperature around 1800°C. Subsequently, we have proved the performance of the furnace through the measurement of the tem- perature dependence of a-lattice constant of 4H-SiC in the range from room temperature to 1500 °C.

The formation of basal plane stacking faults in highly nitrogen-doped 4H-SiC crystals was theoretically investigated. A novel theoretical model based on the so-called quantum well action (QWA) mechanism was proposed; the model considers several factors, which were overlooked in a previously proposed model, and explains well the annealing-induced formation of double layer Shockley-type stacking faults in highly nitrogen-doped 4H-SiC crystals. We further revised the model to consider the carrier distribution in the depletion regions adjacent to the stacking fault and were successful in explaining the shrinkage of stacking faults during annealing at even higher temperatures.

Basal plane bending and stress distribution in physical vapor transport-grown n-type 4H-SiC crystals were investigated. High resolution X-ray diffraction measurements were performed on commercially available 3-inch-diameter 4H-SiC substrates and along the growth front surface of as-grown 1-inch-diameter 4H-SiC boules. The measurements revealed that structural parameters such as the c-lattice constant, basal plane tilting, and FWHM showed characteristic variations across the substrates and as-gown boules, indicating that the crystals had a non-uniform distribution of dislocations comprising domain structures. Residual stress measured by micro Raman spectroscopy showed a similar behavior, which was an oscillatory spatial variation. On the basis of these results, defect structures in the crystals are elucidated.

Surface morphologies on the (000 1 ¯ ) C facet of 4H–SiC boules grown using the physical vapor transport method were examined in various scales (from millimeter to nanometer) using different types of microscopies such as differential interference contrast (DIC) optical microscopy and atomic force microscopy (AFM). DIC optical microscopic observation revealed that there are three distinct morphological regions at the growth front of the 4H–SiC boules: the facetted region, non-facetted region and intermediate region between them. The local inclination of the facet surface from the {0001} basal plane increases toward the edge of the facetted region and then decreases in the intermediate region. AFM observations revealed that characteristic step structures were established in these two regions and that nitrogen doping significantly influenced the stability of the step structures. Based on the results, the formation mechanism of surface morphologies on the (000 1 ¯ ) C facet of 4H–SiC boules is discussed.

The variation in nitrogen doping concentration on the ( 000 1 ¯ ) facet of 4H-SiC single crystals grown by the physical vapor transport (PVT) growth method was investigated using Raman scattering microscopy. The observed variation was well correlated with the terrace-width variation in the step–terrace structure on the facet; regions with unevenly distributed terrace-widths always showed a lower nitrogen concentration compared to those having evenly distributed terrace-widths. It was revealed by low-energy electron channeling contrast imaging that in lower nitrogen concentration regions, terraces with two dangling bonds per silicon atom at the step riser in the step-down direction always became wider. On the basis of these experimental results, the nitrogen incorporation kinetics during PVT growth of 4H-SiC crystals was discussed.

A novel characterization method using Raman scattering microscopy for basal plane stacking faults (SFs) and associated partial dislocations (PDs) in 4H-SiC crystals is proposed. Atomically thin SFs were successfully detected through the peak shift of the A1t transverse optical phonon mode in 4H-SiC, which could induce an electrical dipole moment within the SFs. The A1t mode was also sensitive to the strain field around the PDs encompassing SFs. Raman scattering microscopy imaging was able to determine the core type of PDs and their line direction within the crystals. Numerical simulation of Raman scattering microscopy images of PDs was also conducted and showed a quantitative agreement with the experimentally obtained images. Finally, the method was applied to the investigation of expansion behaviors of double Shockley SFs (DSSFs) in heavily nitrogen-doped 4H-SiC crystals during high temperature annealing and revealed several important aspects of DSSF expansion.

A novel characterization method for the nitrogen doping concentration in heavily nitrogen-doped (more than 1 × 1019 cm−3) 4H-SiC crystals using Raman scattering microscopy is proposed. The method utilizes the integrated intensity of the longitudinal optical phonon-plasmon coupled (LOPC) mode signal arising at 1100 cm−1, which exhibits marked changes in the peak position, intensity, and line shape when the nitrogen concentration in 4H-SiC crystals changes. The proposed method showed a much higher sensitivity to the nitrogen doping variation in 4H-SiC crystals compared to the conventional characterization methods using the LOPC peak shift and the Fano interference, and detected a small variation (~1%) in the nitrogen doping concentration across the (000) facet of heavily nitrogen-doped 4H-SiC crystals.

A mosaic substrate is a promising candidate to create large size single crystal diamonds for various types of applications. In this study, the crystal orientations of the joint areas of four single crystal plates of a mosaic substrate were measured using the high resolution electron backscatter diffraction method. The lattice rotation of [110] to [–110] (X-Y direction) was less than 0.5° and the lattice rotation mappings of [001] to [110] (Z-X direction) and [1–10] (Z-Y direction) were less than 0.2°. Considering that plate alignment is not particularly considered during the fabrication process, further improvement can be expected to realize a well- aligned mosaic substrate.

Defect formation during the initial stage of physical vapor transport (PVT) growth of 4H-SiC crystals in the [000-1] and [11-20] directions was investigated by x-ray diffraction, defect-selective etching, and micro Raman scattering imaging. X-ray diffraction studies showed that the growths in the [000-1] and [11-20] directions exhibited markedly different behaviors with respect to the defect formation during the initial stage of growth. While a characteristic lattice plane bending was observed for the PVT growth along [000-1], a tilted domain structure was revealed near the grown crystal/seed interface for the growth in the [11-20] direction. Micro Raman scattering imaging revealed that nitrogen enrichment occurred near the grown crystal/seed interface and was associated with compressive stress parallel to the interface. Based on the results, the defect formation mechanisms during the initial stage of PVT growth of 4H-SiC are discussed.

The surface morphology on the (000-1)C facet of 4H-SiC boules grown by the physical vapor transport method was examined in various scales (from millimeter to nanometer) using different types of microscopies such as differential interference contrast (DIC) optical microscopy and atomic force microscopy (AFM). The DIC optical microscope observation revealed that there exist three distinct morphological regions at the growth front of the 4H-SiC boules; they are facetted, non-facetted, and the intermediate region between them. The local inclination of the facet surface from the (000-1) basal plane increases toward the facet edge and then decreases over the intermediate region. AFM observations revealed characteristic step structures in these two regions and also that they are significantly influenced by nitrogen-doping. Based on the results, the formation mechanism of the facet morphology on 4H-SiC boules is discussed.

Defect formation during the initial stage of physical vapor transport (PVT) growth of 4H–SiC in the [ 11 2 ¯ 0 ] direction has been investigated by x-ray reciprocal space mapping (RSM), defect-selective etching, and low-voltage scanning electron microscopy. RSM studies showed that 4H–SiC crystals grown in the [ 11 2 ¯ 0 ] direction showed a significant degradation of crystalline quality during the initial stage of PVT growth, compared with crystals grown in the [ 000 1 ¯ ] direction; the growth in the [ 11 2 ¯ 0 ] direction resulted in a misoriented domain structure near the grown crystal/seed interface. At the interface, high densities of basal plane dislocations, extending parallel to the interface, and threading edge dislocations along the c-axis were observed. The former caused a tilt domain boundary around an axis parallel to [ 1 1 ¯ 00 ] . Based on the results, the paper discusses the defect formation mechanism during the initial stage of PVT growth in the [ 11 2 ¯ 0 ] direction and points to the difference in nitrogen concentration between the seed and the grown crystal as a cause of the observed domain and dislocation structure at the grown crystal/seed interface.

Defect formation during the initial stage of physical vapor transport (PVT) growth in the [000-1] and [11-20] directions has been investigated by x-ray reciprocal space mapping (RSM) and defect-selective etching. RSM studies showed that, while the crystal grown in the [000-1] direction did not show a significant degradation of the crystalline quality during the initial stage of growth, the growth in the [11-20] direction resulted in misoriented subgrain structure near the grown crystal/seed interface. The cause of the domain formation is assumed to be the difference in nitrogen concentration between the seed and the grown crystal, and the results indicate that the growth in the [11-20] direction is greatly affected by the nitrogen doping difference.

Structural investigation of the seeding process for the physical vapor transport (PVT) growth of 4H–SiC single crystals was conducted by high-resolution x-ray diffraction (HRXRD) and synchrotron x-ray topography. Characteristic lattice plane bending behavior was observed in the near-seed regions of the grown crystals. The bending of the (1120) lattice plane was localized near the seed/grown crystal interface, and the (0001) basal plane bent convexly in the growth direction near the interface, indicative of the insertion of extra-half planes pointing toward the growth direction during the seeding process for PVT growth. This study discusses a possible mechanism for the observed lattice plane bending and sheds light on defect formation processes during the PVT growth of 4H–SiC single crystals.

The orientation of growth sectors of highly B doped p+ high pressure and high temperature (HPHT) substrates were investigated by high angular resolution electron backscatter diffraction (HR-EBSD). The lattice rotation mapping images of the areas across the 〈111〉 growth sector boundaries were measured. The crystal orientation was found to be inclined approximately 0.03° for [001] to [-1-10], compared with the [-1-11] growth sector area at a given location. Other orientation inclinations were observed such as 0.02° for [001] to [110] and 0.025° for [001] to [1-10]. The same phenomena were confirmed for the other two crystals. Conventionally, it has been recognized that a HPHT-grown crystal is a perfect single crystal, however, our results indicate that "twin boundaries" are formed between the growth sectors for p+ HPHT substrates. To meet the requirements for the power device wafer specifications, the establishment of growth technology is desirable in the near future.

The formation of single Shockley stacking faults (SSSFs) in 4H-SiC crystals under non-equilibrium conditions (e.g., the forward biasing of PiN diodes and ultraviolet light illumination) is a key phenomenon in the so-called bipolar degradation of SiC power devices. This study theoretically investigated the physical mechanism of this phenomenon based on the concept of quantum well action. As a first approximation describing the non-equilibrium state of the material, we employed quasi-Fermi level approximation. We then made improvements by considering several physical effects governing the carrier distribution near and in the SSSF. The improved model accounts well for the excitation threshold and the temperature dependence of SSSF expansion. Thus, the model provides useful insights into the driving force of SSSF expansion under non-equilibrium conditions.

Wide bandgap (WBG) semiconductors, especially silicon carbide (SiC) and gallium nitride (GaN), promise transformational advances in electrical power systems, with increased power conversion efficiency for transportation and renewable energy utilization. Compared with the semiconductor silicon, SiC and GaN materials offer significantly higher electrical and thermal conductivities, increased avalanche breakdown field strength, and improved ruggedness under extreme environmental operating conditions. These basic material properties translate into more compact and lightweight power conversion systems with significant energy savings, much improved signal sensing and transmission characteristics, and longer operating life in the field.

The morphological and electrical properties of surface growth pits caused by dislocations in 4H-SiC epitaxial layers were characterized using tunneling atomic force microscopy. The characteristic distribution of the tip current between the metal-coated atomic force microscopy tip and the SiC was observed within a large surface growth pit caused by a threading screw dislocation. The current was highly localized inside the pit and occurred only on the inclined surface in the up-step direction near the pit bottom. This paper discusses the causes and possible mechanisms of the observed tip current distribution inside surface growth pits.

We report a new approach to produce high quality epitaxial graphene based on the concept of controlling Si sublimation rate from SiC surface. By putting a mask substrate to suppress Si sublimation from the SiC surface in ultrahigh vacuum, epitaxial graphene growth at 4H-SiC (0001) was locally controlled. Spatially graded surface graphitization was confirmed in a scanning electron microscopy contrast from the outside unmasked region to the inside masked region. The contrast was discussed with Raman characterization as the increase of graphene thickness and the surface compositional change of SiC. Results indicate two types of growth processes of epitaxial graphene at 4H-SiC (0001) step-terrace structures.

Defect formation during the early stages of physical vapor transport (PVT) growth of 4H-SiC was investigated using high resolution x-ray diffraction (HRXRD). Characteristic lattice bending behaviors were revealed in the nearby seed crystal regions of grown crystals. The lattice bending was localized in close proximity to the seed/grown crystal interface, and the (0001) basal planes bended convexly toward the growth direction, indicative of the insertion of extra-half planes pointing toward the growth direction during the initial stages of crystal growth. This paper discusses the possible mechanisms of the observed lattice bending and sheds light on the defect formation processes during PVT-growth of 4H-SiC single crystals.

Sublimation-recrystallization processes occurring during PVT are investigated from the viewpoint of quasi-equilibrium phase transitions of SiC. In addition to the elemental reaction processes of PVT, other phenomena such as silicon droplet formation and in-situ etching are also discussed based upon the Si-C binary phase diagram, and possible mechanisms are proposed.

The morphological instability appeared at step-free 4H-SiC (0001) surfaces was investigated. The step-free surfaces were fabricated at the bottom of inverted-mesa structure by the method combining a laser digging and Si-vapor etching. By repeated Si-vapor etching treatments, randomly created crater and maze structures were cyclically appeared at the step-free surfaces. These structures were distinctly classifiable by their depths from the step-free surfaces. Crater structures have 0.2 - 0.3 nm depth and maze structures have 0.5 nm depth. The morphological evolutions indicate the process of destruction of the step-free (0001) basal plane and generation of steps from step-free surfaces in the Si-vapor etching process.

Step-terrace structures were observed at on-axis/4o off 4H-SiC {0001} surfaces after Si-vapor etching which we have been supposed as an original technique to planarize and etch the SiC surfaces by utilizing a TaC crucible in temperature ranged from 1600 to 2200 oC. The structures obtained after the Si-vapor etching obviously indicated temperature dependence. There were two types of step-terrace structures in terms of the step height and the shape of the step edges at on-axis surfaces. Step bunched surfaces consisting of full unit cell height (= 1.0 nm) steps with {1-10n} facets at the step edges were observed at 4H-SiC (0001) in lower temperatures below 2000 oC, while smooth isotropic surfaces with half unit cell height (= 0.5 nm) steps and without any stable facets at the step edges were observed at 4H-SiC (0001) in higher temperatures above 2000 oC and in all temperature conditions (1600 - 2200 oC) at 4H-SiC (000-1). Similar tendency was also confirmed at 4o off 4H-SiC {0001} surfaces. From the comparison with 6H-SiC, macro step bunching (~10 nm height) was revealed to be a unique phenomenon at 4H-SiC (0001) surface in the etching.

Reduction of the performance-limiting defects in SiC epitaxial substrates is indispensable to further penetration of SiC devices into the power device markets. This paper provides an opportunity to overview the current issues in SiC epitaxial substrates, focusing on dislocation-related problems. Structural defects such as threading screw dislocations (TSDs) and basal plane dislocations (BPDs) as well as surface morphological defects on the epitaxial layers due to dislocations are of current concern. The paper reviews recent important results regarding these defects from the viewpoint of crystal growth and discusses their causes and formation mechanisms.