Materials Science Forum Vols. 600-603

Abstract: 4H-SiC was grown on 4H-SiC (1100) substrates by sublimation boule growth, and transmission electron microscopic investigation was carried out. Two basal-plane-dislocations in the same basal plane (the BPD pair), whose dislocation line extend toward the [1100] growth direction, were observed as aligned along [0001]. The density of the BPD pairs along [0001] was in the same order with that of the stacking faults in the sample. A threading screw-dislocation was observed in between aligned BPD pairs. It is proposed that the interaction between stacking faults and threading screw-dislocations on the grown surface generates the BPD pairs. Since a high density of stacking faults is inherent to the growth on the substrates perpendicular to (0001), keeping an atomically flat grown surface is important to prevent the generation of the threading screw-dislocations, and thus to suppress the generation of the BPD pairs in case of the growth on (1100) and/or (11 2 0) substrates.

Abstract: Etching of 4H-SiC wafers in molten KOH as a method for micropipe and dislocation density analysis was investigated. The obtained results were correlated with those of the synchrotron white beam x-ray topography. Heavily nitrogen-doped SiC shows a significantly different etching behavior in comparison with the low-doped material. This complicates identification of different types of threading defects. In particular, it is difficult to separate Threading Screw Dislocations (TSD) from Threading Edge Dislocations (TED). Depending on the level of doping and thermal history of the crystal, some of the etch pits emerging due to the 1c screw dislocations can be as large as those due to the micropipes.

Abstract: We have presented a combined method of microscopic measurements between Raman scattering and polarizing optical microscope to characterize inhomogeneous residual stress distributions around dislocations in 4H- and 6H-SiC wafers. First, stressed portions were found in wafers by an optical polarizing microscope under a crossed Nicole arrangement. Then, the portions were examined by Raman-imaging technique for lateral variations of phonon spectra. The residual stresses were quantified from the phonon-peak frequency shift using a known frequency-shift rate for 6H-SiC. Characterization to the depth direction was also conducted by surface etching with molten KOH. The stresses typically amounted to the order of 100 MPa. In a 4H-SiC homoepitaxial wafer sample, we observed threading dislocations transferred from the substrate to the epitaxial layer, and found that larger stress fields existed in the epitaxial layer than the substrate. We also observed stress distributions around compressively stressed sub-grain boundaries.

Abstract: The stacking fault formation in highly nitrogen-doped n+ 4H-SiC single crystal substrates during high temperature treatment has been investigated in terms of the surface preparation conditions of substrates. Substrates with a relatively large surface roughness showed a resistivity increase after annealing at 1100°C, which was confirmed to be caused by the formation and expansion of double Shockley-type basal plane stacking faults in the substrates. The occurrence of the stacking faults largely depended on the surface preparation conditions of the substrates, which indicates that the primary nucleation sites of stacking faults exist in the near-surface regions of substrates. In this regard, mechano-chemically polished (MCP) substrates with a minimum surface roughness (< 0.3 nm) exhibited no resistivity increase and very few stacking faults after annealing even when the nitrogen concentration of the substrates exceeded 1×1019 cm-3.

Abstract: Material defects such as Si-core and C-core partial dislocations (PDs) and threading screw dislocations (TSDs) and threading edge dislocations (TEDs) are being investigated for their contributions to device performances in 4H-SiC. Non-destructive electroluminescence and photoluminescence techniques can be powerful tools for examining these dislocations. In this report, these techniques were used to reveal the different spectral characteristics for the mentioned dislocations. At higher injection levels, both the Si-core and C-core PDs possessed a spectral peak at 700 nm. However, at lower injection levels, the spectral peak for the Si-core PD remained at 700 nm while the peak for the C-core moved to longer wavelengths. For the threading dislocations, TSDs possessed a peak between 800 and 850 nm while the TEDs possessed a peak at 600 nm independent of the injection levels.

Abstract: We investigated the expansion of single Shockley stacking faults (SSFs) in a 4H-SiC epitaxial layer under high-intensity scanning laser beam during room temperature photoluminescence mapping, which is similar to the degradation of bipolar pin diodes during forward current injection. In an epitaxial layer on an 8 off-axis (0001) substrate, the SSF-related intensity patterns induced by scanning high-intensity laser beam were classified into two types. The first one was a triangular pattern and the second a pattern which expanded in accordance with the motion of the scanning laser beam. The origins of the SSFs responsible for both patterns are presumably due to the preexisting basal plane dislocations and the dislocation-loops on the basal plane in the epitaxial layer, respectively. On the other hand, most of the SSF-expansion in on-axis (11 2 0) epitaxial layers were similar to the second type in the (0001) epitaxial layer. We, therefore, suggest that the dislocation-loops, which were located close to the surface, were dominant nucleation-sites of the SSFs in the (11 2 0) epitaxial layers.

Abstract: The influence of electron-beam irradiation on defects in 4H-SiC diode structures was investigated by cathodoluminescence (CL) microscopy and spectroscopy. In addition to threading edge and screw dislocations, two types of stacking faults (SFs) were characterized by their emission energy, geometric shape, and the sensitivity of electron-beam irradiation. The SFs at λ = 425 nm (2.92 eV) expand from the surface of basal plane dislocation with line direction [11-20] and change their geometric shape by electron-beam irradiation. The SFs at λ = 471 nm (2.63 eV) are only slightly influenced by electron-beam irradiation. The former corresponds to the Shockley-type SFs previously observed in the degraded p-i-n diodes, and the latter to in-grown SFs with 8H structure. The panchromatic CL images constructed by the sum of monochromatic CL images suggest that there are nonradiative recombination centers in the vicinity of Shockley-type SFs. The nucleation sites and the driving force for SF expansion are discussed.

Abstract: Electron-hole recombination activated Shockley partial dislocations bounding expanding stacking faults and their interactions with threading dislocations have been studied in 4H-SiC epitaxial layers using synchrotron x-ray topography. The bounding partials appear as white stripes or narrow dark lines in back-reflection X-ray topographs recorded using the basal plane reflections. Such contrast variations are attributable to the defocusing/focusing of the diffracted X-rays due to the edge component of the partial dislocations, which creates a convex/concave distortion of the basal planes. Simulation results based on the ray-tracing principle confirm our argument. The sign of the partial dislocations can be subsequently determined.

Abstract: X-ray rocking curve characterization is a relatively fast and nondestructive technique that can be utilized to evaluate the crystal quality of SiC substrates. The contribution of lattice curvature to rocking curve broadening is estimated, and shown to be the major contribution to the measured broadening (FWHM). The feedback on lattice quality is used to optimize our SiC growth process. In the optimized growth runs, the typical variation in rocking curve sample angle Ω across the entire 3” diameter wafer is about 0.2 degrees. Possible mechanisms leading to changes in the lattice curvature are discussed.

Abstract: This article describes the analysis of the polytype transformation of SiC ingot. We analyzed the sample by Raman spectroscopy and TEM observation. The result of the analysis shows the polytype is transformed from 4H-SiC to 6H-SiC, and then returned to 4H-SiC. We found that the direction of the c-axis is not the same as the growth direction of the ingot. And also we found the existence of 8H-SiC at the interface between 6H-SiC and 4H-SiC region by the selected area diffraction pattern and confirmed it by HR-TEM observation.