Papers by Author: Katsunori Danno

Abstract: We performed top-seeded solution growth of 4H-SiC for obtaining longer length crystal. Si-Cr and Si-Ti melts were used as solvents. Meniscus formation technique was applied to the present study. Special attention was paid to improve the process stability during long-term growth. One of major technological problems in the solution growth is that the precipitation of polycrystalline SiC which hiders the stable single crystal growth. Another problem is the fluctuation of supersaturation at the growth interface during the growth. Through the optimization of growth process conditions, we have successfully grown 4H-SiC single crystals up to 14 mm long with three-inch-diameter, and evaluated their crystalline quality.

Abstract: A threading dislocation (TD) in 4H-SiC, which was currently interpreted as a perfect threading edge dislocation (TED) by synchrotron monochromatic-beam X-ray topography (SMBXT) and molten KOH etching with Na₂O₂ additive, was performed comparative characterization using weak-beam dark-field (WBDF) and large-angle convergent-beam electron diffraction (LACBED) methods. The TD was suggested to be dissociated into a dislocation pair which can be observed in the WBDF image of g=-12-10. The TD, which was identified as b//[-12-10] by SMBXT observation, was unambiguously determined as b=1/3[-12-10] by LACBED analysis. In the case of perfect TED, it was found that the direction of Burgers vector derived from SMBXT observation corresponds to LACBED analysis.

Abstract: In this paper, we report a newly developed dislocation-revealing etch pit method for 4H-SiC single crystal, which can distinguish edge (TED, Burgers vector b=a), elementary screw (TSD, b=1c) and mixed (TMD, b=c+a) threading dislocations. In this method, vaporized NaOH gas was used to etch the Si-face of a SiC wafer at substrate temperature around 950 °C. By a side-by-side comparison between the optical images of the etch pits and the X-ray topographic (XRT) images, it has been found that threading dislocations (TDs) in SiC could be revealed as hexagonal etch pits with distinct geometrical features (shape, size and facet orientation) depending on their Burgers vectors. Based on these results, we consider this etch pit method as an easily-operated and inexpensive technique to categorize TDs, and it may help to promote our understanding on the different roles that these types of TDs have played in the performance degradation of SiC power devices.

Abstract: To solve the problem that no preferential chemical etching is available for dislocation revelation from the carbon-face (C-face) of 4H-SiC, a novel etching technique using vaporized KOH has been developed. It was found that this etching technique can reveal the three commonly found dislocation types, i.e., threading screw dislocations (TSDs), threading edge dislocations (TEDs) and basal plane dislocations (BPDs) as large hexagonal, small hexagonal and triangular, respectively. Centimeter-scale dislocation mapping has been obtained, and the pit positions on the C-face were compared with those on the Si-face, to study the dislocation propagation behaviors across the sample thickness. We have found one-to-one correlation for nearly 96% of the TSDs, indicating a dominant proportion of TSDs penetrate the whole wafer thickness. The vaporized KOH etching technique has provided an effective and inexpensive method of making inch-scale mapping of dislocation distribution for the C-face epitaxial and bulky 4H-SiC.

Abstract: We developed the transmission electron microscopy (TEM) sample preparation technique for the low dislocation density of 4H-SiC by combining the KOH+Na₂O₂ (KN) etching and the focused ion beam (FIB) microsampling technique. The dislocation under sea-shell pit was then characterized by large-angle convergent-beam electron diffraction (LACBED). It is demonstrated that this method is powerful for evaluating Burgers vectors of dislocations. Burgers vector of the measured basal plane dislocation (BPD) is determined to be b=1/3[-12-10]. Two-beam bright-field (TBBF) imaging identified the rotating direction of the threading screw dislocation (TSD) is counter-clockwise.

Abstract: Electron beam induced current (EBIC) and etch pit method have been used to study the dissociation behavior of basal plane dislocations (BPDs) in 4H-SiC under electron beam irradiation. When 20 kV scanning electron beam was applied for 1 h, it has been found that BPDs whose dislocation lines were along [11-20] off-cut direction dissociated into partial dislocations (PDs) forming a stacking fault (SF) between them; while no dissociation was found for BPDs extending along other directions. These results are discussed in terms of different formation energy of SFs expanding from a pure screw type and a mixed type BPD. In addition, the angle between dislocation line of a BPD and the [11-20] off-cut direction might also play a role in determining the minimum energy for SF formation.

Abstract: The wide size distribution of the hexagonal etch pit of screw dislocations (SD) in 4H-SiC wafer was found in spite of the narrow size distribution of the SD pit in epitaxial film. Calculation on the basis of the strain energy equation indicated that etch pit size depends on the Burgers vector and dislocation tilt. Size variation of SD etch pits in 4H-SiC wafer fabricated by sublimation method is explained to be caused by the dislocation tilt by observing the sizes and the positions of etch pits from the surface of the epitaxial film to the inside of 4H-SiC wafer. The SDs in 4H-SiC wafer fabricated by sublimation method propagate to c-axis direction in macroscopic but changing tilt in microscopic.

Abstract: Diffusion of transition metals in 4H-SiC has been investigated by secondary ion mass spectroscopy using epilayers and substrates implanted with titanium (Ti), chromium (Cr), iron (Fe), or nickel (Ni). In the epilayers, Cr, Fe, and Ni atoms have diffused by argon (Ar) annealing at 1780°C for 30 min. In n⁺ substrates, the diffusivity of the metals is smaller than that in the epilayers, and only Ni has diffused by the annealing. By the Ar or helium implantation following the implantation of transition metals, diffusion of transition metals can be successfully suppressed.

Abstract: Dislocations in highly doped n-type 4H-SiC (n+-SiC, n>1019 cm-3) substrate have been studied by means of electron beam induced current (EBIC). Ni/n-SiC/n+-SiC/Al structure was fabricated in order to simultaneously observe the dislocations in n-SiC epilayer and n+-SiC substrate. We have found that dark dots in the EBIC image correspond to threading screw dislocations (TSDs) and threading edge dislocations (TEDs) with the former being relatively darker. Short dark lines along off-cut are attributed to basal plane dislocations (BPDs) in the epilayer; and the randomly oriented long dark lines are caused by the BPDs in the substrate. The classification of the dislocations by EBIC has been examined by wet etching in KOH+Na2O2.

Abstract: We have proposed a new wet etching recipe using molten KOH and Na2O2 as the etchant (“KN etching”) for dislocation revelation in highly doped n-type 4H-SiC (n+-4H-SiC). Threading screw dislocations (TSDs) and threading edge dislocations (TEDs) have been clearly revealed as hexagonal etch pits differing in pit sizes, and basal plane dislocations (BPDs) as seashell-shaped pits. This new etching recipe has provided a solution to the problem that conventional KOH etching is not effective for dislocation identification in 4H-SiC if the electron concentration is high (>mid-1018 cm-3). We have investigated the effect of SiC off-cut angle on KN etching and it has been shown that the “KN etching” is applicable for the n+-SiC substrate with off-angle from 0o to 8o.