Takayuki Shirai

Diffusion of Transition Metals in 4H-SiC and Trials of Impurity Gettering

The diffusion of transition metals in 4H-SiC has been investigated by secondary ion mass spectrometry using epilayers and substrates implanted with titanium (Ti), chromium (Cr), iron (Fe), or nickel (Ni). Implanted Cr, Fe, and Ni atoms diffuse by subsequent Ar annealing at 1780 °C in n-type 4H-SiC epilayers. In n+-type substrates, the diffusivities of Ti, Cr, and Fe are almost negligible, while only Ni diffuses. By the helium implantation following the implantation of transition metals, no diffusion of Ti, Cr and Fe is observed in epilayers. The diffusion of transition metals in SiC is discussed based on the results of first-principles calculation.

Solution Growth of p-Type 4H-SiC Bulk Crystals with Low Resistivity

P-type 4H-SiC bulk crystals have been grown at a high growth rate of 1.0 mm/h by solution growth using Si-Cr-Al based melt. The crystals grown from solution with an Al content of 10at% show low resistivity of 35 mcm, which is two orders of magnitude lower than commercialwafers (Resistivity: 2500 mcm). The low-resistivity crystals have flat surface and few solvent inclusions. These results indicate that solution growth is a suitable method for fabricating low-resistivity p-type substrates with low cost.

Development of Solvent Inclusion Free 4H-SiC Off-Axis Wafer Grown by the Top-Seeded Solution Growth Technique

This study reports our newly developed technology for SiC solution growth. In particular, we succeed in completely suppressing solvent inclusions, which have been a serious technological problem peculiar to the solution growth method. Then, we fabricate two-inch-diameter 4° off-axis SiC wafers without solvent inclusions. Moreover, we performed their crystal defects evaluation. It was found that our wafers were low resistance n-type 4H-SiC and contain almost no basal plane dislocation. As a result, the superior quality of our solution-grown crystal was confirmed.

Dislocation Behavior in Bulk Crystals Grown by TSSG Method

The dislocation behavior during bulk crystal growth on the 4H-SiC (000-1) C-face using the solution method was investigated. A 2 inch wafer with a 4° off angle was fabricated from a bulk crystal grown by the TSSG method, and the dislocations in the crystal were evaluated using synchrotron X-ray topography and TEM observation. From the topograph images, it was found that the TSD density remarkably decreased as the growth progressed. Furthermore, the TEM observation suggested that TSD decreases as the threading dislocations convert to in-plane defects toward the center of the crystal. Conventionally, it was considered that conversion of threading dislocations hardly occurs in solution growth on the C-face. However, it is thought that this phenomenon was not observable because the conversion efficiency is remarkably low. We speculate that dislocations may be transformed by suddenly making macrosteps during bulk growth.

Dislocations in SiC Revealed by NaOH Vapor Etching and a Comparison with X-Ray Topography Taken with Various g-Vectors

Threading dislocations (TDs) in 4H-SiC have been studied by comparing etch pits formed by NaOH vapor etching with results of synchrotron monochromatic-beam X-ray topography (XRT) taken under different g-vectors. Burgers vectors determined based on XRT results were utilized to investigate the etch pit characteristics of edge (TED), screw (TSD) and mixed (Burgers vector b=c+a, TMD) threading dislocations. It has been found that pit formation by NaOH vapor etching was very different to that by conventional molten KOH etching. We discuss the possibility of using NaOH vapor etching to distinguish TMDs from TSDs, and report a variety of characteristic etch pits formed by this method and their correlations to dislocation behavior.

Cross-sectional observation of stacking faults in 4H-SiC by KOH etching on nonpolar

To evaluate the evolution of stacking faults (SFs) in 4H-SiC along the c-axis growth direction, techniques that can be used to determine the precise position of SFs and their fault types from cross-sectional nonpolar faces are urgently required. In this research, we have studied the feasibility of using cathodoluminescence (CL) imaging and face molten KOH etching to obtain information on the SF density and SF types. Particular attention has been paid to the possibility of determining the stacking sequence of SFs using their CL signatures or the geometrical properties of their KOH etch figures. Transmission electron microscopy (TEM) has been employed to clarify the atomic arrangement of SFs beneath the KOH etch figures, and a model is proposed to explain the formation of linear etch patterns during KOH etching due to the existence of SFs.

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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.