Hirokatsu Yashiro

Mechanism of Molten KOH Etching of SiC Single Crystals : Comparative Study with Thermal Oxidation

The etching mechanism of SiC single crystals by molten KOH has been investigated. The etching process is significantly affected by the etching ambience: the etching rate is greatly reduced by a nitrogen gas purge. This result clearly suggests an essential role of dissolved oxygen in the melt. SiC{0001} surfaces show a large surface polarity dependence, where the etching rate of SiC(0001)C is about four times larger than that of SiC(0001)Si. The etching rate of SiC(0001)C exhibits an Arrhenius type temperature dependence with an activation energy of 15–20 kcal/mol. The obtained activation energy and selectivity between the (0001)C and the (0001)Si surfaces are quite similar to those for thermal oxidation, which implies that the surface oxidation process occurs during molten KOH etching of SiC and is the rate-limiting step for the etching. We have conducted a comparative study of molten KOH etching with thermal oxidation in regard to the crystal orientation, polytype and carrier concentration dependence.

Growth of large high-quality SiC single crystals

Abstract The availability of large high-quality silicon carbide (SiC) single crystals is a key issue in the development of the full potential of SiC-based device technology. In this paper, recent achievements in bulk crystal growth of SiC are reviewed. We present results on the physical vapor transport growth of SiC bulk single crystals by highlighting the crystal diameter enlargement and the quality improvement of SiC crystals. The causes and formation mechanisms of crystallographic defects, such as micropipes and low-angle grain boundaries, in SiC crystals are discussed. The results of the growth perpendicular to the c -axis are also reported, where stacking faults are defects of major concern. We present an atomistic surface model for the stacking fault generation and discuss a possible way to circumvent this problem.

Growth of Crack-Free 100mm-Diameter 4H-SiC Crystals with Low Micropipe Densities

The theromoelastic stress in post-growth SiC crystals has been investigated in order to suppress the cracks which were frequently observed in SiC crystals with larger diameters. Optimizing the temperature distribution in growing crystals lead to reduction of tensile stress components, and thus resulting in crack-free 100mm diameter SiC crystals with micropipe (MP) densities of 0.025/cm2. The concept of process optimization we established is confirmed to be effective to the growth of large diameter SiC crystals with mechanical stability.

Impurity incorporation kinetics during modified-Lely growth of SiC

The impurity incorporation kinetics during modified-Lely growth of silicon carbide (SiC) have been studied in terms of several growth parameters. It was found that the nitrogen incorporation is well described by a Langmuir isotherm type equation, implying that dynamic equilibrium between the vapor phase and the adsorbed nitrogen is established. The polytype of grown crystal and the seed orientation influence the impurity incorporation. For growth on (0001)C, 6H-SiC crystals always incorporate more nitrogen and less boron than 4H-SiC crystals, while no clear polytypic dependence of impurity incorporation is observed for growth on (1100) and (1120). Atomic force microscope observations revealed that there is a marked difference in the growth morphology between 6H-SiC(0001)C and 4H-SiC(0001)C. The origin of the polytypic dependence of impurity incorporation during growth on (0001)C is discussed with reference to the growth surface morphology.

Nitrogen Incorporation Kinetics during the Sublimation Growth of 6H and 4H SiC

Nitrogen incorporation kinetics during the sublimation boule growth of SiC have been studied in terms of several growth parameters. 6H and 4H SiC crystals were heavily doped with nitrogen as a donor. It was found that the growth rate has little influence on the doping concentration, indicating that nitrogen incorporation is not kinetically limited at normal growth rates in the sublimation growth. On the other hand, surface polarity and polytype were found to influence the nitrogen incorporation kinetics at the growth front. By optimizing the growth conditions, bulk resistivities as low as 7.6×10-3 Ω cm and 5.3×10-3 Ω cm were obtained for 6H and 4H SiC, respectively.

Development of large single-crystal SiC substrates

This article reviews the recent development of large single-crystal silicon carbide (SIC) substrates. The technological potential of SiC for high-power, high-temperature, and high-frequency electronic devices has been recognized for several decades; however, such applications have been largely hindered by problems related to bulk crystal growth. The SiC bulk crystal growth technology has recently achieved drastic improvement and enabled the growth of large high-quality single crystals. Due to the availability of large high-quality substrates, progress in SiC thin-film epitaxy and devices has been rapid, and application of SiC to many fields is fast reaching its real potential.

Development of Lapping and Polishing Technologies of 4H-SiC Wafers for Power Device Applications

The development of lapping and polishing technologies for SiC single crystal wafers has realized the fabrication of an extremely flat SiC wafer with excellent surface quality. To improve the SiC wafer flatness, we developed a four-step lapping process consisting of four stages of both-side lapping with different grit-size abrasives. We have applied this process to lapping of 2-inch-diameter SiC wafers and obtained an excellent flatness with TTV (total thickness variation) of less than 3 μm, LTV (local thickness variation) of less than 1 μm, and SORI smaller than 10 μm. We also developed a novel MCP (mechano-chemical polishing) process for SiC wafers to obtain a damage-free smooth surface. During MCP, oxidizing agents added to colloidal silica slurry, such as NaOCl and H2O2, effectively oxidize the SiC wafer surface, and then the resulting oxides are removed by colloidal silica. AFM (atomic force microscope) observation of polished wafer surface revealed that this process allows us to have excellent surface smoothness as low as Ra=0.168 nm and RMS=0.2 nm.

Stepped structure on the {0001} facet plane of α-SiC

Abstract The surface topography of the {0001} facet of as-grown 6H and 4H SiC boules was observed ex-situ by atomic force microscopy (AFM). Peculiar stepped structures of SiC{0001} to the growth face polarity and the polytype were detected. Height steps equal to the c-lattice parameter (1.5 nm) were observed to be dominant on the 6H-SiC(0001)Si surface. They were very regularly arranged, i.e. straight and almost equally spaced. On the other hand, meandering macrosteps of height more than 10 nm were observed on the 6H-SiC(000 1 )C and the 4H-SiC(000 1 )C surfaces. Based on the results, the mechanism of formation of these surface morphologies is discussed.

Defect Formation During Sublimation Bulk Crystal Growth of Silicon Carbide

The defect formation during sublimation bulk crystal growth of silicon carbide (SiC) is discussed. SiC bulk crystals are produced by seeded sublimation growth (modified-Lely method), where SiC source powder sublimes and is recrystallized on a slightly cooled seed crystal at uncommonly high temperatures (≥2000°C). The crystals contain structural defects such as micropipes (hollow core dislocations), subgrain boundaries, stacking faults and glide dislocations in the basal plane. The type and density of the defects largely depend on the crystal growth direction, and many aspects are different between the growth parallel and perpendicular to the c-axis. Micropipes are characteristic defects to the c-axis growth, while a large number of stacking faults are introduced during growth perpendicular to the c-axis. We discuss the cause and mechanism of the defect formation

Analysis of Basal Plane Bending and Basal Plane Dislocations in 4H-SiC Single Crystals

4H-SiC single crystals were grown by the physical vapor transport (PVT) growth method under different thermoelastic stress conditions, and the degree of basal plane bending in the crystals was characterized by the peak shift measurement of X-ray rocking curves. The results indicate that the degree of basal plane bending largely depends on the magnitude of the thermoelastic stresses imposed on the crystals during PVT growth. Quantitative analysis of basal plane bending revealed that the density of basal plane dislocations (BPDs) estimated from basal plane bending is much smaller than that obtained from defect-selective etching. It was also found that the BPD density is correlated with the threading screw dislocation (TSD) density in PVT-grown SiC crystals. These aspects of BPDs were discussed in terms of the BPD multiplication process triggered by the intersection of BPDs with a forest of TSDs extending along the c-axis.