Hosei Hirano

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.

Low-Thermal-Conductive

The thermal conductivity kappa(T), thermal diffusivity alpha(T) and thermal dilatation dL(T)/L of the DyBaCuO bulk superconductor have been measured. The ab-plane kappa of the bulk at 50 K is about one third of that of the YBaCuO bulk, and moreover, the superconducting characteristics (critical current density Jc and superconducting transition temperature Tc) compare favorably with those of the YBaCuO bulk. The current-voltage (I-V) characteristics of the current lead fabricated from DyBaCuO bulk and the durability against the thermal quenching are as good as those of the YBaCuO lead. The DyBaCuO bulk is a promising material for the power current lead use to be replaced for the YBaCuO bulk

rm DyBaCuO

We have fabricated ten pieces of Y-Ba-Cu-O bulk current leads with a superconductor cross-sectional area 4 mm /spl times/4 mm and investigated the transport properties in liquid nitrogen at an applied magnetic field of 0.5 T. All current leads could carry larger than 2500 A. Furthermore, we presented a transport model of this type of current leads for predicting the I-V characteristics and the critical current, and confirmed that the predicted behavior agrees well with the experimental results.

Bulk Superconductor for Current Lead Application

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.

Critical current of Y-Ba-Cu-O bulk current leads

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.

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

A ferromagnetic material can be levitated by the pinning effect of a field-cooled superconductor. This paper presents two methods for modeling this effect: 1) an approximate calculation to determine the relationship between attractive force and air gap at both room temperature and superconductive temperature (77 K) and 2) a novel way of modeling the pinning effect by a finite-element method (FEM). A comparison of analytical and FEM results with experimental results verifies the validity of the methods. The methods can be used to estimate the systems behavior when the cylindrical yoke is replaced by a ring yoke. The stiffness of the system will increase by 70% (to 5.3 N/mm) when a ring yoke with the same surface area is used instead of a cylindrical yoke

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

A possible mechanism of hexagonal void movement during Physical vapor transport (PVT)-growth is proposed in terms of quasi-equilibrium phase transition process based upon the Si-C binary phase diagram. The hexagonal void movement can be realized when two different reactions occurs simultaneously: (1) SiC(s) solidification and (2) decomposition without graphitization. Further, the kinetic instability of the void movement observed is also discussed, and found to be explainable if the effect of the temperature gradient existing in the crystal grown in conventional PVT-process is included.

Numerical Modeling of Iron Yoke Levitation Using the Pinning Effect of High-Temperature Superconductors

Time-dependent evolutions of single and quadruple Shockley stacking faults (sSSF and 4SSF) in 4° off 4H-SiC epitaxial layers have been investigated. UV illuminations using an Hg-Xe lamp light source generate dissociations of basal plane dislocations (BPDs) into sSSFs whereas for 4SSFs no significant changes in shape occur. Detailed analyses of Photo-luminescence (PL) signals suggest that Si- and C-core partials have different PL spectrum distributions in the wavelength range larger than 750 nm, giving rise to images with different contrasts in PL mappings.

A Possible Mechanism for Hexagonal Void Movement Observed during Sublimation Growth of SiC Single Crystals

This paper describes the numerical results of electrical, thermal and mechanical properties in a model current limiting device with QMG bulk superconductor reinforced by various kinds of metal bypass. The electric circuit analysis for simulation of current sharing is coupled at each time step with the finite difference method to calculate the temperature distribution inside the device. The obtained results are compared with experimental data carried out previously, and they have a good agreement with each other. The profile of internal stress in the limiting devices is also estimated by means of a commercial code of finite element analysis, and it is concluded that the magnitude of mechanical stress applied to the superconductor is less than an allowable level for each model device.

Time Sequential Evolutions of Optically-Induced Single Shockley Stacking Faults Formed in 4H-SiC Epitaxial Layers

Dislocations in highly nitrogen-doped (N > 1×1019 cm-3) low-resistivity ( < 10 mcm) 4H-SiC substrates were investigated by photoluminescence imaging, synchrotron X-ray topography, and defect selective etching using molten KOH. The behavior of dislocations is discussed particularly in terms of their glide motion in the presence of a high concentration of nitrogen. The results indicate that nitrogen impurities up to mid 1019 cm-3 concentration do not show any discernible influence on the glide behavior of basal plane dislocations (BPDs) in 4H-SiC crystals grown by physical vapor transport (PVT) method.