Ichiro Nagai

Highest conductivity oxide SrMoO3 grown by a floating-zone method under ultralow oxygen partial pressure

Single crystals of a highly conductive oxide SrMoO3 have been grown by a floating-zone method under argon atmosphere with ultralow oxygen partial pressure p(O2)∼10−25atm. The obtained single crystals of SrMoO3 reveal quite low resistivity at 300 K ρ(300K)=5.1μΩcm, which is the lowest to date in the values of ρ(300K) of all the oxides. Resistivity and specific heat data suggest that electrons in SrMoO3 behave as the Fermi liquid (correlated electron), and that its low resistivity is due to the extremely small electron-phonon interaction in SrMoO3.

Influence of Processing and of Material Defects on the Electrical Characteristics of SiC-SBDs and SiC-MOSFETs

The influences of processing and material defects on the electrical characteristics of large-capacity (approximately 100A) SiC-SBDs and SiC-MOSFETs have been investigated. In the case of processing defects, controlled activation annealing is the most important factor. On the other hand for material defects, the number of epitaxial defects must be decreased to zero for both SBDs and MOSFETs. The dislocation defects in SiC wafers are dangerous for the breakdown voltage of MOSFETs. However, they are not killer defects. If the epitaxial defect density is sufficiently low and the dislocation density is in the order of 10000cm-2, the long- term reliability of the gate oxide at the electric field of 3MV/cm can be guaranteed.

Observation of Misfit Dislocations Introduced by Epi-Layer Growth on 4H-SiC

4H-SiC substrate wafers with epi-layers were observed using monochromatic synchrotron X-ray topography in grazing incidence geometries, to investigate the defects in the epi-layer. Misfit dislocations with b=+1/3[11 2 0] caused by the difference in lattice parameter between the epi-layer and the substrate were observed. The misfit dislocations are located near the interface as edge dislocations, and appear at the top surface as screw dislocations on basal planes. It was observed that more than half of them were introduced from the growing epi-layer surface. The misfit dislocations and some screw dislocations with b=+1/3[11 2 0] are observed to remain as basal plane dislocations at the surface, while other basal plane dislocations were converted to threading edge dislocations in the epi-layer.

Contrast of Basal Plane and Threading Edge Dislocations in 4H-SiC by X-Ray Topography in Grazing Incidence Geometry

Dislocations in a substrate wafer of 4H-SiC with an epi-layer were observed using technique of monochromatic synchrotron X-ray topography in a grazing incidence geometry. Six different Burgers vectors of basal plane dislocations and threading edge dislocations were identified by changing the Bragg reflections, and by analysis of images of dislocation. We identify some relations of the Burgers vector and the dislocation contrast observed for g=11 2 8. Some of these relationships are discussed in this report.

Magnetic and transport properties of bilayered perovskite Sr3Ir2O7

We report magnetic and transport properties of polycrystalline samples of bilayered perovskite Sr3Ir2O7, which is synthesized by using high pressure technique. Magnetic susceptibility shows weak ferromagnetism below 290K with small size of remanent moment (10−3μB/Ir). Resistivity shows insulating behavior below 290K. We discuss the magnetism and conductivity in Sr3Ir2O7 comparing with those of Sr3Ru2O7, which has a quite similar crystal structure to Sr3Ir2O7, from the view point of rotation angle of MO6 (M=Ir, Ru) octahedra.

Quasi-Two-Dimensional Fermi-Liquid State in Sr2RhO4-δ

Single crystals of layered perovskite Sr 2 RhO 4-δ (δ=0.0 and 0.1) are successfully grown by the floating-zone method. Stoichiometric single crystals (Sr 2 RhO 4.0 ) are obtained by O 2 -annealing the as-grown crystals (Sr 2 RhO 3.9 ). Sr 2 RhO 4.0 and Sr 2 RhO 3.9 show quasi-two-dimensional Fermi-liquid behavior at low temperatures, whereas there are large differences in the anisotropy of electrical resistivity ρ c (3 K)/ρ a b (3 K) and Wilson ratio R w between Sr 2 RhO 4.0 and Sr 2 RhO 3.9 : ρ c (3 K)/ρ a b (3 K) = 2400 (19000) and R w =3.8 (6.4) for Sr 2 RhO 4.0 (Sr 2 RhO 3.9 ). The differences observed between the temperature dependence of the in-plane electrical resistivity ( T <15 K), magnetic susceptibility ( T <300 K) and specific heat ( T <10 K) of Sr 2 RhO 4.0 and Sr 2 RhO 3.9 are mainly derived from those between the density of states and band structure near the corresponding Fermi level. This indicates that the changes in these physical properties, which are accompanied by oxygen defects in th...

Dislocation Contrast of 4H-SiC in X-Ray Topography under Weak-Beam Condition

We have investigated dislocation image of 4H-SiC wafers projected on synchrotron X-ray topographs taken under different positions in the rocking curve of a diffraction peak. The diffraction geometry was grazing-incidence extremely asymmetric and the diffraction vectors were g = 1 1 2 8 and 112 8. The weak-beam images were demonstrated for basal-plane dislocations and threading-screw dislocations. The basal-plane dislocation images became narrower in width at the off-Bragg conditions, and they were decomposed to separate lines under the weak-beam condition. The threading-screw dislocations showed changes in their shape and contrast as the crystal set was tilted from the rocking-curve peak, and finally the characteristic images near the dislocation core were observed under the weak-beam condition. The origin of these weak-beam images is unclear, but it will offer detailed analysis of the dislocations.

AlN bulk single crystal growth on SiC and AlN substrates by sublimation method

Aluminum nitride (AlN) bulk single crystals up to 45 mm in diameter and 10 mm in thickness have been successfully grown on 6H-SiC(0001) substrates by the sublimation method. Moreover, high growth rate of 200μm/h has been achieved by high temperature homo-epitaxial AlN growth on AlN(0001) substrate. The Raman spectrum indicates that the polytype of the grown AlN single crystals is a Wurtzite-2H type structure, and the crystals do not include any impurity phases. In both the hetero- and homo-epitaxial AlN bulk single crystal growth, the quality at the top of the crystal improves as the crystal thickness increases along the <0001> direction during the growth: low etch pit density 7 × 104 cm−2 and 1 × 104 cm−2 have been achieved at a thickness of 8 mm and 2 mm in the hetero- and homo-epitaxial growth, respectively.

Voltage-Current (V-I) Characteristics of 1.5kV Class pn Junctions with p-Well Structures on (0001) 4H-SiC

We have fabricated the four pn-type junction TEGs (Test Element Groups) having different structure. Those TEGs are close to the double-implanted (Di) MOSFETs, step by step from the simple pn diode. Voltage-current (V-I) characteristics of the hundred TEGs having p-well structure show similar blocking characteristics of those of simple pn diodes on the same wafer. This indicates that the p-well structure itself does not cause a significant deterioration on the blocking yield. On the other hand, the yield is significantly influenced by the annealing condition for ion-implanted layer. The oxide-related hard breakdown on the JFET region dominates the blocking yield. The reach-through breakdown of the TEGs having the n+ region within each p-well becomes largely suppressed by the high-temperature and short-time annealing.

Magnetic Properties of Ca3Ru2O7 Grown by a Floating Zone Method

We have succeeded in growing single crystals of Ca3Ru2O7 by a floating zone method. The temperature dependence of magnetic susceptibility was measured in the temperature range from 2 to 300 K for the field along ab-plane and c-axis. The magnetic susceptibility shows anisotropic behavior. For T>55 K, the susceptibilities of both directions increase like the Curie–Weiss behavior with decreasing temperature. In the temperature range between 55 and 48 K, the susceptibility along ab-plane decreases steeply, while the one along c-axis is almost constant, which implies that the magnetic moment is aligned antiferromagnetically in the ab-plane. The susceptibilities drastically decrease in both directions at 48 K and are almost independent of temperature below 30 K.