Manabu Ishimaru

Critical current properties in HTS tapes

Abstract The author’s recent studies on the critical current properties in Bi-2223 multifilamentary tapes as well as YBCO coated IBAD tapes have been reviewed. Extended electric field vs. current density ( E – J ) characteristics were studied over wide range of temperature, T , magnetic field, B , and angle, θ . Based on the analysis of statistic J c distribution and scaling, we derived an analytical expression for J as a function of E , T , B , and θ . This method allows us to describe and extrapolate the nonlinear E – J characteristics even in extremely low E region and/or high B region based on rather simple measurements.

Recrystallization of MeV Si implanted 6H‐SiC

Microstructures of recrystallized layers in 8 MeV Si3+ ion implanted 6‐H‐SiC (0001) wafers have been characterized by means of transmission electron microscopy. Epitaxial recrystallization of buried amorphous layers was observed at annealing temperature as low as 1000u2009°C. Layer‐by‐layer epitaxy of 6H‐SiC initially occurred and it was changed to columnar growth when layer‐by‐layer growth exceeded 100 nm in thickness. From the microdiffraction analysis, it was found that the columnar regions are defected 6H‐SiC with crystal orientations different from the substrate. In addition to 6H‐SiC, epitaxial 3C‐SiC was also confirmed in the recrystallized layer. Based on these results, we have proposed a structure model of the recrystallized layer in which stacking faults in the columnar regions are induced by mismatched connections between the columnar and layered 6H‐SiC regions.

Estimation of E–J characteristics in a YBCO coated conductor at low temperature and very high magnetic field

Abstract E – J characteristics in a YBCO coated conductor at low temperature and very high magnetic field up to 27 T have been estimated using experimental E – J characteristics below 12 T along with a scaling analysis based on the percolation model. 1 mm thick YBCO films was deposited on a biaxially aligned Y 2 O 3 /YSZ/Hastelloy substrate fabricated by the IBAD technique. The critical current I c for the 1 cm wide tape was 85 A at 77 K in self-field. We measured E – J characteristics in perpendicular field up to 12 T using 100 μm wide and 1 mm long bridge patterned in the tape. Statistic J c distribution and pinning properties were analyzed from the E – J characteristics. Using the pinning parameters determined by the scaling analysis, we can calculate the critical current properties as a function of temperature and magnetic field. E – J characteristics at low temperature and very high magnetic field up to 27 T were estimated analytically and compared with the experimental data. Estimated value of J c and n -value, and their error analysis were also discussed.

Percolative transition and scaling of transport E-J characteristics in YBCO coated IBAD tape

We have investigated extended electric field-vs.-current density E-J characteristics in YBCO coated IBAD tapes. The results of a Monte-Carlo study on the depinning in a random pin medium were compared with measurements. Using a low temperature scanning laser microscope, we examined the percolative behavior of the local resistive transition in the YBCO tape. It was also shown that the depinning probability, which is proportional to the dynamic resistance of the E-J curves, is scaled as a function of reduced current density with the aid of a power index. Consequently, the E-J characteristics in the tapes can be described by the combination of the two kinds of scaling laws: one is the scaling law of the depinning probability predicted in a random network and the other is the scaling law for the flux pinning force. These properties agree well with the percolation model of depinned clusters.

Amorphization and solid phase epitaxy of high-energy ion implanted 6H-SiC

Abstract We have investigated microstructures of damaged and recrystallized layers in MeV-ion implanted 6H-SiC (0001) wafers by means of cross-sectional transmission electron microscopy. The substrate surfaces were implanted at 160°C with 1 × 1017/cm2 8 MeV Si3+ ions using a tandem accelerator. A buried amorphous layer was formed ranging from ∼ 1.6 μm to ∼ 3.4 μm in depth. The amorphous/crystalline transition regions consisted of many stacking faults perpendicular to the [0001] direction, and their density increased toward the amorphous region. The amorphous layer regrew epitaxially from the undamaged substrate at an annealing temperature of ∼ 1000°C. This epitaxial 6H-SiC layer changed to columnar 6H-SiC with crystal orientations different from the substrate. In addition to these crystalline 6H-SiC, the existence of polycrystalline 3C-SiC was confirmed in the middle part of the recrystallized layer.

Microstructural evolution of oxygen implanted silicon during annealing processes

Abstract Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and electron energy loss spectroscopy (EELS) have been employed to characterize the microstructures, oxygen distributions, and chemical bonding states of SIMOX (Separation by IMplanted OXygen) wafers. Silicon substrates were implanted at 550°C with 2xa0×xa010 17 and 4xa0×xa010 17 cm −2 180 keV oxygen ions, followed by annealing at various temperatures. EELS spectra indicated that the chemical bonding state between Si and oxygen changed at 1000°C anneal, though there was no appreciable change in the microstructures observed by TEM below 1000°C anneal. Above 1100°C, oxygen precipitates were preferentially formed around the peak positions of the defect and oxygen concentrations in the as-implanted samples. These precipitates aggregated to reduce the surface energy, and their size increased with temperature. After 1350°C anneal, a continuous buried oxide layer was formed in the higher-dose specimen, while the upper- and lower-precipitates remained separately in the lower-dose one. The XPS profiles did not change at the later stage of annealing processes in the 2xa0×xa010 17 cm −2 sample, though the precipitates became larger in size. This suggests that the oxygen diffusion mostly occurs along the lateral direction but does not along the vertical one in this sample

Transmission electron microscopy studies of crystal-to-amorphous transition in ion implanted silicon

The microstructure of 5 MeV ion implanted silicon at room temperature has been investigated in detail by means of cross-sectional transmission electron microscopy. Buried amorphous layers were observed in the specimens obtained by ion doses of 1×1017 and 2×1017/cm2 with abrupt amorphous-to-crystal interfaces. Damaged layers adjacent to the amorphous layers included many dislocation loops and the concentration increased toward the amorphous region. Microdiffraction patterns and high-resolution images showed that this damaged region is defective crystalline silicon, suggesting that homogeneous amorphization occurs due to an accumulation of defects. The atomistic structure of the damaged regions was analyzed by comparing high-resolution electron microscopy images with those calculated on the basis of a model for amorphization processes proposed previously [T. Motooka, Phys. Rev. B 49, 16u2009367 (1994)].

Dose dependence of microstructural evolution in oxygen-ion-implanted silicon carbide

Transmission electron microscopy (TEM) and Rutherford backscattering spectroscopy and ion channeling (RBS/C) have been used to characterize the microstructures and damage distributions in oxygen-ion-implanted silicon carbide (SiC). 6H–SiC single crystals with [0001] orientation were irradiated with 180 keV O+ ions at 650u200a°C to fluences ranging from 0.2×1018 to 1.4×1018/cm2. No continuous buried amorphous layer was formed in the specimen with 0.2×1018u200aO+/cm2, although there were striated regions consisting of amorphous and crystalline structures at depth between 200 and 300 nm. A continuous buried amorphous layer appeared above 0.4×1018u200aO+/cm2, and the amorphous regions grew in size with increasing fluence. TEM and RBS/C results indicated that microstructures and elemental distributions change drastically between 0.7×1018 and 1.4×1018u200aO+/cm2.

Monte Carlo simulation of CuPt-type ordering in off-stoichiometric III-V semiconductor alloys

CuPt‐type ordering in off‐stoichiometric III‐V semiconductor alloys has been investigated by a Monte Carlo method based on an Ising‐like crystal growth model for the layer‐by‐layer stacking on a (001) substrate. (110) Fourier power spectra of the atom configurations thus obtained exhibit superlattice spots due to the CuPt ordering. In the (110) spectra, satellite reflections arise, flanking the fundamental lattice spots of the zinc‐blende structure and their intensity increases with the progress of ordering. The results are in good agreement with the experimental results previously obtained by transmission electron microscopy. The calculated atomic arrangement confirmed that the compositional modulation is the cause of the satellite reflections along the [110] direction.

Monte Carlo simulation of L11-type ordering due to surface step migration in the epitaxial growth of III-V semiconductor alloys

We investigated L11-type ordering due to surface step migration in III-V semiconductor alloys of (A0.5B0.5)IIICV, using an Ising-like crystal growth model. A surface step along the [110] direction (the [110] step), which has a preference for unlike-atom pairs along the step, bears (111) and (111) variants (CuPtB-type) of the four possible L11 variants. On the other hand, a surface step along the [110] direction (the [110] step), which has a preference for like-atom pairs, produces either of the two CuPtB-type variants depending on the direction of the step motion. Antiphase boundaries are inserted parallel to (001) planes in the former case, and (111) or (111) in the latter case. The ordering rate in the case of the [110] step is larger than that for [110] step migration. These results explain well the previous experimental results obtained by transmission electron microscopy.