arXiv subjects

Yukari Ishikawa

Publications and source records attributed to Yukari Ishikawa.

7 recordsLinked to original sources

arXiv2026

Groove-shaped defects in as-grown (001)-oriented $β$-Ga$_2$O$_3$ epilayers prepared by halide vapor phase epitaxy

Groove-shaped defects (GSDs) degrade the surface flatness of as-grown (001)-oriented $β$-Ga$_2$O$_3$ epilayers and necessitate chemical mechanical polishing before device fabrication, increasing processing costs and the risk of damage. We investigated the morphology, subsurface structure, and formation mechanism of GSDs in a homoepitaxial layer grown by halide vapor phase epitaxy using synchrotron X-ray topography and electron microscopy. The GSDs extended several millimeters along [010] and consisted predominantly of (-102) basal facets bounded by steep (100) sidewalls. Their wafer-scale distribution showed a spatial correspondence with variations in wafer curvature, suggesting that local surface orientation influences GSD formation. Careful alignment of surface images with transmission X-ray topographs revealed no one-to-one correspondence between GSDs and substrate defects, providing no evidence that substrate dislocations serve as their nucleation sites. Instead, transmission electron microscopy revealed planar defects localized near the terminal boundaries where the faceted GSD sectors met the surrounding (001) growth region; no such defects were observed in specimens extracted from the middle of GSDs. These defects exhibited α-fringe contrast characteristic of inclined translational planar defects, with the dominant segments assigned to the (1-21) plane. The observations suggest that variations in local surface orientation and step supply may promote three-dimensional faceted growth, producing persistent (-102)/(100) sectors. The localized planar defects are therefore interpreted as consequences of growth-sector coalescence rather than the origins of GSD nucleation. These findings provide insight into the roles of wafer curvature and surface step supply in GSD formation.

cond-mat.mtrl-sci↗

arXiv2026

Dislocations in (011)-oriented vertical Bridgman $β$-Ga$_2$O$_3$ substrates

Dislocation in (011)-oriented $β$-Ga$_2$O$_3$ substrates grown by the vertical Bridgman method was investigated using X-ray topography (XRT), combined with X-ray reticulography. Transmission XRT reveals dislocations lying on the (001) plane and extending along [010], forming arrays associated with domain boundaries. Dislocations on the (011) plane were also identified but differ from those responsible for line-shaped pits on (001) epilayers. Reflection XRT shows good agreement with transmission XRT and enables classification of dislocation types based on contrast features. Reticulography confirms domain boundaries with misorientation on the order of 1E-5 rad, providing insight into defect formation relevant to epi-growth and device performance.

cond-mat.mtrl-sci↗

arXiv2026

Three-dimensional visualization of threading dislocation in GaN by polarized-light microscopy

We demonstrate high-throughput three-dimensional imaging of threading dislocations in ammonothermal GaN wafers using polarized-light microscopy with collimated LED illumination. Threading dislocations exhibited focal-depth-dependent in-plane shifts, enabling visualization of their three-dimensional inclination behavior over large areas. Synchrotron radiation X-ray topography indicated that dislocations with identical in-plane Burgers vector components tended to exhibit similar trace geometries and inclination directions. The threading dislocations were found to be tilted by approximately 3.3° from the c-axis in directions perpendicular to their Burgers vectors, indicating climb-mediated motion associated with strain relaxation during crystal growth. These results demonstrate a simple nondestructive approach for large-area three-dimensional characterization of dislocations in GaN wafers.

cond-mat.mtrl-sci↗

arXiv2026

Three-dimensional visualization of lattice defects in $β$-Ga$_2$O$_3$ via synchrotron-radiation Borrmann-effect X-ray topo-tomography

beta-Ga2O3 is a promising material for next-generation power electronics; however, its performance is strongly affected by lattice defects such as dislocations. In this study, we demonstrate three-dimensional (3D) visualization of dislocations in \b{eta}-Ga2O3 using synchrotron-radiation X-ray topo-tomography under a two-beam Borrmann-effect condition in transmission X-ray topography. By rotating the sample about the diffraction vector and acquiring a series of topo-tomographic images at different rotation angles, the evolution of dislocation contrast is captured, providing intuitive, depth-resolved visualization of dislocations. This method enables clear separation of dislocations in the epilayer and substrate in Schottky barrier diode structures, offering insight into dislocation propagation and their impact on epitaxial growth and device performance. This study represents the first demonstration of 3D dislocation reconstruction in beta-Ga2O3.

cond-mat.mtrl-sci↗

arXiv2026

Determination of Burgers vectors of dislocations in monoclinic $β$-Ga$_2$O$_3$ crystals by large-angle convergent-beam electron diffraction

We demonstrate the applicability of large-angle convergent-beam electron diffraction (LACBED) for Burgers vector determination in monoclinic $β$-Ga$_2$O$_3$. The inner product $g \cdot b$ in this non-orthogonal system can be evaluated without a metric tensor by using the dual relationship between real and reciprocal lattice bases. Based on this framework, Burgers vectors of dislocations introduced by nanoindentation were unambiguously determined from LACBED node counts. The results are consistent with weak-beam dark-field imaging, confirming the effectiveness of LACBED for $β$-Ga$_2$O$_3$.

cond-mat.mtrl-sci↗

arXiv2026

Synchrotron-radiation X-ray topography and reticulography of bulk $β$-Ga$_2$O$_3$ crystals grown by the cold crucible method

The structural properties of a $β$-Ga$_2$O$_3$ single crystal grown by the oxide crystal growth from cold crucible (OCCC) method were investigated using synchrotron radiation X-ray topography and X-ray reticulography. The region grown beneath the seed exhibits high crystalline quality with a rocking curve full width at half maximum of about 26 arcsec. During diameter enlargement, a twist-type lattice misorientation develops between the central and laterally expanded regions, originating near the shoulder and propagating along boundaries parallel to the $\langle010\rangle$ growth direction. Dislocation analysis reveals that $\langle010\rangle$-oriented screw dislocations dominate the defect structure with densities of ~$10^{5}$cm$^{-2}$, while higher densities (~$10^{6}$cm$^{-2}$) appear in the wing region. These results clarify defect formation in OCCC-grown $β$-Ga$_2$O$_3$ and provide insights for optimizing growth conditions.

cond-mat.mtrl-sci↗

arXiv2026

Kinematical and dynamical contrast of dislocations in thick GaN substrates observed by synchrotron-radiation X-ray topography under six-beam diffraction conditions

Dislocations in a thick ammonothermal GaN substrate were investigated using synchrotron-radiation X-ray topography (SR-XRT) under six-beam diffraction conditions. The high brilliance of the synchrotron source enabled the observation of the super-Borrmann effect, which markedly enhanced the anomalous transmission of X-rays through the 350~$μ$m-thick crystal. Systematic variation of the deviation angle~$Δω$ revealed a clear transition from kinematical to dynamical diffraction, consistent with theoretical predictions based on dynamical diffraction theory. By selectively exciting five equivalent two-beam diffraction conditions near the six-beam configuration, the Burgers vectors of individual threading edge dislocations (TEDs) were determined according to the $g\cdot b$ invisibility criterion. The measured dislocation image widths agreed well with calculated values derived from the extinction distance and $|g\cdot b|$ dependence, confirming that most dislocations possess Burgers vectors containing an $a$-type component of $\frac{1}{3}\langle 11\bar{2}0\rangle$ or $\frac{2}{3}\langle 11\bar{2}0\rangle$. These results demonstrate that SR-XRT under multibeam diffraction provides a powerful, nondestructive method for quantitative dislocation analysis in thick GaN crystals, offering valuable insights into defect structures critical for high-performance GaN-based electronic devices.

cond-mat.mtrl-sci↗