On the basis of a novel crystal hardness control, we successfully realized macrodefect-free, large (2–6 in.) and thick +c-oriented GaN bulk crystals by hydride vapor phase epitaxy. Without the hardness control, the introduction of macrodefects including inversion domains and/or basal-plane dislocations seemed to be indispensable to avoid crystal fracture in GaN growth with millimeter thickness. However, the presence of these macrodefects tended to limit the applicability of the GaN substrate to practical devices. The present technology markedly increased the GaN crystal hardness from below 20 to 22 GPa, thus increasing the available growth thickness from below 1 mm to over 6 mm even without macrodefect introduction. The 2 and 4 in. GaN wafers fabricated from these crystals had extremely low dislocation densities in the low- to mid-105 cm−2 range and low off-angle variations (2 in.: <0.1°; 4 in.: ~0.2°). The realization of such high-quality 6 in. wafers is also expected.
In this paper, progress in the Na-flux point seed technique (SPST) will be reviewed. Bulk GaN crystals with a diameter of 2.1 cm, a height of 1.2 cm, and large dislocation-free areas were successfully produced by SPST. Panchromatic cathodoluminescence images of a wafer sliced parallel to the c-face from the crystal showed the lack of dark spots due to dislocations over a large area of the wafer. Structural properties were evaluated using synchrotron X-ray diffraction analysis at SPring-8. The full width at half maximum of the 006 rocking curve was found to be 2.1 arcsec, close to the calculated value of 2.0 arcsec for a perfect GaN crystal, indicating that crystals grown by SPST have an almost perfect structure. In addition, we have extended the use of SPST to the coalescence growth of GaN crystals to increase the wafer diameter and obtained a 2 in. GaN wafer with a low dislocation density and a low curvature by this technique.
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The characteristics of structural defects observed on (100) wafers in β-Ga2O3 single crystals grown by directional solidification in a vertical Bridgman furnace were studied in terms of crystal growth conditions. No high-dislocation-density regions near the wafer periphery were observed owing to the lack of adhesion between the as-grown crystal ingot surface and the crucible inner wall, and directional solidification growth in a crucible with a very low temperature gradient resulted in β-Ga2O3 single crystals with a low mean dislocation density of 2.3 × 103 cm−2. Line-shaped defects up to 150 µm long in the  direction were detected at a mean density of 0.5 × 102 cm−2, which decreased with decreasing growth rate. The line-shaped defect structure and formation mechanism were discussed.
Quartz wafer is an important part of the force sensor of piezoelectric multi-dimensional force sensor. Its structural form affects the overall measurement performance of force-sensitive components and sensors. In this paper, the shape of the quartz crystal group was first analysed, and then two different types of quartz crystal group models were established using finite element software, and their simulation analysis was performed. The corresponding relationship between the deformation of the two different shapes of quartz crystal set under the same external load and the limitation of external space size was studied. The same axial load and tangential load were applied to two different types of quartz crystals respectively, and the corresponding maximum equivalent stress and stress distribution regions were obtained. This paper provides an important reference for the fabrication of quartz crystal arrays and the selection of quartz wafer shape for piezoelectric multi-dimensional force sensors.