Mamoru Imade

Growth of Large GaN Single Crystals on High-Quality GaN Seed by Carbon-Added Na Flux Method

Seeded growth on high-quality GaN seed was studied using a carbon-added Na flux method. A GaN single crystal (8.6 mm long, 5 mm high) was grown in 96 h without polycrystal formation on a crucible. Under a carbon-added condition, dependence of the growth rate of the seed and the growth habit on the flux composition was studied. We found that the growth rate was higher with Ga-poor flux, resulting in a high growth rate of 30 µm/h along the c-axis and 33 µm/h along the a-axis (66 µm/h for both sides). The growth habit changed from prismatic to pyramidal with increasing Ga composition.

Fabrication of low-curvature 2 in. GaN wafers by Na-flux coalescence growth technique

Low-curvature and large-diameter GaN wafers are in high demand for the development of GaN-based electronic devices. Recently, we have proposed the coalescence growth of GaN by the Na-flux method and demonstrated the possibility of enlarging the diameter of high-quality GaN crystals. In the present study, 2 in. GaN wafers with a radius of curvature larger than 100 m were successfully produced by the Na-flux coalescence growth technique. FWHMs of the 002 and 102 GaN X-ray rocking curves were below 30.6 arcsec, and the dislocation density was less than the order of 102 cm−2 for the entire area of the wafer.

Coalescence Growth of Dislocation-Free GaN Crystals by the Na-Flux Method

We have recently shown that dislocation-free GaN crystals could be grown on a GaN point seed by the Na-flux method. To enlarge the diameter of dislocation-free GaN crystals, we propose here the coalescence of GaN crystals grown from many isolated point seeds. In this study, we found that two GaN crystals grown from two point seeds arranged along the a-direction coalesced without generating dislocations at the coalescence boundary, and the c-axis misorientation between two crystals around the coalescence boundary gradually diminished as the growth proceeded. These results indicate that coalescence growth may become a key technique for fabricating large-diameter dislocation-free GaN crystals.

Ultraviolet laser-induced degradation of CsLiB_6O_10 and β-BaB_2O_4

We investigated ultraviolet (UV) laser-induced degradation of nonlinear optical crystal CsLiB6O10 (CLBO) and β-BaB2O4 (BBO) using a high-repetition-rate pulsed laser. In this research, we found that the degradation of CLBO was caused by a UV-induced refractive index change at a peak power density higher than a few tens of MW/cm2. On the other hand, BBO exhibited lower UV transmittance at a higher intensity of 10 MW/cm2 and the UV absorption increased with time due to the formation of an absorption center. Moreover, we confirmed that the degradation resistance of CLBO with fewer light scattering defects was improved.

Growth of Prismatic GaN Single Crystals with High Transparency on Small GaN Seed Crystals by Ca–Li-Added Na Flux Method

The addition of Ca–Li to Na flux was attempted in order to control the growth habit and further improve transparency of GaN crystals. As a result, the growth habit changed to prism shape by the addition of Ca. Furthermore, we succeeded in growing prismatic GaN crystals with high transparency by adding appropriate amounts of Ca and Li to the flux. The optical absorption coefficient at 450 nm wavelength obtained from the crystal was 1.07 cm-1. This result suggests that the addition of Ca–Li to Na flux is a promising method of growing transparent GaN single crystals.

Synthesis of GaN Crystal Using Gallium Hydride

A new method for synthesizing a large area of c-axis GaN film was developed. The gallium hydride which was formed by the reaction between metal-Ga and hydrogen gas was used as the Ga source, and reacted with NH3 gas to grow GaN crystals on the sapphire (0001) substrate. 2.7 µm-thick GaN film with c-axis orientation could be grown on the substrate. Stable conditions for the synthesis of gallium hydride by the reaction of H2 gas with metal-Ga were examined. As a result, gallium hydride is stable at about 1000°C which is a temperature commonly used for the growth of GaN. These results show that the use of gallium hydride as the predominant species of the Ga source can provide a relatively inexpensive method of growing GaN crystals at a high level of purity.

Growth of bulk GaN crystals by the Na-flux point seed technique

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.

Effects of Solution Stirring on the Growth of Bulk GaN Single Crystals by Na Flux Method

Recently, we succeeded in fabricating centimeter-sized bulk gallium nitride (GaN) crystals with large dislocation-free areas on a GaN point seed. However, problems of polycrystal formation, skeletal growth, and low growth rate still remained. In this study, to suppress skeletal growth, polycrystals formation and increase the growth rate, we introduced two types of solution-stirring techniques – rotating stirring and swinging stirring – in the growth on point seeds by the Na flux method. We found that increasing the reversal frequency of the rotating stirring and increasing the rate of the swinging stirring increased the growth rate and suppressed the formation of polycrystals and skeletal growth. Moreover, the maximum c-direction growth rate of 46 µm/h was achieved without the formation of polycrystals and skeletal growth. We conclude that solution stirring may be an effective technique for fabricating high-quality large bulk GaN crystals.

Al Doping of CsLiB6O10 for High Resistance to Ultraviolet-Induced Degradation

We investigated the ultraviolet-induced degradation of a nonlinear optical crystal CsLiB6O10 (CLBO). High-repetition-rate pulsed lasers with an output wavelength of 266 nm induced degradation of CLBO at a lower peak power density than the bulk laser-induced damage threshold. This degradation behavior is similar to photorefractive damage. We found for the first time that Al-doped CLBO heated at 150 °C exhibits a longer lifetime for the degradation than undoped CLBO.

Dislocation confinement in the growth of Na flux GaN on metalorganic chemical vapor deposition-GaN

We have demonstrated a GaN growth technique in the Na flux method to confine c-, (a+c)-, and a-type dislocations around the interface between a Na flux GaN crystal and a GaN layer grown by metalorganic chemical vapor deposition (MOCVD) on a (0001) sapphire substrate. Transmission electron microscopy (TEM) clearly revealed detailed interface structures and dislocation behaviors that reduced the density of vertically aligned dislocations threading to the Na flux GaN surface. Submicron-scale voids were formed at the interface above the dislocations with a c component in MOCVD-GaN, while no such voids were formed above the a-type dislocations. The penetration of the dislocations with a c component into Na flux GaN was, in most cases, effectively blocked by the presence of the voids. Although some dislocations with a c component in the MOCVD-GaN penetrated into the Na flux GaN, their propagation direction changed laterally through the voids. On the other hand, the a-type dislocations propagated laterally and c...