Hiroki Imabayashi

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.

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.

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.

Mechanism for enhanced single-crystal GaN growth in the C-assisted Na-flux method

First-principles molecular dynamics simulations are used to examine the effect of C addition in Na-flux growth of GaN. The mechanism for suppression of polycrystalline growth and the enhancement of single-crystal growth was identified by systematically calculating activation free energies for the formation and dissociation of C–N bonds. The energy barrier for C–N dissociation in a Ga–Na melt is ≥3 eV; thus, dissociation is inhibited and the growth of polycrystals is suppressed. However, at kink sites at a Na/GaN interface with excess Ga atoms, the barrier is only ~1.0 eV, allowing C–N dissociation and growth of GaN single crystals.

Homoepitaxial growth of GaN crystals by Na-flux dipping method

The realization of low-dislocation-density bulk GaN crystals is necessary for use in the fabrication of future high-power devices with low power consumption. In this study, we attempted the regrowth of low-dislocation-density (104–105 cm−2) GaN substrates to fabricate thick and low-dislocation-density GaN crystals using the dipping technique with the Na-flux method. In the growth using this dipping technique, the generation of dislocations at the interface between the GaN substrate and the regrowth layer was prevented, and we succeeded in fabricating thick and low-dislocation-density GaN crystals. In the growth without the dipping technique, the surface of the GaN substrate demonstrated meltback immediately before the growth, and dislocations were newly generated. These results indicate that the Na-flux dipping technique has potential use for the fabrication of low-dislocation-density bulk GaN crystals.

Structural Analysis of Carbon-Added Na–Ga Melts in Na Flux GaN Growth by First-Principles Calculation

We investigated the fundamentals of the effect of C addition on Na flux GaN growth by first-principles calculation. We simulated C-added Na–Ga melts using molecular dynamics (MD) simulations to examine the local melt structure around a N atom. We also calculated C–N bond energy using constrained MD simulations. Results show that a N atom bonded to a C atom and there were no Ga atoms around the N atom because C–N bond energy was larger than Ga–N bond energy. This is the reason for the suppression of heterogeneous nucleation by C addition. It was also found that the C–N bond energy was affected by surrounding Ga atoms and that the C–N atomic distance increased with the Ga coordination number around the N atom.

The Effects of Substrate Surface Treatments on Growth of a-Plane GaN Single Crystals Using Na Flux Method

Nonpolar GaN substrates are necessary for the improvement of GaN device performance. The growth of high-quality nonpolar GaN crystals, however, has not yet been achieved. In this study, we grew a-plane GaN crystals using the Na flux method and investigated the effects of the substrate surface treatment on the crystallinity of grown GaN crystals. A-plane GaN substrates with chemical mechanical polishing (CMP) and with chemical etching using pyrophosphoric acid were used as the seed substrates. We found that full width at the half-maximum (FWHM) of the X-ray rocking curve (XRC) of GaN crystals grown on the substrate with chemical etching was smaller than that on the substrate with CMP. The results show that chemical etching is more effective than CMP for improving the crystallinity of a-plane GaN crystals.

High-Temperature Growth of GaN Single Crystals Using Li-Added Na-Flux Method

The Na-flux method is a promising for fabricating GaN crystals with high quality. In our previous study, we found that the surface morphology and transparency of these crystals were improved by raising the growth temperature. Increasing the threshold pressure of nitrogen for GaN growth, however, made GaN growth at high temperatures difficult. In this study, we attempted to grow GaN crystals by the Na-flux method with the addition of Li to the flux, which promoted the solubility of nitrogen in the flux. As a result, the threshold pressure of nitrogen for GaN growth decreased, and GaN crystals with high crystallinity were grown at 900 °C. In addition, we found that the crystallinity of the grown GaN crystals was improved and the concentration of impurities in the grown GaN crystals was decreased by raising the growth temperature.

The Effects of Ba-Additive on Growth of a-Plane GaN Single Crystals Using Na Flux Method

Large-area nonpolar GaN substrates with high crystallinity are necessary to improve the performance of GaN devices. Nonpolar GaN substrates of 2-in. diameter have been commercially fabricated by growing along the nonpolar direction on heterogeneous substrates. However, the crystallinity of the nonpolar GaN substrates requires improvement. Here, we grew a-plane GaN crystals using the Na flux method and investigated the effects of a Ba-additive on surface morphology and crystallinity. We found that the crystallinity of the crystals grown by the Na flux method was greatly improved compared with that of seed substrates. Moreover, the use of the Ba-additive suppressed the formation of voids that occurred during the Na flux growth without the Ba-additive. As a result, a-plane GaN crystals with high crystallinity were produced using the Na flux method with the Ba-additive.