Tomoya Iwahashi

Growth of a Large GaN Single Crystal Using the Liquid Phase Epitaxy (LPE) Technique

We grew a 12×5×0.8 mm GaN single crystal using the liquid phase epitaxy (LPE) method with Na flux. This GaN single crystal was grown on a 3 µm-thick GaN thin film synthesized on sapphire using the metal organic chemical vapor deposition (MOCVD) method. It grew in a polyhedral form larger than the MOCVD-GaN substrate. Results indicate that a MOCVD-GaN thin film on a sapphire substrate functions as a seed crystal in Na flux. The use of mixed nitrogen gas containing 40% ammonia instead of pure N2 gas also enabled the growth of a 10 µm-thick GaN homo-epitaxial film on an MOCVD-GaN film under 5 atm, the lowest reported pressure for growing GaN in Na flux. In this paper, we describe the liquid phase epitaxy (LPE) technique for growing bulk GaN single crystals, as well as the results of photoluminescence (PL) measurements. We also compare the PL intensity of the bulk GaN obtained in this study and the MOCVD-GaN. PL measurements revealed that the peak intensity of GaN single crystal grown by LPE indicates 40 times larger than MOCVD-GaN film. Also, dislocation density of bulk GaN crystals could be drastically reduced by the LPE growth technique.

Growth of Transparent, Large Size GaN Single Crystal with Low Dislocations Using Ca-Na Flux System

Growth of large and transparent GaN single crystals was carried out by applying the liquid phase epitaxy (LPE) method in a Ca-Na mixed flux system. We have previously reported LPE growth of GaN in a Na flux system, and that GaN crystals grown by LPE have extreme low dislocations and show excellent photoluminescence characteristics. In this study, use of a Ca-Na mixed flux system enabled us to grow transparent GaN crystals under low nitrogen pressure and to further improve the photoluminescence (PL) characteristic. The dislocation density of this crystal is very low (2 ×105 cm-2 in highest point).

Synthesis of bulk GaN single crystals using Na-Ca flux

We grew GaN single crystals in Na–Ca flux and found that the presence of Ca in a high-temperature flux system has the following advantages for growing GaN single crystals. First, Ca in solution drastically increased the yield of GaN crystals. Second, transparent GaN single crystals are easy to grow around the gas-liquid interface. Third, the pressure required to synthesize the GaN is reduced. These effects can be interpreted as resulting from increased nitrogen solubility in the flux. In this paper, we report the effects of Ca on the yield of GaN and threshold pressure for growing GaN in Na–Ca flux.

Liquid Phase Epitaxy Growth of m-Plane GaN Substrate Using the Na Flux Method

We report the fabrication of {10-10} (m-plane) GaN single crystal substrates grown by the Na flux method. First, we investigated the growth conditions for which the m-plane is stable. Then, an m-plane GaN substrate, of size 10 ×10 ×0.5 mm3, was obtained employing the Na flux method and liquid phase epitaxy (LPE) technique using an m-plane GaN thin films grown by metal organic chemical vapor deposition (MOCVD) as a seed. The full width at half maximum (FWHM) of the rocking curve of the LPE m-plane GaN crystal was 231 arcsec, which was markedly improved compared with that of the MOCVD-grown m-plane GaN template (3248 arcsec) used as the seed crystal.

Growth of Bulk GaN Single Crystals Using Li-Na Mixed Flux System

We found the yield of GaN grown in a Li-Na flux system to be much higher than that grown in Na flux. Nitrogen dissolution seemed to increase with Li in solution, which promotes the growth of GaN crystals. In this paper, we report that the yield of GaN and habit modifications greatly depend on the composition of the Li-Na flux system and that GaN can be grown in a pure Li flux at about 50 atm.

Influence of pressure control on the growth of bulk GaN single crystal using a Na flux

We investigated the influence of N 2 pressure control on the growth of bulk GaN crystals by using a Na flux. GaN single crystals with a maximum dimension of 3 mm could be obtained by this method. though smaller crystals of 0.5-2 mm in size have been grown without pressure control. This suggested significant effects of additional NH 3 gas on growth of GaN crystals in the Na flux method.

Fabrication of a-Plane GaN Substrate Using the Sr–Na Flux Liquid Phase Epitaxy Technique

We report the fabrication of a-plane GaN single crystal substrates grown by the Na flux method. In this research, Sr was added into the flux system as additive to control the crystal habit of GaN single crystals. As the amount of Sr in the melt increased, it was found that the GaN crystal shape changed from pyramidal to prismatic crystals elongated parallel to the direction. Additionally, liquid phase epitaxy (LPE) GaN crystals were grown in Sr–Na solution on a-plane GaN templates fabricated by metal organic chemical vapor deposition (MOCVD) on sapphire substrates. ω-scan X-ray diffraction measurements showed that the full width at half maximum (FWHM) of the (11-20) plane of the LPE GaN crystal was smaller (236 arcsec) than that of the a-plane of the GaN template (1152 arcsec).

Optical Property of GaN Single Crystals Grown by Liquid Phase Epitaxy (LPE)

The photoluminescence spectra of GaN single crystals were investigated at the temperature range of 20K to 300K. The GaN single crystals are grown using the liquid phase epitaxy technique with Na or Na-Ca flux. These crystals are grown on 3 µm GaN thin films. The photoluminescence spectra of the GaN single crystal grown by Na flux method shows a strong band edge emission at low and room temperature. The emissions from the deep levels of the Na-flux GaN were very weak at low temperature. This reveals the GaN single crystal grown in Na flux contains few impurities imported by the high temperature solution system. On the other hand, the GaN single crystal grown in the Na-Ca mixed flux system shows the donor-acceptor pair emission. The origin of the acceptor seems to be Ca or Mg included in the Ca mixed flux. The yellow luminescence from the GaN single crystal grown in the Na-Ca mixed flux was not observed even at room temperature. The acceptor impurity (Ca and/or Mg) seemed to suppress the formation of the Ga vacancies. These results were consistent with the data of secondary ion mass spectroscopy (SIMS).