The work reported herein demonstrated that nanopipes can be formed via a surfactant effect, in which boron impurities preferentially migrate to semipolar and nonpolar facets. Approximately 3 μm-thick GaN layers were grown using halogen-free vapor phase epitaxy. All layers grown in pyrolytic boron nitride (pBN) crucibles were found to contain a high density of nanopipes in the range of 1010 to 1011 cm−2. The structural properties of these nanopipes were analyzed by X-ray rocking curve measurements, transmission electron microscopy, and three-dimensional atom probe (3DAP) tomography. The resulting 3DAP maps showed nanopipe-sized regions of boron segregation, and these nanopipes were not associated with the presence of dislocations. A mechanism for nanopipe formation was developed based on the role of boron as a surfactant and considering energy minima. A drastic reduction in the nanopipe density was achieved upon replacing the pBN crucibles with tantalum carbide-coated carbon crucibles. Consequently, we have confirmed that nanopipes can be formed solely due to surface energy changes induced by boron impurity surface segregation. For this reason, these results also indicate that nanopipes should be formed by other surfactant impurities such as Mg and Si.

Here, we propose a halogen-free vapor phase epitaxy (HF-VPE) technique to grow bulk GaN single crystals. This technique employs the simplest reaction for GaN synthesis (reaction of Ga vapor with NH3) and can potentially achieve a high growth rate, a prolonged growth duration, a high crystal quality, and a low cost. The analyses of thick HF-VPE-GaN layers grown under optimized growth conditions revealed that high-quality crystals, both in terms of dislocation density and impurity concentration, are obtained at high growth rates of over 100 µm/h.

This paper describes the self-assembled fabrication of single-crystal GaN with a bimodal pore (meso/macropore) size distribution (BiPS-GaN). A 4.7 μm-thick BiPS-GaN layer was grown spontaneously using halogen-free vapor phase epitaxy in conjunction with boron impurity doping (>1 × 10 19 atoms/cm 3 ) on a GaN template fabricated via metalorganic chemical vapor deposition (MOCVD-GaN). The boron impurity acted as a surfactant, and its segregation generated a dense (>1 × 10 10 cm −2 ), homogeneous distribution of mesopores with sizes of 30− 40 nm in GaN during growth. In addition, macropores with sizes of 0.1−2 μm were produced by the fusion of mesopores in close proximity to one another. As a result, BiPS-GaN exhibited a high density of both meso-and macropores, all aligned in the vertical direction (that is, along the c axis). BiPS-GaN showed good electroconductivity and almost the same high degree of crystallinity as the MOCVD-GaN template. Furthermore, the hybrid meso/macropore structure of BiPS-GaN imparted excellent photoabsorption properties and allowed this material to work as an efficient support for a nanosized IrO x catalyst. The photocurrent density in BiPS-GaN was enhanced by as much as a factor of 5 compared to planar GaN by effective absorption due to the hybrid meso/macropore structure of BiPS-GaN. Moreover, the oxygen generation efficiency of BiPS-GaN with the IrO x catalyst was approximately doubled, compared to that of BiPS-GaN without IrO x , while maintaining long-term stability. These results demonstrate that BiPS-GaN fabricated in this facile manner has significant potential in applications such as photoelectrochemical reactions and catalysis.

No abstract

The GaN growth rate during halogen-free vapor phase epitaxy (HF-VPE) is significantly increased by the use of an evaporator made of a porosity-controlled TaC ceramic. A fin-shaped evaporator, which is immersed in a molten Ga source at temperatures above 1373 K, effectively pumps molten Ga by capillary action and provides a fivefold increase in the surface area of the source. This results in a 3–5 times increase in both the Ga supply rate and the GaN growth rate.