Lateral Growth Behavior of GaN Grown by the Na-Flux Method

Na-flux is a promising growth method for producing large-diameter GaN substrates with ultralow dislocation density, required for high-power-density vertical GaN power devices. However, the lateral growth behavior of GaN under near-thermodynamic-equilibrium conditions remains insufficiently understood. In this work, circular apertures were introduced into the GaN substrate by laser processing to deliberately induce lateral growth during Na-flux growth. GaN originating from the aperture edges coalesced toward the center, with coalescence preferentially along the ⟨112̅0⟩ in-plane direction, with {112̅2} semipolar facets exposed at the advancing growth front. Notably, GaN grown above the apertures exhibited an enhanced lateral growth rate independent of the underlying seed, termed freestanding epitaxy. Furthermore, the stress distribution and crystallographic orientation within the lateral growth regions were systematically investigated by Raman spectra and Lang X-ray topography, respectively. Defect-selective etching with molten NaOH-KOH revealed a drastic reduction in the threading dislocation density from ∼106 cm–2 to below 104 cm–2 in the freestanding epitaxial regions. These results demonstrate that GaN lateral growth can be effectively enhanced in the Na-flux method, providing a physical basis for the fabrication of ultralow-dislocation-density GaN substrates.