Systematic thermal processing of GaN epilayers: Temperature-Driven surface evolution and kinetic analysis

Controlling GaN surface morphology under high-temperature processing is critical for tailoring the structural and functional properties of GaN epilayers. In this work, we systematically investigate the influence of post-growth thermal treatment on the evolution of the GaN surface under a nitrogen ambient. A series of annealing experiments was performed over the temperature range 1110−1200 °C for multiple thermal cycles (one to four). The results identify distinct temperature-dependent regimes governing surface decomposition, grain nucleation, saturation, coalescence, and re-decomposition processes. Arrhenius analysis of nanograin density yields an activation energy of ∼3.77 eV, consistent with thermally activated nanograin formation under N₂ ambient. Correlations between surface chemical composition (gallium accumulation and oxidation), nanoscale morphology, structural stability, and optical response are established, elucidating the interplay between thermally induced surface evolution and partial relaxation of residual compressive strain. The established temperature−cycle processing map provides a framework for controlled thermal modification of GaN epilayers without compromising bulk crystalline quality.