We investigated the structural evolution of GaN nucleation layers in the initial growth stages on commercial c-plane sapphires with atomic steps at the surface, using field-emission scanning electron microscopy, synchrotron x-ray scattering, and high-resolution electron microscopy. GaN nucleates into islands preferentially on the atomic steps. The initial small islands of 25 Å high have well-ordered cubic sequences and nearly coherent interfacial structures with a large compressive strain of ∼10%. As the islands grow to 50 Å high, the strain is drastically reduced, to less than 1%, by generating misfit dislocations at the interface and forming the six-to-seven matched interfacial structure. Interestingly, stacking faults are developed from the GaN/sapphire interface, which induces a cubic–hexagonal transformation. The changes in the stacking order during the initial growth are investigated quantitatively.

We investigated the structural behavior of the Ni/Au contact on GaN(0001) and found the catalytic role of Au during annealing, using in situ x-ray scattering. The oxidation kinetics of Ni during annealing in air was greatly affected by the presence of Au. The accelerated GaN decomposition and Ni nitride formation during annealing in N2 also provide evidence of the catalytic role of Au. The results suggest that oxidized Ni/Au ohmic contact exhibit better thermal stability than that of the N2 annealed Ni/Au contacts.

Interfacial microstructure in GaN nucleation layers was investigated using synchrotron x-ray scattering and transmission electron microscopy. We find that tensile-strained, aligned, interfacial domains coexist with misaligned domains in an as-grown nucleation layer of mostly cubic stacking. The tensile strain originates in a 6/7 matched interfacial structure, wherein 6-Ga atomic distances in GaN match to 7-Al atomic distances in sapphire. The tensile state of the aligned, interfacial domains is preserved during annealing to 1100 °C, while the stacking sequence changes from cubic to hexagonal order. The correlation length of the stacking order is rather short, ∼9 Å in the hexagonal phase, compared to that of the cubic phase in the as-grown nucleation layer, ∼25 Å, due to stacking faults generated during the kinetically limited transformation.

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