From Pd to Pd/Au and Pd/Ni/Au: Structural and interfacial characterization of ohmic contacts to p-GaN

Interfacial chemistry and reaction pathways of Pd-based metallization on p-type GaN were investigated for Pd, Pd/Au, and Pd/Ni/Au contact systems using transmission electron microscopy, X-ray diffraction, X-ray Reflectivity, secondary ion mass spectrometry, and electrical characterization. Thermal annealing activates coupled diffusion and reaction processes involving hydrogen and gallium transport from GaN into the metallization. These processes lead to the formation of Pd–Ga intermetallic phases and to chemical and structural modification of the near-interface semiconductor region. The reaction sequence and evolution of interfacial microstructure strongly depends on metallization composition. In the Pd-only system, annealing produces PdO and PdGa phases together with void formation. These processes promote interface roughening and morphological instability, which correlate with increased contact resistivity. In contrast, an Au capping layer alters the reaction thermodynamics by promoting the formation of an PdAu alloy that suppresses Pd oxidation while maintaining chemical interaction with GaN. This stabilizes the interfacial microstructure and results in the lowest contact resistivity among the studied systems. Insertion of a Ni interlayer between Pd and Au further modifies interfacial chemistry, resulting in the formation of NiO, PdAu, and PdGa phases. This configuration enhances interdiffusion and is associated with extensive void formation and pronounced modification of the near-surface GaN region. Although interfacial reactions are more extensive, the resulting microstructural degradation correlates with reduced contact resistivity. The mechanistic insights demonstrate that the balance between interfacial reaction, oxidation, and void formation governs microstructural evolution and, consequently, the electrical performance of Pd-based contacts to p-GaN.