Microstructure, electrical properties, and thermal stability of Al ohmic contacts to n-GaN

As-deposited Al contacts were ohmic with a room-temperature contact resistivity of 8.6 x 10(-5) Ohm . cm(2) on Ge-doped, highly n-type GaN (n = 5 x 10(19) cm(-3)). They remained thermally stable to at least 500 degrees C, under flowing N-2 at atmospheric pressure. The specific contact resistivities (rho(c)) calculated from TLM measurements on as-deposited Al layers were found to range from 8.6 x 10(-5) Ohm . cm(2) at room temperature and 6.2 x 10(-5) Ohm . cm(2) at 500 degrees C. Annealing treatments at 550 degrees C and 650 degrees C for 60 s each under flowing N-2 resulted in an overall increase of contact resistivity. Cross-sectional, high-resolution electron microscopy (HREM) revealed that interfacial secondary phase formation occurred during high-temperature treatments, and coincided with the degradation of contact performance. Electron diffraction patterns from the particles revealed a cubic structure with lattice constant n = 0.784 nm, and faceting occurring on the {100} faces. Spectroscopic analysis via electron energy loss spectroscopy (EELS) revealed the presence of nitrogen and small amounts of oxygen in the Al layer, but no appreciable amounts of Ga. The results of microstructural and crystallographic characterization indicate that the new interfacial phase is a type of spinel Al nitride or Al oxynitride.