Evolution of surface microstructure of Re-Al-Ni-Au based ohmic contacts on n-type GaN

Recently, rhenium (Re) based ohmic contacts to GaN have been studied for their low resistivity, smooth surface morphology, and sharp edge acuity at low annealing temperatures. In this work, we discuss the evolution of surface microstructures for Re-Al-Ni-Au ohmic contacts on n-GaN as a function of Re layer thickness and annealing temperature. For all Re thicknesses, the Al and Ni segregate into agglomerates that increase in size with increasing annealing temperature. These agglomerates are surrounded by Al-Au films. Along with the underlying Re layer, they form different crystallographic phases of Re-Al-Ni, Al₆Re, AlAu₂, and Al₂Au₅. This, along with the formation of Re-N phases at the metal-semiconductor interface leads to low resistivity ohmic contacts on n-GaN. Investigating the evolution of the contact microstructure is an important step in understanding the behavior of the Re-based ohmic contact system.