Mo/Al/Mo/Au-based ohmic contacts to AlGaN/GaN heterostructures

Abstract

Mo/Al/Mo/Au metallization scheme as an ohmic contact to undoped AlGaN/GaN heterostructures was investigated. The optimal thicknesses of the metal layers were determined: Mo (10 nm)/Al (60 nm)/ Mo (50 nm)/Au (50 nm). The specific contact resistance of the fabricated ohmic contact is 4.7 × 10^–7 Ohm cm² (0.14 Ohm mm). The microstructure of the contact after annealing was investigated using scanning and transmission electron microscopy, X-ray diffractometry and energy-dispersive X-ray spectroscopy. It is shown that a noticeable alloying of metallization into semiconductor upon annealing does not occur, but strong mixing of metals takes place. X-ray diffraction analysis demonstrated the presence of interfacial compounds, namely, Al₂Au, Al_{3 + x}Mo_1–x, AlMo₃, Al₁₂Mo, GaMo₃ and GaAu₂. Investigations of the phase composition of films depending on the thickness ratio of the metallization layers have shown that the formation of Al₂Au phase has a negative effect on the contact surface morphology, and the formation of GaMo₃, Al_xMo_y phases likely plays the most important role in the ohmic contact formation, which was also confirmed by the method of energy-dispersive analysis.