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, Al3 + x Mo1–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 x Moy phases likely plays the most important role in the ohmic contact formation, which was also confirmed by the method of energy-dispersive analysis.