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

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 cm 2 (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, Al 2 Au, Al 3 + x Mo 1– x , AlMo 3 , Al 12 Mo, GaMo 3 and GaAu 2 . Investigations of the phase composition of films depending on the thickness ratio of the metallization layers have shown that the formation of Al 2 Au phase has a negative effect on the contact surface morphology, and the formation of GaMo 3 , Al x Mo y phases likely plays the most important role in the ohmic contact formation, which was also confirmed by the method of energy-dispersive analysis.