V/Al/V/Ag contacts to n -GaN and n -AlGaN

Low-resistance V/Al/V/Ag contacts have recently been reported to Al0.27 Ga0.73 N/GaN heterostructures with a thin GaN cap. These contacts had smooth surfaces and good edge definition. In this study, the V/Al/V/Ag metallization was adapted to other compositions of n -Alx Ga1-x N, and it was found to provide low specific contact resistances as well as smooth surfaces on both n -GaN and n -Al0.58 Ga0.42 N. Another advantage of these contacts is that limited changes in specific contact resistance and morphology were observed when changing the metal layer thicknesses and processing conditions of the V/Al/V/Ag metallization on these semiconductors. The V (20)/Al (95)/V (20)/Ag (120 nm) contact provided a specific contact resistance of (2.1±0.9) × 10-6 cm2 when annealed at 825°C for 30 s on n -GaN and a V (20)/Al (95)/V (5)/Ag (120 nm) contact provided a value of (2.4±0.3) × 10-4 cm2 when annealed at 875°C for 60 s on n -Al0.58 Ga0.42 N. In each case, transmission electron microscopy revealed that the reaction between the semiconductor and metallization was limited and the majority of the interface was contacted by a composite of primarily Ag-bearing grains. Measurements of the specific contact resistance as a function of temperature revealed that field emission is the dominant current transport mechanism in low resistance Ohmic contacts to n -GaN and n -Al0.58 Ga0.42 N.