Investigation of V-Ti/Al/Ni/Au Ohmic contact to AlGaN/GaN heterostructures with a thin GaN cap layer

In this paper, we report on a novel V-Ti/Al/Ni/Au metal stack as an Ohmic contact to AlGaN/GaN heterostructures with a thin GaN cap layer. The thin vanadium layer (10 nm) is employed as the first layer under the conventional Ti/Al/Ni/Au contact. This layer is shown to play a crucial role in achieving low specific contact resistance (ρ_c) and smooth surface morphology. The V-Ti/Al/Ni/Au contact exhibits significantly improved surface roughness when compared with Ti/Al/Ni/Au contacts with a root mean square value of 11 nm. The specific contact resistivity is measured to be 2.3 × 10^-6 Ω cm². According to cross-sectional transmission electron microscopy (TEM) analysis, very limited reaction is observed between the V-Ti/Al/Ni/Au contact and the AlGaN surface. It is demonstrated that the thin vanadium layer prevents excessive formation of TiN protrusions, which is considered to be the key to improving surface morphology with low contact resistance.