The role of barrier layer on Ohmic performance of Ti/Al-based contact metallizations on AlGaN/GaN heterostructures

Ohmic performance and surface morphology of Ti/Al/metal/Au schemes, where metal is Ti, Mo, Pt, Ir, Ni, Ta, or Nb, have been studied to identify the role of the barrier layer on contact formation and reliability of multilayered metallizations. Results show a strong dependence of contact resistance and specific contact resistivity, as well as surface morphology, on the type of metal layer used. Annealing temperature dependent Ohmic performance was observed for the different schemes. The existence of distinct reaction domains with discernable compositional contrast is evidenced on the surface of annealed contacts. The observed variations in Ohmic performance and surface morphology did not have any correlation with the physical properties of the elemental metal layer. This suggests that contrary to the generally accepted opinion of the layer acting as a mere barrier layer, which serves to prevent indiffusion of Au and outdiffusion of Al, results from the present study provided insights on the active role this layer plays on the characteristics of metallization schemes. Although thermal annealing optimization for any of the Ti/Al/metal/Au scheme can enable low-resistance contact formation, however, surface morphology and reliability (e.g., lateral encroachment) considerations limit the practical usage of the schemes to a few such as Ti/Al/Mo/Au and Ti/Al/Pt/Au.