Microstructural evidence on electrical properties of Ta/Ti/Al and Ti/Ta/Al ohmic contacts to n-AlGaN/GaN

Electrical properties and microstructures of Ta/Ti/Al and Ti/Ta/Al contacts to n-AlGaN/GaN heterostructure field-effect transistor structures were investigated using the transmission line method and transmission electron microscopy. The specific resistivity (5.3×10−7 Ω cm2) of Ta/Ti/Al contacts is much lower than that (5.1×10−4 Ω cm2) of Ti/Ta/Al contacts, for the same heterostructure and similar metallization. The contact resistivity was found to depend on the thickness of the AlGaN layer, interfacial phase, and interface roughness. The formation of interfacial phases by solid-state reactions with the metal layer during annealing appears to be essential for ohmic behavior on n-III-nitrides suggesting a tunneling contact mechanism.