Structure and properties of Ta/Al/Ta and Ti/Al/Ti/Au multilayer metal stacks formed as ohmic contacts on n-GaN

Formation of ohmic contacts to GaN is of high practical importance for device fabrication. Due to the wide band gap, formation of multilayer metal structures is required to make electrical connections with low contact resistance. The paper presents a study on structure, composition, adhesion and electrical properties of Ti/Al/Ti/Au and Ta/Al/Ta metal stacks fabricated by e-beam evaporation and thermal annealing in order to provide ohmic contacts to n-type GaN films grown on Si. For the Ti-based case, an interdiffusion of Au and Ga into the stack is found, which is probably caused by a granular structure of the top Ti layer making no proper barrier. Ti of the bottom layer is observed to diffuse into GaN, forming a thin layer of titanium nitride with a low Schottky barrier at GaN interface allowing ohmic contact as shown by electrical measurements. The Ta-based stacks have the expected layered structure with minor interdiffusion of Al and Ta at the interfaces of these two metals. No direct microscopic evidences for Ta diffusion into GaN is observed. However, the formation of a thin tantalum nitride layer at the GaN interface can be deduced from the current–voltage measurements showing ohmic electrical contacts with low contact resistivity of 1.2 × 10⁻³ Ω cm². Four-point bending tests on the both types of samples show that cracks always develop at the interface between Si and buffer layers of GaN. No exfoliation of the metallization layers is observed allowing us to conclude about good adhesion of the metal stacks. Thus, the fabricated Ti- and Ta-based multilayer structures show good electrical performance and reliable adhesion making them promising for the formation of ohmic contacts to n-type GaN.