Influence of Thermal Annealing on the PdAl/Au Metal Stack Ohmic Contacts to p-AlGaN

In this study, a PdAl (20 nm)/Au (30 nm) metal stack scheme is used for forming low-ohmic-resistance contact on Mg-doped (1.5 x 10(17) cm(-3)) p-type AlGaN at various annealing temperatures. Using a circular-transmission line model, the specific contact resistance ((rho)(c)) of PdAl/Au/p-AlGaN ohmic contact is determined via the current-voltage (I-V) characteristics. As-deposited contacts demonstrate non-linear behavior. However, the contact exhibits linear I-V characteristics with excellent ohmic contact of (rho c) = 1.74 x 10(-4) (omega)cm(2), when annealed at 600 degrees C for 1 min in a N-2 atmosphere. The Ga and Al vacancies created at the PdAl/Au and p-AlGaN interfaces, which act as acceptors to increase the hole concentration at the interface. The out-diffusion of Ga as well as in-diffusion of Pd and Au to form interfacial chemical reactions at the interface is observed by X-ray photoelectron spectroscopy (XPS) measurements. The phases of the Ga-Pd and Ga-Au phases are detected by X-ray diffraction (XRD) analysis. Morphological results show that the surface of the contact is reasonably smooth with the root-mean-square roughness of 2.89 nm despite annealing at 600 degrees C. Based on the above experimental considerations, PdAl/Au/p-AlGaN contact annealed at 600 degrees C is a suitable p-ohmic contact for the development of high-performance electronic devices.