Electrical and interface properties of PdAl/Au metal alloyed ohmic contacts on p-type GaN for high-temperature MEMS devices

In this work, a PdAl/Au (20/30 nm) metal alloyed scheme was investigated for obtaining low resistance ohmic contacts to Mg-doped p-type GaN. The specific contact resistance (ρ_SCR) was determined using the circular-transmission-line pattern method between the metal contacts and p-type GaN by current–voltage (I–V) measurements. It is noted that the ρ_SCR of the as-deposited contact (1.23 × 10⁻² Ω cm²) was enhanced upon rapid thermal annealing (RTA) at 600 °C (7.82 × 10⁻⁴ Ω cm²) for 1 min under N₂ ambient. The effective Schottky barrier heights (SBHs) of the various annealed contacts were determined using the Norde and I–V methods. It is observed that the effective SBHs were dependent upon the RTA conditions. According to the X-ray diffraction and X-ray photoelectron spectroscopy results, the gallide-related phases were formed at the PdAl/Au/p-GaN interface such as Au₇Ga₂ and Ga₃Pd₅ upon RTA at 600 °C. These phases were responsible for obtaining low contact resistivity of the PdAl/Au contact. Atomic force microscopy results show that the surface morphology (root-mean-square, RMS) of the contact was reasonably smooth even after RTA at 600 °C with an RMS roughness of 0.714 nm. Observations indicate that the PdAl/Au metal alloyed contact was a suitable ohmic contact to p-type GaN for the development of commercially viable large-scale GaN-based microelectromechanical system applications.