Degradation Mechanism of Pd/p-GaN Ohmic Contacts

GaN-based wide bandgap semiconductors and devices have been quickly emerging as a new frontier and cutting-edge technique. It is necessary to fabricate Ohmic contact for GaN device application. Pd is one of candidate metals for fabricating metal/p-GaN Ohmic contacts. Under the identical preparation conditions, different thickness Pd films were sputtered on p-GaN epilayers in sputtering chamber directly connected with MOCVD growth system. After the metal deposition, the samples were then annealed at 300℃ and 600℃ in ambient atmosphere of N2: O2=4: 1 or under vacuum conditions for 90 s. They were measured and characterized by four-probe Van de Pauw, X-ray photoelectron spectroscopy(XPS), and atomic force microscopy(AFM). It is found that the electrical properties and surface morphologies of samples were dependent on annealing ambient atmosphere and heat treatment temperature conditions. When the samples were annealed at 600℃ under high vacuum conditions, Ga can be diffused out from Pd/p-GaN interface. It would be helpful to make Ga and Pd reactive to become alloy as well as to produce Ga vacancy for Ohmic contact. While the samples were annealed in N2: O2=4: 1 environment, however, oxidation of metal Pd and Ga becomes a dominant factor resulting in degradation of the Ohmic contacts.The higher the annealing temperature in oxygen environment, the worse the electrical properties. Surface morphologies of the annealed samples were completely changed from a smooth atomic step morphology to a dendritic polycrystal grain state if oxygen was participated into the interface during the annealing process. Therefore, avoiding oxygen incorporation at Pd/p-GaN interface shall be a key role in forming alloy and Ohmic contact, which favors the stability and reliability of device performance. © 2021, Science Press. All right reserved.