On the importance of Ni-Au-Ga interdiffusion in the formation of a Ni-Au / p-GaN ohmic contact

TL;DR

This study investigates Ni-Au-Ga interdiffusion during rapid thermal annealing of Ni-Au/p-GaN contacts, revealing that Ga vacancies and Au-Ga interfacial layers are key to achieving ohmic behavior, independent of Ni or NiOx presence.

Contribution

It provides new insights into the interdiffusion mechanisms and clarifies the role of Ga vacancies and Au-Ga layers in forming ohmic contacts on p-GaN.

Findings

Ga vacancies facilitate Schottky barrier reduction

Au-Ga interfacial layer is crucial for ohmic contact formation

Ni or NiOx presence is not the main factor for ohmic behavior

Abstract

The Ni-Au-Ga interdiffusion mechanisms taking place during rapid thermal annealing (RTA) under oxygen atmosphere of a Ni-Au/p-GaN contact are investigated by high-resolution transmission electron microscopy (HR-TEM) coupled to energy dispersive X-ray spectroscopy (EDX). It is shown that oxygen-assisted, Ni diffusion to the top surface of the metallic contact through the formation of a nickel oxide (NiOx) is accompanied by Au diffusion down to the GaN surface, and by Ga out-diffusion through the GaN/metal interface. Electrical characterizations of the contact by Transmission Line Method (TLM) show that an ohmic contact is obtained as soon as a thin, Au-Ga interfacial layer is formed, even after complete diffusion of Ni or NiOx to the top surface of the contact. Our results clarify that the presence of Ni or NiOx at the interface is not the main origin of the ohmic-like behavior in such contacts. Auto-cleaning of the interface during the interdiffusion process may play a role, but TEM-EDX analysis evidences that the creation of Ga vacancies associated to the formation of a Ga-Au interfacial layer is crucial for reducing the Schottky barrier height, and maximizing the amount of current flowing through the contact.