Téo Baptiste

The formation of an ohmic p-contact represents a critical step in the processing of GaN-based devices such as light-emitting diodes (LEDs). Thermal annealing in an O2-containing atmosphere at high temperature of a GaN/Ni–Au contact is mostly used today, while nonannealed contacts have been reported to provide much lower electrical performances. We experimentally evidence that the quality of the contact before annealing strongly depends on the nature of the Ni/GaN interface. Electrical results are correlated to microstructural and physicochemical analysis using high-resolution transmission scanning electron microscopy coupled with energy dispersive x-ray spectroscopy for this purpose. Using an annealed contact formed in standard conditions as a reference, we evidence from in situ treatment of the interface before metal evaporation that interfacial Ga-Oxide plays a detrimental role. We show that the metal evaporation conditions may strongly impact the physicochemical composition of the interface before annealing. We obtain nonannealed contact with electrical characteristics close to that of an annealed, standard one, using a reduced evaporation rate, in our evaporation conditions. This result is of interest for the processing of p-GaN contacts at a low thermal budget.

High-resolution transmission electron microscopy (TEM) coupled to energy dispersive X-ray spectroscopy (EDX) is used to clarify the exact role of Ni–Au–Ga interdiffusion mechanisms taking place during rapid thermal annealing under an oxygen atmosphere of a Ni–Au/p-GaN contact. 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 a transmission line method 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 with 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.