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

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.

💡 Research Summary

This paper investigates the interdiffusion mechanisms of nickel, gold, and gallium in Ni‑Au/p‑GaN contacts during rapid thermal annealing (RTA) performed in an oxygen atmosphere, and correlates these mechanisms with the electrical performance of the contacts. Using a standard InGaN/GaN blue‑LED structure (460 nm emission), a 20 nm Ni layer followed by a 200 nm Au layer was deposited by electron‑beam evaporation. Samples were subjected to RTA at temperatures ranging from 350 °C to 650 °C for 2 – 10 minutes under a constant O₂ flow (500 sccm). Electrical characterization employed the Transmission Line Method (TLM) to extract specific contact resistance (rc), maximum current (Imax) at 5 V, and Schottky barrier height (ΦB).

Unannealed contacts displayed a high Schottky barrier (~0.70 eV), a large rc (~1.5 × 10⁻¹ Ω·cm²), and a modest Imax (~0.30 mA). After a brief 2‑minute anneal at 450 °C, ΦB dropped below 0.45 eV, rc fell to 7.5 × 10⁻³ Ω·cm², and Imax rose to ~0.80 mA. Extending the anneal to 10 minutes further increased Imax to ~1 mA while only modestly reducing rc to 3.7 × 10⁻³ Ω·cm², indicating that most of the electrical improvement occurs early in the anneal.

High‑resolution TEM, HAADF‑STEM, and energy‑dispersive X‑ray spectroscopy (EDX) were used to probe the structural evolution. In the as‑deposited state, a ~3 nm interfacial layer of nickel oxide (NiOx) was observed at the Ni/GaN interface, together with nanograined Ni and Au layers. Upon annealing, Ni oxidizes further, forming NiOx that migrates toward the top surface of the metal stack. Simultaneously, Au diffuses downward, reaching the GaN surface, while Ga atoms out‑diffuse from the GaN/metal interface. This results in the formation of a thin Au‑Ga interfacial alloy layer (a few nanometers thick). Importantly, the emergence of this Au‑Ga layer coincides with the transition from Schottky‑like to ohmic behavior, even when Ni or NiOx are no longer present at the interface.

The authors argue that the key factor behind the reduced contact resistance is not the presence of NiOx, as previously suggested, but rather the creation of gallium vacancies (Ga‑vacancies) associated with the Au‑Ga interfacial layer. The out‑diffusion of Ga creates a high concentration of Ga‑vacancies near the p‑GaN surface, which effectively raises the acceptor (hole) concentration, lowers the Schottky barrier, and enhances carrier injection. The Au‑Ga alloy also provides a high work function (~5 eV), further aligning the metal Fermi level with the valence band of p‑GaN. While “auto‑cleaning” of surface contaminants during annealing may contribute, quantitative EDX mapping shows that Ga‑vacancy formation dominates the electrical improvement.

Methodological limitations are acknowledged: (i) quantification of light elements (O) in Au‑rich regions is prone to overestimation due to Bremsstrahlung background; (ii) Ga contamination from FIB preparation can artificially raise measured Ga content, but this effect is limited to 2‑3 % and is accounted for; (iii) surface oxidation of the TEM slice can affect oxygen signals, prompting the use of thicker windows for accurate bulk composition.

In summary, the study provides compelling evidence that the formation of a thin Au‑Ga interfacial layer, accompanied by Ga‑vacancy generation, is the primary driver for achieving low‑resistance ohmic contacts in Ni‑Au/p‑GaN systems. This insight shifts the design focus from merely promoting NiOx formation to deliberately engineering Au‑Ga interfacial alloys and controlling Ga out‑diffusion during oxygen‑ambient annealing. The findings have practical implications for the fabrication of high‑performance p‑type contacts in GaN‑based LEDs and laser diodes, suggesting new annealing recipes and metal stack optimizations that prioritize Au‑Ga interlayer formation.