Superior p-GaN ohmic contacts
A collaboration led by Hiroshi Amano’s group at Nogoya University is pioneering a process for producing very thin p-GaN layers with an ultra-low contact resistance.
According to the team – a partnership between Nagoya University, Cornell University, the University of Illinois Urbana-Champaign, MIT, City University of Hong Kong, and Suzhou Institute of Nano-Tech and Nano-Bionics – their simple low-thermal-budget post-growth approach, involving magnesium deposition, annealing, and chemical cleaning, creates high-quality ohmic contacts on thin p-GaN layers.
A wide range of devices should benefit from the team’s process that realises a specific contact resistance of just 1-3 x 10-4 Ω cm2. This ultra-low resistance reduces operating voltage, boosts efficiency, and trims power consumption.
Note that there are additional benefits for devices that require a very thin p-GaN layer, or are vulnerable to process-induced damage.
That means that HBTs will benefit, because the p-GaN base is very thin – during device fabrication the exposed base surface can suffer significant damage from dry etching, which is difficult to repair.
“In contrast, our magnesium-annealing approach can be applied directly after processing, and provides a relatively simple way to recover the electrical performance of the exposed p-GaN surface and improve the overall device characteristics,” remarks team spokesman, Haitao Wang.
The researcher from Nagoya University points out that UV LEDs and lasers could also be improved with the team’s process, as it enables the p-GaN to be very thin, aiding light extraction and optical efficiency.
“Our technique is attractive in this situation, because it can improve the contact properties of very thin p-GaN without introducing severe surface roughening. This provides an important combination of electrical and optical advantages.”
The team’s process for producing ohmic contacts is markedly different from conventional approaches, which involve epitaxial growth of an extremely thin, heavily doped p+-GaN contact layer.
According to Wang, traditional approaches place stringent requirements on epitaxial growth and doping control, and leave the p-GaN surface susceptible to damage from a dry-etching plasma process, required before metal-electrode formation. Since 2021, Amano’s team has been investigating the formation of low-resistance ohmic contacts through the deposition of magnesium films on GaN surfaces. Initially, researchers produced magnesium films with a thickness of several tens of nanometres, using relatively high annealing temperatures, but this led to substantial roughening of the GaN surface after annealing. Now they have refined the process, depositing a sub-10-nm-thick metallic magnesium film on p-GaN, before undertaking a 5 minute ‘soft anneal’ at 600 °C, followed by a HCl clean.
X-ray photoelectron spectroscopy measurements have revealed that the energy separation between the valence-band maximum and the Fermi level has decreased from 2.2 eV to 1.3 eV. This suggests that soft annealing reduces the effective hole-injection barrier at the metal/p-GaN interface while narrowing the depletion region and enhancing hole tunnelling.
Using their process, the team produced quasi-vertical p-i-n diodes with a flat p-GaN surface, favourable hole-transport characteristics, a high breakdown voltage, and a low leakage current.
According to Wang, the team’s next goals are to demonstrate the universality of their technique on more GaN devices, understand the underlying physics in much greater detail, and establish a systematic and standardised magnesium-annealing process platform.
Reference
H. Wang et al. Appl. Phys. Lett. 129 063306 (2026)
