In lab, ultrathin magnesium cuts p-GaN contact resistance

At Nagoya University, the decisive step lasted five minutes. Haitao Wang, Jia Wang and their colleagues heated an ultrathin magnesium film deposited on p-type gallium nitride, or p-GaN, to 600°C (1,112°F). The resulting electrical contact reached a resistivity of (1–3) × 10⁻⁴ Ω cm², among the lowest reported for thin p-GaN, according to the researchers' paper in Applied Physics Letters.

The contact is an ohmic contact: a connection designed to let current pass between a metal and a semiconductor without a large energy penalty. In p-GaN, the positive charge carriers are holes—effectively empty spaces left by electrons. Magnesium helps create those holes, but too few are mobile at room temperature near the metal interface, producing a wide barrier and high resistance.

The Nagoya team first explored thicker magnesium layers, but annealing left the surface too rough for reliable thin-device fabrication. The new approach uses a film no more than 10 nanometers thick. During the shorter, lower-temperature treatment, magnesium diffuses into the surface and is rapidly consumed, creating a highly concentrated doped layer that narrows the barrier and promotes hole tunneling. The surface stays smoother, and the researchers found that a protective cap layer was not necessary; capped samples had shown unwanted impurities.

So what changes in practice? For chip makers, the attraction is not only the lower resistance but the process itself: the “top-down” treatment is described as simpler, quicker and cheaper than growing a heavily doped GaN layer from the bottom up. It can also be applied after device processing, which could make it more compatible with existing manufacturing steps. Lower-loss contacts could help improve the efficiency of LEDs and transistors used in electric vehicles and data centers.

The boundary is clear. This is a laboratory result, not a semiconductor product in service. Haitao Wang says the team is now applying the method to different devices, including LEDs and electric-vehicle transistors; those applications remain under development.