Scientists have developed a simple way to reduce electrical resistance in gallium nitride semiconductors, potentially paving the way for more efficient LEDs, electric vehicles, data centers and other high-power electronics.
The advance comes from researchers at Nagoya University in Japan, who used an extremely thin layer of magnesium to improve one of the troublesome electrical connections in gallium nitride, or GaN.
Their findings were published in Applied Physics Letters.
Semiconductor devices such as LEDs and transistors typically contain two types of material.
N-type material carries electrical charge mainly through electrons, while p-type material carries positive charge through “holes,” which can be thought of as spaces where electrons are missing.
Both need electrical contacts that allow current to enter and leave efficiently. These are called ohmic contacts.
For decades, however, making low-resistance contacts for thin p-type GaN has been difficult. High resistance means more energy is wasted when electricity passes through the device.
The Nagoya team, led by Haitao Wang and Jia Wang, found that depositing an ultrathin layer of magnesium onto p-type GaN could dramatically improve the contact. They then heated the material to 600°C for just five minutes.
The resulting contact resistivity was between 1 and 3 × 10⁻⁴ ohm-square centimeters, among the lowest values reported for thin p-type GaN. Importantly, the treatment did not significantly damage or roughen the material’s surface.
Magnesium already has an important history with GaN. Adding small amounts of magnesium to GaN helped make practical p-type GaN possible, contributing to the development of efficient blue LEDs. That work was recognized with the 2014 Nobel Prize in Physics.
But magnesium-doped GaN has a problem. At room temperature, relatively few mobile holes are available near the boundary between the metal contact and semiconductor. This creates a barrier that makes it harder for electrical current to pass through.
One solution is to grow another heavily doped GaN layer containing many more holes. However, this approach can be expensive, complicated and vulnerable to damage during later manufacturing.
The new method takes a simpler approach.
The researchers had previously experimented with much thicker magnesium layers. Although those experiments produced scientifically interesting structures, heating caused the surface to become too rough for reliable use in thin devices.
This time, they reduced the magnesium layer to no more than about 10 nanometers. Surprisingly, they also found they didn’t need a protective coating to prevent the highly reactive magnesium from oxidizing. It appears that only the uppermost magnesium oxidizes, leaving enough material underneath intact.
During the short heat treatment, magnesium moves into the surface of the GaN, creating an extremely concentrated magnesium-rich region. This narrows the electrical barrier and allows positive charge carriers to pass through more easily.
The technique could also be cheaper and more flexible than growing additional semiconductor layers because it can be performed after other device manufacturing steps.
The researchers are now testing the approach in different technologies, including LEDs and transistors designed for electric vehicles. If successfully commercialized, the tiny magnesium layer could help GaN devices waste less electricity while delivering better performance.
