A new way of depositing very thin films of magnesium followed by soft annealing lowers the contact resistivity in p-type GaN semiconductor.
Image Credit: Jia Wang & Haitao Wang, Nagoya University.
Scientific Frontline: Extended "At a Glance" Summary: Low-Resistance Contacts for p-Type Gallium Nitride
The Core Concept: Researchers have developed a novel, top-down method to significantly lower the electrical resistance of contacts in p-type gallium nitride (GaN) semiconductors by depositing and heating an ultrathin layer of magnesium.
Key Distinction/Mechanism: Traditional methods to lower resistivity involve growing a heavily doped GaN layer, a costly process vulnerable to damage. This new technique deposits a capless, ultrathin (less than 10 nanometers) magnesium layer directly onto the p-GaN surface, followed by "soft annealing" (600 degrees Celsius for five minutes). The magnesium diffuses into the surface, creating an ultrahigh concentration magnesium-doped layer that narrows the depletion region and promotes hole tunneling, drastically reducing contact resistance to (1–3) × 10⁻⁴ Ω cm² while maintaining surface smoothness.
Major Frameworks/Components:
- Gallium Nitride (GaN): A wide-bandgap semiconductor material.
- p-Type Doping: Introducing magnesium into GaN to create "holes" (positive charge carriers) by providing one less valence electron than the replaced gallium.
- Ohmic Contacts: The electrical connections required to move current into and out of the semiconductor with minimal energy loss.
- Depletion Region: A wide barrier at the metal-semiconductor boundary in p-GaN caused by magnesium's stubborn acceptance of electrons at room temperature, which typically creates high electrical resistance.
- Quantum Tunneling: The mechanism promoted by the ultrahigh magnesium concentration, allowing holes to pass through the narrowed depletion region barrier.
Branch of Science: Materials Science, Solid-State Physics, Electrical Engineering.
Future Application: The accelerated commercialization and improved energy efficiency of a wide range of electronic devices, particularly thin p-type GaN components like LEDs and power transistors used in electric vehicles and data centers.
Why It Matters: High contact resistance has been a decades-long efficiency bottleneck for p-type GaN semiconductors. This simpler, faster, and cheaper top-down process overcomes that barrier and is compatible with existing post-fabrication processes, paving the way for more energy-efficient power electronics globally.
Researchers from Nagoya University in Japan have found that depositing an ultrathin (less than 10 nanometers) magnesium layer onto gallium nitride (GaN) and gently heating it for a short time creates the best electrical contacts yet reported for the thin p-type layers used in devices such as LEDs and power transistors.
Semiconductor devices such as LEDs and transistors are generally made up of two halves: an n-type, which carries a negative charge via electrons, and a p-type, which moves positive charge carriers called holes, which are essentially electron voids. Both halves rely on contacts that can move electric current in and out of them with minimal energy loss. These connections, known as ohmic contacts, have been an efficiency bottleneck in thin p-type GaN semiconductors for decades owing to their high resistance.
Now, a team led by Haitao Wang and Jia Wang at the Institute of Materials and Systems for Sustainability (IMaSS) at Nagoya University has developed a new method for lowering the resistance of p-type GaN contacts. They deposited an ultrathin magnesium layer onto the p-GaN surface and applied a heat treatment at 600 °C for five minutes, thereby achieving a contact resistivity of \((1\text{--}3) \times 10^{-4}\ \Omega\ \text{cm}^2\) without damaging the surface. This is among the lowest reported contact resistivities for thin p-type GaN.
Their findings, which are expected to make a wide range of electronic devices used in applications such as electric vehicles and data centers more energy-efficient, have been published in the journal Applied Physics Letters.
The Challenges of p-GaN Contacts
Magnesium-doped p-type GaN was developed at Nagoya University by Isamu Akasaki and Hiroshi Amano in work recognized by the 2014 Nobel Prize in Physics for blue LEDs. In this material, holes are produced by adding small quantities of magnesium, which has one fewer valence electron than the surrounding gallium, in a process known as doping.
However, magnesium is a stubborn acceptor of electrons at room temperature. As a result, the metal-semiconductor boundary lacks enough mobile holes, resulting in a wide barrier called a depletion region. This barrier creates high resistance to the flow of charge, and thus, more energy is needed to drive currents across it.
One way to counter this high resistivity is to grow a heavily doped GaN layer with a rich concentration of holes directly at the interface. However, this is a costly and laborious process that is vulnerable to damage during later device fabrication stages, such as plasma etching.
A Thin Magnesium Layer Does the Job, No Cap!
To look for alternatives a few years earlier, the same group tried depositing a layer of metal magnesium directly onto a GaN wafer and processed it with a heat treatment called annealing. This magnesium layer was relatively thick, measuring dozens of nanometers. In doing so, they also discovered the emergence of a remarkable superlattice structure.
"This superlattice was very interesting for fundamental science," said Jia Wang, who led the 2024 study published in Nature. "But at that time, the surface after annealing was too rough, raising concerns about device reliability, particularly for thin p-type GaN, so some of the benefits of this approach could not be fully demonstrated."
To solve the roughness issue, Haitao Wang pivoted to depositing a much thinner magnesium film. The problem with this approach is magnesium's high reactivity, which makes it oxidize easily in air. To prevent this, a protective cap layer is typically used to isolate the magnesium from air. However, all samples using a cap showed unwanted impurities.
"I then tried [a capless sample] as a reference, and it came out as a surprise," Haitao Wang said. "I found that the cap didn't make any difference." It is likely that only the very top layer of magnesium gets oxidized, preserving the rest even if the total thickness is no more than 10 nanometers.
The researchers subjected this thin layer to a heat treatment called "soft annealing," where the sample is heated to a lower temperature (600 °C) and for a shorter time (five minutes) compared to normal annealing. During this process, the ultrathin magnesium layer gets rapidly consumed as the magnesium diffuses into the surface region of the p-type GaN. To the researchers' delight, this new surface turned out to be much smoother than in their previous work.
This ultrathin, ultrahigh-concentration magnesium-doped layer narrowed down the contact depletion region and promoted the tunneling of holes across it, thereby lowering contact resistance.
Furthermore, this "top-down" process is simpler, quicker, and cheaper to implement than "bottom-up" crystal growth. It can also be applied after device processing, making it more flexible and compatible with existing fabrication processes. As a result, it holds promise for the accelerated commercialization of higher-efficiency semiconductors.
"We are now applying this method to different types of devices, such as LEDs and transistors in electric vehicles," said Wang.
Funding: This work was supported by the Japan Science and Technology Agency (JST) Adopting Sustainable Partnerships for Innovative Research Ecosystem (ASPIRE) program (Grant No. JPMJAP2311), the Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research (KAKENHI, Grant No. 24K17305), and the Young Researchers Activation Support Project of the Center for Integrated Research of Future Electronics (CIRFE) at Nagoya University.
Published in journal: Applied Physics Letters
Authors: Haitao Wang, Shumeng Yan, Zhiyu Xu, Yingying Lin, Peirong Yu, Joshua Jaehyung Park, Joseph E. Dill, Hei Wong, Qingyuan Han, Xigen Li, Qian Sun, Tomás Palacios, Debdeep Jena, Huili Grace Xing, Jia Wang, and Hiroshi Amano
Source/Credit: Nagoya University
Edited by: Scientific Frontline
Reference Number: ms082426_01