New semiconductor synthesis leads to more efficient sources

28 Oct 2021

NC State improves the performance of III-nitride materials and addresses the green gap.

North Carolina State University (NC State) has announced the development of a new synthesis process for III-nitride semiconductor materials that could lead to increased efficiency levels from lasers and LEDs, alongside research into improving green LED emissions.

Published via two papers, in Applied Physics Letters and Superlattices and Microstructures, the projects offer a route to ensuring that more of the energy input to the materials is converted to light in LEDs, and to tackling the long-established "green gap."

NC State's synthesis method was designed to address "two of the main issues facing long wavelength emitting LEDs, mainly the low growth temperature and high values of strain in the quantum wells," according to the APL paper.

The project employed the manufacturing technique termed semibulk growth, a method for enhancing the eventual transport of carriers through a completed semiconductor through careful layering of indium gallium nitride with gallium nitride during manufacture of the semiconductor's n-type region. This is known to reduce defects in the structure as well as fill in pits that form on the surface.

NC State demonstrated that the semibulk growth approach can be used for the p-type layer in LEDs to increase the number of holes for electrons to transport into, and hence the efficiency of the LED. This should mean that LED devices can be made in one growth via metal organic chemical vapor deposition (MOCVD) techniques without a lengthy processing time in between, according to the team.

"We have developed a process that produces the highest concentration of holes in p-type material in any III-Nitride semiconductor made using MOCVD," said Salah Bedair from NCState. "And this is high quality material - very few defects, making it suitable for use in a variety of devices."

Going greener on InGaN

The green gap refers to long-standing efforts aimed at enhancing the efficiency of green-emitting LEDs, a parameter which has continued to pose technical challenges even as the performance of red and blue-violet emitters has improved.

Efforts to tackle the problem go back some time, focused in particular on the epitaxial structure and the substrate material. Optics.org reported back in 2006 that a project backed by the US Department of Energy intended to develop gallium nitride materials suitable for improved green LEDs so that nitride LEDs "can begin to realize their full commercialization potential."

The NC State project concluded that one of the main reasons for the green gap is the large lattice mismatch between the light emitting part of the material, the quantum well, when gallium nitride substrates are used. Replacing a gallium nitride substrate with an indium gallium nitride template was found to bring significant improvements.

Comparing the LED emission spectrum for the same quantum well emitting in blue when grown on a gallium nitride substrate but emitting in green or yellow when grown on indium gallium nitride showed that a 100 nanometer shift in emission could be achieved, due to application of the InGaN templates.

"Relaxed InGaN templates shift blue LED emission from blue to the green gap spectral range," commented the project in its paper. "The results indicate the potential of the InGaN template approach in addressing problems facing long wavelength InGaN LEDs."