III-nitride semiconductors are incredibly useful wide-bandgap semiconductors used to produce LEDs and lasers emitting light in the visible bandwidth range. Two major limitations for making more advanced optical equipment on a large scale through metal organic chemical vapor deposition (MOCVD) are the limited concentration of holes that can be made on p-type material for electrons to move into and the “green gap,” in which LED output deteriorates in the green and yellow range of the visible spectrum. Researchers at North Carolina State University have proposed a new III-nitride semiconductor synthesis method that tackles both the hole concentration and green gap problems through the optimized use of indium gallium nitride (InGaN) semibulk templates.
Aiming to improve the efficiency of semiconductors through greater p-type hole density, the researchers grew the InGaN semibulk templates with different indium concentrations ranging from 2.4% to 15.2% through MOCVD and assessed the impact of indium content on hole formation. The team found that a relatively high content around 15.2% increased the hole density to 5 x 1019 cm-3, about an order of magnitude greater than the highest concentration previously achieved in p-type III-nitride materials using MOCVD.
The semibulk approach, which places layers of GaN between layers of InGaN, reduced defects in the material from lattice mismatch between the template and GaN substrate. The increased hole density allows for improved efficiency by converting more energy input for LEDs into light and preventing energy from being wasted as heat by reducing metal contact resistance in laser diodes. This synthesis technique is also cost-effective as it uses existing MOCVD methods for large-scale manufacturing of p-type materials without any need for additional processing. This study was published in Applied Physics Letters.
“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, a co-author of the paper. “And this is high quality material - very few defects - making it suitable for use in a variety of devices.”
The researchers’ semibulk growth strategy was also applied to LEDs to address the green gap problem. A large lattice mismatch with quantum wells is often to blame when materials are grown on a GaN substrate. Growing the semiconductor materials on InGaN templates with an indium content of up to 12% reduced strain on the quantum wells and shifted the wavelength by 100 nm, from 470 nm (blue) to 570 nm, which falls within the green gap range. This study was published separately in Superlattices and Microstructures.
Photo: Electroluminescence measurements of (a) Blue LED on GaN, (b) Green LED on InGaN template, (c) Near yellow LED on InGaN template. The insets of Fig. 1(b) and Fig. 1(c) show the image of the emission at 1.5 mA injection current. Credit: Salah Bedair