The catch, says Tansu, is the relative inefficiency of the green light produced by an LED. When combined with red and blue LED light to produce white light, this shortcoming limits the overall “radiative efficiency” of the white LED light.
The inefficiency of green LED light, says Tansu, stems from a phenomenon called the charge-separation effect, which occurs when electron and electron hole carriers are spatially separated inside the nanoscale quantum-well active region of LED devices.
In nitride LEDs, says Tansu, a large internal polarization field exists in the InGaN semiconductor quantum well that is required to generate green light. But the polarization field creates an electrical field inside the active region of the semiconductor, and the electrical field in turn promotes the separation of electron and electron hole carriers in the active region.
“This leads to a reduction in the radiative transition probability rate for electron and electron hole carriers in the active region that is responsible for generating light radiation,” says Tansu.
|Tansu’s work has been featured in Laser Focus World and Applied Physics Letters.|
To achieve a higher efficiency of green LED light and boost the radiative efficiency of the overall LED, Tansu is attempting to engineer a 2- or 3-nanometer structure which allows the electron and electron hole carriers to align more precisely.
“We need to engineer the spatial position of both the electron and the hole carriers to the center of the nanostructure-active region to increase the chance that they will overlap,” says Tansu. “To do this, we use quantum mechanics to design the nanostructures for engineering the electron and hole wave functions, which will result in a significantly enhanced radiative efficiency.”
Tansu reported the results of his work in 2007 in the Proceedings of the IEEE/OSA Conference on Lasers and Electro-Optics (CLEO) and in Applied Physics Letters (APL). The APL article was coauthored with Ronald A. Arif and Yik-Khoon Ee, Ph.D. candidates and members of Tansu’s group. Laser Focus World has also reported on Tansu’s use of staggered InGaN quantum wells to enhance the radiative efficiency of nitride LEDs.
Tansu and his group, including Hongping Zhao, a Ph.D. candidate, are pursuing other approaches based on type-II nitride-based quantum wells and InGaN quantum dots. The results of these studies have been published in APL and the Journal of Crystal Growth.
In a related effort to improve the efficiency of LEDs, Tansu and his group are experimenting with arrays of microlenses to extract, with greater efficiency, the light that is generated in quantum-well nanostructures but then trapped inside the structures in part because of the large refractive-index difference between gallium nitride and air.
The group has published the results of this research in APL in an article that was coauthored with James Gilchrist, assistant professor of chemical engineering at Lehigh, and Pisist Kumnorkaew, a graduate student in chemical engineering.
The group’s work with microarrays was featured in 2008 in Laser Focus World, which said the arrays promised to improve light-extraction efficiency of LEDs while overcoming three challenges to previous extraction techniques – cost, scalability and process control.
Tansu is also collaborating with professors Rick Vinci and Helen Chan of materials science and engineering on research aimed at reducing defect density by decreasing the non-radiative recombination process in gallium-nitride semiconductors, which in turn will improve the radiative efficiency of the LED. This project is supported by NSF. Tansu’s group has also received funding from NSF’s Electronics Photonics Devices Technology research program. Tansu has filed patent applications on the microlens array and staggered quantum wells, as well as the type-II nitride-based quantum wells.
Tansu, Dierolf, Arif, Ee and Zhao are affiliated with the COT, while Gilchrist, Chan, Vinci and Kumnorkaew are affiliated with the university’s Center for Advanced Materials and Nanotechnology.