Research groups at the University of California at Santa Barbara (UCSB), the University of Tsukuba and the Tokyo University of Science have developed a new class of gallium nitride (GaN) semiconductor thin films, which are used to make blue and green LED chips. The new materials are known as semipolar and nonpolar GaN.
The research was funded by the Japanese Science and Technology Corporation (JST) under its Exploratory Research and Advanced Technology (ERATO) program. The ERATO "Nakamura Inhomogeneous Crystal" project is headed by UCSB's Shuji Nakamura, formerly of Nichia.
The ERATO researchers have demonstrated that the optical output power of LEDs fabricated using nonpolar and semipolar GaN saturates at much higher drive currents than for conventional LEDs, which is promising for demanding high-brightness applications such as automobile headlights.
Also, LEDs fabricated on non-polar and semipolar GaN exhibit little or no wavelength shift when drive current is increased. This is expected to have benefits for system integrators who utilize RGB mixing.
The resistance of p-type GaN layers is improved in non-polar films, which could lead to reduced heating, a limiting factor in device performance. Further, semipolar GaN-based LEDs have a lower turn-on voltage than typical conventional LEDs.
Also, semipolar and nonpolar GaN-based LEDs emit polarized light, which is required by liquid crystal displays (LCDs). Conventional LEDs used in LCD backlights require a polarizing filter.
Semipolar and nonpolar GaN films have a different crystal structure from conventional (hexagonal) GaN, with reduced or zero built-in electric fields. This makes it easier for the process leading to light emission – the combination of electrons and holes - to occur.
Download the ERATO press release (PDF, 16k)
