Record performance figures for nonpolar and semipolar LEDs have been reported by researchers at the Solid State Lighting & Display Center (SSLDC) at UC Santa Barbara (UCSB), and the Japan Science & Technology Agency’s Exploratory Research for Advanced Technology program (JST ERATO).
Steve DenBaars, part of the UCSB team that also includes Shuji Nakamura, James Speck and Umesh Mishra, says that the team believe this to be "one of the biggest fundamental breakthrough in GaN emitters in several years."
Nonpolar and semipolar LEDs are a new class of gallium nitride (GaN) based devices based on non-standard GaN material orientations. Compared with conventional GaN-based LEDs, the nonpolar and semipolar versions are expected to exhibit higher external quantum efficiency (EQE) at high current densities, as well as emitting polarized light.
The new non-polar LEDs have an external quantum efficiency of 41% and radiant powers as high as 25 mW for standard size (300 x 300 µm) and operating current (20 mA).
Semipolar LEDs of the same size exhibited external quantum efficiency of 30% and radiant powers as high as 18 mW, also at 20 mA.
"The wavelength range is 400-415nm now, but we will be making longer wavelength LEDs shortly," says DenBaars. "The long term goal is higher EQE at higher current densities and longer wavelengths."
The UCSB groups have also reported conventional c-plane LEDs with EQE of 66% and 35 mW radiant power. These LEDs have been used to make white LED lamps which boast a luminous efficacy of 116 lm/W.
While high efficiency is desirable in many applications, the polarized emission is also likely to prove important. "The nearest term application [for these devices] is LCD backlighting using polarized light from non-polar LEDs," says DenBaars.
Research funding was provided by JST ERATO and also by SSLDC, which is focused on advancing new semiconductor-based energy efficient lighting and display technologies through partnerships with industry leaders.
Nonpolar and semipolar LEDs
Conventional GaN-based LEDs are usually grown with the crystal structure in a c-plane orientation. While such devices benefit from decades of research, the structure itself is susceptible to polarization-induced fields that limit device performance.
In simple terms, these fields cause the electrons and holes to be separated from one another, reducing the chances that they will combine to produce a photon of light. This effect is more severe in longer wavelength devices, explaining why the best green LEDs have a much lower performance than blue LEDs.
Another effect of polarization-induced fields is a shift to shorter emission wavelengths as the drive current is increased.
Polarization-induced effects can potentially be reduced or eliminated by growing the GaN films in different crystal orientations. Nonpolar GaN LEDs can be grown in the a-plane or m-plane directions, and there are also several semipolar crystal planes.
Nonpolar and semipolar LEDs are expected to exhibit a much lower dependence of emission wavelength on drive current.
Also, higher levels of p-type doping – one of the toughest challenges in GaN device fabrication – will lead to higher current densities and enhanced efficiency.