Kopin samples CyberLite blue LEDs
Although the wavelength was not specified, the new LEDs demonstrated a radiant power of 10 mW at 20 mA (unpackaged), which is comparable to commercially available high brightness blue devices. The emitters also provide electrostatic discharge (ESD) resistance of over 4000 V, while the resistance of commercially available LEDs is 2000 V or below. Kopin says the high ESD resistance is important for automotive applications.
"Getting below 3.0 V has been a scientific hurdle for nearly a decade," said Kopin founder and chairman John Fan. "We believe we can move CyberLites into large-volume production for the mass market. We have already begun shipping evaluation samples of CyberLites to prospective customers towards this goal."
Light from NanoPockets
The new LEDs are described in the July 29 edition of Applied Physics Letters. Researchers from Kopin and North Carolina State University grew several InGaN/GaN-on-sapphire devices under various MOCVD growth conditions. Devices with high optical efficiency were found to have thickness variations in the InGaN quantum wells that occurred at certain of growth rates and temperatures. The indium in the well induces compressive stress, which is relieved by a periodic thickness variation.
In the paper, the QWs were characterized using TEM under conditions that gave diffraction and atomic number contrast. This allowed both the crystalline quality and composition of the wells to be estimated. The technique showed QWs with periodic variations in thickness occurring over distances of 50 to 80 nm, in addition to localized (or short range) indium compositional fluctuations occurring every 3 to 4 nm.
Processed devices containing such long range variations produced up to 10 mW of optical power, while devices with InGaN layers that feature little or no variation in thickness delivered around 3.8 mW. The authors concluded that the long-range thickness variations are more effective than localized indium composition fluctuations, which occur over short distances, in providing quantum confinement for carrier recombination.
Previously, localized carrier recombination induced by indium phase separation in the QWs has been widely used to explain the efficient light emission in highly defective InGaN LEDs. These fluctuations are believed to separate regions of carrier recombination from regions containing dislocations, which are line defects typically present at densities up to 1010/cm2 in GaN-based LEDs. Kopin is calling the efficient recombination regions caused by long range thickness variations in its devices “NanoPockets”. The company says the pockets contain defect-free material that does not interact with dislocations.
"Realizing that defects cannot be completely eliminated [from InGaN/GaN-on-sapphire LEDs], Kopin provided confinements for production of light away from the defects, and inside these NanoPockets," said Jagdish Narayan of NCSU, a co-author of the APL paper and director of National Science Foundation Center of Advanced Materials and Smart Structures.
