First light not so bright from non-polar LEDs
By Richard Stevenson in Montpellier, France
GaN LED and laser pioneer Shuji Nakamura has revealed details of his team s first light emitters grown on non-polar and semipolar substrates.
Addressing delegates at the International Symposium on Blue Lasers and LEDs (ISBLLED) in Montpellier, France, Nakamura said that the output powers of these new types of devices were significantly lower than for standard LEDs.
The result may disappoint some, because semi- and non-polar LEDs have been expected to deliver higher powers than devices grown on conventional polar material, which suffer from internal electric fields that reduce emission intensity.
The best blue devices made by the University of California, Santa Barbara professor's team had an output of 1 mW at 20 mA drive current, while the best green LEDs produced 0.26 mW at 250 mA.
Nakamura believes that these low output powers are due to either stacking faults or point defects.
Although the output powers may be lower than expected, the novel devices are showing other benefits that Nakamura had predicted.
For example, the non-polar LEDs are not subject to the quantum Stark effect that causes a change in emission wavelength at different drive currents, which means that their color is less sensitive to changes in output power.
In addition, these devices have a lower series resistance, while they also feature p-doped layers with a carrier concentration that is an order of magnitude higher than standard devices.
And their polarized emission makes them suitable for direct backlighting of LCD displays, which normally require a polarizing light filter to operate with conventional LED backlights.
"The only problem is output power," concluded Nakamura.
