Green LEDs Take a Semipolar Route
Green LEDs, which comprise thin films of GaN doped in various ways, present problems when the upper films are grown along a polar direction of the GaN crystal. For example, strong polarization-induced electric fields are produced and lead to a blueshift in the peak emission at high LED drive currents. Unfortunately, it is much harder to grow the film along a nonpolar direction, so most commercial LEDs take the polar approach. A group from the University of California, Santa Barbara, now has demonstrated an alternative in the use of a semipolar direction.
The green LED is grown along a semipolar plane, offering benefits such as lower polarization-induced electric fields and reduced blueshift with increasing bias. Image by Hisashi Masui, University of California, Santa Barbara.
“Growth along semipolar planes can give us most of the advantages of nonpolar planes, while also giving us a ‘workaround’ to some of the issues associated with nonpolar planes,” said graduate student and research team member Rajat Sharma.
He explained that benefits include lower polarization-induced electric fields and a reduced blueshift with increasing bias that is characteristic of nonpolar films. Semipolar-grown films also incorporate indium — a crucial element in the LED — more efficiently and have a wider growth window than nonpolar films.
In their studies, the researchers grew the films atop a sapphire substrate, first laying down a 20-μm-thick semipolar GaN template using a process of hydride vapor phase epitaxy. They then used metallorganic chemical vapor deposition to lay down a stack of N- and P-type GaN films sandwiching several layers of InGaN separated by GaN.
They characterized the LEDs, measuring the emission wavelength as a function of drive current, and found that they emitted at about 525 nm and shifted to the blue by about 5 nm as drive current went from 50 to 250 mA. A commercial device used for reference shifted from more than 520 nm to less than 510 nm as the drive current went from about 25 to 100 mA. The researchers also found that the semipolar LEDs turned on at a lower voltage and emitted partially polarized light, which the commercial device did not.
Plans call for optimizing the crystal quality and reducing the defect density of the semipolar GaN template, which should help confirm the findings and improve commercial prospects. Other research is aimed at laying down the semipolar GaN directly using metallorganic chemical vapor deposition. The result could be more efficient LEDs as well as improvements to other devices that can benefit from the higher P-type conductivity predicted for semipolar films, such as GaN laser diodes, Sharma said.
Applied Physics Letters, Dec. 5, 2005, 231110.
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