Making white-light-emitting diodes without phosphors

Dong-Ming Yeh
2008 SPIE Newsroom  
Stacking quantum wells of different indium contents and using II-VI semiconductor nanocrystals for color conversion are two solutions to the problem of generating broad-spectrum visible light. Solid-state lighting technology has attracted attention recently because it offers the potential to save energy and protect the environment by producing light more efficiently. In particular, high-efficiency indium gallium nitride/gallium nitride (In-GaN/GaN) quantum-well (QW) white-light-emitting diodes
more » ... LEDs) look promising. The major techniques for fabricating semiconductor-based white-light devices use phosphors to convert UV or blue LED output into longer-wavelength colors to mix to form white light. So far, the most widely used phosphor is cerium-doped yttrium aluminum garnet (YAG:Ce), which can effectively convert blue light at wavelengths around 460nm into yellow light near 560nm. The conversion quantum efficiency can be larger than 60%. 1 Based on this technique, the efficiency of a white-light LED as high as 190lm/W has been reported. 2 Because of the broad spectrum of YAG:Ce emission and the twocolor nature of such an LED, however, the color-rendering index is not high, only in the range from 66 to 80. Recently, another technique that offers a higher color-rendering index has been widely implemented: output from a UV LED has been converted through three phosphors into blue, green, and red light, which are then mix to create white light. However, the conversion efficiencies of the phosphors-particularly the one that generates red light-are not as high as we would like. 3 For this reason, technologies that avoid phosphors deserve intensive study. Two alternative approaches for implementing white-light LEDs have been demonstrated. The first uses a novel technique for growing high-indium InGaN/GaN QW structures that allow efficient emission in the green-yellow band, or even in the orange-red range. 4 If a high-quality yellow-or red-emitting QW LED can be grown, we could achieve white-light generation by stacking two QW layers that emit in the blue and yellow, or three layers emitting in the blue, green, and red. Continued on next page
doi:10.1117/2.1200802.1069 fatcat:jdjqkgopjvc7zj26ymi6lngtgi