INDUSTRY LEDs
Figure 6: Spectra of two different approaches for green LEDs. The emission generated by the phosphor is broader than that resulting from a direct green, InGaN-based LED
to a record 150 lm (280 mW) at 350 mA (see Figure 5 for plots of this 533 nm LED). This corresponds to an efficacy of 135 lm/W – compared with 108 lm/W for the 1 mm2
chip.
, is an enabling technology for high-performance projection systems based on red, green and blue LEDs. Efficacies at very low drive currents are particularly impressive. They exceed 190 lm/W at 100 mA, and are in excess of 300 lm/W below 2 mA.
Increasing currents to higher values leads to far greater output at slightly shorter wavelengths: Driven at 700 mA, the chip emits 248 lm and 480 mW at a peak wavelength of 531 nm; and cranking up the current to 1A propels the output to 313 lm and 620 mW, with peak wavelength shifted to 529 nm. The latter figure, which equates to more than 310 lm (600 mW) at a current density of 50 A cm-2
Pumping phosphors An alternative approach for making a green emitter is to take a blue LED and add a green phosphor. We have investigated this, using a ceramic platelet of the green phosphor lutetium aluminium garnet (LuAG). This pumping approach creates a significantly different green emission profile: the emitter features a 531 nm peak wavelength, a Gaussian peak at 525 nm and a full- width half maximum (FWHM) of 33 nm;
while the chip-phosphor combination produces a peak wavelength of 529 nm, has a central wavelength of 557 nm and produces a FWHM of 99 nm (see Figure 6). A broader emission profile has its pros and cons. It’s favourable for general lighting because it offers a high CRI, but a narrower emission is preferred in applications such as projection. There, the smaller spectral bandwidth of direct- green LEDs quashes cross talk, leading to higher system efficiency. What’s more, if direct-green LEDs are used for projection, they can cover a wider colour range than a converted green solution (see Figure 7).
However, a blue LED and a green phosphor is still an attractive option, because it avoids issues associated with the green gap. Although there are inevitable losses associated with the Stokes shift, pumping a phosphor with a blue chip leads to higher efficiencies, because droop is not as strong at shorter wavelengths (see Figure 8). In addition, internal piezoelectric fields are weaker in blue LEDs, leading to lower electrical losses. We have compared the luminous flux and the efficacy of the two different
Figure 8: Current-dependent luminous flux and efficacy of two different approaches to generate green light. Whereas the green InGaN/GaN LED shows significant droop at high operating currents, a blue LED in combination with a phosphor converter yields higher efficacy and luminous flux at a typical driving current
approaches, using ThinGaN chips 1mm2 in size. At lower current densities, the green LED is more efficient than its blue cousin, there are no conversion losses, and efficacy is 291 lm/W at 1 mA. However, efficacy falls rapidly as current increases, and is just 108 lm/W and 66 lm/W at 350 mA and 1 A, respectively. Blue LEDs, in comparison, are more efficient at higher current densities, with efficacy peaking at a current of 20 mA. Driven at 350 mA, the blue LED and green phosphor combination emits 194 lm at 191 lm/W, and at 1A delivers 462 lm at 145 lm/W.
Figure 7: The CIE 1931 colour space chromaticity diagram shows two red-green- blue approaches with given red (610 nm) and blue (450 nm) LEDs used in combination with a direct green InGaN LED or a phosphor- converted green LED. The narrower emission spectra of a direct green InGaN LED, compared to green generated by phosphor conversion, makes this device better suited to projection
Further Reading M. Peter et. al. Phys. Stat. Sol. C 5 2050 (2008) A. Laubsch et. al. IEEE Trans. Electron Dev. 57 79 (2010) D. Schiavon et. al. Phys. Status Solidi B 250 283 (2013) Y. Narukawa, et. al. J. Phys. D: Appl. Phys. 43 354002 (2010) D. Steigerwald et. al. JOM 49 18 (1997) J. Piprek, phys. stat. sol. (a) 207 2217 (2010) Y. C. Shen et. al. Appl. Phys. Lett. 91 141101 (2007)
Several routes are available for increasing the efficiency of the direct green LED, so that it closes the performance gap with the blue-chip-and-phosphor combination: Carrier density could be cut by increasing the volume of the active region via the addition of more wells; internal quantum efficiency could be increased through improvements to material quality; and the active area could be increased by optimising the design of the chip and its dimensions. In our view, the pathway with most potential is to improve the epitaxial growth process, because this could lead to a lower forward voltage and superior carrier transport.
£ We gratefully thank the German Federal Ministry of Education and Research (BMBF) for financial support (grant number 13N9974, ‘‘High Quality LED’’) for the development of direct green LEDs. We also appreciate the support of numerous colleagues at Osram and Osram Sylvania.
© 2013 Angel Business Communications. Permission required.
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www.compoundsemiconductor.net October 2013
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