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Wavelength-stable rare earth-free green light-emitting diodes for energy efficiency

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Abstract

Solid state lighting seeks to replace both, incandescent and fluorescent lighting by energy efficient light-emitting diodes (LEDs). Just like compact fluorescent tubes, current white LEDs employ costly rare earth-based phosphors, a drawback we propose to overcome with direct emitting LEDs of all colors. We show the benefits of homoepitaxial LEDs on bulk GaN substrate for wavelength-stable green spectrum LEDs. By use of non-polar growth orientation we avoid big color shifts with drive current and demonstrate polarized light emitters that prove ideal for pairing with liquid crystal display modulators in back light units of television monitors. We further offer a comparison of the prospects of non-polar a- and m-plane growth over conventional c-plane growth.

©2011 Optical Society of America

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Figures (7)

Fig. 1
Fig. 1 Cross sectional transmission electron micrographs of the active region in homoeptiaxlly grown GaIn/GaN LED structures revealing absence of structural defects and highly uniform quantum well regions. a) green a-plane structure; b) the same viewed over a length of 7.4 μm; c) a cyan m-plane structure. After [17,18]
Fig. 2
Fig. 2 Peak wavelength versus current density in (a) polar c-plane, (b) non-polar a-plane, and (c) non-polar m-plane LEDs. In the polar structures, wavelength stability is satisfactory only for blue LEDs, while non-polar structures offer the same also in the green. After [17,18].
Fig. 3
Fig. 3 Non-polar 520 nm green LED die (700 x 700 μm2) on m-plane bulk GaN substrate. There is only a minimal change of color as the current is varied from 20 mA to 100 mA.
Fig. 4
Fig. 4 CIE 1931 color loci of a) c-plane polar and b) a- and m-plane non-polar LEDs as current is varied in the range of 0.1 – 100 A/cm2. Perceived cyan and green colors remain stable only in the non-polar LED structures. Symbols correspond to those in Fig. 2.
Fig. 5
Fig. 5 Linear polarization analyzed emission spectra of the top emission of a green m-plane LED. Spectra differ in peak wavelength and intensity revealing a polarization ratio ρ = 0.85. Inset: the LED in operation which schematics of the polarization geometry.
Fig. 6
Fig. 6 Emission polarization ratio versus peak wavelength in PL and EL of this and other work for various growth orientations. m-plane structures offer the highest values and a trend to increase with wavelength. After [13].
Fig. 7
Fig. 7 Upper limit of the internal quantum efficiency as a function of incident 408 nm optical beam power density. Room temperature data was obtained by scaling to the respective PL maximum at T = 4 K. Most interestingly, under highest excitation, values in a-plane material surpass those of c-plane material at similar wavelength.
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