An overview of planar resonant-cavity light-emitting diodes is presented. Letting spontaneous emission happen in a planar cavity will in the first place affect the extraction efficiency. The internal intensity distribution is not longer isotropic due to interference effects (or density of states effects). The basics of dipole emission in planar cavities will be shortly reviewed using a classical approach valid in the so called weak-coupling regime. The total emission enhancement or Purcell

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... r, although small in planar cavities, will be explained. The design of a GaAs/AlGaAs RCLED is discussed. We review the state-of-the-art devices in different semiconductor material systems and at different wavelengths. Some advanced techniques based on gratings or photonic crystals to improve the efficiency of these devices are discussed. RCLEDs are not the only candidates that can be used as high-efficiency light sources in communication and non-communication applications. They compete with other high-efficiency LEDs and with VCSELs. The future prospects of RCLEDs are discussed in view of this competition. INTRODUCTION High radiance, modulation capabilities, spectral purity and efficiency are no longer exclusively attributed to lasers. Since the invention and first demonstration 1 in 1992 of the Resonant-Cavity LED (RCLED) which uses photon quantisation in microcavities to enhance spontaneous emission properties, directionality, intensity and purity can as well denote key performance characteristics of LEDs. These assets make RCLEDs particularly suited for optical communication applications, more specifically data communication via Plastic Optical Fiber (POF) and infrared wireless communication. The internal isotropic spontaneous emission field distribution represents the main limitation to acquire high efficiencies in standard LEDs. Due to the large refractive index of the light emitting medium, the efficiently generated photons can only be extracted when they impinge on the interface with an incidence angle smaller than the critical angle defined by total internal reflection (Snell's law). For a semiconductor LED (e.g. n 1 ≈3.5) in air (n 2 =1), the critical angle θ c =asin(n 2 /n 1 )≈16 o . Consequently, only a fraction ≈2 % of the isotropically emitted photons can be extracted (singleside extraction). Several solutions have been presented that successfully outsmart Snell's law. Geometrical issues like slanted interfaces, surface roughening, etc. enhance the extraction probability of the isotropically emitted photons. RLCEDs, on the other hand, cancel the intrinsically isotropic emission profile. Conceptually, a RCLED consists of a high reflective mirror, a cavity with a thickness in the order of the wavelength including the active layer with several quantum wells for light generation, and a semi-transparent mirror for light extraction. The mirrors make up a (Fabry-Perot) resonator in which constructive and destructive interferences dictate the possible emission directions. The latter corresponds with resonant modes. With an appropriate cavity design, the preferential propagation direction of the photons can thus be forced from total internal reflection regime towards the extraction cone, benefitting to the extraction efficiency. Together with this increase of directivity and/or efficiency due to a redistribution of the photons, the spontaneous emission rate will be enhanced due to the Purcell-effect. However, because of the planar geometry and the rather small reflectivity coefficients of the cavity mirror(s) in practical applications, the Purcell-factor is close to one, resulting in a negligible spontaneous emission rate enhancement (see section 2.2). Spontaneous emission in a layered medium is summarised in section 2. For more details, the reader is referred to 2,3 . Application of this theory is illustrated in section 3 by the design of a GaAs/AlGaAs RCLED. Section 4 gives an idea of the state of the art of the RCLED in different material systems. Further improvement by advanced techniques is discussed in section 5. to 18.62.2.90. Terms of Use: http://spiedl.org/terms Proc. of SPIE Vol. 4996 81 Downloaded from SPIE Digital Library on 22 Mar 2011 to 18.62.2.90. Terms of Use: http://spiedl.org/terms