Conductive omni-directional reflectors for spontaneously emitting light sources

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0401075SchubertSpontaneously-emitting light sources based on semiconductors are used in applications as diverse as lighting, sensing, communications, and signage. The efficient extraction of light from the light-emitting material into free space is essential for the future advancement of such light sources. There are two fundamental approaches enabling efficient light extraction in spontaneous emitters, namely the employment of structures with near-perfect transparency or the employment of optical reflectors with near-perfect reflection characteristics. Such reflectors should reflect light with very low optical losses irrespective of the angle of incidence and wavelength. A question of great intellectual interest and technical relevance concerns the optimum type of reflector that meets these requirements.Here a novel type of reflector is proposed that possesses (i) high reflectivity, (ii) omni-directional characteristics, (iii) electrical conductivity, and (iv) broad spectral range. The reflector consists of a semiconductor, a low-refractive-index dielectric layer perforated by an array of micro-contacts, and a metal layer. It is shown that such a triple-layer omni-directional reflector (ODR) has reflectivities 99 % with a weak anglular dependence thereby significantly increasing the light-extraction efficiency in spontaneously-emitting sources. The new type of ODR compares very favorably with the well-known metal reflector and distributed Bragg reflector. The demonstration of performance improvements in GaInN and AlGaInP LEDs using the new ODR will be an integral part of the project. It is known that these LEDs suffer from low metal-contact reflectivity (GaInN LEDs) and the lack of reflectivity at oblique angles of incidence in distributed Bragg reflectors (AlGaInP LEDs).