1 Introduction Over the last decade phosphor converted light emitting diodes, pcLEDs, have impacted most lighting applications, including general illumination, backlighting of displays, automotive and specialty lighting. In many applications, high efficiency at high power density is desired to maximize LED "lumens per dollar", and leverage greater source brightness to enable novel optical system design or reduced cost from smaller optics. So far, the focus on efficiency vs power density has been predominantly on the primary light source, the blue emitting InGaN LED. The word 'droop', uncommon a decade ago, describes the decrease in efficiency with drive which can be traced back to rather fundamental processes [1, 2], of which, Auger recombination involves three charge carriers interacting non-radiatively, subtracting from the radiative recombination, a two-carrier process. A similar kind of higher order recombination process, in this case a process closely quadratic in the concentration of excited activators, as will be shown, appears to decrease quantum efficiency of otherwise highly efficient phosphors, pumped with high brightness blue LEDs. This decrease of efficiency with (optical) excitation, will be referred to as 'droop in phosphors'.
Deep ultraviolet light emitting diode structures with a peak wavelength of 275 nm were grown by metalorganic chemical vapor deposition on (0001) silicon carbide. Despite its strong ultraviolet light absorption, silicon carbide was chosen as a substrate rather than sapphire for its improved thermal conductivity and the potential for vertically conducting devices. An output power of 0.11 mW was observed at 300 mA DC during single device on-wafer testing, and output powers of 2.09 mW at 1.3 A were obtained from a packaged, silicone encapsulated array of five devices. Forward voltages as low as 4.9 V at 20 mA were obtained. The injection profile of Cp2Mg during the p-AlGaN blocking layer was instrumental in the suppression of emission at undesired wavelengths and the realization of peak-to-defect level ratios greater than 100.
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