The performance of InGaN LEDs in terms of photon extraction efficiency is analyzed by the Monte Carlo photon simulation method. Simulation results show that the sidewall slanting scheme, which works well for the AlInGaP or InGaN/SiC system, plays a very minimal role in InGaN/sapphire systems. In contrast to InGaN/SiC systems, a lower refractive index sapphire substrate restricts the generated photons to enter the substrate, minimizing the chances for the photons to be deflected by the slanted sidewalls of the epitaxial semiconductor layers that are usually very thin. The limited photon transmission to the sapphire substrate also degrades the photon extraction efficiency, especially in the epitaxial side down mount. One approach to exploit the photon extraction potential of the epitaxial side down mount may be to texture the substrate-epitaxy interface, by possibly growing the epitaxial layers on a sapphire substrate that is either appropriately surface textured or patterned and etched. In this case, randomized photon deflection off the textured interface directly increases the number of photons entering the sapphire substrate, from which they easily couple out of the chip, thereby improving the photon extraction efficiency drastically.
We propose a new electrode design for InGaN/Sapphire LED chips. In the new design, the thin p-ohmic metal layer on the top surface of the chip is partitioned into standardized multiple patches. Each patch is then connected to the p-electrode pad by a metal-film type of series resistor whose value is tailored to its own patch such that the current density distribution in the active region under the patch is almost the same, eliminating the severe current crowding phenomenon observed in the conventional design. As a consequence, both the maximum output power achievable from a unit InGaN/sapphire LED chip and the device reliability would be significantly improved.
To improve the escape of photons from an LED structure, we fabricated nano-sized cavities on a p-GaN surface utilizing Pt self-assembled metal clusters for an etch mask. Wet and dry etching processes were employed to produce nano-sized cavities on the p-GaN surface. The dry etching process produced cavities with diameters ranging from 200 nm to 450 nm and from 30 to 80 nm in depth, respectively. The wet etching process, however, produced small size cavities with a size of 5 ~ 6 nm. Electroluminescence measurement showed that the relative optical output powers are increased by 88% as evidenced by frontside measurement compared to those of LEDs with no nano-sized cavities. In addition, the electrical performance was also improved as evidenced by the I-V characteristic curves. This enhanced performance can be attributed to an enhancement in light escaping due to the increased light emitting area as the result of the surface cavities and also to the reduced contact resistance due to the increased contact area.
In this presentation, basic elements of light-emitting diode (LED) lamp design are discussed. In practical applications of LED lamps, the far-field photon distribution pattern is one of the important considerations. Both the reflecting cup and lens surface profile employed in the design can be flexibly adjusted by a few parameters such that the far field photon distribution pattern is rather easily manipulated. For simulation of LED lamps, we have used Monte Carlo photon simulation method. Based on simulation results, we can verify or explain the effect of the various LED lamp design parameters on far-field patterns. Some of the important design examples are LED lamps with far-field patterns that are either tilted by certain angle in the vertical direction or double- lobed in the horizontal direction. LED lamps of this type of far-field patterns may find some application in some special outdoor displays, for instance, in a large stadium.
We propose LEDs based on geometrically deformed chips of which the horizontal cross-section is either rhomboidal or triangular and, in addition, side walls may also be slanted. In such deformed chips, the photon trajectory changes with each reflection off the wall and, as a result, the continued total internal reflections as observed in conventional rectangular cubic chips are suppressed.
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