Light extraction improvement of GaN-based light-emitting diodes using patterned undoped GaN bottom reflection gratings

Abstract

ABSTRACT The Gallium Nitride (GaN) Light-Emitting-Diode (LED) bottom refection grating simulation and results are presented. A microstructure GaN bottom grating, either conical holes or cylindrical holes, was calculated and compared with the non- grating (flat) case. A time monitor was also pl aced just above the top of the LED to measure both time and power output from the top of the LED. Many different scenarios were simulated by sweeping three parameters that affected the structure of the micro-structure grating: unit cell period (u ) from 1 to 6 microns, unit cell width ( w ) from 1 to 6 microns, and unit cell grating height ( d) from 50 to 200nm. The simulation results show that the cylindrical grating case has a 98% light extraction improvement, and the conical grating case has a 109% light extraction improvement compar ed to the flat plate case. Keywords: Gallium Nitride, light-emitting-diode, grating 1. INTRODUCTION As a result of our energy conversation efforts, lighting sources have become one of the hot areas of research due to their applications in a variety of fields such as lighting displays, bulb technology, and photonics. The demands of these applications require low power consumption, yet a high brightness and luminosity with minimal heat. We can even control the color contrast of the device and create a full color set with red, green, and blue Light-Emitting-Diodes (LEDs) [1]. LEDs have been used in many appli cations, however the light extract ion efficiency is still very low for GaN LEDs due to several factors: Gallium Nitride (G aN) has a low critical angle that traps light inside the device [2], absorption of light within the device due to dislocations and defects within the GaN crystal [3], and device design and structure has not been optimized (ie. epitaxial side up vs. epitaxial side down chip structures) [4]. It is crucial that we improve GaN LED light extr action efficiency and redu ce energy consumption. The major limitation to the light extraction efficiency is the light trapping due to GaN’s low critical angle. This applies to any large change of the refraction index existin g between layers, such as between the solid state LED and air, and the solid state LED and a resin. It has been show n that resins increase the extraction efficiency due to the more gradual change in refraction, allowing more light emitting out due to the larger escape angle [5].Many methods of improving LED efficiency are currently being explored. Almost all of these methods are seeking to extract the trapped light in greater quantity and faster speed. Those methods being explored are placing photonic crystals or nanostructure grating on one of LED layers to modify the effective index of refraction at the boundary [6-7], randomized roughening on the surface of the device [7-8], slanted device conf igurations that result in pyramidal shapes [7], and inverted “flip-chip” designs that put the epitaxial side upwards or downwards [4] [9]. The second inefficiency of GaN LEDs is the loss of light from absorption due to dislocations and defects within the GaN crystal. These are impurities that absorb the light, an issue when light is trapped inside the LED. The longer it takes to extract the trapped light, the more the photons suffer from absorption. So, it is critical that we find methods to extract light quickly from the LED before the energy is taken by absorption [10-11]. A grating structure helps