| III-Nitride light emitting diodes (LEDs), often combined with phosphors to fill in longer wavelengths of the visible spectrum, have become the preferred method to produce high-efficiency white lighting. These LEDs offer the advantages of lower electricity consumption, longer lifetimes, and no toxic components. One of the major barriers to maximizing the efficiency of these LEDs, however, is the total internal reflection of light at the surface of the LED, causing a large majority of the light to be trapped in the material and absorbed before it can escape. While current enhancement methods, particularly surface roughening and encapsulation, can increase the light extraction efficiency to ~80%, there is still room for improvement.;We investigated the method of light extraction by air-gap photonic crystals (PhCs) that are embedded within the epitaxial layers of the LED during metal-organic chemical vapor deposition (MOCVD) growth using a new method that was developed for this work. Two device designs were fabricated and studied. These were the double embedded PhC LED (with an n-side PhC and a p-side PhC) and the thin-film flip-chip (TFFC) LED with a p-side embedded PhC.;Both LEDs were characterized in angle-resolved electroluminescence, and the external quantum efficiency (EQE) of the TFFC PhC LEDs was measured in an integrating sphere. Angle-resolved data were compared with optical simulations of the structures. Efficiency improvements for TFFC PhC LEDs compared to reference devices without a PhC were found to be limited to 70% due to plasma etch damage---necessary to create the PhC but detrimental to internal quantum efficiency (IQE). Measurements on the double embedded PhC LED showed that the extraction length of guided light was very short: 21-39 mum. The high rate of extraction in this device contributed to its increase in vertical output power after embedding the second PhC, despite expected losses in IQE from etch damage and current crowding. |
