Increased light extraction and directional emission control in gallium nitride photonic crystal light emitting diodes.

摘要

GaN has become the prominent material for blue-green light emitting diodes (LEDs) and efficient white light sources. Advancements in LED efficiency for lighting has the potential to dramatically impact energy consumption world wide. A limiting factor to achieving high efficiencies in GaN LEDs is the light extraction efficiency. This work addresses many key issues pertaining to the use of PhCs to increase the extraction efficiency and emission directionality of GaN LEDs. Limitations in extraction efficiency of GaN photonic crystal light emitting diodes (LEDs) are addressed by implementing an LED design using both 2D photonic crystals (PhCs) in-plane and index guiding layers (IGLs) in the vertical direction. The effects of PhCs on light extraction and emission directionality from GaN LEDs are studied experimentally. Angular resolved electroluminescence clearly shows the combined effect of controlling the vertical mode profile with the IGLs and tailoring the emission profile with the periodicity of the PhC lattice. Various materials are used to increase the index contrast of the IGL and the effects are measured. Increases in vertical emission as high as 3.5x are achieved for PhC LEDs with an Al0.12Ga0.88N IGL over non-PhC LEDs with a ∼30% improvement attributed to the incorporation of the AlGaN IGL. This enhancement is achieved by tailoring both the directionality and guided mode control.;The impact of incorporating PhCs and IGLs on LED device design and performance are addressed. Effects of etching the PhCs near the QWs have been observed and explanations for this behavior will be discussed. It will be shown that an un-doped IGL can severely limit current spreading in the n-type side of the device and have a detrimental impact on device performance. Finally, a method of patterning PhCs with periodicities as small as 230nm by laser interference lithography and imprint lithography has been developed to provide a fast, inexpensive method of pattering PhCs over large areas.