Enhancement of light extraction from light emitting diodes

The fabrication studies of silica/polystyrene (PS) colloidal microlens arrays with various aspect ratios were performed on the III-nitride light-emitting diodes (LEDs). The use of colloidal-based microlens arrays led to significant enhancement in light extraction efficiency for III-nitride LEDs. In varying the aspect ratios of the microlens arrays, the engineering of various PS thicknesses was employed by using high-temperature treatment and redeposition process. The effects of PS thickness on the light extraction efficiency and far-field emission patterns of InGaN quantum-well (QW) LEDs were studied. The total output powers of microlens LEDs with various PS thicknesses exhibited 1.93-2.70 times enhancement over that of planar LEDs, and the use of optimized PS layer thickness is important in leading the enhancement of the light extraction efficiency in large angular direction.

http://chaos.cc.lehigh.edu/pubs/Li_XH_Microlens_LEDs_IEEE_Photonics_Journal_June_2011.pdf

Light Extraction Efficiency and Radiation Patterns of III-Nitride Light-Emitting Diodes With Colloidal Microlens Arrays With Various Aspect Ratios

A light emitting diode is disclosed that includes a support structure and a Group III nitride light emitting active structure mesa on the support structure. The mesa has its sidewalls along an indexed crystal plane of the Group III nitride. A method of forming the diode is also disclosed that includes the steps of removing a substrate from a Group III nitride light emitting structure that includes a sub-mount structure on the Group III nitride light emitting structure opposite the substrate, and thereafter etching the surface of the Group III nitride from which the substrate has been removed with an anisotropic etch to develop crystal facets on the surface in which the facets are along an index plane of the Group III nitride. The method can also include etching the light emitting structure with an anisotropic etch to form a mesa with edges along an index plane of the Group III nitride.

Improved External Extraction Light Emitting Diode based upon Crystallographic Faceted Surfaces

Improvement of light extraction efficiency of InGaN quantum wells light emitting diodes (LEDs) using SiO2/polystyrene microspheres was demonstrated experimentally. The utilization of SiO2/polystyrene microlens arrays on InGaN quantum wells LEDs, deposited via rapid convective deposition, allows the increase of the effective photon escape cone and reduction in the Fresnel reflection. Improvement of output power by 219% for InGaN quantum wells LEDs emitting at peak wavelength of 480nm with SiO2/polystyrene microspheres microlens arrays was demonstrated.

Enhancement of light extraction efficiency of InGaN quantum wells light emitting diodes using SiO2/polystyrene microlens arrays

Improvement of light extraction efficiency of InGaN light emitting diodes (LEDs) using polydimethylsiloxane (PDMS) concave microstructures arrays was demonstrated. The size effect of the concave microstructures on the light extraction efficiency of III-Nitride LEDs was studied. Depending on the size of the concave microstructures, ray tracing simulations show that the use of PDMS concave microstructures arrays can lead to increase in light extraction efficiency of InGaN LEDs by 1.5 to 2.0 times. Experiments utilizing 2.0 micron thick PDMS with 1.0 micron diameter of the PDMS concave microstructures arrays demonstrated 1.70 times improvement in light extraction efficiency, which is consistent with improvement of 1.77 times predicted from simulation. The enhancement in light extraction efficiency is attributed to increase in effective photon escape cone due to PDMS concave microstructures arrays.

http://chaos.cc.lehigh.edu/pubs/YE_PK_RAA_HT_JFG_NT_Optics_Express_2009.pdf

Light extraction efficiency enhancement of InGaN quantum wells light-emitting diodes with polydimethylsiloxane concave microstructures.

In this letter, we will report on a nitride-based light emitting diode with a mesa sidewall roughening process that increases light output power. The fabricated GaN-based light-emitting diode (LED) wafers were first treated through a photoelectrochemical (PEC) process. The Ga/sub 2/O/sub 3/ layers then formed around the GaN : Si n-type mesa sidewalls and the bottoms mesa etching regions. Selective wet oxidation occurred at the mesa sidewall between the p- and the n-type GaN interface. The light output power of the PEC treated LED was seen to increase by about 82% which was caused by a reduced index reflectance of GaN-Ga/sub 2/O/sub 3/-air layers, by a rough Ga/sub 2/O/sub 3/ surface, by a microroughening of the GaN sidewall surface, and by a selective oxidation step profile of the mesa sidewall that increases the light-extraction efficiency from the mesa sidewall direction. Consequently, this wet PEC treated process is suitable for high powered nitride-based LEDs lighting applications.

Enhanced light output in nitride-based light-emitting diodes by roughening the mesa sidewall

The InGaN-based light-emitting diode (LED) with an inclined undercut structure is fabricated through the photoelectrochemical two-step process to increase light extraction efficiency. In the first step the sidewall-undercut structure at the p-type and n-type GaN interface is created by selective wet oxidation on an n-type GaN surface in pure H 2 O solution. In the second step an inclined undercut structure through a crystallographic wet-etching process is formed by immersion in hot KOH solution. This crystallographic wet-etching process can remove the Ga 2 O 3 layer and form a {1011} p-type GaN stable plane, {1010} n-type GaN stable plane on the mesa sidewall. This inclined p-type GaN plane of LED structure can provide the higher overlap of incident light beam core and extraction core overlap on the mesa sidewall, and the total light output power of the treated LED is 2.10 times higher than the standard LED. Consequently, this inclined undercut LED structure is suitable for high-efficiency nitride-based LED application.

Enhanced Light Output Power in InGaN Light-Emitting Diodes by Fabricating Inclined Undercut Structure

Photoelectrochemical etching and oxidation of the sidewall structures of InGaN light-emitting diodes has been utilized to significantly improve optical emission efficiency. Only with an H 2 O solution can n-type InGaN/GaN multi-quantum-well (MQW) and n-type GaN:Si layers be selectively etched off from the p-type GaN:Mg epitaxial layer. In addition, the sidewall of n-type MQW/n-type GaN layers will be oxidized and etched simultaneously. This is attributed to the interposing of a Ga 2 O 3 layer with a low reflective index between the GaN and air as well as with the rough Ga 2 O 3 /GaN structures. Under a 20 mA operating current, and after a 10 min photoelectrochemical (PEC) oxidation process, the light output power increased by 42%. This PEC oxidizing and etching process is suitable for high-power nitride-based light-emitting diode lighting applications.

Jing-Hui Zheng

Papers

Enhanced light output in nitride-based light-emitting diodes by roughening the mesa sidewall

Enhanced Light Output Power in InGaN Light-Emitting Diodes by Fabricating Inclined Undercut Structure

High-Efficiency InGaN Light-Emitting Diodes Via Sidewall Selective Etching and Oxidation

Self-Assembled GaN:Mg Inverted Hexagonal Pyramids Formed Through a Photoelectrochemical Wet-Etching Process

The self-assemble GaN:Mg inverted hexagonal pyramids formatted by photoelectrochemical wet-etching process