Enhancement of light extraction from light emitting diodes

The large amount of light emitted from a light emitting diode (LED) being trapped inside the semiconductor structure is the consequence of the large value of the refractive index. The total internal reflection (light incident on a planar semiconductor/air interface is totally internally reflected if the angle of incidence exceeds the critical value determined by Snell's law) is the major factor responsible for the small light extraction efficiency (other important contributions to the losses are the internal absorption and blocking of the light by contacts). The typical LED structure comprising a number of layers most of which have high refractive index could be considered as a multilayer waveguide that could support a large number of trapped guided modes. The paper reviews approaches to enhanced light extraction grouped into two sets depending on whether their application results in the change in the spontaneous emission (either the spontaneous emission rate or the angular distribution, or both): (1) molding of the flow of light emitted from the active region by the modification of the chip shape or the surface morphology to increase the light intensity: and (2) modification of spontaneous emission, for example, by placing of the light emitting region inside the optical cavity. Special attention is given to LEDs made from nitrides of elements of group III (InAlGaN) that cover a large part of visible and ultraviolet (UV) spectra and are considered as a major candidate for sources for the solid-state general illumination. An Appendix contains review of numerical models used to study the light extraction. (C) 2010 Elsevier B.V. All rights reserved.