Could embedded photonic crystals be the way?

High-efficiency LEDs are considered the best choice for future lighting due to their remarkable efficiency in transforming electricity into light. High-efficiency LEDs have only recently been demonstrated thanks to improvements in semiconducting GaN quality leading to better internal quantum efficiency, and to the implementation of high-efficiency light extraction concepts. The latter are made necessary by the fact that light is normally trapped inside the semiconductor by total internal reflection when it attempts to escape the LED. Recent advances in photonic crystals (PhCs) show that they could become the best technique to yield high-efficiency LEDs.

To obtain high light extraction in LEDs, a number of schemes have been investigated, such as the use of patterned substrates, or shaped, flip-chip, and thin-film rough-surface LEDs. While these schemes can offer high extraction efficiency, they provide little control over the direction of the light emission in the device.

Newer designs for light extraction rely on PhCs. From the beginning, PhCs have been suggested as a way to achieve high-efficiency light sources. While the early concept of controlling light emission by an omnidirectional 3D photonic band gap (PBG) has had limited success due to practical difficulties, such as achieving highly ordered 3D-PhCs, a more successful approach has been based on the use of 2D-PhCs, most often used as surface diffractive elements for the light trapped within the LED structure.

The enhancement of the extraction efficiency in LEDs through the use of PhCs, however, requires a structure design that optimizes their interaction with the guided optical modes in the LED.

In a recent work published in Journal of Physics D: Applied Physics, three approaches are described to increase this interaction: introduction of an AlGaN optical confining layer, use of thin-film LEDs and use of embedded PhCs. In the latter approach, the higher interaction between optical modes and the embedded PhCs yielded close to unity extraction efficiency for such devices.

The investigation and optimization of these three different approaches made use of high-resolution angle-resolved measurements to experimentally determine the PhC extraction parameters, which was an essential tool for corroborating the theoretical models and quantifying the competition between absorption and extraction phenomena in LEDs.

About the authors

The work was performed at University of California, Santa Barbara and partially supported by the 'Center for Energy Efficient Materials' at UCSB, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DESC0001009. Dr Matioli (right) is a post-doctoral researcher in Professor Weisbuch's group, in the Materials Department at University of California, Santa Barbara.

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