Research: The Interdisciplinary Center for Wide Bandgap Semiconductors and The Solid State Lighting and Display Center

The Interdisciplinary Center for
Wide Bandgap Semiconductors
and
The Solid State Lighting & Display Center

R e s e a r c h

O p t o e l e c t r o n i c s

R C L E D s

RCLEDs offer opportunities to create solid-state lighting systems with greater efficacies and higher lumen packages than existing systems using ‘traditional’ LEDs. As a result of incorporating a gain medium within a resonant cavity, RCLEDs emit a highly compact and directional light beam. The higher extraction efficacy of these devices is the main advantage of these technologies over conventional LEDs. Extraction efficacy refers to the ability of the photons generated by a particular system to actually exit the system as ‘useful’ radiation. Currently, more than half of the photons generated by conventional LEDs are trapped within the device, where they are absorbed and converted into heat, reducing LED performance. RCLEDs are much more efficient than traditional LEDs at generating ‘useful’ (extracted) light, resulting in higher output powers.

Although light source efficacy plays an important role for widespread acceptance of this new technology, it is not the only factor. Improved optical control of illumination from RCLEDs will also result in integrated luminaires having smaller ‘footprints’ and narrower profiles, avoiding detrimental interactions with other common components such as electrical systems, data communications, and HVAC systems.

In general, when forming device arrays, a vertical device geometry is preferable to a planar structure. Emission normal to the substrate eliminates the need for dicing, allows for closer packing of diodes, facilitates coupling with phosphors, and simplifies packaging and integration with fiber optic elements. Fabrication of vertical devices also tends to be simpler, allowing for on-wafer testing which shortens the feedback process and reduces overall costs.

Vertical cavity surface emitting lasers (VCSELs) and RCLEDs both provide improved output efficiency over standard LEDs while occupying a smaller footprint and still providing the vertical advantage over edge-emitting LDs. However, the drawbacks of the VCSEL are the strict limits placed on allowable losses in the laser cavity. Due to the short gain length in a vertical cavity, the mirror reflectivities must exceed 99% and scattering and absorption losses must be kept to a minimum. RCLEDs have the advantage of requiring only one high reflectivity mirror (typically 80-90%), with the second mirror reflecting at about 50%. Placing an active region within a resonant cavity results in enhanced emission (by a factor of 2-10 times), greater spectral purity (2-5 times narrower), greater temperature stability, and a more directional emission [1]. The improved output power and spectral purity that RCLEDs provide over LEDs implies that they can be used to transmit light along a fiber optic bundle.


		2 Possible RCLED designs

[1] Schubert, E.F., Wang, Y.-H., Cho, A.Y., Tu, L.-W., Zydzik, G.J., Appl. Phys. Lett., 60 921 (1992)

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