Novel layers enhance red-LED extraction efficiency

Many of today s AlGaInP-based LEDs are used to provide red sources for traffic signals and automobile brake lights. However, this type of device could enjoy even greater commercial success if its cost-per-lumen was lower, as this would spur deployment in projectors, LCD TV backlights and color-temperature tunable lighting fixtures.

One approach to cutting the cost-per-lumen involves the introduction of new technologies that boost LED efficiency. Improvements in epitaxial growth and device processing have pushed this device s internal quantum efficiency close to its theoretical limit, so any further gains must come from increases in the emitter s extraction efficiency.

Several methods have already been developed for this purpose, but none is ideal. Adding a distributed Bragg reflector (DBR) to an LED reduces light absorption in the GaAs substrate, but reflectivity at oblique angles of incidence is relatively low and this results in significant optical loss. Replacing the substrate with a transparent one, such as sapphire or GaP, also has downsides, because this approach can t deliver the significant improvements in thermal conductivity that are needed to boost maximum drive current and lumen output. Surface texturing, meanwhile, can increase light output, but it is difficult to control the shape and feature sizes with this conventional chemical-etching process.

The thermal conductivity issue has recently been addressed by transferring epitaxial layers to electrically and thermally conductive substrates. However, even with this advance, optical efficacies of many of the best commercial 620 nm LEDs are only about 50 lm/W. This means that the high-brightness LEDs produced by bonding techniques can t fulfill customer expectations in terms of efficiency.

However, at Epistar Corporation, Taiwan, we have just unveiled a new series of AlGaInP LEDs that can deliver far higher efficacies. These products – which are named the P- and A-series (although they were initially launched as Phoenix and Aquarius LEDs, respectively) – feature improvements in light extraction efficiency of at least 50%, thanks to the addition of multilayer structures with an undulating surface and a graded refractive index. Fabrication takes place on existing equipment and the production capacity is hardly impacted at all.

We call these proprietary multilayer structures "Lambertian transmittance and reflectance surfaces" because they obey Johann Heinrich Lambert s cosine emission law. Such structures reflect or emit with their greatest intensity in the direction normal to the surface, and with least intensity at the most oblique angles (see figure 1 for a definition).

Our P- and A-series LEDs feature Lambertian transmitters and reflectors on the top and bottom of the device, respectively. The transmitter boosts extraction by directing the majority of the light in forward directions and reflecting very little back into the device, where it could be absorbed by the quantum well. The reflector, meanwhile, directs most of the light heading towards the substrate back into the device at angles that prevent multiple internal reflections within the chip.

We manufacture our P-series chips by creating a Lambertian reflector on the GaP surface, which is the top layer of the AlGaInP-on-GaAs epiwafers (figure 2a). This wafer is then bonded to silicon, before the GaAs substrate is removed. Next we etch the n-type cladding layer to form the Lambertian reflector and define a gold p-type contact on the back of the silicon substrate. Wafer probing follows, before the bonded device wafer is diced into individual chips.

A-series LEDs have a slightly different design (figure 2b), with the wafer bonded to sapphire and a transparent adhesive film used as the bonding agent. Aside from that, the fabrication process is similar to that for the P-series.

Our A-series chips emit at 615–620 nm and have a forward voltage of just over 2 V (see table 1 for details). The 620 nm version delivers 107 lm/W at 20 mA and the 615 nm equivalent produces 130 lm/W at the same drive current (see figure 3 for efficacies at other drive currents). This product s maximum forward current rating is 40 mA, and the device is intended for backlighting and architectural, entertainment and decorative lighting.

The P-series chips are the same size as their A-series cousins but operate at higher currents, thanks to the silicon substrate s superior thermal conductivity. At a current of 250 mA they can deliver 25 lm (figure 4), but we recommend that the drive current is kept below 70 mA. This LED is suitable for the same applications as our A-series chips, but we are also targeting deployment in traffic lights, signage and channel letters.

Both of our products are highly reliable. 1000 hour tests revealed very stable light outputs for P-series chips driven at 80 mA under conditions of 85 °C and 85% humidity, and A-series chips at 40 mA under the same conditions.

We believe that our red LEDs deliver record-breaking efficacies at 20 mA and our customers say that they offer an improvement of 30–50% over other products in the market. We will be manufacturing these devices on our production line this year and we are confident that they will help to drive greater deployment of the red LED.