Technical Insight
Surface texturing boosts light emission in new-generation AlInGaP LED chips (High-brighted LEDs)
Controlling how light reflects at the surfaces of an LED chip has led to a doubling of the extraction efficiency of high-power red and yellow AS-AlInGaP LEDs, say Norbert Linder and co-workers from Osram Opto Semiconductors.
LEDs have begun to penetrate the lighting component market, replacing both light bulbs and fluorescent lamps and at the same time opening up a whole range of new applications. Currently LEDs are principally used where monochromatic light sources are needed or in situations where the available space is restricted in some way. Examples include the interior and exterior lighting fixtures of automobiles, traffic signals and display applications. To increase the market penetration of high-brightness LEDs, highly efficient emitters must be made available at low cost. With this in mind Osram Opto Semiconductors has developed a new generation of MOVPE-grown AlInGaP-LEDs for the red (620 nm) to yellowish-green (560 nm) wavelength range. Getting the light out The most severe constraint on the performance of LEDs is poor light extraction from the chip. This is a result of the small critical angles of total internal reflection, which are due to the large differences in refractive index between the semiconductor and the epoxy resin used in encapsulation. In the new device design an etched surface structure enhances the light emission through the window layer of the chip (see ). The basic elements of the surface texture of the chip are triangular-shaped structures which have tilted sidewalls. Within these structures closed reflection loops, such as are also present in rectangular-shaped chips, are broken by the non-rectangular geometry. Photons which are generated in the active layer can then be extracted more efficiently, either directly through the various surfaces or after multiple reflections with changing angles of incidence. Furthermore the contact geometry is chosen such that current is injected primarily under the light extraction structures. Using this design, an improvement in the external efficiency of more than 50% is obtained depending on the thickness of the window layer. Absorbing substrates The devices are grown on 4 inch absorbing GaAs substrates and the epitaxial structure is optimized for maximum electro-optical conversion efficiency using an appropriate choice of the layer composition and widths. For longer wavelengths such as 620 nm (red), the internal quantum efficiency is estimated to be close to 100%, and drops to about 60% at 590 nm for yellow devices. Different layer structures are used for red and yellow LEDs, leading to similar values for the luminous flux for both colors with a maximum for reddish-orange (610 nm). A specially designed wide-angle Bragg reflector is employed to recover a large part of the backward radiation of the chip. Scalability The wall-plug efficiencies obtained for surface-textured packaged chips exceed 30 lm/W. This is about twice as high as that obtained for conventional absorbing-substrate LEDs based on GaAs, and comparable to transparent substrate (TS) LEDs. TS-LEDs contain a device that has the original substrate removed and is typically wafer-bonded to a GaP substrate. This technique is currently limited to 3 inch wafers. Absorbing substrate (AS) LEDs omit this process. The angular radiation characteristic of the chip is almost unaffected by the surface structure and deviates little from the Lambertian shape. This particular feature allows standard square chips to be replaced without changing the package, even in applications where the radiation pattern is critical. A major attraction of the concept is the easy scaling to larger chip sizes. At present there are three versions of the new-generation LED chip available for different types of applications: a high-current version, a standard chip and a low-cost design (see ). For all chip types the typical operation voltage remains below 2.2 V. The LEDs are fabricated using standard chip processes, leading to high production yields and low manufacturing costs. The new chip is compatible with conventional mounting techniques and is available in a variety of SMT packages.
