Siegmar Kugler

High-efficiency resonant-cavity LEDs emitting at 650 nm

We fabricated resonant-cavity light-emitting diodes (LEDs) emitting at 650 nm. Compressively strained GaInP quantum wells were used as an active layer embedded between AlGaAs-AlAs Bragg mirrors. The Bragg mirrors formed a one-wavelength optical resonator. Two devices with different light-emitting areas were compared: 1) a large area chip (300 /spl mu/m/spl times/300 /spl mu/m) with a conventional LED contact and 2) a small area chip with an 80-/spl mu/m light opening with an annular contact. Large devices are more suitable for high output power whereas the smaller devices might be useful for data transmission e.g., via plastic optical fibers. For epoxy-encapsulated large area devices, we achieved a maximum wall-plug efficiency of 10.2% and maximum output power of 12.2 mW at 100 mA. The small area LEDs yielded 2.9 mW at 20 mA and a maximum wall-plug efficiency of 9.5%.

High-Brightness AlGaInP Light-Emitting Diodes Using Surface Texturing

There is a large number of new applications in lighting and display technology where high-brightness AlGaInP-LEDs can provide cost-efficient solutions for the red to yellow color range. Osram Opto Semiconductors has developed a new generation of MOVPE-grown AlInGaP-LEDs to meet these demands. Our structures use optimized epitaxial layer design, improved contact geometry and a new type of surface texturing. Based on this technology we achieve luminous efficiencies of more than 30 lm/W and wallplug efficiencies exceeding 10% of LEDs on absorbing GaAs substrates. The epitaxial structure does not require the growth of extremely thick window layer and standard processes are used for the chip fabrication. This allows for high production yields and cost-efficient production.

Fast LEDs for polymer optical fiber communication at 650nm

Data rates above 250 MBit/s via polymer optical fiber (POF) require specially designed light emitting diode (LED) chips. Because of the absorption minimum of the POF, the red wavelength region around 650 nm is of special interest. LED chips in this region comprise active regions out of the AlGaInP material system. To achieve low rise and fall times in the optical output, the current density has to be increased to levels about 400A/cm2. Hence the chip design needs ways to confine the current injection to a region significantly smaller than the chip itself. Conventional methods use epitaxially grown layers to ensure the lateral current spreading over the active region. But to confine the current to this region, the current spreading has to be eliminated locally. Typically, this is achieved by ion-implantation, mesa etching or selective oxidation of an AlGaAs layer of high Al-content. In this paper we present a new, planar, and very cost effective chip design for data rates around 250 MBit/s. No current spreading layer is included in the epitaxial growth, but is supplied during chip process using a transparent conductive oxide. Optical power around 2 mW at 20 mA without epoxy encapsulation, and rise and fall times around 2.5 ns have been reached.

100-lm/W InGaAlP Thin-Film Light-Emitting Diodes With Buried Microreflectors

Thin-film light-emitting diodes (LEDs) belong to the most successful LED concepts for achieving high efficiencies. The incorporation of buried microreflectors with inclined facets prevents the light generation under the top contact and bondpad and offers an additional light extraction scheme. As a result, an external quantum efficiency of 50% could be demonstrated at a wavelength of 650 nm, and a luminous efficiency of more than 100 lm/W could be achieved in the wavelength range from 595 to 620 nm

Red and orange resonant-cavity LEDs

Usually resonant-cavity light-emitting diodes (RCLEDs) are used as emitters for plastic optical fiber communication. However, there are some arguments that may lead to the introduction of RCLEDs in a much wider range of applications. A typical high-brightness AlGaInP LED consists of a Bragg mirror, the active region and some layers for current spreading and light extraction. The thickness of these layers can add up to several ten microns which causes long epitaxial growth times. The total thickness of a RCLED can be significantly lower. Furthermore, since no lattice mismatched layers such as GaP are involved, the total incorporated strain is low which simplifies wafer handling and device processing. For this reason we studied RCLEDs with a dominant wavelength around 632 nm (superred) and 605 nm (orange). The processes for epitaxial growth and chip fabrication were optimized for homogeneity on 4 inch wafers and suitability for low-cost mass production, respectively. Possible applications for our RCLEDs are optical scanners, indicators, signal lights and other applications which benefit from the enhanced directionality of RCLEDs.