Ralph Wirth

High brightness AlGaInP light-emitting diodes

This paper reviews the recent progress of AlGaInP high brightness light-emitting diodes. After the discussion of some basic material properties and the general problem of light extraction we will discuss several approaches of high efficiency devices.

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%.

Photonic crystal structure effect on the enhancement in the external quantum efficiency of a red LED

The enhancement in external quantum efficiency of a red light-emitting diode (LED) from photonic crystal (PhC) hole patterns was investigated. A red LED was chosen because its epitaxial layers are relatively free from defects as compared to GaN-based LEDs. The peak emission wavelength was 642 nm, and a triangular-lattice PhC was designed with a hole diameter to lattice distance ratio of 0.5. The lattice distance to wavelength ratio (a/lambda) was varied from 0.2 to 4.6 in order to evaluate the enhancement in the external quantum efficiency. An improvement in efficiency greater than 75% was obtained for a/lambda between 0.6 and 2.0. This improvement of the optical characteristics occurred with unchanged electrical properties

Directional light extraction from thin-film resonant cavity light-emitting diodes with a photonic crystal

We report directional light extraction from AlGaInP thin-film resonant cavity light emitting diodes (RCLEDs) with shallow photonic crystals (PhCs). Diffraction of guided modes into the light extraction cone enhances the light extraction by a factor of 2.6 compared to unstructured RCLEDs, where the farfields still show higher directionality than Lambertian emitters. The external quantum efficiency is 15.5% to air and 26% with encapsulation, respectively. The PhC-RCLEDs are also more stable to a temperature induced wavelength shift than unstructured RCLEDs.

Enhanced light extraction efficiency from AlGaInP thin-film light-emitting diodes with photonic crystals

We investigate the use of photonic crystals for light extraction from high-brightness thin-film AlGaInP light-emitting diodes with different etch depths, lattice constants, and two types of lattices (hexagonal and Archimedean). Both simulations and experimental results show that the extraction of high order modes with a low effective index neff is most efficient. The highest external quantum efficiency without encapsulation is 19% with an Archimedean A7 lattice with reciprocal lattice constant G=1.5k0, which is 47% better than an unstructured reference device.

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.

Strong High Order Diffraction of Guided Modes in Micro-Cavity Light-Emitting Diodes With Hexagonal Photonic Crystals

Photonic crystals (PhCs) have now been firmly established as an efficient means for light extraction from light emitting diodes (LEDs). We analyze the diffraction properties from thin GaN micro-cavity LEDs with hexagonal lattices that feature three guided TE modes only. In contrast to common design rules, we find that high order diffraction contributes significantly to the light extraction and increases the directionality of the emitted light. The implementation of the PhC leads to an enhancement in light extraction by a factor of up to 1.8 and the directionality of the light is greatly improved with a radiant intensity enhancement factor of 4.3, which can only be explained by the higher order diffraction that has been hitherto neglected. Furthermore, we show that higher order diffraction contributes significantly to the high azimuthal extraction uniformity we observe, suggesting that the use of quasi-crystal lattices is not necessary. We use a model including mode absorption where each in-plane angle of the guided modes is treated separately in order to explain the experimental results.

Recent progress of AlGaInP thin-film light-emitting diodes

The concept of an AlGaInP thin-film light emitting diode includes a structure of semiconductor layers with low optical absorption on which a highly reflective mirror is applied. After bonding this wafer to a suitable carrier, the absorbing GaAs substrate is removed. Subsequently, electrical contacts and an efficient light scattering mechanism for rays propagating within the chip is provided. To achieve high efficiency operation it is crucial to optimize all functional parts of the device, such as the mirror, contacts, and active layer. Different mirrors consisting of combinations of dielectrics and metals have been tested. New chip designs have been evaluated to reduce the absorption at the ohmic contacts of the device. For efficient light scattering, the surface roughness of the at the emission window has to be optimized. Using these structures, and a thin active layer consisting of five compressively strained quantum wells, an external quantum efficiency of 40% is demonstrated at 650 nm. Further improvement is expected. Since the AlGaInP material system can provide only poor carrier confinement for active layers emitting in the yellow wavelength regime, the internal efficiency of these LEDs is comparably low. In order to reduce the problem of carrier leakage, a yellow active region usually consists of some hundred nanometers of active material. To circumvent the problem of this highly absorbing active layer, a separation of the light generation and the area of light extraction is suggested for yellow thin-film LEDs. First results are presented in this paper.

Beam-shaping properties of InGaN thin-film micro-cavity light-emitting diodes with photonic crystals

Photonic crystals (PhCs) are known to diffract guided modes in a light-emitting diode into the light extraction cone according to Bragg´s law. The extraction angle of a single mode is determined by the phase match between the guided mode and the reciprocal lattice vector of the PhC. Hence, light extraction by PhCs enables strong beam-shaping if the number of guided modes can be kept to a minimum. InGaN thin-film micro-cavity light-emitting diodes (MCLEDs) with photonic crystals (PhCs) emitting at 455 nm have been fabricated. The GaN layer thickness of the processed MCLEDs with a reflective metallic p-contact was 850 nm. One and two-dimensional PhCs were etched 400 nm into the n-GaN to diffract the guided light into air. The farfield radiation pattern was strongly modified depending on the lattice type and lattice constant of the PhC. Two- six- and twelve-fold symmetry was observed in the azimuthal plane from 1D lines, hexagonal lattices and Archimedean A7 lattices, respectively. The emission normal to the LED surface was enhanced by up to 330% compared with the unstructured MCLEDs. The external quantum efficiency was enhanced by 80% for extraction to air. The flux from PhC-MCLEDs in a radial lens was 15.7 mW at 20 mA and 36% external quantum efficiency was measured at 3 mA. High order diffraction was found to contribute significantly to the enhancements in efficiency and directionality. The experimental results are compared with FDTD simulations. Keywords: light-emitting diodes, photonic crystal, cavity, InGaN

Far-field radiation pattern of red emitting thin-film resonant cavity LEDs

AlGaInP thin-film resonant cavity light-emitting diodes (RCLEDs) show an improved performance compared to standard red emitting RCLEDs. External quantum efficiencies at 650 nm of 23% and 18% with and without encapsulation, respectively, have been obtained for devices showing a maximum emission in the normal direction. Thanks to the high angle-averaged reflectivity of the bottom hybrid mirror, a strong photon recycling effect occurs in these structures. The decrease of the absorption with increasing injection level reduces photon recycling and increases extraction of lateral guided modes. The redirection of part of the emission from the vertical to the lateral direction with increasing current density is reflected in the evolution of the far-field radiation pattern.