Johannes Baur

High-Power and High-Efficiency InGaN-Based Light Emitters

In this paper, we report on the latest advancements in improving AlGaInN-based visible-light-emitting-diode (LED) efficiency in epitaxy, chip, and package designs. We investigate the fundamental origin of the typical high current ¿droop¿ of efficiency observed in such LEDs. We show that this effect is most likely not caused by incomplete carrier injection or carrier escape but that it is rather a fundamental material property of InGaN/GaN-heterostructure-based light emitters. The droop can be reduced in improved epitaxial LED active-layer designs. We show how this can be achieved by lowering InGaN volume carrier density in multiple quantum wells (MQWs) and thick InGaN layers. Improved epitaxial MQW structures are then combined with a new advanced chip concept. It is optimized for high efficiency at high current operation and arbitrary scalability and can be manufactured at low cost. This is accomplished by improving light-extraction efficiency, homogenizing the emission pattern, reducing forward voltage, and lowering thermal resistance. The improved high current efficiency can be fully exploited by mounting the chip in the highly versatile new OSLON SSL package. It features very stable package materials, a small footprint, and an electrically isolated design decoupling electrical and thermal contacts.

Advanced technologies for high-efficiency GaInN LEDs for solid state lighting

Solid state lighting has seen a rapid development over the last decade. They compete and even outperform light sources like incandescent bulbs and halogen lamps. LEDs are used in applications where brightness, power consumption, reliability and costs are key parameters as automotive, mobile and display applications. In the future LEDs will also enter the market of general lighting. For all of these new applications highly efficient, scalable and cost efficient technologies are required. These targets can be matched by SiC based flip chip LEDs which enable the design of high current chips with efficiencies of up to 28 lm/W in white solderable packages. An alternative approach is the implementation of thinfilm technology for GaInN. The LED is fabricated by transferring the epilayers with laser lift off from sapphire to a GaAs host substrate. In combination with efficient surface roughening and highly reflective p-mirror metalization an extraction efficiency of 70% and wall plug efficiency of 24% at 460 nm have been shown. The chips showed 16 mW @ 20 mA with a Voltage of 3.2 V. The technology is scalable from small size LEDs to high current Chips and is being transferred to mass production.

InGaN on SiC LEDs for high flux and high current applications

We investigate the influence of chip size, substrate shaping and mounting techniques on the light extraction efficiency of large area InGaN-LED chips grown on 6H-SiC substrates. New techniques to achieve good light extraction for large chip areas are demonstrated and discussed. Applying these techniques to InGaN on SiC chips with 1 mm 2 size, we generate 150 mW of blue light and 33 lm of white light at a forward current of 350 mA. For efficient light extraction from the chip and for good thermal coupling the chip is soldered up-side down into a newly developed SMT package with a thermal resistance below 10 K/W.

First European GaN-Based Violet Laser Diode

We report on the realization of room temperature pulsed operation of GaInN multiple quantum well laser diodes. The devices were grown by organometallic vapor phase epitaxy on SiC substrates. Gain guided laser structures with a 8 μm wide resonator show a threshold current density of 17 kA/cm2. For decreasing stripe width the threshold current density increases due to decreasing overlap of electrically pumped area and the lateral extension of the optical wave. The devices were operated at temperatures up to 90 °C with characteristic temperatures of 200 and 290 K for emission wavelengths of 418 and 428 nm, respectively.

High-power InGaN LEDs : present status and future prospects

The ThinGaN® technology of OSRAM Opto semiconductors enables high power LEDs with wall plug efficiencies of currently up to 50%, enabling efficacies of > 100lm/W for white and green LEDs. The good scalability of the technology enables devices which deliver high luminous flux. The future limitations regarding efficacy of white LED can be estimated to be 150lm/W for high color rendering. Besides efficiency long term stability and high temperature capability are requirements for market adoption

GaInN LEDs: straight way for solid state lighting

With the new Generation of InGaN-based thinfilm Chips efficacies of 110/lm/W and output power of 32 mW at 20 mA (5 mm Radial lamp, 438nm, chip-size 255&mgr;m x 460&mgr;m) are reached. Due to the scalability of the ThinGaN concept chip brightness and efficiency are scalable to larger chip sizes: the brightness achieved for a 1 mm2 ThinGaN Power chip at 350 mA were 495mW (445nm) and 202mW or 100 lm (527nm). White LEDs with phosphorus achieved 102 lm at 350mA, mounted in an OSTAR module with six LED chips 1200 lm were demonstrated at 1000 mA driving current. White emitting automotive headlamp modules with 620lm (5x 1mm2 chip at 700mA) and 41 MCd/m2 as well as green emitting projection modules with 57 MCd/m2 at 2A/mm2 drive current and 12mm2 chip area are realized. These technological improvements demonstrate the straight way of GaInN-LEDs for Solid State lighting.

Recent progress in high efficiency InGaN LEDs

InGaN LED efficiency depends significantly on current density, emission wavelength and junction temperature. Phosphor-converted blue LEDs are still more efficient than RGB multi-LED solutions despite inevitable Stokes losses: 104 lm/W at 350 mA have been demonstrated for a warm-white (TC = 2950 K) phosphor-converted blue InGaN LED with 1 mm2 chip size.